Heathrow Director: as I understand it, a directional antenna with four slightly overlapping 90+ degree segments, is used to reduce overlapping transmissions/garble, multipath interference and/or other interesting things I know little about. TCAS II (current) uses a directional antenna on top of the a/c and most installations also a directional option at the bottom. Very neat and well engineered. A side effect is that there is some directional information which can't be spoofed easily...
see Introduction to TCAS II V 7.1 from the FAA, p 11, 12, 18, 19, much more details
cue "a firm grasp of the non-essential" for checkers.
Edit: TCAS doesn't derive angle/bearing from directional antennas (this was intented in TCAS III, but precision isn't good enough), yet the directional antennas could be an obstacle to a "hacker"
