I'm working on a robotics project that I believe could greatly benefit from the use of differential GPS. The robot will be navigating within a known 100m by 100m area and will need to have a positional accuracy of about 25cm. These two things lead me to believe that DGPS is a good fit. I know that with DGPS, I need a receiver that lets me see the pseudoranges to the visible satellites. These receivers tend to be expensive, but I'm not sure why. I was looking at the u-blox LEA-6T as a potential candidate, but I'm definitely open to other suggestions. Will I need pseudorange-outputting units for both the base station and rover, or just the base station? If I at least know which satellites the rover unit is calculating its position with, then that should be enough, right? Is knowing that information available in cheaper units? As you might guess, I'm pretty much just starting my research on this, and I'd appreciate any thoughts and recommendations. Thanks.