Why and how can TCAS and SSR transponder work at the received frequency of each other?

Question

The TCAS sends interrogation pulses at 1030 MHz and receives replies at 1090 MHz. And the transponder replies interrogations at 1030 MHz receiving at 1090 MHz. There could be interference between them, right?

Answer 1

To clarify:

• A transponder replies on 1090 MHz to ATC interrogations received on 1030 MHz.

• A TCAS elicits nearby transponders replies by sending interrogations on 1030 MHz, and listens for replies on 1090 MHz.

So each one transmits on the reception frequency of the other. This is what allows a TCAS to detect threats, even when there is no TCAS on the other aircraft. As you assumed, both transmitters need to be synchronized to prevent the saturation of the receivers.

For TCAS able to coordinate with the other TCAS, continuous data exchange occurs on 1090 MHz using the mode S transponder datalink, thus this transmission is not performed by the TCAS itself.

Radio equipment global synchronization

There is a dedicated avionics mutual suppression bus/line used to report radio equipment status (transmit/receive), to allow different units to be synchronized.

Source.

Interference prevention specific to TCAS

Several techniques are used, both to limit local radio interferences, and spectrum occupancy.

• “whisper/shout mode” used by TCAS on 1030 MHz, to use the lowest possible transmission power to not interfere with aircraft which are too far to be threats (to lessen FRUIT on ground stations)

• Beam-steering for top antenna in order to elicit replies only from transponders in a certain azimuth, and adapt the transmission power for this azimuth.

• Number of active interrogations limit based on the number of TCAS in the vicinity, to not increase FRUIT beyond limits defined by ICAO equations. This reduces the detection range.

• This limit also prevents transponders from being suppressed a too long time. Transponder inhibition occurs after transmission, to reduce the spectrum occupancy. Suppression can also be explicitly triggered by a mode S interrogation from a TCAS.

For a detailed view of the interference limiting procedures, see ICAO Doc 9863)

The next two sections are more about TCAS use of frequencies than a direct answer to your question.

TCAS-to-TCAS coordination on 1090 MHz

In order to coordinate resolution advisories, in particular the vertical rates, both aircraft must be equipped with a TCAS implementing ACAS II protocol (i.e. a TCAS 7.1 or higher), this implies a mode S transponder.

A TCAS sends interrogations on 1030 MHz and listens for replies on 1090 MHz. For this phase the TCAS is relatively independent.

But as soon as a thread is detected, the resolution is coordinated by exchanging data messages between TCAS on 1090 MHz. The TCAS prepares the data to be sent on 1090 MHz, the transponder performs the transmission.

ADS-B Out also uses the transponder datalink the same way.

Overview of ACAS II protocol

We assume the aircraft looking for intruders uses ACAS II.

Observation phase

• The TCAS passively listens to transponder replies to ATC (in modes A/C and S) and to DF17 messages (ABS-B broadcast), transmitted by potential intruders on 1090 MHz.

• TCAS also elicits direct transponder responses by broadcasting interrogations on 1030 MHz (modes A/C and S). Aircraft in the vicinity reply, and a TCAS can determine each aircraft range and azimuth, and for replies in mode C and S, altitude and vertical rate, identifying possible threats.

Threat detection and resolution

• When an intruder enters the closest protection envelope, the TCAS elaborates a resolution advisory, based on its maneuvering possibilities at the moment, and pushes the advisory parameters into register 30 (hexadecimal) of the aircraft SSR transponder.

• The transponder sends the resolution advisory in a DF16 message (on 1090 MHz), this message is received by the TCAS of the intruder, and, assuming the intruder has an operational TCAS, is used to determines a complementary maneuver.

• If the intruder TCAS implements only ACAS I, the process cannot be coordinated and stops here. Else a coordinated TCAS-to-TCAS exchange continues on 1090 MHz via the transponders. The resolution advisory may be adjusted by each TCAS, until the threat is cleared.

More

• ACAS protocol in ICAO Annex 10, vol 4.
• Mode S in ICAO Doc 9688.
• Mode S messages in The 1090 Megahertz Riddle.
• My answer to Why is a Mode S transponder required to be installed and working for TCAS II to be operational?