Maintenance of the Geodetic Reference Frame in the Global Positioning System

Beryl Brodsky, John LaBrecque, James J. Miller, Robert A. Nelson, A.J. Oria, Michael Pearlman

Abstract:	The Global Positioning System (GPS) satellite ephemerides are derived from pseudorandom noise (PRN) signals and carrier phase referenced to on-board atomic clocks and a ground network of GPS reference stations and expressed in the WGS 84 reference frame. The WGS 84 reference frame is periodically realigned to the International Terrestrial Reference Frame (ITRF) by the National Geospatial-Intelligence Agency. The ITRF is, in turn, obtained from the combined analysis of GPS, Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), and Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) data. The current realization of the reference frame could be described as circular in that an independent method of external verification is currently not available. In fact systematic errors exist in both the estimated satellite coordinates and the two reference frames that limit the accuracy of positions and timing derived from the GPS system. These systematic errors must be estimated through the analysis of observations independent from current radiometric observables. In practice, accuracy of the measurement standards should be better than the required operational requirements by a factor of ten to ensure that the desired requirement is met. The experience of the last three decades indicates approximately a factor of ten per decade improvement in the performance of our geodetic systems. This improvement has spawned a host of new applications in civilian, military, and scientific endeavors that are likely to require a continuation in the improvement of geodetic system accuracy. Currently, the accuracy of both the ITRF and the WGS 84 is estimated to be on the order of 1 part per billion (6.4 mm at the Earth’s surface) with observed regional drifts on the order 1.8 mm/yr and errors in the collocation of geodetic stations exceeding 5 mm/yr. There is also little to verify this estimated accuracy of the reference frames because successive estimates of the ITRF are retrospective and utilize the same historical data sets except for the addition of more recent data and new analysis approaches. All determinations of the ITRF are therefore inter-related and not independent. GPS is both a critical component in the determination of the ITRF geodetic reference frame and serves as the principal means of positioning within the geodetic reference frame. Though the current accuracy of the ITRF and WGS 84 reference frames marginally meet most civilian and military requirements, current scientific challenges in Earth observation demand more accuracy than the geodetic systems including GPS and the geodetic reference frames are able to deliver. Because scientific requirements are often the vanguard of future civilian and military requirements, it is very likely that within the lifetime of GPS III, the accuracy of the reference frames will be unable to meet the anticipated broader societal requirements. This is the emerging GPS capability gap. If GPS is to continue as the primary geodetic reference system, then we must ensure that the GPS continues to evolve in order to fill an emerging capability gap in system accuracy. This report examines options to address the emerging capability gap. These include: (1) Increased Frequency of Satellite Clock Updates; (2) Radiometric Tracking at Higher Frequency; (3) Laser Tracking of GPS Satellites; (4) Astronomical Geolocation of GPS Reference Stations; and (5) GPS Intersatellite Links.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	3036 - 3047
Cite this article:	Brodsky, Beryl, LaBrecque, John, Miller, James J., Nelson, Robert A., Oria, A.J., Pearlman, Michael, "Maintenance of the Geodetic Reference Frame in the Global Positioning System," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 3036-3047.
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