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About Transponders and TCAS

To be seen by the TCAS collision-avoidance system on a large aircraft, you must have a transponder. Nothing other than a transponder will, for at least the next decade, protect you from an accident like the 2006 collision between a jet and a glider near Minden. While a transponder detector or ADS-B receiver (as in PowerFLARM) will alert you to an approaching jet, you still must have time to spot the traffic and get out of the way. Again, you will remain invisible to the jet's TCAS without a transponder.

Transponders started out as simple devices to help ground controllers identify aircraft on their radars, but have evolved into something quite complicated to support collision avoidance and modern ATC. Gliders in high-jet-traffic areas often carry transponders to be visible to ATC and visible to the collision avoidance systems on larger aircraft. Below are the (very highly condensed) facts you need to understand to follow the ADS-B discussion.

Mode A/C Transponders

Mode A/C (often just referred to as Mode C, or more technically ATCRBS) transponders reply only when they receive an interrogation message. Interrogations are sent either by an FAA radar ground-station, or by an airborne TCAS collision-avoidance system on a large aircraft. Depending on the kind of interrogation message received, your transponder replies with either the transponder code you set on the instrument panel (for a Mode A interrogation message), or the altitude from your altitude encoder (for a Mode C interrogation message). The transponder code reply also contains a flag indicating whether you have pressed the "Ident" button (this helps ground-controllers sort out different aircraft).

Mode A/C transponders create serious issues for ATC and collision-avoidance systems. Because there are lots of jets flying around with TCAS systems firing out interrogations, and each aircraft may be interrogated by multiple ground stations, transponders can get very busy in high-traffic areas (see TCAS discussion below). Thousands of transponder replies per second are not uncommon in certain areas, and replies can and do step on each other (think lots of planes all shouting at the same time). This can lead to ATC being unable to hear the transponder reply for an aircraft ATC is tracking on radar.

Mode S Transponders

To reduce problems associated with Mode A/C transponders, Mode S transponders provide "selective" replies with additional information. Mode S transponders work better in the current and future ADS-B environment, though they are also compatible with older interrogators (that is, they behave like a Mode A/C transponder when they receive old-style Mode A/C interrogations). Mode S is already required in many controlled airspaces around the world and will gradually replace Mode A/C over time, for example Mode A/C is effectively outlawed in Germany and The Netherlands.

Mode S adds selective response. Each Mode S transponder has a unique ICAO aircraft identifier programmed into it during installation. Two new kinds of interrogations can request replies from:

  1. only a specific aircraft as specified by its unique identifier (that is a selective interrogation), or
  2. any Mode S transponders, in which case all transponders reply with just their unique IDs

Thus, its possible to find out what aircraft are near, and selectively interrogate them.
Mode S transponder selective replies include:

  1. the pilot-set transponder code, and the altitude from the altitude encoder
  2. the aircraft's unique ICAO identifier (yes, big brother is watching)
  3. the aircraft registration or scheduled airline flight identifier

Because Mode S transponders reply selectively, they don't transmit as often - so they use less power.

Mode S Transponders with Extended-Squitter (also referred to as 1090ES and ADS-B-Out via 1090)

ADS-B-Out requires an aircraft to periodically broadcast its identification, position, and track - no interrogation required. In addition to basic Mode S transponder function, some Mode S transponders (for example the low-cost Trig transponders) are also ADS-B-Out transmitters when coupled with a GPS. Every couple seconds they transmit an Extended Squitter (ES) message which includes identification, pressure altitude, and GPS position and track.

In the US, the FAA has announced that effective on January 1, 2020, any aircraft operating in airspace where a Mode C transponder is required today will also be required to carry an ADS-B-Out transmitter. Other than the glider exemption, this typically means in class A-C airspace, above 10,000', or in a mode C veil. This effectively will require replacement of Mode C transponders with a Mode S units that include 1090ES (ADS-B-Out via 1090) plus a GPS to feed position to the transponder.

[ Editorial: Many of the newest and lowest cost transponders are Mode S and include 1090ES (aka ADS-B-Out, e.g. Trig, Garrecht, Funkwerk/Filser transponders). Many of these new units include an internal altitude encoder as well, further reducing purchase and installation costs. If you're worried about getting run over by a jet and think you need a transponder, get a Mode S unit with 1090ES. Without ADS-B-Out you will usually not receive TIS-B or ADS-R traffic information. And Mode S has a better chance of being heard by a TCAS in high-traffic areas. If you already have a Mode C transponder it will make you visible to jets and ATC, but for new installations get Mode S. ]

TCAS II Collision Avoidance

In the US, the TCAS II collision avoidance system has been required in large aircraft since 1994, and is now carried on most corporate jets. TCAS II works by interrogating transponders and giving the pilot instructions to avoid a collision when a hazard exists. To see if a reply presents a collision hazard, TCAS II combines directional radio antennas (to determine range and bearing and speed) with altitude received from the transponder.

What is PCAS ?

PCAS stands for Portable Collision Avoidance System, for example Zaon. A PCAS receives transponder replies and alerts the pilot if there is a potential conflict. Zaon's XRX model has a directional antenna and gives direction as well as estimated range, but this unit is generally too large for gliders. The Zaon MRX unit is popular in gliders due to its smaller size and lower cost, and gives range only. For Mode C transponders, PowerFLARM functions similarly to the Zaon MRX. Unlike the Zaon MRX, PowerFLARM also receives ADS-B.


Next, read about ADS-B here...


For Techies Only, More Details about Transponders and TCAS

  1. Not all Mode S transponders support 1090ES (ADS-B-Out via 1090). Some transponders that do support 1090ES require an expensive ARINC GPS input; not currently practical for gliders. The low cost Trig Mode S transponders support an NMEA GPS input and use the NMEA GPS information for 1090ES messages, so can be fed by any flight recorder including PowerFLARM.
  2. Mode A, C, and S transponder replies are all transmitted on 1090 MHz, which is why "1090" is used as a qualifier in ADS-B discussions. Interrogations are transmitted on 1030 MHz.
  3. Mode S greatly reduces simultaneous replies, which is why Mode C transponders have already been phased out in many countries and airspaces and why this phase-out continues outside USA...
  4. Eurocontrol measured in excess of 33,000 transponder replies in a peak second at Frankfurt, prior to when Mode S transponders became mandatory in Germany.
  5. Given all the simultaneous replies from Mode A and Mode A/C transponders, the reason any of this works at all is the power from a given transmission decreases as the square of the distance. Thus, closer transponders drown out transponders further away. This works great for collision avoidance, where you really only care about the planes close to you. It doesn't work so well for the traffic control system when it tries to interrogate aircraft at some distance.
  6. As of TCAS II version 7.1, TCAS II does not take advantage of 1090ES GPS information (it only uses radio signal-strength ranging and encoder altitude, not GPS coordinates and track). TCAS II does use Mode S selective addressing when communicating with Mode S transponders to reduce interrogation rate.
  7. 1090ES information is used by NextGen systems (ground stations and airborne traffic displays).
  8. A transponder is generally not an ADS-B-In receiver - they do not typically include the additional hardware and software required for general 1090 reception. For example the Garmin GTX330 provides TIS reception (received on 1030), but does not have a 1090 receiver so cannot receive TIS-B over 1090.
  9. A posting by Darryl Ramm discussing details about 1090 MHz RF congestion and collisions
  10. When two Mode C transponders are interrogated at the same time (as when they are close), they respond at the same time. This can lead to garbled responses as the replies step on each other. This is referred to as synchronous garbling.
  11. To avoid synchronous garbling, formation flights work with ATC and activate only one Mode C transponder in the formation.
  12. To avoid garbling in extremely high traffic areas, VFR traffic may be instructed to turn off Mode C transponders. For example, VFR traffic arriving and departing the Oshkosh airshow are instructed to turn off Mode C transponders.
  13. A gaggle of gliders with more than a few Mode C transponders may produce synchronous garbling. The probability of garbling is affected by number of gliders, their antenna orientations, altitudes, and proximity.
  14. A gaggle with multiple Mode C transponders active may be invisible to TCAS collision avoidance, because TCAS must receive ungarbled Mode C altitude replies to function.
  15. Replies for Mode A and Mode C interrogations use the same encoding. There is no (simple, foolproof) way for a receiver to tell whether it just heard a transponder code or an altitude, nor associate an altitude with a specific transponder code when there are lots of replies. Its a little easier for the station doing the interrogation and using directional antennas, but hard for a passive receiver to make sense of the cacophony.
  16. Some Bonanza-class aircraft carry TCAS I, which is less sophisticated than TCAS II. TCAS I interrogates transponders and provides traffic warnings, but does not provide collision avoidance instructions like TCAS.

References for Further Detail

Mode S Overview from Eurocontrol
Eurocontrol Mode S FAQ, including deployment requirements around the world
TCAS overview

Please send comments, questions, and corrections regarding this page to Dave.Nadler@Nadler.com