No Rata, Nigel on Draft is correct. Closure rates are calculated . To do this the relative position of a target is determined first.
Distance is established by time interval. 12.35 microseconds is a 'radar mile', time for radar waves to travel one nautical mile and a reply to return. Deduct a processing interval.
Relative bearing is determined by the TCAS receiving aerial being directional.
Altitude of target referenced to 1013mb is announced in its reply.
These three forms of data can fix a target in relative position. Repeating the process gives a change in position and relative vertical and horizontal closure rates are easily calculated.
Multiple targets, interrogation rates, message formats, degarbling, collision avoidance envelope, time to go to closest approach point....the details are very complicated but happily irrelevant to the user.
Last edited by avoman; 7th Sep 2003 at 19:55.