In this short post for beginners, Oren Gal shares the base concept of the GPS module of his HLS (Homeland security) application.
Source: Oren Gal's GIS Website
Each force sends its GPS location to a server and this server sends over the IIS (websync is a great software to use in this scenarios) the location data. All Silverlight clients listen to this channel and when a client receive new location (and more info) from a force, he locate the Graphic object inside the map and just change the geometry property to the new value.