forked projects?

I am embarking on a project to build a GPS receiver in software around my
hardware, the Universal Software Radio Peripheral (USRP, see
http://www.ettus.com for more info).  I wanted to use OpenSourceGPS to do the
position fixes, but I'll have to do my own tracking, etc.

My question is this -- it seems that there are several disparate efforts around
the same code base.  There's a PCI one, RTAI linux-based one, and a couple for
some different hardware.  Is there a common code repository for these?  Or have
they forked?  It would be nice to adapt the main code base so that different
hardware and operating systems could be used without duplicating effort on the
common parts.

Thanks,
Matt Ettus
Ettus Research LLC