there are 2 way radios that can monitor where other people on the channel are at the time, and incorporate GPS capability, but something to consider is OPSEC. By using this on the other teams channel (of dubious legality for in game rules) but you COULD use this to pinpoint enemy movement by their radios.
Of course I don't know what range of freqs the thing can pick up, be it just within a small range (2 way radios) or if it could branch higher to pick up military grade radios.