I think you can easily do two primitive things: * Receivers in the metal-reinforced concrete canyons of a big city suffer from multi-path reflections of buildi

I think you can easily do two primitive things: * Receivers in the metal-reinforced concrete canyons of a big city suffer from multi-path reflections of buildings, leading to bigger uncertainty. You can improve those signals with a (still pretty uncertain) rural reference. This reference typically also sees low angle satellites, and it receives all signals without reflections. * You can improve the uncertainty of some of your references by running analysis on it's position history and compare it with (highly certain) map data. One of your rural signals is moving at 78 mph? High likelihood to be in the left lane on that highway. Moved with 30mph, now suddenly stopped? Waiting at that specific traffic light, probably first car. With crosschecks between enough references, some averaging and smoothing, you can produce a usable reference. Add a couple of absolute references (no reason for the company to not have a fixed antenna in the 50 largest cities each), and you're competing with current commercial reference services. It's also worth noting, that for many of the uncertainties of the GPS signal (satellite clock jitter, atmospheric conditions), you don't need to update your position delta every second. It's already an improvement if you get a good reference update every minute or so.