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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