When a TCAS system issues an RA, it emits an inhibit command to the conflicting a/c over the mode-S link, causing the second TCAS to issue a preventive RA rather than another corrective one, which could potentially nullify the one issued by the first system.
If they both detected the conflict simultaneously and issued a corrective RA each, the TCAS system with the highest Mode-S address will have priority and the RA for the system with the lower address will be cancelled and changed to a preventive.
If you are in TA&RA mode while unable to climb, this can clearly lead to a situation where another TCAS system 'nhibits or cancels the corrective RA that could have saved the day and instead issues a preventive one that would have saved the day as well - if you had been able to climb as your TCAS told you.
Oh, on the technical side. Distance is determined by the reply times as per usual ATCRBS operation, altitude by the altitude encoding in the replies and bearing by using a directional antenna.
Does anyone have handy the details of what information mode-S is capable of transmitting?
With all this possible with plain oleŽ mode-S, imagine what will be doable with ADS-B. Wow.
Cheers,
/ft