Transponder antennas

[SE210]

Commercial aircraft have two Mode S antennnas.

Is this due to the fact, that there are two transponders installed, or does each transponder need both antennas?

[greenspinner]

SE210, excerpt from AMM

An airborne transponder provides coded reply signals in response to interrogation signals from the ground secondary radar and from aircraft which will eventually be equipped with the TCAS.
The ATC/Mode S system has the capability for Modes A, C and S.
Mode A: identification
Mode C: altitude range
Mode S: In a Mode S environment, the transponder responds to ATCRBS-formatted interrogations with ATCRBS-formatted replies, but, in addition, it can be discretely addressed directly by Mode S interrogators. Discrete addressing means that only the specifically addressed Mode S transponder will reply. Top and bottom antennas and associated antenna switching and signal processing features provide the Diversity antenna features that allow compatibility with TCAS II. The diversity antenna allows selection of signal receptions from either the top or the bottom antenna based on the characteristics of the received interrogation signals. The transponder selects the antennas (lower or upper) which receive the best transmission signal from the ground ATC secondary radar. This improves air-to-air surveillance and communication. The ATC/Mode S transponder, is also used in collision avoidance. In this role, the transponder performs as an ATCRBS and Mode S transponder for ATC requirements, while simultaneously serving as an integral part of TCAS. The Mode S transponder has two basic functions related to TCAS:
(1) Approximately every second, a Mode S transponder makes a transmission known as squitter. These transmissions alternate between the top and bottom Mode S antennas. Squitter is coded with the aircraft Mode S address and equipment capabilities. A ground station and airborne TCAS listen for squitter as the initial indication that a Mode S-equipped aircraft has entered the surveillance area.
(2) The communication between two TCAS-equipped aircraft occurs from the TCAS antenna of one aircraft to the Mode S transponder antenna of the other aircraft. Aboard the TCAS-equipped aircraft, data received by the Mode S transponder is passed to a TCAS computer, and the TCAS computer determines the content of reply messages.
The Mode S reply includes the flight number transmitted by the Flight Management, Guidance and Envelope Computer (FMGEC).
The ATC system operates with two frequencies:
-
interrogation: 1030 MHz
-
reply: 1090 MHz

Modes A, C and "All call" interrogations are transmitted in the form of coded pulses PPM (Pulse Position Modulation).
An "All call" interrogation is an interrogation which generates Mode A or C and Mode S replies.

[ATCast]

The two antennas are there because the airframe would otherwise shield the radio signal causing blind spots for TCAS, Radar and ADS-B. The transponder replies to interrogations through the antenna on which the strongest interrogation signal was received. Squitters (transmission unsolicited by interrogations used for TCAS and ADS-B) are send more or less randomly out of either the top or bottom antenna. It is called "antenna diversity"

Aircraft fitted with a Mode S transponder and with a MTOM > 5700kg or maximum cruising speed > 250 KTAS must have antenna diversity (ICAO Annex 10, vol IV, 2.1.5.3)

As far as I know, there is no requirement to carry two transponders. The reason for carrying two transponders is redundancy; if you wouldn't have a backup system, a transponder failure would mean landing at the nearest suitable airport in most airspaces.