Can TCAS calculate the altitude of its aircraft and that of others in vicinity?
TCAS doesn’t calculate altitude. It listens to the transponders of other aircraft as they broadcast their own altitudes; that’s how it knows the altitude of the other aircraft. When two aircraft with TCAS approach each other, the TCAS systems negotiate at high speed to determine which aircraft should descend and which should climb. At very low altitudes, integration of ground proximity warning systems with TCAS allow the former to override the latter, so that a pilot is warned of terrain and does not attempt to follow a resolution advisory that tells him to descend. The other aircraft will still be advised to climb, so it should still be possible to resolve any conflict. Simple versions of TCAS (often installed in small aircraft, because they are less expensive) simply advise of potential conflicts, but do not issue resolution advisories. With these systems, it’s up to the pilot to decide what to do. Airliners have the more advanced TCAS II systems that always provide RAs if required.
