A secondary surveillance radar can be either the good old ATC Beacon Interrogator or the Mode S (for Select). TCAS is TCAS
If a ground sensor interrogates a Mode S equipped aircraft on the first time it either has the target and track information
handed off from another sensor or uses an all call. Subsequent interrogation responses are campared with the track data and single anomolous replies rejected. If the ground sensor is a ATCBI interrogator and the aircraft has the old ATCBI aboard the interrogator will receive the transponder transmissions caused by the TCAS interrogation as well as the ground sensor interrogation if the ground station antenna is pointed close enough to the aircraft replying to the TCAS interrogation. I don't recall what the beam widths and stuff for the old ATCBI-5 or 6 are. The Mode S . Through time (range) gating features in the sensor it will reject returns that are not from it's interrogations mostly. It will then provide the target information to the ATC facility which will then correlate the information with other track information. If the ground sensor is a Mode S sensor this correlation willl be done at the sensor site.
Mode S interrogations are either 56 bit or 112 bit and though I do not have the specifications for TCAS I believe they use a very similar data format with the same, or many of the same fields.
By the way, the Mode S specification describes performance using a 12.5 RPM scan rate.
Hope this helps
The question, of course has nothing to do with Mode S (selective) beacon interrogation or TCAS, which are essentially data radios with data links sending information on the aircraft and interrogation back and forth.
Last edited by Iron City; 16th Jul 2002 at 12:56.