RTKLIB has a number of different algorithms it can use to calculate position. The first two in the list are “Single” and “DGPS”. Both of these methods use only the pseudorange data and not the carrier phase information. Without the carrier phase information, the precision of these methods is quite low, and probably worse than what the GPS receiver would provide without RTKLIB. In general they are not very interesting other than possibly for some initial debug during setup. The rest of the algorithms can be divided into two groups, differential and PPP. The differential algorithms determine position relative to a known nearby location while the PPP (precise point positioning) algorithms determine absolute position. In general, the data quality of the low cost hardware we are using is only good enough to use with the differential methods and not the PPP methods, so we will focus on those. RTKLIB supports four differential modes: Static, Kinematic, Moving-Base, and Fixed. The kinematic mode is designed to calculate the relative position between a fixed base and a moving rover and that is what we will use. It can also be used with a moving base if we are concerned only with finding the distance between the two and not trying to translate that to a fixed position, and we will use it this way as well.
In my particular application, I am interested only in the distance between two receivers and not in any absolute locations. In this case I will collect data from two low-cost receivers and use one as the base and the other as the rover. In many cases, though, it is useful to use an existing ground station as the base and the low cost receiver as the rover. I use this method for testing and verification, even though I don’t plan to use it in my final solution.
For the ground stations I have used, it is generally available online less than an hour after it is taken. It is fairy easy to pull it manually using a user-friendly form from the CORS webpage, or you can use the RTKGET utility in RTKLIB. Be aware that some stations have only GPS data, and some have both GPS and GLONASS data.