Some of the error in GPS measurements of closely located receivers is correlated. There is more information in the relative measured location of two units than in the absolute measured position of one. (Differential GPS is based on this: an accurate measure of a receiver's location is made by comparing its relative position to a known receiver location.)

So... rather than estimating the location of each user independently, maybe they fold in the relative position of each user and solve their positions jointly. Or put another way, the systematic error in the users' direct GPS coordinates might be estimated jointly rather than independently. In this way any other information used to more accurately locate individuals could be combined together to the benefit of all.

Did you guys come up with any more honking great ideas like namespaces?

("Namespaces are one honking great idea -- let's do more of those!" is the last line of wisdom printed when importing "this" in Python.)