I'm guessing there is an internal API exposed thru BREW (which is Qualcomm's "User mode, poor man's OS"). I think the Verizon RAZR's VZNavigator is a BREW application. BREW also has a PC emulator. If one can somehow "debug" VZNavigator on the BREW simulator, that may offer some more insight into the AGPS interface. Of course, I don't have ANY tools to create/debug stuff, so unfortunately I can only talk
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Getting the A (=Assisted) of AGPS working might be tough since it requires a network component to cooperate (i.e. a local base station tells your unit "you are in 'this' state"). Without the assisted component the time to first lock is a few minutes instead of a few seconds. But hey - given the level of interest from devs/hackers I'd be glad if we even got to GPS, let aside AGPS.