One has to be careful that time and money are allocated correctly. For several years I've been contemplating developing a lower cost GPS Differential/RTK system using commercial receivers.
While an open source solution would be a nice to have, it really makes no sense. I can purchase off the shelf differential GPS solutions that are inexpensive given any realistic valuation of my time. Rather than spend months developing a GPS solution I've decided to just purchase one off the shelf. I'm going to use the hemisphere GPS crescent oem board with their RTK and 20 Hz options. I'll integrate the RTK base into my Telemetry suitcase and the rover goes on the Rocket/Helicopter. This is just so much easier than developing a either a GPS solution or a fancy interial/GPS Kalman filter. The data sheet accuracy for the RTK solution is 2cm. If it can keep that in a dynamic environment that will be amazing, even the specified L-diff solution at 28cm is plenty good enough.