I'm building a unit that needs to head away from its point of origin
until it gets to a preset distance. At the moment the distance calc is
based on time elapsed. This doesn't work very well because speed varies.

I like the idea of building a UHF transponder/beacon set. The unit sends
a unique pulse train then listens for a reply. The beacon receives and
decodes the pulse train, waits a preset time, then replys. The unit
receives the reply and calculates distance from the signal's time of
flight. Jittering the PRF and range gating would improve signal to
noise.

Max distance is around 1500m so a 2km range would be desireable. I'd
like transmit status information in the pulse. Basically what I'm after
is a low power UHF (~400MHz) version of the air traffic secondary
surveilance radar.

Trouble is I don't know where to start :(

I'm comfortable making my own PCBs and programming PICs but don't know
much at all about RF circuits. Would someone care to point me to a good
starting point? A website or a readily available book would be nice. A
similar project I could examine would be even nicer :)

TIA
Rob

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