h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The TCAS or ACAS function is carried out by the TCAS processor which is separate from the transponder function, although in the latest equipment they may be physically packaged together. Smaller planes may have only the transponder which generates Air Traffic Control replies and replies to TCAS equipped aircraft; commercial transport aircraft have the TCAS processing function which does the collision avoidance calculations and generates appropriate warnings and displays the advisory information as well as exchanging TCAS data with other aircraft. The transponders have omnidirectional antennas; the TCAS processor antenna(s) is (are) electronically steerable and the output power can also be controlled; this helps in directing TCAS interrogations to specific aircraft since the system needs to monitor up to about 40 other aircraft at a time.

Good job on your write-up of this obscure technology!