Hi GrahamC,
3 grams is certainly ambitious, particularly for a GPS logger with remote interrogation or GPRS. As gm mentioned, Microwave Telemetry sell some very small tags, but as far as I know, they’re not GPS, rather they’re PTT tags, with all the inherent accuracy problems. Still, 9.5g is a great achievement even without GPS.
Have you considered “old-school” VHF tracking? I just posted a reply in a thread about tracking turtles (viewtopic.php?t=13805&postdays=0&postor … 724f260924) asking if there was anybody interested in helping to develop a cheap, open source tracking beacon. Could prove of interest for all of us wildlife people (and James Bond aficionados).
I’m all for ambitious ideas so I wish you the best with this project and if anything else jumps to mind I’ll post.