Software libraries exist for turning a low cost GPS receiver into an RTK GPS. They assume you have a receiver capable of providing phase information and a connection to a NTRIP service
- GoGPS: developed at Politecnico di Milano which supports thess hardware/services
- RTKLib: developed at Tokyo University of Marine Science and Technology is written in C and supports different type of receivers. They have also a very deep analysis of GPS antennas as well, unfortunately it is in Japanese so you might prefer to read the paper in English.
Here you can find some documents evaluation the result you might obtain by using these libraries
- Kjeld Jensen, Morten Larsen, Tom Simonsen and Rasmus N. Jørgensen. Evaluating the performance of a low-cost GPS in precision agriculture applications. RHEA-2012 - First International Conference on Robotics and Associated High-Technologies and Equipment for Agriculture September 19-21, Pisa, Italy
You could built your own, but it has to be properly designed since it requires to handle RF parts. These resources might become the useful:
- Evaluation kit from ublox such as the EVK-6T (295 euros on 04/12/2012) or the EVK-6PPP (295 Euros on 04/12/2012). With case and antenna.
- UBLOX NEO-6P Precise Point Positioning Reference Board (169.99 USD on 04/12/2012) with SMA antenna connectors for UAV systems, Robots, RC, FPV or other projects. You can buy also an Active GPS Antenna (19.99 USD on 04/12/2012)
- Michele Bavaro produces evaluation kits for several low-cost GPS receivers including the S1315F-RAW, the NV08C-CSM, and the uBlox NEO-6X. He works in cooperation with optimalsystem.