Inverse differential GPS (IDGPS) is a variant of DGPS in which a central location collects the standard GPS positioning information from one or more mobile units and then refines that positioning data locally using DGPS techniques. With IDGPS, a central computing center applies DGPS correction factors to the positions transmitted from each receiver, tracking to a high degree of accuracy the location of each mobile unit, even though each mobile unit only has access to positioning data from a standard gps asset tracking device

This technique can be more cost-effective in some ways than standard DGPS, since there is no requirement that each mobile unit is DGPS-enabled, and only the central site must have access to the DGPS correction data. However, there is an additional cost for each mobile device, since each unit must have a means of communicating position data back to the central computer for refinement. For applications such as delivery fleet management or mass transit, IDGPS may be an ideal technique for maintaining highly accurate position data for each vehicle at a central dispatch facility, since the communication channel is already available, and the relative cost of refining the positioning information for each mobile unit at the central location is minimal. Of course, with the discontinuation of S/A, DGPS refinement may no longer be necessary for many of these applications.

Server-assisted GPS is a positioning technique that can be used to achieve highly accurate positioning in obstructed environments. This technique requires a special infrastructure that includes a location server, a reference receiver in the mobile unit, and a two-way communication link between the two, and is best suited for applications where location information needs to be available on demand, or only on an infrequent basis, and the processing power available in the mobile unit for calculating position is minimal.

In a server-assisted GPS system, the location server transmits satellite information to the mobile unit, providing the reference receiver with a list of satellites that are currently in view. The mobile unit uses this satellite view information to collect a snapshot of transmitted data from the relevant satellites, and from this calculates the pseudorange information. This effectively eliminates the time and processing power required for satellite discovery and acquisition. Also, because the reference receiver is provided with the satellite view, the sensitivity of the mobile unit can be greatly improved, enabling operation inside buildings or in other places where an obstructed view will reduce the capabilities of an autonomous electronic tracking device.

Once the reference receiver has calculated the pseudoranges for the list of satellites provided by the location server, the mobile unit transmits this information back to the location server, where the final PVT solution is calculated. The location server then transmits this final position information back to the mobile device as needed. Because the final position data is calculated at the location server, some of the key benefits of DGPS can also be leveraged to improve the accuracy of the position calculation. An illustration of the relationship between the reference receiver and the location server in a server-assisted GPS system can be seen in Figure 8.6.