How does TCAS determine distance?
 

Watching the little blips the other day and it occurred to me - how does my little unit tell how far away the other blips are?

Height I understand - and mode C is height at 1013 datum, but the distance?

The Pilots Guide for my Honeywell KMD550 does not describe the operation either.

Like any radar!

Send out message, wait for reply, and time it. Probably has a fixed "addition" for receiver to process signal and return it...

NoD

I may be wrong but I don't think it is so much distance that it looks at, but closure rate. (vertical and horizontal)

... and how does it determine whether the other aircraft is getting closer?

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.