A Global, High-Accuracy, Differential GPS System

R. Hatch, T. Sharpe, P. Galyean

Abstract:	NavCom Technology, Inc. (NavCom) and Ag Management Solutions (AMS), both components of John Deere and Company, have completed the development of a global Differential GPS system. This system, referred to as StarFire, is operated by NavCom. StarFire provides high accuracy navigation with sub-decimeter horizontal accuracy. This unprecedented accuracy is made possible by several important developments. Significantly, the predecessor WADGPS system, previously developed by NavCom and deployed on several continents for Deere, had already put in place several of the required components. These included: (1) development of highquality, dual-frequency GPS receivers for use as both reference and mobile receivers; (2) dual-frequency extended smoothing of the code with the carrier to remove the two largest error sources in a WADGPS system, i.e., ionospheric refraction and multipath effects; and (3) development of a new L-band communication module and the associated multi-band antenna which was designed to receive the GPS L1 and L2 frequencies together with the Inmarsat L-band communication frequencies (1525-1565 MHz.). The principal source of error in the predecessor version of StarFire, which limited the solution to continental size areas, was the inaccuracy in the broadcast satellite orbits. To convert the separate WADGPS regional systems into a single, unified, global DGPS system required the generation of accurate, real-time GPS orbits. NavCom licensed the technology to compute real-time orbit and clock corrections from the Jet Propulsion Laboratory (JPL). JPL had developed for the National Aeronautics and Space Administration (NASA) the software to compute in real time the orbit and clock corrections needed for a global DGPS system but had no means of distributing them to isolated users. NavCom now broadcasts the correction stream over three Inmarsat satellites to provide global coverage. In addition to the JPL Hub software to generate the corrections, NavCom contracted with JPL to receive the data from the JPL/NASA global reference sites to augment the NavCom reference sites. The result is a global DGPS system, which can supply high accuracy results anywhere the geosynchronous Inmarsat satellites are visible. StarFire is accurate enough to challenge local Real-Time Kinematic (RTK) GPS systems in many applications, but does not require local base stations, and its positioning results are absolute, not relative. While the accuracy in any given 24 hours is typically sub-decimeter, the variation over shorter intervals can be significantly less since there is very little high-frequency noise in the position solution. This allows StarFire to be used in semiautomatic and assisted steering applications which formerly required much more expensive RTK solutions. Many applications, which can effectively use this new level of global accuracy, are obvious. In addition to agricultural applications, StarFire is already finding significant use in offshore positioning applications. Many other uses for this new level of global accuracy are expected to develop in the near future.
Published in:	Proceedings of the 2003 National Technical Meeting of The Institute of Navigation January 22 - 24, 2003 Disneyland Paradise Pier Hotel Anaheim, CA
Pages:	562 - 573
Cite this article:	Hatch, R., Sharpe, T., Galyean, P., "A Global, High-Accuracy, Differential GPS System," Proceedings of the 2003 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2003, pp. 562-573.
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