Engineering the USCG Differential System for Sub-meter Performance

M. Parsons, A. Cleveland, C. Treib, D. Wolfe

Abstract:	As each of the segments that make up the Standard Positioning System (SPS) continue to improve and access to additional codes become available to the civil user on L2 and L5, the future usefulness of GPS augmentation systems may hinge on their ability to provide superior integrity and accuracy to the user. The Coast Guard is engaged in an effort to identify ways of improving the current Differential Global Positioning System (DGPS) positioning accuracy. This paper explores improvement some of the improvement efforts currently underway to enhance DGPS system accuracy. Atmospheric Corrections: NOAA proposes to extract highly localized, but extremely accurate atmospheric corrections from their prediction models. These atmospheric corrections will improve on existing dual frequency ionospheric data collected by traditional CG Reference Stations by incorporating wet and dry tropospheric data. This data is collected by NOAA weather monitoring equipment currently deployed on all Nationwide Differential GPS (NDGPS) sites nationwide. Conceptually, NOAA's extremely high quality atmospheric corrections may be able to produce positions with accuracy that may approach 10cm at the center of coverage and potentially de-correlate to less than 30cm accuracy at the fringe of beacon coverage (450km). Dual Beacon Receivers / Enhanced Almanac: RTCM SC-104 is currently developing an RTCM Type 27 message. The RTCM 27 is an enhanced beacon almanac message that contains enough information to enable a user's receiver to intelligently choose from which available beacon it will accept differential corrections. Sky wave is a significant problem for DGPS receivers, particularly at night when the ionosphere is extremely effective at reflecting medium frequency MF sky wave back to earth. The MF sky wave problem manifests itself in areas where the MF ground wave from the intended "closest" beacon site may be diminished or masked by terrain. The RTCM type 27 message is expected to virtually eliminate the current sky wave interference issues. The dual frequency concept helps in coverage areas served by two or more sites where users may benefit by using corrections received from several nearby Nationwide and Maritime Differential GPS (N/DGPS) broadcast sites to generate DGPS corrected position solutions. Network Synergy: The Coast Guard is investigating the potential improvement in N/DGPS system robustness gained by "sharing" correction information between adjacent sites. The continental grid of beacon transmitters is filled to a point where dual coverage is common. Sharing correction data generated by adjacent sites has the potential to allow users to significantly mitigate local masking issues and temporary beacon signal loss periods. It is a way of ensuring the user will always be able to obtain an adequate number of corrections to produce a DGPS corrected position solution. Receiver Enhancements: The Coast Guard is investigating whether accuracy compromises made during initial N/DGPS system design to mitigate the impact of Selective Availability (S/A) can be revisited in a post-S/A environment to offer users higher accuracy. Any added benefit to users gained by correcting the WAAS pseudorange will be evaluated and quantified. Improvements such as the Wide Area Network (WAN) transition from X.25 to Frame Relay and the implementation of the new Nationwide Control Station have opened the doors to engineering possibilities that may bring N/DGPS to decimeter level accuracy. Providing this level of accuracy would attract whole new groups of users that require greater precision, as well as offering better service for the current users.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	45 - 58
Cite this article:	Parsons, M., Cleveland, A., Treib, C., Wolfe, D., "Engineering the USCG Differential System for Sub-meter Performance," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 45-58.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In