As I understand, according to this FAA booklet on TCAS, transponders will engage in cycles of interrogations and responses to establish if a collision may occur.

The next step is for both TCAS systems to negotiate respective resolution advisories. My question is how the negotiation takes place in the TCAS system internally, and in particular what communication protocol takes place for the negotiation.

To elaborate, as an example, a browser can make an HTTP request to a server (like this site) which follows protocols as to what information should be provided and how it should be encoded.

Along the same vein, how is information encoded for TCAS systems to negotiate? Since I don't know of any open source TCAS used in commercial aviation, I can't inspect code.

  • $\begingroup$ Related $\endgroup$
    – Pondlife
    Mar 25 '18 at 13:39
  • $\begingroup$ Also related $\endgroup$
    – DeltaLima
    Mar 25 '18 at 14:05
  • 1
    $\begingroup$ @DeltaLima Just because it is about TCAS does not make it related; my question is about how data is encoded for a TCAS negotation, and I do not find that question pertinent. It details how the algorithm works, rather than what is actually being sent for negotiations. $\endgroup$ Mar 25 '18 at 14:06
  • $\begingroup$ @user1997744 If you look at the accepted answer to that question, under the header RA coordination you will find a short description of what is being sent. $\endgroup$
    – DeltaLima
    Mar 26 '18 at 9:13
  • $\begingroup$ @DeltaLima yes but it’s quite obvious its intent is being sent - what else could it be? It has to say what action it wants to take one way or another. It’s the encoding that is pertinent. $\endgroup$ Mar 26 '18 at 9:43

TCAS air-to-air messages use the same format and frequencies as "Mode S" surveillance radar. They're defined (in great and overwhelming standardesque detail) by a document called "ICAO Annex 10".

ICAO wants serious money for a copy of it -- and of course if you're producing actual avionics nothing less than the official source will do -- but for satisfying idle curiosity it's often possible to find a draft or bootleg by persistent googling.

The Mode S message format is also used for ADS-B broadcasts, so equipment to receive and decode them is relatively easily available to hobbyists. The lower layers of open-source ADS-B software will also be generally relevant.

  • $\begingroup$ So ICAO Annex 10 would be what would be used to encode say, “I want to descend while you climb”, so to speak in layman’s terms? Oh and which volume of Annex 10 is relevant here? $\endgroup$ Mar 25 '18 at 14:58
  • $\begingroup$ @user1997744: If I remember correctly, the algorithm is constructed such that in most cases the two TCASes would produce complementary RAs without waiting for such an explicit negotiation, but instead comparing each other's reported altitudes and airframe addresses. But in order to get all the corner cases there are also explicit negotiations of the kind you describe, and somewhere in Annex 10 their exact format will be defined. I don't remember whether Annex 10 also contains a high-level description of the algorithm or you need to go elsewhere for that. $\endgroup$ Mar 25 '18 at 15:08
  • $\begingroup$ Okay, I’ll try to get a copy and grep for some keywords to try to find that part. Thanks. $\endgroup$ Mar 25 '18 at 15:12
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    $\begingroup$ @user199744 Take a look at RTCA DO-185B, the Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance System II (TCAS II) for all the details of TCAS II algorithms and message formats. It is more detailed than annex 10 (which repeats part of DO-185B). $\endgroup$
    – DeltaLima
    Mar 25 '18 at 20:10

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