Here's maybe more than you wanted to know:
http://www.gtwn.net/~keith.peshak/TexasRanger.htm
I had his Altitude Nag in my Bellanca Cruisair back in '90-92, and it was a great product; still is.
He wrote a white paper back then about the Iran Air A300 that was shot down by the USNavy in what, 1987? He postulated that it was the fault of the Navy radar confusing the 6400 ft. altitude reply with an IFF code that was sometimes used by the Iran AF. You know, it's only the spacing of 21.3 microseconds vs 8 microseconds that distinguishes an altitude interrogation from a code interrogation.
I'm fairly new to Pprune; has that accident been discussed?
GB