Global Navigation Space Systems: reliance and vulnerabilities

Global Navigation Space Systems:
reliance and vulnerabilities
Global Navigation
Space Systems:
reliance and vulnerabilities
© The Royal Academy of Engineering
ISBN 1-903496-62-4
March 2011
Published by
The Royal Academy of Engineering
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London SW1Y 5DG
Tel: 020 7766 0600 Fax: 020 7930 1549
Registered Charity Number: 293074
Cover: A Lockheed Martin engineer checks out a GPS IIR spacecraft
(courtesy of Lockheed Martin)
A copy of this report is available online at
2 The Royal Academy of Engineering
Executive summary
GNNS overview
The range of applications
3.1 Some critical applications of GNSS
3.2 System-level criticalities
Vulnerabilities of GNSS services
4.1 System vulnerabilities
4.2 Propagation channel vulnerabilities
4.3 Accidental interference
4.4 Deliberate interference
Resilience to disruption of GNSS services
5.1 Position and navigation
5.2 Timing
5.3 Vulnerability mitigation
Conclusions and recommendations
6.1 Reliance on GNSS for PNT is high and increasing
6.2 GPS, Galileo, Compass and GLONASS common vulnerabilities
6.3 Recommendations
Annex A – Current and planned PNT applications using GNSS
Annex B – GNSS failure modes and characteristics
Annex C – Some commercial jammers
Annex D – Jamming trial example
Annex E – Acknowledgements
As technologies become easier to use and more cost effective their use can
become almost ubiquitous. If they present a more convenient solution to an old
problem, they can usurp older technologies very quickly, forcing obsolescence on
otherwise excellent technologies and taking a dominant position. The use of
Global Navigation Satellite Systems (GNSS) for deriving position, navigation and
timing (PNT) data is such a case. The Global Positioning System (GPS) is currently
the most widely used and best known example of GNSS.
Dr Martyn Thomas CBE FREng
Today, the relative ease of use of GPS in-car navigation systems means that many
motorists rely entirely on GPS for navigation and if they have a road map as a
back-up, it is not likely to have been used or updated in a long time. This is a trivial
example of reliance on GPS with neglect of back-up systems, but the use of GPS
signals is now commonplace in data networks, financial systems, shipping and air
transport systems, agriculture, railways and emergency services. Safety of life
applications are becoming more common. One consequence is that a surprising
number of different systems already have GPS as a shared dependency, so a
failure of the GPS signal could cause the simultaneous failure of many services
that are probably expected to be independent of each other.
The European Commission has estimated that, already, 6-7% of GDP in Western
countries, that is to say €800 billion in the European Union, is already dependent
on satellite radio navigation, so this study into our reliance on GNSS and potential
vulnerabilities is both important and timely. Such widespread use of GNSS
derived data within our economies means that the secure provision of PNT data is
now a matter of national security as well as a major economic asset.
Dr Martyn Thomas CBE FREng
Study Chairman
Global Navigation Space Systems: reliance and vulnerabilities 3
4 The Royal Academy of Engineering
Executive summary
Executive summary
In an ever more connected world, society’s reliance on high integrity positional,
navigational and timing (PNT) data is growing. The easy and cheap availability of
Global Positioning System (GPS) and other global navigation satellite systems
(GNSS) has meant that their use as primary sources of data can be found in an
increasing number of products and services. The range of applications stretch
from highly accurate surveying to in-car navigation, and from network
synchronisation to climate research.
The Academy’s study has identified an increasing number of applications where
PNT signals from GNSS are used with little, or no, non-GNSS based back-ups
available. The trend is for GNSS to be used in a growing number of safety of life
critical systems. Unfortunately, the integrity of GNSS is insufficient for these
applications without augmentation (see below). Non-GNSS based back-ups are
often absent, inadequately exercised or inadequately maintained.
The original implementation of GNSS, the US operated GPS comprises ground
based, space based and receiver segments, all of which are susceptible to failures
of various types. There are also some common mode failure mechanisms which
can affect whole classes of receiver or even the entire satellite constellation.
Photo: ESA - S. Corvaja 2008
The Soyuz-Fregat launch vehicle carrying
GIOVE-B on launch pad, 2008
A failure, or loss of signal due to some outside influence, can result in a range of
consequences depending on the application; in a telecommunications network,
a small loss in the efficiency of data handling may occur while the system
‘freewheels’ until a signal is restored: in a surveying application where timing is
not critical, some delays may occur before the survey can be properly completed.
In such applications, a temporary loss of GNSS signals might be considered an
inconvenience. However, where systems are used in safety of life critical
applications, the consequences can be more severe – in some situations, even if
operators are well versed in procedures for a loss of GNSS signals, the number of
interlinked systems simultaneously activating alarms can lead to eroded
situational awareness of operators in what could well be an emergency situation.
Some systems which rely on PNT signals from GNSS are robust in themselves and
procedures are in place to deal adequately with any GNSS based system faults
that occur. However, disruptive interference can occur unintentionally and, worse
still, deliberate interference is a real and growing possibility. As opportunities arise
for criminals to make money, avoid costs or avoid detection, it is known that
significant effort will be directed towards attacking GNSS based systems. The
banking infrastructure has already seen such an increase in high-tech attacks and
now devotes considerable time and expense to countermeasures.
Potential and already known mechanisms for deliberate interference include:
Jamming GNSS based vehicle tracking devices to prevent a supervisor’s
knowledge of a driver’s movements, or avoiding road user charging.
Rebroadcasting (‘meaconing’) a GNSS signal maliciously, accidentally or to
improve reception but causing misreporting of a position.
Spoofing GNSS signals to create a controllable misreporting of position, for
example to deceive tracking devices.
As the use of GNSS for revenue raising purposes increases through road user
charging or vehicle tracking, the prevalence of cheap jamming devices will
increase. Because the signal received at ground level from the GNSS satellites is
weak – it may be as low as -160dBW (1 x 10–16W) – jamming over a small area is
Global Navigation Space Systems: reliance and vulnerabilities 5
easily achieved and it is known that dedicated kit is already readily available for
purchase over the internet even though use of that equipment in the UK is illegal. In
the United States, monitoring for GPS signal anomalies is routine and the
occurrence of jamming incidents, both deliberate and accidental is growing. In the
UK, the Technology Strategy Board is supporting a project to establish a service to
verify the extent to which GNSS signals can be trusted by users.
We have therefore made a number of recommendations with the aims of (a)
raising awareness of the nature and magnitude of the issues; (b) proposing some
policy interventions that could reduce the risks; and (c) increasing the resilience of
services that rely on GNSS.
a) Raising awareness and analysing impact
Critical services should ensure that GNSS vulnerabilities are included in
their risk registers and that the risks are reviewed regularly and
mitigated effectively.
National and regional emergency management and response teams should
review the dependencies (direct and indirect) on GNSS and mitigate the risks
Services that depend on GNSS for PNT, directly or indirectly, should
document this as part of their service descriptions, and explain their
contingency plans for GNSS outages (say, of duration 10 minutes, 2 hours, 5
days, 1 month)
b) Policy responses
It is already illegal to place GNSS jamming equipment on the market in the
EU, as it cannot be made compliant with the EMC Directive. The Directive is
transposed into UK national legislation. The use of jammers is also a serious
offence under the UK Wireless Telegraphy Act 20061. Ofcom also has the
ability to close remaining loopholes by putting in place a banning order
under the 2006 Act which would prohibit import, advertisement and mere
possession of jammers. The case for this is easily justified given the clear
danger to safety of life services, which present a clear priority for Ofcom. We
recommend that Ofcom should introduce such a banning order, ideally in cooperation with other European legislators.
5. The Cabinet Office Civil Contingencies Secretariat should commission a
review of the benefits and cost-effectiveness of establishing a monitoring
network to alert users to disruption of GNSS services, building on the results
of the GAARDIAN and similar projects and the US experience with JLOC.
6. The Cabinet Office should consider whether official jamming trials of GNSS
Services for a few hours should be carried out, with suitable warnings, so that
users can evaluate the impact of the loss of GNSS and the effectiveness of
their contingency plans.
Widely deployed systems such as Stolen Vehicle Tracking or Road User
Charging should favour designs where the user gains little or no advantage
from the jamming of signals that are so important to other services.
8. The availability of high quality PNT sources is becoming a matter of national
security with financial transactions, data communication and the effective
operation of the emergency services relying on it to a greater or lesser extent.
6 The Royal Academy of Engineering
Executive summary
Greater cross-government coordination of science and technology issues
related to national security should explicitly recognise the importance of PNT,
treating it as an integral part of the operation of national infrastructure.
c) Increasing resilience
9. The provision of a widely available PNT service as an alternative to GNSS is an
essential part of the national infrastructure. It should be cost effective to
incorporate in civil GNSS receivers and free to use. Ideally it should provide
additional benefits, such as availability inside buildings and in GNSS blindspots. We are encouraged by progress with eLORAN in this context.
10. The Technology Strategy Board (TSB) and the Engineering and Physical
Sciences Research Council (EPSRC) are encouraged to consider the merits of
creating an R&D programme focused on antenna and receiver improvements
that would enhance the resilience of systems dependent on GNSS.
Global Navigation Space Systems: reliance and vulnerabilities 7
1 Introduction
The Royal Academy of Engineering became alarmed by a report2 in May 2009
from the United States General Accounting Office (GAO) which concluded that
the United States Air Force would have difficulty in maintaining its investment in
the GPS system and in launching new satellites to schedule and cost. The
importance of the GPS system to the US and global economies led the Academy
to the belief that, although the GAO has sounded alarm bells, it would be
politically unacceptable for the United States Government to allow the GPS
system fail or become degraded through lack of funding*.
Nevertheless, having concluded that the ongoing funding by the United States
Government of the GPS system was, for the time being, safe, the Academy
became concerned that the use of GPS for positional, navigational and timing
services had become highly ubiquitous, often with little or no consideration of
GPS independent back-up systems. In many cases, although GPS independent
back-up systems did exist, their use was not adequately exercised and users were
not always fully competent in their use.
The Academy, therefore, decided to carry out a study of GNSS, our reliance on
them and the consequences of failure or degradation of the GPS service or the
associated ground based systems. The study identified a number of threats to GPS
as it is currently configured. These range from catastrophic loss of the entire
system due to solar activity, bad data uploads to individual satellites or the entire
constellation through to local jamming or spoofing of signals or incompatibility
of older receivers following system updates.
When a technology such as GPS becomes so useful, so easy to use and universally
available, users tend to take the technology for granted and concern themselves
less with maintaining and practicing the use of alternatives. At a trivial level, this
means that many millions of us now routinely drive around the UK guided by GPS
navigation systems, but rarely carry a road map as back-up and, if we do, have
probably not invested in an updated road map for many years. Many, more safety
critical applications have inadequately exercised back-ups.
“It is estimated that, already, 6-7% of GDP in Western countries, i.e. € 800 billion in
the European Union, is dependent on satellite radio navigation.” - Report from the
Commission to the European Parliament and the Council, Mid-term review of the
European satellite radio navigation programmes dated 18 January 2011.
The accuracy, pervasiveness and convenience of GPS mean that its application
has moved far beyond navigation and the list of applications continues to grow.
The time signals from the GPS system in particular have found application in
managing data networks and mobile telephony. The consequences of failure
in these applications may only be a loss of efficiency, but the knock-on effects
of data congestion to the many users of these networks could cause
significant difficulties.
* The US GAO published a follow-up report in 2010 called GPS: Challenges in
Sustaining and Upgrading Capabilities Persist. The first IIF satellite was finally
launched on 27 May 2010, almost 3.5 years late. They conclude: “The IIIA
schedule remains ambitious and could be affected by risks such as the
program's dependence on ground systems that will not be completed until
after the first IIIA launch ... a delay in the launch of the GPS IIIA satellites could
still reduce the size of the constellation to below its 24-satellite baseline, where
it might not meet the needs of some GPS users.”
8 The Royal Academy of Engineering
1 Introduction
With the implementation of the European Galileo system, the resilience of the
combined GPS / Galileo system will be considerably improved, but many of the
categories of vulnerabilities identified in this report will remain.
The study working group considered that the techniques and equipment needed
to interfere with GNSS signals were readily available to anyone who cared to
research them. Deliberate interference in GPS signals is already a significant issue
with many instances of jamming for criminal purposes and at least one (in Korea)
for political motives3. With the planned introduction of Galileo, we expect the
system to be used for increasingly sophisticated revenue raising purposes, such as
road user charging. It is known that organised criminals are willing to invest
significant resources in exploiting system vulnerabilities when the potential rewards
justify it and we therefore expect a significant increase in attempts to exploit such
vulnerabilities over time. We conclude that deliberate interference in GNSS systems
will be an emerging threat that needs to be monitored and managed.
During the course of the study our concerns over redundancy and resilience were
exacerbated when we became aware that the United States Government had
closed its LORAN-C stations. Considerable research and development has been
undertaken by the General Lighthouse Authorities and others on an enhanced
LORAN (eLORAN) which has been demonstrated to provide a robust alternative to
satellite based systems, but the funding of the eLORAN system is not yet secure.
Photo: ESA - P. Müller
GIOVE-B arrives in the HPF
As part of the study, the working group examined the likely future developments
of GNSS both in terms of the satellite systems themselves, the receivers and the
uses they are put to. At each stage, threats to the continued use of GNSS both
locally and globally were examined. Threats were generally categorised as isolated
system failures, global system failures and external threats which, except in the
case of solar activity, were localised.
Global Navigation Space Systems: reliance and vulnerabilities 9
2 GNSS overview
Global Navigation Satellite Systems (GNSS) is the generic term for space-based
systems that transmit signals that can be used to provide three services: Position,
Navigation, and Timing - known collectively as PNT. The best known and most
popular of the GNSS is the US Global Positioning System (GPS), although the
Russian GLONASS system is regaining its strength and other systems are being
developed, most notably Galileo in Europe and Compass in China. The systems all
work in approximately the same way so only a description of GPS follows for brevity.
GPS can be split up into three areas, the ground, space and user segments.
The ground, or control, segment is used to upload data to the satellites, to
synchronize time across the constellation and to track the satellites to enable
orbit and clock determination.
The space segment consists of the GPS satellites in six orbital planes. 24
satellites make a full constellation, although there are currently (January 2011)
32 in service, 2 of which have been declared unusable until further notice.
The satellite’s code is used to identify it in orbit (it should be noted that this is
the fundamental difference between GPS and GLONASS which differentiates
satellites by frequency channel).
The user segment consists of the receivers and associated antennas, used to
receive and decode the signal to provide PNT information.
GPS is a ranging system with three available carrier frequencies, all multiples of a
fundamental frequency (Table 1). The distance is derived primarily through
measuring the time difference between the transmission from the satellite and
reception at the receiver of a coded signal. This range is more properly known as the
pseudorange since it is affected by a number of system unknowns including clock
biases and propagation delays which must be solved for or estimated. The carrier
phase of the signals can also be used to derive the range, providing for a more
accurate position fix, but with inherent ambiguity. Ranges to at least four satellites
are required to determine position and time. (Timing applications can function with
a single satellite in view, although for verification reasons, two are preferred.)
L1 1575.42 MHz
L1 1602.00 MHz
L2 1227.6 MHz
L2 1246.00 MHz
E5A 1176.45 MHz
L5 1176.45 MHz
1575.42 MHz
1191.795 MHz
E5B 1207.14 MHz
1278.75 MHz
Table 1: GNSS RF Carrier Frequencies
The navigation message is transmitted from the satellite to the user and gives the
satellite identifier together with information on satellite health, predicted range
accuracy, ionospheric and clock correction coefficients as well as orbital
ephemeris to allow the receiver to calculate the satellite position. The message
also contains an almanac which gives status, location and identifier information
for all satellites in the constellation.
Errors or biases occur within GPS as a matter of course and are independent of
the interference or denials of service outlined within this report (Table 2).
10 The Royal Academy of Engineering
2 GNSS overview
Error Source
Satellite orbit
Orbital biases occur within the ephemeris transmitted,
mostly as a result of un-modeled gravitational forces.
Satellite clock
The satellite clocks experience drift and noise which are
modeled and included as part of the broadcast message,
although residual error remains.
Ionosphere and
The signals are delayed in the region above an altitude
of 80km by an amount proportional to the number of
free electrons. The effect is lower when the satellite is at
the zenith than when it is near the horizon and it is
frequency dependent. Uncorrected this is the largest
error source.
Delay in the signal caused by varying temperature and
humidity levels at up to 12km in height. Basic models
can correct up to 90%.
Receiver noise
the signal.
Inherent noise within the receiver which causes jitter in
In addition to the direct satellite-to-receiver path, the
signals are also reflected from the ground and other
objects. These cause multiple copies of the signal or a
broadening of the signal arrival time both of which
reduce precision.
Table 2: Primary GPS system error sources.
In normal standalone operation, GPS will give a three-dimensional position
accuracy of around 5-10m, and also provides velocity to approximately 20 cm/s
and time to within 1 microsecond. These accuracies are dependant on the user
equipment, error sources present and the configuration of the satellites that are
being tracked. If the satellites tracked are all in one portion of the sky for example,
the geometry is poor and attainable accuracy will be affected.
Different GPS applications require varying degrees of positional accuracy. In-car
and personal navigation, for example, require only the standard GPS positioning
accuracy, whereas more demanding applications require augmentation of the
standard GPS data, be it in terms of integrity or correction information. Differential
GPS (DGPS) is widely used to improve upon GPS accuracies. Here corrections to the
pseudoranges (and / or carrier phases) are computed to improve the positional
accuracy of a user’s GPS receiver. DGPS corrections can be applied in either post
processing or real-time. DGPS can generally improve positional accuracy to
between a metre and a centimetre depending on the signals used, user equipment
and methodologies adopted. Some examples of DGPS services are:
The Wide Area Augmentation System (WAAS) was developed as an air
navigation aid by the Federal Aviation Administration to improve GPS
accuracy, integrity, and availability. WAAS uses a network of ground-based
reference stations to monitor the GPS satellites signals, and geostationary
satellites to transmit information to users.
The European Geostationary Navigation Overlay Service (EGNOS) is a satellite
based augmentation system developed by the European Space Agency, the
Global Navigation Space Systems: reliance and vulnerabilities 11
European Commission and EUROCONTROL. It is intended to supplement GPS,
and potentially GLONASS and Galileo by providing integrity messages,
corrections and additional ranging signals.
The General Lighthouse Authorities’ DGPS service is a network of 14 groundbased reference stations providing corrections to GPS via MF radio
transmissions out to at least 50 nautical miles from the coast around the
United Kingdom and the Republic of Ireland.
Ordnance Survey runs the national OS Net GNSS infrastructure of 110 base
stations. OS Net provides free GPS and GLONASS data products as well as a
commercial high accuracy DGPS service.
Augmentation can also take the form of a data service enabling fast acquisition,
where the receiver is sent orbital and timing information to enable almost
immediate tracking when the receiver is switched on. This technique is one form
of Assisted GPS. High-sensitivity GPS receivers exist and are used in difficult
environments, for example to aid tracking of the very weak signals indoors.
The next five years will see a massive boost to GNSS with the introduction of
satellites from up to four systems, plus the extra, more powerful, signals being
added to GPS and GLONASS. 100 plus satellites could be available to the user.
Apart from very low-cost applications, GPS only receivers will probably become a
thing of the past. This will be partly driven by the plan to move GLONASS from
frequency to code division multiple access, and partly driven by the development
of the Galileo and Compass systems. Multi-GNSS tracking will deliver improved
availability, accuracy and integrity. There are some concerns, however, that the
increased number of signals will raise the level of noise leading to lower resilience
against other sources of interference.
12 The Royal Academy of Engineering
3 The range of applications
Photo: ESA
3 The range of applications
The free availability and accuracy of the GNSS signals for location and timing,
combined with the low cost of receiver chipsets, has made GNSS the chosen
solution for a very wide and growing range of applications. These include transport
(rail, road, aviation, marine, cycling, walking), agriculture, fisheries, law enforcement,
highways management, services for vulnerable people, energy production and
management, surveying, dredging, health services, financial services, information
services, cartography, safety monitoring, scientific and environmental studies,
search and rescue, telecommunications, tracking vehicles and valuable or
hazardous cargoes, and quantum cryptography. Some applications are described
below; a fuller list of applications is contained in Annex A.
ESA expert inspecting GIOVE-A in clean room
At present, road transport applications are the majority users of GNSS signals, for
in-car navigation, commercial fleet management, taxi services, public transport
monitoring and passenger information, and emergency vehicle location, dispatch
and navigation. GNSS-based road user charging schemes have been introduced
in other countries and are under consideration in the UK.
In aviation, most commercial aircraft now use GNSS for en-route navigation and
several States have licensed GNSS for initial approach and non-precision
approach to specified airfields. Automatic Dependent Surveillance – Broadcast
(ADS-B) is increasingly used in areas of the world where there is no radar
coverage; this involves aircraft calculating their position using GNSS and other
sources and broadcasting it to other aircraft.
Maritime applications include ocean and inshore navigation, dredging, port
approaches, harbour entrance and docking, vessel traffic services (VTS),
Automatic Identification System (AIS) hydrography, and cargo handling.
Railway applications include the management of rolling stock, passenger
information, preventing doors opening unless they are alongside the platform,
cargo tracking signalling, train integrity and level crossing approach.
The use of GNSS for navigation of civil unmanned vehicles includes Unmanned
Aerial Vehicles (UAVs), and autonomous land vehicles (from lawnmowers to
agricultural machinery).
Scientific applications of GNSS are widespread and include surveying,
environmental and atmospheric monitoring, animal behaviour studies, botanical
specimen location, meteorology and climate research.
GNSS are used in agriculture and fisheries for land area mapping, yield
monitoring, precision planting and spraying, autonomous vehicle control and to
monitor fishing limits.
Security applications include tracking of vehicles and valuable cargoes, and
covert tracking of suspects.
GNSS timing is important for telecommunications applications. Synchronous
technologies are much more efficient than asynchronous technologies but require a
time source with appropriate accuracy, stability and reliability to operate effectively
or at all, and GNSS can provide this. While ground-based clocks are accurate enough
for this purpose (especially with the availability of chip scale atomic clocks (CSAC)),
the synchronisation of many such clocks is problematic. GPS allows the derivation of
synchronised UTC through resolving the signals from a number of satellites at a
known position. Only a ‘good guess’ of the current time is required and quartz clocks
have therefore been adequate for this process making synchronous time keeping
significantly more cost effective.
Global Navigation Space Systems: reliance and vulnerabilities 13
The use of time can be split into three clear and separate aspects: frequency
control, time of day and common epoch (usually UTC) time slot alignment (also
known as ‘Phase’).
Stability of radio communications transmission, constant digital traffic flow, time
slot alignment and traditional services over next generation Ethernet based
infrastructure are some of the features that good time and timing bring to
communications networks.
Financial systems increasingly need precise time stamping to prioritise trades and
to provide an audit trail.
3.1 Some critical applications of GNSS
In road transport, emergency vehicle location, dispatch and navigation require
medium availability and accuracy. Future applications such as automated
highways and lane control will need very high availability, integrity and accuracy.
In aviation, search and rescue already uses GNSS, and control of the movement of
aircraft and other vehicles on airports and precision landing approaches are
being considered.
Safety of Life maritime applications include search and rescue, synchronisation of
flashing navigational aids, and navigation in crowded waterways under low
visibility conditions.
The Rail Safety and Standards Board forecasts that GNSS will be in use this decade
for railway signalling and train movement control and monitoring.
The police use GNSS for tracking suspects and generating evidence to use in
prosecutions and for situational awareness for armed response units.
Annex A contains a more complete list of existing and planned GNSS applications
together with their required PNT accuracy.
3.2 System-level criticalities
The wide range of applications dependent on GNSS signals provides many ways
in which seemingly unrelated services could fail simultaneously as a result of
disruption to the GNSS signals resulting from the vulnerabilities described in the
next section of this report. Dependence on GNSS connects many otherwise
independent services to form an accidental system with a single point of failure.
Erroneous GPS signals in an urban area could cause road accidents whilst
disrupting the dispatch and navigation of emergency vehicles and causing their
communications systems to fail. At sea in fog or at night, jamming could cause
collisions between ships or with obstructions whilst causing emergency beacons
to broadcast false positions, delaying search and rescue.
No-one has oversight of the increasing range of services that are dependent on
GNSS or the complex ways in which they interact, and therefore no-one can
reliably fully predict the consequences of a significant disruption of GNSS signals.
This is an unsatisfactory situation for important services that are dependent on
GNSS. This can be addressed through analysis of the effect of different types of
disruption to the service, then making the decision between investing in
detection, adding extra robustness or supplementing with an independent
source of PNT data that has been shown to be unaffected by the types of GNSS
failure that cause concern. In carrying out such an analysis, it will be important to
consider whether the service that is being analysed forms part of a larger service
and whether GNSS failures might simultaneously disrupt other services that are
relied on to provide resilience.
14 The Royal Academy of Engineering
4 Vulnerabilities of GNSS services
4 Vulnerabilities of GNSS services
All GNSS are vulnerable to failure, disruption and interference, and much work has
been done to assess the possible failure modes and their effects on services, and
to develop strategies to detect failures and correct them. One important analysis
of possible failures in GPS dependent systems is given in Annex B.
The vulnerabilities of GNSS can broadly be classified into three different
1) System related (including signals and receivers).
2) Propagation channel related (atmospheric and multipath).
3) Interference related (accidental or intentional).
GNSS have system-level vulnerabilities: GPS satellites have on rare occasion
broadcast dangerously incorrect signals, a reduced number of satellites visible
could prevent availability of a position fix, and GNSS receivers can incorrectly
process valid signals to give unpredictable results.
GNSS signals are very weak: typically less than100 watts transmitted from a
distance of 20,000 km to 25,000 km. When received at the surface of the earth,
the signal strength may be as low as –160 dBW (1 x 10–16 ) watts, with a spectrum
spread out effectively below the noise floor in the receivers. Deliberate or
unintentional interference with this signal can easily defeat the signal recovery or
overload the receiver circuitry.
Furthermore, signals are vulnerable to disruptions in the atmospheric medium
they pass through, and receivers can also unintentionally lock onto reflections of
the signals, known as multipath, giving unexpectedly large errors.
These causes can have quite different effects on users, such as partial or complete
loss of the positioning and timing service, poorer accuracy, very large jumps in
position, velocity or time, and ‘hazardously misleading information’ (HMI) that is
to say, believable data that is dangerously wrong in safety critical applications.
4.1 System vulnerabilities
Each GNSS has three segments: ground, space and user, and each of these
segments have vulnerabilities to problems or failures.
Ground and satellite segment vulnerabilities
The ground segment is responsible for maintaining the system time, controlling
the satellites, uploading navigation data (Clock prediction data and Almanac and
Ephemeris orbit prediction data) that will be broadcast to users from the satellites,
and monitoring the signals broadcast across the globe. The satellites carry clocks,
signal generation units and amplifiers and antennas to broadcast the signals,
modulated with the navigation data.
These systems are designed with high reliability in mind, and in the case of GPS,
the ground segment and satellites are designed to be resistant to military attack,
but nevertheless there are vulnerabilities, including the following:
Too few satellites – The US Government Accountability Office has identified
a long term risk of a shortage of GPS satellites due to potential simultaneous
failure of old spacecraft and late delivery of next generation satellites4. As the
number of satellites drops down to or below the specified minimum of 24
Global Navigation Space Systems: reliance and vulnerabilities 15
satellites, users would experience a reduction in service, more position
outages and periods of worse accuracy due to less favourable satellite
geometry. Although the original GAO report was perhaps overly pessimistic
and the warnings have been toned down in the more recent version, it
highlights how the availability of satellite navigation requires long-term
political and financial commitment from the responsible governments, and
careful programmatic management. An example of where such
governmental commitment became unsustainable was the temporary
decline of GLONASS in around 19995. Although the GLONASS constellation
has since largely recovered, all GNSS have a long-term dependence on stable
financing and good management.
Upload of bad navigation data – Pages of navigation data are uploaded to
GPS satellites in advance of applicability, including the clock predictions and
precise orbit predictions (ephemeris) required to achieve full position
accuracy. If a page of bad data is uploaded to a satellite, the clock and
position knowledge of that satellite may suddenly be in error when the page
becomes applicable (typically the Ephemeris set is changed every 2 hours), or
the error could grow slowly as the time from epoch of applicability increases.
Examples of bad GPS data uploads, fortunately without too serious
consequences, occurred in June 2002, March 2000 and March 19935. A
feasible but extremely unlikely scenario is that bad data is uploaded in
advance such that the whole constellation simultaneously transmits
erroneous navigation data causing all GPS receivers to fail across the world.
Jump or drift of clock on satellite – GNSS depends on the predictable
performance over 48 hours of precise atomic clocks carried onboard the
satellites. On occasion, these clocks behave unpredictably, and produce
errors that grow slowly in a potentially dangerous way before the operators
can spot it and mark it as unhealthy. On 1st January 2004, the clock on GPS
satellite SVN-23 drifted for 3 hours before the command centre marked it
unhealthy, by which time the range error had grown from 0 to 300 km6.
Another similar case happened in July 20017.
Bad signal shapes – If there is a fault in the signal modulation or generation
process on a satellite, an unusual signal envelope can be transmitted that
causes unpredictable behaviour in receivers, potentially causing dangerous
errors in some cases, while possibly undetectable in others, subject to signal
tracking loop implementation. Two examples are the so-called ‘evil waveform’
produced by GPS satellite SVN 19 in 1993 that caused an error of up to 8
metres8, and GPS SVN 49 launched March 2009 which carried a piggy-back L5
signal generator unfortunately causing an onboard multipath effect on the
main signals, and resulting in variable, sometimes substantial errors in
receivers at different elevation angles9.
Service interruption or loss of satellite due to orbital environment –
GNSS satellites pass through the radiation belts around the Earth, and are
consequently subject to intense radiation. When solar storms occur (flares,
Coronal Mass Ejections (CMEs), etc.), the satellite is subjected to bursts of
highly energetic and disruptive particles. These effects are mitigated by the
spacecraft design and are not normally a cause for concern, although they
reduce the satellite lifetime. Unusually intense events could, however, cause a
temporary shut down of the satellites. In the worst case, if a super-storm, or a
so-called Carrington event occurs, multiple satellites could be disabled
causing major widespread disruption on the ground. Such an event has not
occurred since the advent of satellites and although unlikely, it cannot be
ignored as a potential threat to society.
16 The Royal Academy of Engineering
4 Vulnerabilities of GNSS services
Attack on ground segment – The GPS ground segment is designed to
withstand military attacks, but could feasibly still have some vulnerabilities to
terrorist or cyber attacks. As more GNSS systems become available, there are
more targets available to attack (remote monitoring stations could be
especially vulnerable) although relative independence of the systems
provides some protection.
GNSS user segment vulnerabilities
Unlike the ground and space segment, the GNSS user segment is extremely
diverse and uncoordinated, comprising all GPS receivers. These include
decryption-capable military receivers, certified safety-of-life aviation receivers,
scientific survey receivers and receivers embedded in mass-market mobile
phones, and numerous others. Different manufacturers design GNSS receivers to
different levels of quality, but even the simplest GNSS receiver is a highly complex
mixture of radio and digital hardware and software. The common thread is that all
receivers are designed to interface with the broadcast GNSS signals, as
documented in the ‘Signal-in-Space Interface Control Document’ (ICD) definition,
for example GPS Open Service10. This diversity means that systematic
vulnerabilities in GNSS receivers will not affect all users, but still could affect one
particular user sector badly, possibly globally, where receivers from one
manufacturer with a latent software bug are used widely. Vulnerabilities could
include the following:
Leap seconds and roll-overs – Generally the correct ongoing function of a
GPS receiver can be tested and verified by manufacturers and users within
the course of hours or days. GPS, however, is subject to events which occur
rarely and may not be accounted for properly and only discovered after the
event has occurred. GPS had a numerical roll-over in August 199911 that
upset some receivers, and also the derivation of UTC from GPS time is subject
to leap seconds once every few years that must be correctly handled.
Although the ICD200 describes the correct way to handle leap seconds,
some receivers still handle them wrongly and can cause a timing error.
System upgrades – Even though GPS has been operating stably for at least
the last decade, system upgrades do occur and can cause unexpected
behaviour in receivers due to ambiguities in the ICD or mistakes by
manufacturers. In April 2007 a 32nd GPS satellite was added to the
constellation which caused problems in some receivers only designed to
handle 31 satellites. In January 2010, an upgrade to the GPS ground segment
software caused problems with military and timing receivers12 believed to be
connected with acquisition and signal lock behaviour.
Receiver bugs – In some sectors such as military and aviation, the design
requirements for a GNSS receiver may be prescriptive and demand
certification testing. In other sectors, there are no external requirements to
meet, and units need only pass the manufacturers’ production test regime. It
is unlikely any such receiver would be completely free from software bugs
that might somehow affect the performance of the unit. There are particular
system circumstances where bugs might be unearthed, such as the handling
of satellites labelled unhealthy, the behaviour of receivers when tracking nonstandard codes, and the behaviour when signals are stronger or weaker than
usual. Depending on the design and configuration of the receiver, different
behaviour may be expected if one satellite is in error, or a jamming signal
causes a jump in one range measurement, but not others. Receiver future
proofing is also key to ensure that the firmware is capable of handling
changes in the GNSS system.
Global Navigation Space Systems: reliance and vulnerabilities 17
Overlay vulnerabilities
In time, users will become increasingly dependent on the various overlay systems
that are required to enhance the performance of GNSS, which potentially
introduces more vulnerability into the system.
Space based augmentation systems – The SBAS systems (EGNOS, WAAS,
etc.) are likely to become vital to aviation and other users who need high
integrity for non-precision approach to landing. SBAS is dependent on a small
number of geostationary communication satellites that carry a piggy-back
navigation payload. When these satellites fail, it may mean the loss of
availability13. SBAS is not operated as a military system, and may have more
vulnerability to attack. SBAS signals are just as vulnerable to ionospheric
disruption as GNSS signals.
AGPS – The use of external Ephemeris data to speed the acquisition of a GPS
receiver (Assisted GPS) brings in new vulnerabilities from the availability and
quality of the assistance information. A bad Ephemeris data-set from AGPS
could make a GPS receiver yield hazardously misleading information, which
could in future have widespread effects.
4.2 Propagation channel vulnerabilities
Atmospheric vulnerabilities
For ground based users the GNSS signals must pass through the atmosphere,
which causes a variety of generally deleterious effects. If the atmosphere were
invariant in time and space due allowance could be made, but unfortunately the
atmosphere is highly variable over many temporal and spatial scales.
The lowest and densest region of our atmosphere, containing our weather
systems, is called the troposphere. Here weather systems can affect GNSS signals
causing modest variations in signal delay which can be largely mitigated using a
model. Hence the troposphere introduces an error term, rather than a threat.
The ionized upper region of the atmosphere known as the ionosphere can also
cause disruptions to GNSS signals and if uncorrected introduce the largest errors
in GNSS. The variability of the ionosphere, especially at high and low latitudes and
at times close to the peak of the sunspot cycle can be highly problematical. This
variability of the ionosphere is one manifestation of space weather. (This is a
general term describing variations in the sun, solar wind, magnetosphere,
ionosphere, and thermosphere, which can influence the performance and
reliability of space-borne and ground based technologies, and can also endanger
human health and safety.) The atmospheric GNSS vulnerabilities include:
Slow variations in total electron count (TEC) in the ionosphere – The
signal from satellite to ground is delayed in proportion to the total electron
count along the signal path and has to be removed or calibrated out.
Variations in TEC are caused by the sun’s effects on the ionosphere, albeit
indirectly. The Earth’s diurnal rotation, the sun’s 27 day rotation and the sun’s
activity cycle of 11 years each cause the electron count to rise and fall.
Superimposed on these cyclic variations are less easily characterised tidal and
other variations. These all introduce a slowly varying error in GNSS range
measurements, which can be mitigated using model estimated corrections,
DGPS corrections or more completely using dual frequency GNSS receivers.
Fast variations in total electron content - Indirectly, these occur as a result
of solar flares and CMEs. The resultant ionospheric gradients cannot always
be corrected by SBAS and other differential systems. SBAS can detect that this
18 The Royal Academy of Engineering
4 Vulnerabilities of GNSS services
has occurred (integrity) and consequently there are no safety critical issues
due to erroneously relying on GNSS, though SBAS signals can also be
disrupted by ionospheric effects. Mitigation is still possible through the use of
dual frequency GNSS receivers.
Scintillation in ionosphere - Small scale perturbations in the ionosphere
can cause the GNSS signals to break up causing signal multipath. GNSS
receivers see rapid variation in both signal phase and amplitude and if not
designed robustly, GNSS receivers can lose signal lock. Scintillation most
often occurs over the equator and near the poles, but can occasionally be
more widespread. The WAAS network was disabled for 30 hours during the
solar storm in Oct/Nov 200314. More disturbances are predicted through
2010-2013 as the solar cycle reaches its peak.
Carrington events – If a large CME aligns with the Earth, a super-storm could
arise with a major and long enduring impact on space and ground electronic
infrastructure. The last such event was in 1859, and experts suggest that the
probability of such an event is of the order of once in 200 years.
Deliberate modification of ionosphere – Nuclear explosions in the upper
atmosphere can cause similar effects to solar storms, potentially affecting the
operation of GNSS for weeks due to propagation anomalies. More particularly,
these events would put the satellites at high risk of malfunction through
being immersed in a high energy particle environment.
Multipath vulnerabilities
Northern Lights over Tromsø, Norway.
The Northern Lights can become much
stronger and be visible at much lower
latitudes than normal during periods of
high solar activity, particularly, solar storms.
Multipath describes the situation when a receiver picks up reflected signals as well
the normal signals direct from the satellite. GNSS signals can reflect off relatively
distant objects, e.g. buildings, and cause gross errors in position accuracy if the
receiver falsely locks onto this reflected signal instead of the direct signal. More
subtle errors are caused when the reflective objects are closer and direct and
reflected signals merge together, causing lower precision code and carrier phase
measurements. Multipath is ubiquitous and a very well known phenomenon to
scientists and receiver manufacturers who have introduced all kinds of measures in
receivers to mitigate the effects, such as multipath rejecting antennas, receiver
filtering and processing techniques. Multipath can still sometimes cause surprising
errors of tens to hundreds of metres to unprepared users, and it is also one of the
most significant barriers to future adoption of autonomous car navigation using
GNSS. Multipath can be significantly reduced through antenna and receiver filtering
and processing techniques. Nevertheless, multipath is considered in this study as
more of a characterisable error source than a threat to GNSS users.
4.3 Accidental interference
Harmonic emissions from commercial high power transmitters, ultra wideband
radar, television, VHF, mobile satellite services and personal electronic devices can
interfere with the GNSS signals. Depending on the magnitude and frequency of
the transgressing signal, there may be complete loss of lock by receivers. An
example of this effect was noticed in 2002 when a poorly installed CCTV camera
in Douglas, Isle of Man, caused GPS within a kilometre to be blocked.
One specific form of accidental jamming may occur when a (typically old) GPS
antenna rebroadcasts the signal on account of poor impedance matching in the
amplified signal path from the low noise amplifier, and this interferes with
reception in an adjacent antenna. To avoid this risk, antennas should be mounted
as far apart as possible and never closer than about 2m.
Global Navigation Space Systems: reliance and vulnerabilities 19
4.4 Deliberate interference
There are three distinct forms of deliberate interference with GNSS signals:
jamming, spoofing, and meaconing.
Jamming is the most likely activity which will impact a conventional industrial use
GPS system. Meaconing (delaying and rebroadcasting) and spoofing (false signal)
which effectively rebroadcasts erroneous satellite ephemeris are currently less
common than jamming, although as mentioned above accidental meaconing could
be caused by the proximity of a GPS antenna with poor impedance matching.
The crudest form of jamming simply transmits a noise signal across one or more
of the GNSS frequencies, to raise the noise level or overload the receiver circuitry
and cause loss of lock. Circuits and assembly instructions for GPS jammers are
widely available on the internet, and commercial jammers can be bought for less
than £20. Commercial jammers are increasingly sophisticated: some are designed
to fit into a pocket, some into car lighter sockets; most jammers are designed to
block GPS, GLONASS and GALILEO (even before GALILEO is operational), others
incorporate jamming of all cellphone frequencies as well, using multiple
antennas. Jammers have been recovered by UK police, examined forensically, and
shown to be very effective.
Powerful jammers are also commercially available, up to at least 25W transmitted
power. A selection of pictures of commercial jammers from internet websites is
shown in Annex C.
Noise jamming can be overcome to some degree by adaptive antennas and
noise filtering in well-designed receivers, but some jammers are now transmitting
GNSS codes rather than noise, to bypass the filters.
Commercial receivers behave unpredictably in areas where there is noise
jamming. Trials by DSTL and Trinity House have shown receivers giving false
information rather than reporting an error: sometimes the errors are too large to
be misleading (ships’ positions shown to be many miles inland, with speeds
approaching Mach 1, for example) but there have also been many instances of
hazardously misleading information (HMI) where vehicles’ positions are offset by a
few tens or hundreds of meters, and courses and speeds are incorrect by a few
degrees and a few knots. The consequences of HMI could be serious if, for
example, ships are navigating in low visibility and broadcasting their (GNSSderived) position to other vessels.
A report on a Trinity House / DSTL jamming trial is reproduced in Annex D.
Jamming can be split into 4 broad areas: Accidental, Criminal, Red Team
Deliberate and Blue Team Deliberate (where Red Team is a generic term for
‘enemy/criminal’ and Blue Team for ‘friendly forces’).
Accidental Jamming is most likely to be caused by harmonics from other RF
signals which sit on the weak GPS signal-from-space. This will typically be
localised and potentially manageable once identified. This may be mitigated by
moving the GPS antenna to screen out the problem and is less likely to be an
issue for a GPS timing system.
Criminal Jamming is caused by people who are looking to defeat GPS tracking
systems. They may be car thieves, road toll evaders, tracker evaders, drivers
seeking to avoid commercial mileage limits or to avoid their bosses’ knowledge of
their movements, etc. This will typically be indiscriminate and both moving and
stationary. It may be fairly low power just to defeat the localised vehicle location
20 The Royal Academy of Engineering
4 Vulnerabilities of GNSS services
system but the car thief is unlikely to be concerned with managing power levels
to minimise impact on additional nearby GPS reception. This is unlikely to be a
problem for most GPS applications if the jamming event is of short duration
and localised.
Red Team Jamming (e.g. Terrorist) is deliberate and may be targeted at some
specific aspect of critical infrastructure, possibly but not necessarily timing
systems. It will be indiscriminate, more likely to be high power and may occur at a
number of locations simultaneously. This is more likely to be a problem and the
impact will be dependent on the back-end filtering and antenna design.
Blue Team Jamming is deliberate – generally to defeat a perceived threat of
covert tracking. It will probably be low power and have a similar impact to
criminal jamming. However there would be an impact if they parked for long
periods near critical infrastructure which used GPS timing – for example a
TETRA** base station.
** Terrestrial Trunked Radio (TETRA) is a digital trunked mobile radio standard
developed by the European Telecommunications Standards Institute (ETSI). The
purpose of the TETRA standard was to meet the needs of traditional Professional
Mobile Radio (PMR) user organisations such as Public Safety, Transportation,
Utilities, Government, Military, and Oil & Gas companies.
Global Navigation Space Systems: reliance and vulnerabilities 21
5 Resilience to disruption of GNSS services
5.1 Position and navigation
Many strategies are available for checking, enhancing or replacing the position
and navigation services provided by GNSS. Many of these are usable only in a
limited range of applications: for example, determining position by clock and
sextant is usable on the open sea, but not realistic for train protection or control
of agricultural machinery.
Alternatives include map or feature matching, inertial navigation (INS),
odometry/pedometry, dead reckoning, radionavigation beacons
(NDB/VOR/DME), eLORAN, and triangulation from cellular telephone, radio or
television transmitters.
Enhanced LORAN (eLORAN) is designed to provide PNT that is accurate enough
for most applications and guaranteed to be independent from GNSS.
The topology of cellular phone masts and other communications transmitters has
not been designed to provide convenient triangulation, and coverage is very
variable. Research by the General Lighthouse Authorities has concluded that such
signals cannot provide PNT of sufficient coverage and accuracy to provide
adequate backup for marine purposes in the event of loss of GNSS PNT.
5.2 Timing
Back-end filtering is a critical aspect of a GPS timing receiver which will define
susceptibility to jamming and interference. To simplify this element – consider
four types of oscillator which are effectively used as flywheels, Temperature
Compensated Crystal Oscillators, (TCXO), Oven Controlled Crystal Oscillators
(OCXO), Rubidium (Rb) atomic clocks and chip scale atomic clocks (CSAC).
TCXO† – Temperature Compensated Crystal Oscillator is a low cost (cents/pence)
component. The TCXO forms part of a phase locked loop or the basis for a
numerically controlled oscillator to offset or compensate for inherent aging and
offset. The TCXO will track the GPS off-air signal but will have no ability to ‘holdover’ in the event of loss of GPS signal. Recent developments in back end signal
processing means that very low cost GPS timing systems fit for purpose
in terms of instantaneous stability are available as super-components for a
few pounds.
OCXO‡ – Oven Controlled Crystal Oscillator is a more expensive oscillator (£10s£100s) with various levels of hold-over stability – usually more stability is more
expensive. Like the TCXO the OCXO forms part of a phase locked loop or the basis
for a numerically controlled oscillator to offset or compensate for inherent aging
and offset but considerably less than the TCXO. The OCXO will track the GPS offair signal and will have reasonable hold-over performance (of the order of hours
depending on stability specification) in the event of loss of GPS signal.
22 The Royal Academy of Engineering
Quartz varies in frequency with temperature. This variation is typically linear at
ambient temperatures and a TCXO makes use of this property to compensate
for accuracy based on knowing the temperature.
At higher temperatures e.g. ~80ºC the variation with temperature flattens off.
Putting the quartz resonator into a single or double oven minimises the impact
that external temperature changes have on the frequency stability.
5 Resilience to disruption of GNSS services
Rb Atomic – Rubidium Atomic Oscillator is a much more expensive oscillator
(£1000+) and forms the heart of major telecom network timing infrastructure. Rb
brings considerable improvement in holdover - moving the ability of the
infrastructure to withstand GPS outage from days to months.
CSAC - Chip Scale Atomic Clocks are just emerging from the R&D labs with first
commercial deliveries recently announced. These will offer stabilities better than
Rb along with power consumption less than a tenth and in a package footprint
smaller than the current miniature Rb oscillators. This represents a clear paradigm
shift in technological innovation and this is a technology to watch since it
promises an ability to withstand outages for many months.
In terms of mitigation, one can also consider the use of two oscillators in a 1:1
resilient architecture (usually OCXO and Rb) and then managing the mean time
to repair (MTTR) to under 24 hours in the event of failure to ensure that system
failures are extremely unlikely§.
1.50 hours/div
Figure 1 - Comparison of time error in holdover between TCXO (cyan), Low Stability OCXO
(Green), High Stability OCXO (Magenta) and Rb (Blue) based GPS timing receivers.
5.3 Vulnerability mitigation
Many receivers incorporate Receiver Autonomous Integrity Monitoring (RAIM).
Where more than four satellites are visible and usable, a RAIM-equipped receiver
will use the additional signals to calculate pseudoranges that should be
consistent with those already calculated. In this way, the receiver can detect and
report a problem with a satellite or with the signal integrity. If six or more satellites
are available, the receiver may be able to exclude a faulty signal from its
calculations and to continue to provide accurate PNT.
In the case of ground and space segments, failures are relatively unlikely but have
been recorded (although operators respond by improving the system in turn),
and upgrades or unusual events can still throw up problems. Some of these
system problems could have serious consequences across the globe. The
availability of more than one GNSS (e.g. Galileo to augment GPS) provides a
robust mitigation against single system failures, if the contingency is properly
handled in the receiver, for example using RAIM, SBAS and other overlay systems.
With MTTR of 24 hours, MTBF – Mean time between failures at the system level
approach 100s if not 1000s of years. Given 1000+ elements in a network (not
unusual with mobile base stations) – the likelihood of a failure that affects the
network could be quite high even with this apparently extraordinarily high MTBF.
Global Navigation Space Systems: reliance and vulnerabilities 23
Receiver design errors harbour the potential for a systematic failure across whole
user sectors, particularly where receivers have been deployed into safety critical
applications when they have not been designed for such.
Ionospheric storms pose a threat to GNSS. Through judicious engineering
decisions and upgrades to transmitted signals, GNSS systems will, in the future, be
more resilient, and extremes of space weather will generally be troublesome
rather than dangerous. However, we note the following very important
exceptions (with probabilities based on historical evidence):
High precision applications (such as autonomous and semi-autonomous
aircraft landing and precision drilling from oil rigs) operating at UK latitudes
will have reduced availability perhaps one to three times per annum in the
years close to the peak of the solar cycle. History would also teach us to
expect still higher precision and integrity requirements in the future which
will, in turn, increase the risk again.
High precision applications operating at high and low latitudes will have
reduced availability many times per annum in the years close to the peak of
the solar cycle. In these regions of the world the impact of space weather is
more problematical.
Carrington Event - Both general and high precision applications, operating
anywhere in the world will have reduced availability for a significant period
(many days), perhaps once per century.
In all cases reduced precision and even outages may occur, but the integrity subsystem should ensure, for example, safety of life.
We also note that:
Ionospheric scintillation impacts are significantly worsened by intentional or
accidental jamming.
Navigation loss can be mitigated by deeply integrated GNSS-inertial
navigation systems.
Resilience to ionospheric effects will be somewhat improved through the use
of signals from multiple constellations
Nevertheless, all GNSS signals share the same frequency band (L-band) and hence
have a common vulnerability to the ionosphere.
Some limited measures against interference can be taken within the GNSS
receiver itself. Many receivers could be configured to detect interference through
monitoring the received signal strength indicator, flagging up a warning if this is
suspected. Some checks in software could be implemented to detect basic
signal spoofing. Some military GNSS receivers use more radical measures against
jamming in their design, such as a very high dynamic range in the signal input
capability, and the use of smart antenna arrays that can detect and attenuate
jammer signals, but these are unlikely to be adopted in civilian receivers owing to
the power requirements and additional cost.
In the USA, the JLOC network has been established to detect jamming
nationwide. They are currently registering thousands of incidents of jamming
each day – many of them legitimate use by authorised agents. A related
European project has recently been funded. The PRS and Operational Tool to
Evaluate and Counteract Threats Originating from Radio-sources (PROTECTOR) study
addresses the problem of detecting interference from radio sources in L-, S-, Ku-
24 The Royal Academy of Engineering
5 Resilience to disruption of GNSS services
and C-bands and improving the resilience of Europe’s Galileo and EGNOS GNSS
systems. A recently published article15 has proposed a cellphone-based network
for detecting and locating sources of GNSS interference.
In the UK, the Technology Strategy Board is supporting a project (GAARDIAN) to
create technology for a mesh of PNT interference detection & mitigation sensors
(IDMs) which will be deployed in the vicinity of PNT dependent infrastructure and
applications. These sensors will monitor the integrity, reliability, continuity and
accuracy of the locally received GPS (or other GNSS) and eLoran Radio Navigation
signals on a 24x7x365 basis creating an alarm network for both natural and
intentional interference to GNSS signals. If the system detects an anomalous
condition, users can be alerted to the problem and investigate.
In December 2010, a follow on project was announced. SENTINEL, which is
funded by The Technology Strategy Board and the Engineering and Physical
Sciences Research Council, will deploy trial GNSS interference detection probes in
a controlled manner to research a service to address the detection of deliberate
or accidental signal jamming and also to detect, quantify and discriminate
between interference and natural phenomena and assess the impact of unusual
multipath in the vicinity.
Global Navigation Space Systems: reliance and vulnerabilities 25
6 Conclusions and recommendations
6.1 Reliance on GNSS for PNT is high and increasing
The use of GNSS for a variety of purposes has become so convenient and
ubiquitous that there is a strong tendency among users to treat it as a given. At
every level, examples of reliance on GPS for positional, navigational and timing
uses without fully tested and exercised non-GPS back-ups have been observed. In
the great majority of cases, the loss of these services in an individual application
will cause only local or isolated inconvenience, but the possibility exists for wider,
single mode or common mode failures with more serious consequences.
Although it is currently rare for safety critical systems to be wholly reliant on
GNSS, related services that are otherwise independent may have GNSS as a
common point of failure, with consequences for the performance of safety critical
tasks. Such conditions occur where, for example in the emergency services,
navigation and positional data is required to help perform safety critical tasks
efficiently, even though its absence would not interfere with the actual
emergency response task once the location of the emergency had been reached.
There are also a number of primary safety critical systems being developed in
fields such as transport.
6.2 GPS, Galileo, Compass and GLONASS common vulnerabilities
The risk of a common mode failure affecting an entire GNSS constellation or even
multiple constellations cannot be ruled out. The Earth is subject to extreme solar
events from time to time and these have the potential to disrupt the GNSS
signals, and the satellites themselves. The disruption may be temporary or may
cause complete satellite failure. Such super-storm events are not predictable, but
studies estimate that these so-called ‘Carrington events’ will occur with a
probability in the order of 1-in-100 per year.
Space Weather events of lesser magnitude will occur more frequently. More than
once per decade, at UK latitudes, there may be an interruption to high accuracy
GNSS services. There should be no direct safety of life issues if the integrity
subsystem informs the users that the navigation solution is degraded, but the
absence of the service will have varying levels of impact, which could be
mitigated if an alternative navigation system is available.
Risk from jamming is growing. As GNSS becomes more widely used for revenue
generation or protection, the rewards from criminal activity aimed at disrupting
the system grow. Already it is known that criminals have used GPS jamming in
connection with theft of high value vehicles and the avoidance of road user
charges16. The cost of jamming equipment is low and while users of such
equipment are concerned only with the jamming of devices on a single vehicle,
the area affected by that jamming signal can be large. It is expected that the
introduction of Galileo, with its additional frequency bands and compatibility
with GPS will make jamming more difficult, but not significantly so for the
determined criminal.
The risk from spoofing is emerging and may become serious.
26 The Royal Academy of Engineering
6 Conclusions and recommendations
6.3 Recommendations
a) Raising awareness and analysing impact
Critical services should ensure that GNSS vulnerabilities are included in
their risk registers and that the risks are reviewed regularly and
mitigated effectively.
National and regional emergency management and response teams should
review the dependencies (direct and indirect) on GNSS and mitigate the
risks appropriately.
Services that depend on GNSS for PNT, directly or indirectly, should
document this as part of their service descriptions, and explain their
contingency plans for GNSS outages (say, of duration 10 minutes, 2 hours,
5 days, 1 month).
Photo: ESA
GIOVE-A mated with Fregat launcher upper stage
b) Policy responses
It is already illegal to place GNSS jamming equipment on the market in the
EU, as it cannot be made compliant with the EMC Directive. The Directive is
transposed into UK national legislation. The use of jammers is also a serious
offence under the UK Wireless Telegraphy Act 20061. Ofcom also has the
ability to close remaining loopholes by putting in place a banning order
under the 2006 Act which would prohibit import, advertisement and mere
possession of jammers. The case for this is easily justified given the clear
danger to safety of life services, which present a clear priority for Ofcom.
We recommend that Ofcom should introduce such a banning order, ideally
in co-operation with other European legislators.
5. The Cabinet Office Civil Contingencies Secretariat should commission a
review of the benefits and cost-effectiveness of establishing a monitoring
network to alert users to disruption of GNSS services, building on the results
of the GAARDIAN and similar projects and the US experience with JLOC.
6. The Cabinet Office should consider whether official jamming trials of GNSS
Services for a few hours should be carried out, with suitable warnings, so that
users can evaluate the impact of the loss of GNSS and the effectiveness of
their contingency plans.
Widely deployed systems such as Stolen Vehicle Tracking or Road User
Charging should favour designs where the user gains little or no advantage
from the jamming of signals that are so important to other services.
8. The availability of high quality PNT sources is becoming a matter of national
security with financial transactions, data communication and the effective
operation of the emergency services relying on it to a greater or lesser extent.
Greater cross-government coordination of S&T issues related to national
security should explicitly recognise the importance of PNT treating it as an
integral part of the operation of national infrastructure.
Global Navigation Space Systems: reliance and vulnerabilities 27
c) Increasing resilience
9. The provision of a widely available PNT service as an alternative to GNSS is an
essential part of the national infrastructure. It should be cost effective to
incorporate in civil GNSS receivers and free to use. Ideally it should provide
additional benefits, such as availability inside buildings and in GNSS blindspots. We are encouraged by progress with eLORAN in this context.
10. The Technology Strategy Board (TSB) and the Engineering and Physical
Sciences Research Council (EPSRC) are encouraged to consider the merits of
creating an R&D programme focused on antenna and receiver improvements
that would enhance the resilience of systems dependent on GNSS.
28 The Royal Academy of Engineering
2 accessed
17 February 2010
National PNT Advisory Board comments on Jamming the Global Positioning
System - A National Security Threat: Recent Events and Potential Cures November
4, 2010
GLONASS: As Good as it Should Be?, E. Rooney, A. Last, Signal Computing Ltd.,
ION GPS-99 page 1363
lavrakas_civil_gps_monitoring.ppt#259,5,What has gone wrong?
Global Navigation Space Systems: reliance and vulnerabilities 29
30 The Royal Academy of Engineering
Developing Chinese GNSS
A GNSS planned by the European Union to be an alternative (or
supplement) to GPS that is non-military and can provide
assured services.
Galileo In-Orbit Validation Element – test satellites launches as
part of the Galileo programme.
Global'naya Navigatsionnaya Sputnikovaya Sistema – Russian
GNSS which is nearing full deployment..
Global Navigation Satellite System – Generic term for space
based navigation systems of which GPS and Galileo are
Global Positioning System – US Military operated satellite
constellation and associated ground segments. Although
originally conceived for military use, position, navigation and
timing signals are fundamental to many civilian applications.
The region of the atmosphere between around 80km – 600km
above the earth. GNSS signals are delayed in the ionosphere
proportional to the number of free electrons given off by
the sun.
Interfering with communications or surveillance
LOng RAnge Navigation – a terrestrial radio navigation system
using low frequency radio transmitters in multiple deployment
(multilateration) to determine the location and speed of the
receiver. eLORAN is an enhanced LORAN system currently
being deployed.
Re-broadcast of GPS signals in such a way as to create a
stronger, but erroneous fix.
The plasmasphere, or inner magnetosphere is a region of the
Earth's near space environment consisting of low energy (cool)
plasma. It is located above the ionosphere.
An approximation of the distance between the GNSS satellite
and a navigation satellite receiver before any corrections
are applied.
Broadcast of signals which can appear to be genuine GNSS
Annex A
Annex A –
Current and planned PNT applications using GNSS
H, horizontal accuracy;
V, vertical accuracy;
S, speed accuracy;
3D, all three spatial dimensions.
Aviation applications
En-route navigation
Low (H)
Initial approach, non precision
approach and departure
Low (H)
Precision approach (Cat I)
Medium (H)
High precision approach (Cat II/III)
High (H
High (V)
Surface movement
V High (H)
Mid-air refuel
V High (3D)
Formation flying
V High (3D)
Helicopter en-route
Low (H)
Helicopter approach
Automatic dependent surveillance –
broadcast (ADS-B)
Low (H)
Low (V)
V High (H)
Road transport applications
In-car navigation
Fleet management
Urban traffic control
Emergency calls
Dynamic route guidance
Selective vehicle priority
Collision avoidance
Automated highway
Road pricing
Intelligent speed assistance
V High
Lane control
V High
Global Navigation Space Systems: reliance and vulnerabilities 31
32 The Royal Academy of Engineering
Stolen vehicle recovery
Restraint deployment
Trip travel information
Road transport applications
Ocean navigation
Low (H)
High (S)
Coastal navigation
Medium (H)
High (S)
Inland waterway navigation
Medium (H)
High (S)
Tugs and pushers
High (H)
High (S)
High (H)
High (S)
Automatic collision avoidance
High (H)
Port approach
High (H)
High (S)
V High (H)
High (S)
Automatic docking
V High (H)
Low to V High (H)
High to V High (V)
V High (H)
V High (V)
V High (S)
V High (H)
V High (V)
Vessel traffic services
High (H)
V High (S)
Cargo handling
V High (H)
V High (V)
V High (S)
Annex A
Rail applications
Signalling and train control
Infrastructure data collection
End of movement authority
Medium (Terminal)
Medium (Busy lines)
Low (Rural lines)
Supervision to buffer stops
Speed profile calculation
Medium (Terminal)
Medium (Busy lines)
Low (Rural lines)
Train location
Medium (Terminal)
Medium (Busy lines)
Low (Rural lines)
Level crossing protection
Medium (Terminal)
Medium (Busy lines)
Low (Rural lines)
High speed warning
Track-side personnel protection
Geographical position of the train
Power supply control
Advisory station stop
Medium (Terminal)
Medium (Busy lines)
Low (Rural lines)
Door control supervision
Train integrity
Train separation
Medium (Terminal)
Medium (Busy lines)
Low (Rural lines)
Passenger information systems
Pre-trip information
On-trip information
Management information systems
Fleet management
Cargo monitoring
Rolling stock maintenance
Infrastructure testing and inspection
Global Navigation Space Systems: reliance and vulnerabilities 33
Autonomous vehicle applications
Unmanned aerial vehicles
Autonomous land-based vehicles
Autonomous underwater vehicles
Timing applications
Network synchronisation
Digital broadcasting
Power generation and distribution
Other applications
34 The Royal Academy of Engineering
Satellite monitoring and
ground based navigation
Frequency/time calibration services
Maintenance of International
time standards
Precision agriculture applications
Yield mapping
Plot mapping
Automatic guidance of farm machines
Fisheries applications
Yield analysis
Fisheries monitoring
Oil and gas applications
Appraisal drilling
Field development
Support to production
Annex A
Emergency services
vehicle applications
Emergency calls
Fleet management for emergency vehicles
Dynamic route guidance for emergency services High
Selective vehicle priority for emergency services
Search and rescue
Maritime emergency and rescue operations
Aviation emergency and rescue operations
Pedestrian emergency and rescue operations
Personnel protection
Aid for blind persons
Persons suffering from Alzheimer’s disease
Transport of the physically
handicapped persons
Protection of very important persons
Scientific applications
Geodesy and surveying
V High (3D)
Global reference systems
V High (3D)
V High (3D)
Geoid determination
V High (3D)
Geographic information system
V High (3D)
Environmental monitoring
V High (3D)
V High (3D)
Climate research
V High (S)
V High (S)
Bridge/dam monitoring
V High (3D)
The information in this table was provided by the GAARDIAN project and is
reproduced with permission.
Global Navigation Space Systems: reliance and vulnerabilities 35
Annex B – GNSS failure modes and characteristics
Impact and remarks
Clock jump
This is a clock misbehaviour that results in an
abrupt change in the transmitted signal without
any notification.
Can result in a range error of up to
thousands of metres.
Clock drift
This type of clock misbehaviour introduces a slow
ramp type error in the transmitted signal. It is
to detect because its signature resembles the
typical relative motion of a satellite and GPS receiver.
PRN 23 on 1 January 2004 experienced
a clock drift error that grew gradually to
a few kilometres.
Incorrect modelling
of orbital parameters
Orbital models consisting of satellite orbits and
clock parameters are constantly updated by a Kalman
estimator maintained at the Master Control Station
in Colorado, USA. These are then uploaded to the
satellites. Any error in these parameters results in
incorrect navigation message. The error in the orbit
parameters increases with the time lapse between
two consecutive uploads and can be in the form of
a slow ramp in the range measurements.
This type of error might be corrected at
the next upload normally after eight
hours provided the error is detected. Its
effect on positioning accuracy depends
on the receiver position and geometry
of available satellites. It can result in a
range error of up to 40 metres.
Radiation damage to The performance of space borne semiconductors
satellite payload
will degrade owing to the high energy particle
environment. There are two types of phenomenon
a) Single Event Upset (SEU) caused by temporary
change in the circuitry and b) aggregated ionising
dose damage which ages the semiconductors or
makes them inoperative. The exposure to
radiation varies with the orbit.
Radiation effects can lead to a variety of
problems including reduced transmitter
power, errors in the navigation data and
faults in the operating system more
broadly. Semiconductors operating in
outer space are radiation-hardened to
minimize damage by radiation.
Block IIR satellites are equipped with Time Keeping
code (NSC) Systems (TKS) to generate a 10.23 MHz
signal. Anomalies can occur in the voltage controlled
oscillator of these systems that are shown to be
correlated to solar eclipses. This results in the
issuance of the Non-Standard Code (NSC). NSC is
also generated when TKS loops are open and telemetry
data are output by the Navigation Data Unit (NDU).
Generation of NSC acts as a warning to
the GPS receiver. A proper GPS receiver
design should remove the relevant
satellite from the position solution as it
is a meaningless measurement.
Otherwise the code lock loop could
become unstable.
Eclipse related
trajectory changes
When a GPS satellite comes out of an eclipse,
its trajectory is perturbed due to the effect of
changing solar radiation pressure.
This can cause range errors of up to 30 m.
Satellite attitude
This results in power fluctuation and changes in
the nominal Signal-to-Noise Ratio (SNR).
This can result in either loss of lock or a
significant signal acquisition/reacquisition time.
Increased solar
Increased solar noise in extreme circumstances.
In severe cases, loss of lock may occur.
Power fluctuations
The transmitted power fluctuations can make it
difficult to lock on to a signal.
Could result in loss of lock.
RF filter failures
Due to filter failure, side lobes may be corrupted.
There can be sudden jumps or slow fluctuation in
signal frequencies.
This makes it difficult for typical
antennae to lock on to signals.
36 The Royal Academy of Engineering
Annex B
Impact and remarks
Onboard multipath, This is due to different transmitting antennae
onboard interferences present on the satellite payload.
and signal reflections
Due to the increase in the number of signals as
a result of GPS modernization, this error might
increase in future. This is usually addressed in
two ways: (a) Multiple antennae on satellites
are positioned in a manner to minimize this
error. However, this is complicated by the
constraint of maintaining all antenna
directions towards the Earth. (b) The multipath
error is calibrated on the ground.
After calibration as in the case of a typical
satellite, attitude error in the range of 10 sec of
arc can be present in the line of sight.
Inter-channel bias
These biases are present between different channels
on the satellite transmitters due to the differences in
the positions of transmitting antennas on the
satellite. Furthermore, antenna phase centre error
is different for different transmitters.
These errors will have more effect when
position solutions are formed using multiple
frequencies. Precise calibration on the
ground for each channel is required to
remove these types of biases. For a typical
satellite the error between L1 and L2
antennae phase centres can be half a metre
in range, for a Block IIA satellite.
between data
and code
This manifests as a constant bias for a
particular satellite.
If there is a de-synchronization error of one
bit between data and code modulation it
can amount to a delay equivalent to a range
error of 1.5 sec.
This is the generation of a powerful radio frequency
in the vicinity of the receiver to either cause loss of
lock (jamming) or degrade navigation accuracy
(interference). Another way is spoofing which is the
intended injection of spurious GPS like signal.
A GPS receiver that locks onto such signals will not
be able to get meaningful measurements.
Interference from amateur radio operators is
a potential threat to GPS signal integrity.
Availability of commercial jammers can
prevent a GPS receiver from tracking signals.
These occur when a GPS receiver is used in the
vicinity of an installation that generates radio
frequencies in the GPS frequency range.
errors (I)
The ionized environment extends from 80 km to
the satellite altitude and is variable in time and
space. It introduces a variable component to the
signal delay and phase.
Harmonic emissions from commercial high
power transmitters, ultra wideband radar,
television, VHF, mobile satellite services
and personal electronic devices can
interfere with the GPS signals. Depending
on the magnitude and frequency of the
transgressing signal, the effect may range
from additional noise in signal to complete
loss of lock.
Uncorrected, range errors of typically 10 m
can occur in single frequency operation. GPS
broadcasts model coefficients in the
navigation message which compensate
typically 50% of the ionospheric delay
during benign conditions. Better
performance is achieved from relative
positioning. Dual frequency receivers correct
virtually all of the ionospheric error.
Global Navigation Space Systems: reliance and vulnerabilities 37
Impact and remarks
errors (II)
Amplitude and phase variations (scintillation)
occur on all frequencies.
In severe cases scintillation causes loss
of lock in carrier and code tracking
loops. As a consequence the PNT
solution is degraded.
Tropospheric errors
The troposphere extends up to an altitude of about
12 km. The GPS signal is delayed in this layer due to
bending and refraction. The error consists of wet and
dry contributions.
Uncorrected range errors of ~0.7m can
occur In normal conditions, the dry part
of the tropospheric delay (90% of the
total) can be compensated for by
conventional models.
These errors are the result of the reception of the
GPS signal by the receiver after reflection from
surrounding surfaces.
This depends on the operational
environment of the receiver and in
extreme cases can result in loss of lock.
Receiver problems
Receiver design and development should be
according to the standard GPS receiver specification.
Departure from these instructions may result in
anomalous situations.
Warnings were issued by the US Coast
Guard that certain receivers are not
integrated properly with other
equipment such as AIS (Automatic
Identification System), radar, etc.. It
might be possible that transmitted
satellites are unhealthy but receivers still
process their data. An example is carrier
phase-only receivers failing to read the
NSC. A receiver cannot lock onto NSC
hence this will affect the acquisition
time in the case of serial receivers and
inappropriate utilization of channels in
parallel receivers.
Human related
GPS as part of cockpit equipment results in
overconfidence of the aircrew.
Fatal accidents have been reported.
Low availability
The number of available satellites may not always
be sufficient to provide good geometry in all areas
of the Earth.
This was reported to have occurred over
the UK and the USA in the past decade.
failure mode
One or both of two critical subsystems – the
satellite bus and the navigation payload are
operating without backup capacity.
At any time, 16 satellites may be
operating in single string failure mode.
The non-availability of backup results in
the shutting down of the satellite signal
in case of failure of the primary critical
Leap Second
This primarily results in a timing error that
degrades navigation accuracy.
On 28 November 2003, a leap second
anomaly was experienced by many
GPS receivers. Receivers might lose
track for a second before recovering.
The above table draws heavily on Failure Modes and Models for Integrated GPS/INS Systems, Umar Iqbal Bhatti and Washington
Yotto Ochieng THE JOURNAL OF NAVIGATION (2007), 60, 327–348. Used with permission.
38 The Royal Academy of Engineering
Annex C
Annex C – Some commercially available jammers
Global Navigation Space Systems: reliance and vulnerabilities 39
Annex D – Jamming trial example
The following extract from GPS Jamming and the Impact on Maritime Navigation,
Alan Grant, Paul Williams, Nick Ward and Sally Basker (The General Lighthouse
Authorities of the United Kingdom and Ireland) is reproduced with permission20.
The US Global Positioning System (GPS) is currently the primary source of
Position, Navigation and Timing (PNT) information in maritime applications,
whether stand-alone or augmented with additional systems. This situation will
continue in the future with GPS, possibly together with other Global Navigation
Satellite Systems (GNSS) e.g. Galileo, being the core PNT technology for eNavigation – the future digital maritime architecture. GPS signals, measured at
the surface of the earth, are very weak. As such, the system is vulnerable to
unintentional interference and jamming, resulting in possible denial of service
over large geographical areas. The result of such interference could be the
complete failure of the mariner’s GPS receiver or, possibly worse, the presentation
to the mariner of hazardously misleading information (HMI) for navigation and
situational awareness, depending on how the GPS receiver reacts to the
jamming incident.
Recognising this, the General Lighthouse Authorities of the United Kingdom and
Ireland (GLA), in collaboration with the UK Ministry of Defence (MOD) Defence
Science and Technology Laboratory (DSTL), have conducted a series of sea-trials
with the aim of identifying the full effects of GPS jamming on safe navigation
at sea.
The trial was conducted over several days during April 2008 at Flamborough
Head on the East coast of the United Kingdom. DSTL provided a professional lowto-medium power jammer, which was controlled remotely by two VHF
transceivers and transmitted a known pseudo-random noise code over the
civilian L1 frequency providing a jamming signal over the whole 2MHz
bandwidth of L1. Although the unit was capable of broadcasting on the P code,
this was not activated. The total power of the signal over the 2MHz bandwidth
was approximately 2dBW (~1.5W) of power.
The Coverage area of the GPS jamming unit at 25m above ground level on
maximum power of 1.58W ERP is shown in the figure below (Image courtesy of
For the dynamic trials, the Northern Lighthouse Board vessel NLV Pole Star steered
a course back and forth between two waypoints on a path that dissected both
the main lobe of the GPS jammer and the two side lobes, but with sufficient
length beyond the jamming region to enable the various GPS enabled units to
reacquire satellites.
The crew of Pole Star was fully briefed prior to the trial and so expecting GPSenabled systems to fail.
40 The Royal Academy of Engineering
Annex D
When Pole Star entered the jamming zone, numerous alarms sounded on the
bridge over a period of approximately 10 minutes. These alarms were all linked to
the failure of different functions to acquire and calculate their GPS position, which
included: the vessel’s DGPS receivers, the AIS transponder, the dynamic
positioning system, the ship’s gyro calibration system and the digital selective
calling system. The crew of the Pole Star was able to recognise each alarm and
silence them but they were expecting the alarms to sound. In the situation where
a crew was not expecting this level of system failure then the distraction caused
by so many alarms sounding at once could have had a significant effect. The
effect could be made worse depending on the time of day (potentially a vessel’s
bridge can be single-manned at night, or with one officer and a look-out) or if the
vessel is performing a manoeuvre or operation demanding high accuracy and a
high degree of human concentration at the time of GPS failure, such as docking
in poor visibility.
Some vessels have integrated bridge systems, which enable automatic execution
of a passage plan on autopilot. If this system is operating at a time that jamming
occurs, then the vessel’s course and heading may change without informing the
crew, potentially leading to extremely hazardous consequences.
Although the Pole Star’s crew was expecting GPS failure, problems were
experienced. The vessel’s Electronic Chart Display & Information System (ECDIS)
was not updated due to the failure of the GPS input, resulting in a static screen.
ECDIS is the normal mode of positioning on board Pole Star (with paper chart
backup,) and during the periods of jamming some crew members became
frustrated when trying to look at the ECDIS. This resulted in the monitor being
switched off!
There are several questions raised by this trial, such as the ability of a vessel’s crew
to quickly revert to traditional means of navigation and also the extent to which
they are able to navigate with these means. Given the greater reliance on satellite
navigation, in particular GPS, these skills are not being used daily and are no
longer second nature. This trial also raised awareness of the number of alarms
that can sound on the bridge and how the sheer quantity can be distracting.
Global Navigation Space Systems: reliance and vulnerabilities 41
Three additional receivers were installed on the trial vessel two of which were
typical marine grade differential GPS receivers, the third was a more expensive
dual frequency surveying receiver (configured to operate on GPS L1 only). Data in
the form of NME sentences was recorded from each receiver throughout the
jamming trial. It should be noted that due to a lack of space on the vessel’s mast,
antennas for the three receivers were installed on the handrail of the main deck,
which meant there was a certain amount of sky obscuration due to the vessel’s
Over the course of the dynamic trials the receivers were monitored and all of
them lost GPS lock. The two differential receivers maintained lock on the medium
frequency broadcast from the nearby Flamborough Head DGPS reference station;
however as the reference station was also affected by the jamming signal, there
were no corrections to apply and their position solution was derived from
standalone GPS. When processing the recorded data from the three receivers, the
NMEA GPRMC (recommended minimum content) sentence was used as this
provides the reported position, speed and time. This sentence also provides an
indication of the validity of the data, setting or clearing a single bit flag. The
decision to set or clear the data valid flag is one that is made by the receiver.
When processing the recorded data from the various receivers only data declared
valid was used, which resulted in the two typical marine grade receivers providing
erroneous positions as they entered and exited the jamming region. The
magnitude of the position error varied, with some small errors, but with others
several tens of kilometres away from the true location. Figures 1 and 2 provide the
reported positions from one receiver, plotted on Google Earth™. The left-hand
plot is from the control run where the jamming unit was disabled; the right-hand
plot is from a run where the jamming unit was enabled and erroneous data was
observed. The colour of each reported position is an indication of the reported
vessel speed at that moment, with blue positions indicating a speed of less than
15 knots; yellow positions indicating a speed of between 16 and 50 knots; orange
positions indicating a speed of between 51 and 100 knots and red positions
indicating a reported speed of greater than 100 knots.
Figure 2 shows that the number of erroneous positions was significant with the
majority of positions coloured red, indicating the reported speed was greater
than 100 knots (the greatest reported speed was over 5000 knots). Clearly, if this
data was being used as input to a navigation system, whether it was an autopilot
or simply an electronic chart the implications are serious. The results shown in
Figure 2 were typical from the two marine grade receivers, although it was noted
that the effect of jamming was more severe when sailing North with the vessel
superstructure between the jamming unit and the GNSS receivers’ antennas.
Therefore, it may be presumed that the jamming signal was attenuated due to
the shadowing effect of the vessel’s superstructure and the ‘moment of
indecision’, that period of time when the strength of the jamming signal was
comparable with that of the GPS satellites, was greater and resulted in an
increased number of erroneous positions. The more expensive survey grade
receiver did not provide any erroneous data positions, rather opting to provide no
position information when experiencing interference from the jamming unit;
clearly this is the preferred situation.
The GPS receivers onboard Pole Star were also affected by the jamming signal and
also reported inflated speeds, albeit on to a smaller degree. The reported position
42 The Royal Academy of Engineering
Annex D
on the vessel’s ECDIS wandered around and the reported speed also increased
above the maximum speed of the vessel. However, the vessel’s receiver did stop
providing position information quite quickly once the vessel had passed into the
jamming area. The implications of providing erroneous positions can be severe
and can greatly affect the safety of the mariner and those around them.
Figure 1: Reported positions with
no GPS jamming active
Figure 2: Reported positions with GPS
jamming switched on.
GPS is vulnerable and this trial has investigated GPS service denial by intentional
interference using low-power jammers. It should be clear that the results can be
extended to GPS service denial by unintentional interference. Unintentional
sources of interference include spurious harmonics from active TV antennas,
damaged GPS antenna cables and ionospheric effects. The latter are correlated
with an eleven-year sun-spot cycle and are particularly prevalent at high latitudes.
This will bring challenges when arctic shipping routes become available.
The main conclusion from this trial is that GPS service denial has a significant
impact on maritime safety:
On shore – the marine picture presented to Vessel Traffic Services /
Management (VTS) will be confused as AIS information with erroneous
positions and high-velocities conflicts with the radar information. Further
study is needed to determine how VTS operators will respond.
Aids to Navigation (AtoNs) – DGPS reference stations can be jammed and the
impact may result in the absence of DGPS corrections and integrity
information broadcast to users over a very large geographical area; AIS used
as an AtoN may broadcast incorrect information; and synchronised lights may
not be synchronised, thus having an adverse impact on visual conspicuity.
On ships – navigation, situational awareness, chart stabilisation and DSC
emergency communications will be lost if they are based on GPS. Some
vessels have integrated bridge systems, which enable automatic execution of
a passage plan on autopilot. If this system is operating at a time when
jamming occurs then, depending on the system design, the vessel’s course
and heading may change without informing the watch-keeper, potentially
leading to extremely hazardous consequences. At this point, continuation of
navigational safety is dependent on mariners’ abilities to recognise that GPS
Global Navigation Space Systems: reliance and vulnerabilities 43
service is being denied and to operate effectively using alternative
techniques (e.g. radar parallel-indexing). Increased use of ECDIS will increase
the attendant risks.
On people – People are conditioned to expect excellent GPS performance. As
a result, when ships’ crews or shore staff fail to recognise that the GPS service
is being interfered with and/or there is a loss of familiarity with alternative
methods of navigation or situational awareness, GPS service denial may make
a significant impact on safety and security. In this trial, despite the fact that
the Pole Star’s crew was forewarned, problems were experienced with the
ECDIS. Moreover, the number of alarms that can sound on the bridge can be
distracting. Moving to other navigation techniques can cause an increase in
bridge workload.
A was unaffected by GPS jamming and demonstrated an accuracy of 8.1m (95%)
which is comparable to stand-alone, single-frequency GPS. Consequently, A can
be used to detect erroneous positions and high velocities that may be
experienced during GPS service denial. Moreover, when GPS is unavailable, A can
provide a PNT input to all maritime systems. Finally, in the future e-Navigation
environment, the combination of GPS, Galileo and A will provide robust and
resilient PNT in order to reduce the impact of human error and to improve the
safety, security and protection of the marine environment.
44 The Royal Academy of Engineering
Annex E
Annex E – Acknowledgements
Working group
Dr Martyn Thomas CBE FREng (Chairman)
Martyn Thomas Associates Ltd
Professor Jim Norton
Independent Director
Alan Jones
Cotares Ltd
Professor Andy Hopper FREng
University of Cambridge
Nick Ward
General Lighthouse Authorities of the UK & Ireland
Professor Paul Cannon FREng
Neil Ackroyd
Ordnance Survey
Paul Cruddace
Ordnance Survey
Martin Unwin
Surrey Satellite Technology Ltd
Richard Płoszek
The Royal Academy of Engineering
Other contributors
Charles Curry
Chronos Technology Limited
Professor Washington Ochieng
Imperial College, London
Printed using
based inks on
paper from
Global Navigation Space Systems: reliance and vulnerabilities 45
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