Mode S Transponders

Mode S Transponders
All About
ith all of the new hype the
past few months concerning Elementary Surveillance and Mode S enhancements, it
would seem appropriate to discuss the
technical characteristics and purpose
behind the new functionality and how
avionics technicians can appropriately
correct problems.
Of course, knowing the purpose
of transponders and what affects enhancements have to the functionality
are extremely important, especially
since this is one system you can count
on to continually evolve. The recent
mandates for this system are just the
tip of the iceberg as the real capabilities and functions have not even been
tapped yet. Believe it or not, transponders in aircraft make up the smallest
segment of their functionality.
According to market forecasts, the
Radio Frequency Identification (RFID)
industry is expected to have double
digit growth over the next few years
and be a multi-billion dollar industry
by 2008. New uses for the military and
commercial applications are literally
staggering. Everything from animal
tracking to ski passes are utilizing transponder technology to communicate
tracking information between sources.
Wal-Mart has issued mandates to their
suppliers to incorporate a version of
the technology in all of their products.
What does this mean for aviation?
Well, the technology is definitely getting refined and enhanced.
So what does a transponder do?
Technically a transponder is a combined radio transmitter and receiver
which operates automatically relaying
data between aircraft and Air Traffic
Control (ATC) on the ground. The sig44
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Air Traffic Control Radio Beacon System (ATCRBS)
nals sent provide a unique “identity”
for each aircraft, essential in crowded
airspace to avoid mid-air collisions.
Transponders were originally developed for military aircraft in WWII for
identification friend or foe (IFF) application but evolved into the commercial
arena. Most air traffic control centers
rely on the range and altitude information returned by commercial aircraft
rather than raw radar data.
All interrogations are made on 1030
MHz, all replies on 1090 MHz. There
are four ways of interrogating, called
Modes. When interrogated in Mode A
an aircraft is asked for its identity, when
Bendix/King KT 73 Transponder
interrogated in Mode C it is asked for
altitude. Modes B and D are not used.
The civil Mode A coincides with the
military Mode 3. That is why Mode A
is often called Mode 3/A. The replies
are called Codes. The Mode A Code
is set in the cockpit in four digits, the
Mode C Code automatically transmits
the altitude, with 1013.2 hPa as the
reference pressure, just like in Flight
Levels. Coupling a dedicated Mode A
Code to a flight plan allows the ATC
controller to see the Aircraft Identification (ID) of a particular aircraft directly on his screen. Mode C does the
same with the addition of altitude. To-
gether with the heading, derived from
the tracking algorithm, the information
available to the radar controller on his
screen has enormously improved.
However, because of the increasing
commercial traffic there is a lack of
Mode A codes. Additionally, the 1090
MHz reply frequency is becoming
saturated, with two typical subsequent
problems: garbling, which means that
the reply pulse trains from more than
one aircraft upon one interrogation
may overlap, sometimes causing confusion, and erroneous replies received
on the ground from transponders that
were interrogated by other interrogators on the ground.
The Mode Select Beacon System,
commonly referred to as “Mode S,”
was developed by Lincoln Labs in
1975 as a way to monitor aircraft to
support an evolutionary addition to the
Air Traffic Control Radar Beacon System (ATCRBS) for automation in dense
traffic areas and to solve the Mode A/C
short comings. In 1986 a mid-air collision between an Aero Mexico DC-9
passenger aircraft and a single engine
Piper over Cerritos, Calif. which killed
82 people placed more emphasis on
the technology. The crash was blamed
on inadequate automatic conflict alert
systems and surveillance equipment
resulting in congress passing into law
the Airport and Airway Safety and Capacity Expansion Act in 1987.
This new law required that all air
carriers (part 121 operators) operating within the United States with more
than 30 seats must be equipped with
TCAS (Traffic Collision Avoidance
System) II by 1993. Air carriers with
10 to 30 seats only had to have TCAS
I. Mode S performs all the functions of
Mode A and C transponders, and has
datalink capability. Mode S transponders are an integral component of all
TCAS II installations and replace the
Mode A and C transponder for TCAS
II equipped aircraft. A Mode S tran-
sponder may be installed to replace a
Mode A or C transponder without necessitating the installation of TCAS.
However, Mode S is required with
TCAS in order to facilitate ID, flight
status (on ground/in flight) and altitude. Different Classes of transponders that identify the capability levels
have been developed over time. However, to meet the EUROCONTROL
requirements for Elementary (Basic)
Surveillance, a Level 2 transponder
is necessary as a minimum capable of
supporting both Interrogator Identifier
(II) and Surveillance Identifier (SI)
codes in accordance with International
Civil Aviation Organization (ICAO)
One new feature of the Mode S
Transponder is that each aircraft is assigned a unique address code, which is
broadcast in unsolicited “SQUITTER”
transmissions occurring approximately
every second. ATC or another Mode S
equipped aircraft can use this address
for interrogation or communication
purposes. Flight ID capability is one of
the functions required by Elementary
Enhanced Surveillance is the new
cat on the block which allows for an
extension to the Squitter Code up to
24 bits containing the International
Civil Aviation Organization (ICAO)
required aircraft address and aircraft
reporting information.
The aircraft parameters required for
Enhanced Surveillance are as follows:
Magnetic Heading
Roll Angle
Track Angle Rate
Vertical Rate
True Track Angle
Ground Speed
Selected Flight Level/Altitude
FMS Selected Altitude
Baro Correction Value
Inertial Vertical Velocity
The standards for Mode S spe-
cific services are described in ICAO
Annex 10 Mode S System. The
minimum operational performance
standards are described in RTCA
A detailed technical definition of all
the parameters required for Elementary and Enhanced Surveillance is
given in the ICAO Manual on Mode
S Specific Services, Doc 9688. These
definitions are further refined in EUROCONTROL document SUR.ET2.
ST03.2000-ID-04, DAP Technical
Characteristics for Mode S Enhanced
Surveillance. The transponder design
and interface requirements are documented in the ARINC Characteristic
There are numerous Mode S Transponders in service and troubleshooting begins with identification. There
are currently nine manufacturers of
aircraft Mode S Transponder systems:
ACSS, AlliedSignal, Bendix/King,
Filser, Garmin, Honeywell, Narco,
Rockwell Collins and ThompsonCSF.
Continued on following page
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Continued from page 45
Bendix/King (Honeywell)
AlliedSignal (Honeywell)
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Rockwell Collins (continued)
Rockwell Collins
Once you have identified the unit, it is important to get the
correct, most current manual (we always do, right?) because
the checkouts are constantly changing. Transponders must
be checked and certified for operation every two years as a
minimum commonly referred to as the “413” check. Federal
Aviation Regulation US FAR § 91.413 states:
ATC transponder tests and inspections.
(a) No persons may use an ATC transponder that is specified in 91.215(a), 121.345(c), or Sec. 135.143(c) of this chapter unless, within the preceding 24 calendar months, the ATC
transponder has been tested and inspected and found to comply with appendix F of part 43 of this chapter; and
(b) Following any installation or maintenance on an ATC
transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and
found to comply with paragraph (c), appendix E, of part 43
of this chapter.
(c) The tests and inspections specified in this section must
be conducted by—
(1) A certificated repair station properly equipped to perform those functions and holding—
(i) A radio rating, Class III;
(ii) A limited radio rating appropriate to the make and
model transponder to be tested;
(iii) A limited rating appropriate to the test to be performed;
Continued on page 49
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Continued from page 47
[(iv) deleted]
(2) A holder of a continuous airworthiness maintenance
program as provided in part 121 or Sec. 135.411(a)(2) of this
chapter; or
(3) The manufacturer of the aircraft on which the transponder to be tested is installed, if the transponder was installed
by that manufacturer.
While most avionics technicians have been performing
91.411 and 91.413 checks for most of their careers, Enhanced
Mode S requires updated test equipment and procedures. The
IFR ATC-601 is the most common test set used in conjunction with a pitot/static test set for testing transponder systems.
However, any test set can be utilized providing it can test the
following transponder responses:
Mode A response
Mode C response with altitude if available
Mode S all-call response
Mode S lockout
Transmitter frequency
Transponder power output
Receiver frequency discrimination
Receiver sensitivity
Receiver interrogation pulse spacing discrimination (all
Individual pulse-width discrimination (all modes)
SLS (Side Lobe Suppression)
Transponder response to invalid address interrogations
SPR on/off
Mode S, UF0 (uplink format 0)
Mode S, UF4
Mode S, UF5
Mode S, UF11
Mode S, UF16
Mode S, UF20
Mode S, UF21
Diversity isolation
Transponder squitter
Airplane identification code
It is very important to test the transponder system in the
right steps with the right equipment. In the future, Automatic
Dependent Surveillance-Broadcast (ADS-B) and Traffic Information Service–Broadcast (TIS-B) are enhancements currently under development that will add complement to radar
for en-route, terminal area and airport surface surveillance,
destination weather, Pilot Reports, add support for certain
airborne separation maneuvers, facilitate the broadcasting of
flight information (e.g. gate, stand, and number of passengers) and the ability to utilize arrival management programs
for airport terminals among hundreds of other combinations
of data. These additions will require properly trained technicians with all of the right tools in order to maintain the strict
tolerances and “stacked” data.
That, in a nut shell is Mode S. Tomorrowʼs transponders
promise to be the backbone of aircraft monitoring and surveillance. It is even possible that transponders are pilots in
the making.
Welcome to the future! ❑
Terms and Definitions
Air Traffic Control
Air Traffic Control Radar Beacon System
Mode Select Beacon System
Electronic Article Surveillance
Traffic Collision Avoidance System
International Civil Aviation Organization
Aircraft Identification
Interrogator Identifier
Surveillance Identifier
Identification Friend or Foe
Side Lobe Suppression
Automatic Dependent Surveillance-Broadcast
Traffic Information Service–Broadcast
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