Improving WAAS Integrity and Availability: UDRE and GIVE Time Updates

Christopher Comp, Ran Gazit, Todd Walter, Per Enge

Abstract:	Redefined UDRE and GIVE parameters for WAAS are proposed that incorporate time updates. The updates are based on the rate of change of the ephemeris and ionospheric errors. Integrity is preserved and avail-ability is improved for users receiving corrections on time. For the UDRE update, the rates of change of GPS ephemeris errors are analyzed from broadcast and precise data. It is shown that the magnitude of the ephemeris rate update is comparable to that of the SA acceleration update that already exists in the WAAS specifications. Additionally, a savings of one or two UDRE index values is achieved in many cases. If ephemeris velocity corrections are provided at regular intervals the UDRE update is not be required. For the GIVE update, temporal changes in ionospheric error are analyzed using NSTB data in conjunction with the IR190 and Klobuchar error models. Several methods of updating the ionospheric delay values or the asso-ciated GIVE values are studied. The recommended GIVE update is based on a ionospheric delay rate ap-plicable over a region of grid points. A GIVE reduc-tion of one or two index values is possible. The use of a solar-fixed frame to predict changes in ionospheric error also shows promise. More data analysis during higher ionospheric activity is advised.
Published in:	Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997) September 16 - 19, 1997 Kansas City, MO
Pages:	1315 - 1234
Cite this article:	Comp, Christopher, Gazit, Ran, Walter, Todd, Enge, Per, "Improving WAAS Integrity and Availability: UDRE and GIVE Time Updates," Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997), Kansas City, MO, September 1997, pp. 1315-1234.
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