(see http://gpspp.sakura.ne.jp/rtklib/rtklib_releasenote.htm )
Basically, we feel as if super-accurate GPS positioning is the sort of tool that makes people have creative ideas of what they can do with that data.. For example, given some time, it may be possible to -say if there is an earthquake, the earth moves.. measure that change, measure continental drift.. measure the rebound in formerly-glaciated areas we are seeing now after the ice age.. build accurate 3D models for archaeology, etc..
It appears that the weak link in the chain for us at this point may be the antennas.
We want an accurate, low-cost antenna that is able to receive righ-hand circularly polarized signals from the entire sky at as close to a single point, electrically as possible, while rejecting reflections well. Its turning out that issues that are not important when the criteria for success is merely getting lots of satellites still make it impossible for a lot of antennas to work well in a high precision application, because they delay the signals coming from some directions enough more than others to make the fixes literally jump around a substantial amount from satellite to satellite. (if you are looking at the raw data coming from the processor)
Ground reflections, probably, are what causes the wildly jumping errors we are seeing. Some antennas (choke ring antennas) cancel them out..
We understand that both in theory and in the real world errors can be reduced to very small distances, in the real world they often are- with expensive antennas.. Thats why they are expensive. The better antennas are also often relatively bulky. They have to be positioned properly, too, to actually be accurate.
What are your experiences with GPS antennas?