USCG NDGPS accuracy and spatial decorrelation assessment

Document Type

Conference Proceeding

Date of Original Version



The United States Coast Guard (USCG) is a military, multi-mission, maritime service within the Department of Homeland Security (DHS). Its core roles are to protect the public, the environment, and U.S. economic and security interests in any maritime region in which those interests may be at risk. The USCG has become increasingly dependent on Global Navigation Satellite Systems (GNSS), especially the US Global Positioning Service (GPS), in order to conduct its missions more efficiently and effectively. The USCG has operated Nationwide Differential GPS (NDGPS) through a network of 85+ remote broadcast sites since 1999 to enhance user receiver position accuracy and integrity. Significant investments were made in recent years to enhance that system by replacing aging reference stations, integrity monitors, and transmitters with state of the art equipment. Results of preliminary and informal studies indicate increases in performance, specifically accuracy and availability, suggesting the need for a more formal review. The current performance standard for NDGPS dates back to the April 1993 Broadcast Standard, which requires a 10-meter (m) 2drms system over the coverage areas. This also is the basis for the US agreeing to Harbor and Harbor Approach requirements found in IALA R-121 and IMO Resolution A.953(23). The specification was derived based on spatial decorrelation, or the degradation of the accuracy of the DGPS correction over increasing baselines between the reference station and rover, as well as in the Horizontal Dilution of Precision (HDOP). The 10m standard also accounted for Selective Availability (S/A), an intentional degradation of the GPS signal applied in order to provide the intended military user with more precise accuracies than was available to civilian users. S/A was turned off by direction of a Presidential Order in 2000. The present GPS L1 Standard Positioning Service (SPS) performance specification is error ≤ 7.8m (2drms) for single frequency Course Acquisition (C/A) code, global average User Range Error (URE) during normal operations. The combination of this current GPS performance statement, preliminary studies of positioning performance following recent NDGPS recapitalization, and the disabling of S/A, together necessitate a detailed re-evaluation of the performance of the current NDGPS system and refinement of the spatial decorrelation model. The primary objectives of this study are to determine the accuracy of the NDGPS for the typical user and the development of an updated spatial decorrelation model for performance as a function of distance from the NDPGS reference site (the current model from the 1993 Broadcast Standard, is 2 meters plus 1 meter per 150 km from the beacon site). Simulation, software processing of existing GPS data sets (available from the Continuously Operating Reference Station or CORS, network) at a collection of spatially dispersed sites with the same NDGPS corrections, and actual field measurements at selected sites provides data for these objectives. This current paper provides results from all testing to date.

Publication Title

25th International Technical Meeting of the Satellite Division of the Institute of Navigation 2012, ION GNSS 2012



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