Traditional way looks very simple

With DGPS support this simplest transform to more complex

Initial state the same as on simple send position to server. But next stay more complex.

After state 1 - server search nearst ground station (via current position, or last known position, or Cell ID information). Algorithm and how to collect ground stations see in step 1. After find near station server connect to STR channel and translate RTCM 2/3 to device via GPRS (2 on diagram). Other device (placed in other region - 3 on diagram) can send own position and other info to server but  has other neast ground station for differential GPS so server must provide info from other station.

Also server must support cash connection - so not open channel to ground station for each device.

Million thank to Peter Neumaier for login/password to euref-ip network,