I am wondering why a Mode S transponder is required to be installed and working for TCAS II to be operational? I read such phrase in Eurocontrol ACAS II guide and I don't understand why mode A/C is not sufficient for TCAS II to be working.


2 Answers 2


From the same guide:

Connection with the Mode S transponder – to issue complementary and coordinated resolution advisories, when both aircraft are equipped with TCAS II.


When the intruder aircraft is also fitted with an ACAS II system, both systems coordinate their RAs through the Mode S data link [...]

This TCAS II feature of communicating the resolution advisories is built around the data link of Mode S. Mode A/C does not have the ability to encode and send the RAC.

Resolution Advisory Complement (RAC) – Information provided by one ACAS to another via a Mode S interrogation in order to ensure complementary manoeuvres by restricting the choice of manoeuvres available to the ACAS receiving the RAC.

Note that "ACAS I" does not generate RAs (see same guide or ICAO Annex 10 Vol IV), and ACAS II needs Mode S, therefore it becomes clearer that two planes capable of generating RAs but without coordination is not good.

  • $\begingroup$ So the reason is to prevent 2 TCAS II airplanes to choose the same RA? $\endgroup$
    – Konrad
    Dec 14, 2021 at 20:24
  • $\begingroup$ @Konrad: yep; the first quotation is about that "connection" (i.e. how Mode S relates to ACAS II). ACAS I not generating RAs makes it even clearer I think. $\endgroup$
    – user14897
    Dec 14, 2021 at 20:57

Short answer

That's a good question since a TCAS II could send and receive coordination messages using its own transceiver and antennas. Why does it use another mean of communication in the first place?

This is to centralize all transmissions done on 1090 MHz, so that they don't overlap with SSR and ADS-B transmissions. The transponder is responsible for managng transmissions on the downlink frequency 1090 MHz.

Frequency management

While a TCAS II is able to listen to other aircraft on 1090 MHz, it doesn't transmit on this frequency. 1090 MHz transmission is completely delegated to the transponder, because:

  • TCAS messages must be correctly sequenced with SSR mode S replies on 1090 MHz and ADS-B messages, in order to prevent overlapping. It makes sense to use the transponder to decide when to send messages originating from these different sources.
  • TCAS II coordinated resolution process involves direct aircraft-to-aircraft communication. A mode S transponder is granted a unique address, and this address is used to select the target aircraft.


  • 1030 MHz

    • is listened to by SSR transponder.
    • is used for transmission by ACAS II to trigger transponder replies from aircraft in the vicinity
    • is used for transmission by ACAS II to broadcast their initial resolution message.
  • 1090 MHz

    • is listened by ACAS II
    • is used by ACAS II to reply to another ACAS II.
    • the actual transmission is delegated to transponder. In order to allow resolution coordination, this transponder must be mode S.


ACAS II protocol, which current implementation is TCAS II 7.1, is used to detect aircraft and exchange resolution data when working with another aircraft equipped with another ACAS II. This exchange is the last phase of ACAS II 3-phase sequence: Detection, Surveillance and Coordination.

The coordination phase includes for ACAS-ACAS resolution the exchange of vertical resolution advisory complement (vertical RAC or VRC), which can tell the intruder, for instance, "do not pass below", because the aircraft has itself prepared/issued a resolution advisory consisting in descending.

  • If no VRC has been received the RA (resolution adivsory) is based on the encounter geometry alone. This is what happens when the resolution is initiated by the first aircraft.
  • If a VRC has been received the RA is determined taking into account the restriction received. In case of conflict, because the two aircraft initiated the resolution at the same time, the one with the lowest ICAO address has the priority.
  • Once the RA has been determined, each aircraft continuously broadcast messages with the VRC restriction, so that the other aicraft can be sure the two RAs are still complementary.

Such VRC are send using messages carried by a transponder mode S 112-bit frame, and therefore a transponder mode S on both sides is required for coordinated resolution. ACAS II capability when the other aircraft is equipped with different systems:



Some details below, and for how mode S transponder is used as a modem by ADS-B and ACAS, see also What's the relation between ADS-B and MODE S.

Mode S messages used

DF and UF are mode S message formats used respectively for downlink and uplink communication, described in mode S protocol.

To detect other aircraft, ACAS II listens to:

  • DF11 messages (56 bits) sent by transponders mode S when interrogated by SSR and by transponders to acquire other aircraft.
  • DF17 messages (112 bits) sent by ADS-B out.

To initiate a session with a detected aircraft, ACAS II sends DF0 messages (56 bits) at short interval until the other aircraft answers, and other DF0 messages are sent as soon as the other aircraft enters the surveillance perimeter.

If the other aircraft enters the resolution perimeter, it is considered a threat, a coordinated resolution sequence is initiated:

  • Message UF16 starts the sequence. It contains a MU field.
  • Messages DF0 (56 bits) and/or DF16 (112 bits), are continuously exchanged. DF16 messages contain a (MV) field.

MU and MV fields have a length of 56 bits and carry:

  • Information about aircraft ACAS resolution capability: No capability, vertical resolution, vertical and horizontal (horizontal resolution is for the future, there are no current implementations).
  • Whether the resolution involve 2 or more aircraft.
  • What the other aircraft can do vertically or horizontal when selecting the resolution action: Passing above or below may be prevented first, and if this is not possible reallowed to continue negociating the resolution. Another information is used for left/right turns. These data are known as vertical (VRC) and horizontal (HRC) resolution adivsory complement (RAC) and the cancellation as CVC an CHC.

MU field:



MU and MV fields with a length of 56 bits are sent using longer 112-bit transponder messages.

ACAS II has a degraded mode based on transponder mode A/C replies, but in that case no RA coordination is possible. ACAS II stops working if the transponder fails.


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