GPS receivers differ considerably in
their anti-jamming effectiveness
Andreas Thiel,
Founder and Executive VP, u-blox
Michael Ammann,
VP Embedded Software Development, u-blox
October 2009
GPS-X-09008
white paper by:
WHITEPAPER
locate, communicate, accelerate
Anti-Jamming
techniques
in u-blox
GPS receivers
Table of contents
Executive Summary 3
Anti-Jamming 4
Capturing extremely weak signals 4
Mitigation measures
5
Field comparisons
7
Conclusion
7
About the Authors 8
About u-blox 9
Anti-Jamming techniques in u-blox GPS receivers / White Paper Published by u-blox AG | 2
Executive Summary
A critical factor when selecting components for a GPS system is the
receiver’s immunity to external noise, or “jamming”. The ability to lock onto
typically faint GPS signals in the presence of noise generated from other
electronic devices has a large influence on the system’s ability to provide
correct location data.
The use of an advanced, proprietary adaptive digital filtering technology
allows the u-blox 5 and u-blox 6 GPS positioning engines to overcome
jamming signals up to 25 dB stronger than conventional GPS receivers
can withstand. The result is the most sensitive and reliable GPS receiver
technology available.
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Anti-Jamming
While underway, whether walking or driving a car, you’ll notice how the
signal strengths from satellites as read by a GPS receiver vary quite a bit, and
in some areas for some unexplained reason you can completely lose satellite
lock. While such reductions in signal strength can be due to attenuation and
signal blockage while in urban canyons, inside building or even underneath
heavy tree cover, it can also be due to unintentional jamming – signals from
everyday electrical and electronic devices that create so much noise that
they mask the satellite signals. These jamming signals can even come from
ordinary battery-operated consumer devices such as portable music players
or mobile phones as well as automobile electronics.
To ensure that their GPS systems provide the optimal functionality, engineers of any consumer appliances incorporating GPS capabilities should not
only follow good engineering practices, they should also select components
designed to mitigate these jamming effects. The proper antenna is certainly
one aspect to keep in mind, but engineers should also be aware that the
abilities of individual GPS receiver chipsets vary widely in their ability to
counteract jamming signals. To help engineers during their component
evaluations, this white paper first reviews some aspects of jamming, and
then shows how u-blox 5 and u-blox 6 uses proprietary technology to
provide anti-jamming capabilities significantly better than conventional GPS
receivers.
Capturing extremely weak signals
GPS Satellites transmit with a
power output of roughly 30 W
and are 20,000 km above us.
One reason that jamming is an issue with any GPS receiver is that the signals
coming from the satellites are so weak. Satellites transmit with a power output of roughly 30 W and are 20,000 km above us. Just imagine: how much
strength would the signal with the strength of a weak light bulb have after
traveling that distance? In fact, a typical signal, when acquired outdoors
with open sky, is in the range of –120 dBm (1 x 10–15 W), and moving inside
a normal residential building can add 20 or 30 dBm of attenuation. With
such a weak signal, other signals in the same GPS frequency band don’t
need to be very strong at all in order to override the satellite signals.
As noted, such unintentional jamming can come from ordinary commercial
electronic appliances. You might expect for consumer electronics that the
FCC, CE and other agencies would regulate against interference of GPS
systems. Unfortunately, that is not the case. First, the regulations don’t
address signals at frequencies higher than 1 GHz whereas GPS carriers are
at 1.575 GHz. Second, the regulations allow emissions with levels of –60 to
–80 dBm, which are many orders of magnitude stronger than GPS signals.
Thus, GPS receivers must deal with unintentional jamming due to the
inadequate design in the electrical world around them. Common sources
of such jamming signals are the clock circuits or switching power supplies
in virtually any electronic device you purchase today and even their displays
due to their driver circuitry and scan rates. For instance, a PDA can generate
noise on its display, through its WLAN port or Bluetooth interface. A mobile
TV can also create heavy interference. There is also considerable potential
for interference in a car – not only from the electronic engine controls but
also from dashboard electronics and entertainment systems.
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Anti-Jamming
If you would like to give yourself a very quick demonstration of such interference, simply take any GPS receiver and place it next to the display of a
desktop or a laptop PC – you will immediately see how the signal strength
of the various satellites drops, and sometimes you can even lose the lock on
the satellites. Further, it’s often difficult to isolate the cause of such unintentional interference. In one famous case (Reference 1), the whole of Moss Landing Harbor
south of San Francisco was being jammed so that GPS reception was impossible even 1 km out to sea. With the help of directional antennas and by
turning off shore power to individual boats, it was possible to determine the
actual emitter: a pleasure boat had a preamplifier built into a commercial
VHF/UHF television antenna that was powered even when the TV was not
on. Even though the TV antenna was stored in a paint locker, its emissions
were strong enough to cause severe problems for both commercial fisherman and pleasure boaters.
Mitigation measures
We’re not going to eliminate all such unintentional jamming signals, so
we have to learn to live with them. Manufacturers of GPS equipment and
chipsets take a variety of approaches in combating jamming signals. For instance, as a first mitigation measure, virtually every GPS receiver places one
or two SAW (surface-acoustic wave) filters into the signal path to attenuate
out-of-band signals (Figure 1).
Next, some manufacturers such as u-blox place a lowpass antialiasing filter
in the RF chip to remove signals that would create aliases during the digitization process in the A/D converter. In addition, a highpass filter just after
the A/D removes DC components that might come from the data conversion and also removes the flicker noise inherent in CMOS circuitry.
Figure 1: This simplified block
diagram of a GPS receiver based
on the u-blox 5 / 6 technology
shows the SAW filter at the signal
input to block out-of-band
interference plus the softwarecontrolled digital filter bank that
blanks out specific interference
sources.
Anti-Jamming techniques in u-blox GPS receivers / White Paper Published by u-blox AG | 5
Anti-Jamming
However, the issue remains of dealing with in-band interference signals.
Each GPS receiver uses its own special methods, but here u-blox 5 / 6 employs some particularly effective techniques. First is the fact that the analog
signals from the LNA are digitized to 5 bits of resolution (thus giving 30 dB
of dynamic range) in contrast to other receivers, which typically digitize to
only 1, 1.5 or perhaps 2 bits (and thus up to 12 dB of dynamic range). With
this extra dynamic range, u-blox 5 / 6 is then able to apply a proprietary
filtering method based on a bank of on-chip digital filters whose configuration can be changed under software control. Specifically, this technique
sweeps across the GPS receiver band looking for strongest signal peaks, and
for each it performs statistical analysis to determine if it is actually a satellite
signal. Upon finding a jamming signal, the scheme puts it into a list so that
this signal is subsequently blanked out. Finally, if a signal for some reason
can’t be removed in this way, the technique builds up a table of thresholds,
and if a real GPS signal drops below this threshold, the detection algorithm
is very cautious about using it so the receiver doesn’t track false signals.
This special jamming-mitigation method requires considerable processing
power, which u-blox 5 / 6 supplies with an ARM® processor. Under control
of this processor, this proprietary combination of hardware and software
can reduce jamming signals by 30 dB compared to conventional products.
To see the results more clearly, examine Figure 2, which shows two curves
comparing the performance of u-blox 5 / 6 with conventional receivers. The
curves show the amount of interference power needed to result in a 3 dB
degradation in the signal at the receiver output (the 3-dB de-sensitization
point), and the curves show results for the +/– 40 MHz around the GPS carrier signal. For example, directly at the carrier, an interference signal of only
roughly –110 dBm is sufficient to degrade the received signal by 3 dB using
conventional methods, whereas u-blox 5 / 6 can handle interference that is
25 dBm stronger before it reaches its 3-dB de-sensitization point.
Figure 2: Measured directly
at the GPS carrier frequency,
an interference signal of only
about –110 dBm is sufficient
to degrade the signal by 3 dB
for receivers using conventional
approaches (blue trace), whereas
u-blox 5 / 6 can handle interference that is 25 dBm stronger
before it reaches its 3-dB desensitization point (red trace).
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Anti-Jamming
Field comparisons
Let’s now translate this talk of dBm into the impact of a jamming signal on
a GPS receiver’s performance in the field. Figure 3 shows results from an
experiment using conventional technology (left) and u-blox 5 / 6 technology
(right) for various signal levels of a continuous wave (CW) jamming signal.
With a CW signal level of 10 dB above thermal noise floor, the u-blox 5 / 6
can receive signals from more satellites. In addition, the large dot on the
floor plan shows that for both systems, each time they read their location,
the results are virtually the same. This gives you confidence in the readings
being accurate. Already with the CW jamming signal level increasing to
20 dB above thermal noise floor the strengths of satellite signals received
using conventional technology are starting to drop so that the GPS receiver
calculates a slightly different position almost every time; as shown with the
spread of position points. In contrast, u-blox 5 / 6 continues to read most
of the satellites with strong signal strength and provides a constant position.
While an even higher CW jamming signal level leads to the failure of the
conventional-technology GPS receiver to read any satellites, u-blox 5 / 6
continues to do so such that its position accuracy remains constant. Only
with a CW jamming signal level of more than 40 dB above thermal noise
floor do signal strengths and number of satellites decrease to where there is
some positional variation – but the system continues to function.
Figure 3: Field tests using conventional technology (left) and
u-blox 5 / 6 technology (right)
at various levels of a singlefrequency, in-band interference
signal. The upper part of each
screen shot shows the signal
strength of the GPS satellites,
and dots in the lower part track
several calculated positions. As
the jamming signal increases, the
number of satellites each technology can track and their signal
strengths decreases.
Conclusion
GPS system designers should consider where their systems will be used,
the type of noise they are likely to encounter, the effects of unintentional
jamming, and the benefits to the end user of selecting the technology with
the most effective jamming mitigation. In any case, a system incorporating a
GPS receiver should always be designed with utmost care avoiding generation of any interference signals in the GPS frequency band. As the data in
this white paper clearly illustrates, innovative jamming-mitigation techniques
as implemented in u-blox 5 and u-blox 6 GPS receivers considerably improve
GPS sensitivity and accuracy in noisy environments.
Reference 1: “Unjamming a Coast Harbor,” GPS World, January 1, 2003.
For more information about Anti-Jamming, visit
http://www.u-blox.com/en/gps-modules.html
or contact: info@u-blox.com
Anti-Jamming techniques in u-blox GPS receivers / White Paper Published by u-blox AG | 7
About the Authors
Andreas Thiel
Andreas has served as Executive Vice President (R&D Hardware) of u-blox
since its incorporation and as Executive Vice President R&D Hardware of
u-blox AG since 1997.
He holds a degree in electrical engineering from Aachen University (RWTH)
in Germany. From 1994 to 1997 he was a research assistant and project
manager at the Swiss Federal Institute of Technology (ETH).
In 1997, he co-founded u-blox AG.
Andreas Thiel
Michael Ammann
Michael Ammann is a vice president embedded software development at
u-blox AG, Thalwil, Switzerland. He obtained his master’s degree in electrical
engineering in 1998 from Swiss Federal Institute of Technology (ETH) Zurich.
He is a member of the Institute of Navigation (ION). His research interests
include Global Navigation Satellite System (GNSS) receiver architectures and
technologies.
Michael Ammann
Anti-Jamming techniques in u-blox GPS receivers / White Paper Published by u-blox AG | 8
About u-blox
u-blox is a leading fabless semiconductor provider of embedded positioning and wireless communication solutions for the consumer, industrial and
automotive markets. Our solutions enable people, devices, vehicles and machines to locate their exact position and wirelessly communicate via voice,
text or video.
With a broad portfolio of GPS modules, cards, chips, and software solutions
together with wireless modules and solutions, u-blox is uniquely positioned
to enable OEMs to develop innovative solutions quickly and cost-effectively.
Headquartered in Switzerland and with global presence in Europe, Asia and
the Americas, u-blox employs 180 people. Founded in 1997, u-blox is listed
on the SIX Swiss Exchange.
Copyright © 2010 u-blox AG
All rights reserved. No part of this publication may be reproduced, stored in
a retrieval system, or transmitted, in any form or by any means, electronic,
mechanical, photocopying, recording, or otherwise, without the prior permission of the copyright owners.
Published by u-blox AG, October, 2009
Any comments relating to the material contained in this document may be
submitted to:
u-blox AG
Zuercherstrasse 68
8800 Thalwil
Switzerland
info@u-blox.com
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