GPS Coverage Issues for Highly Agile Vehicles

Jo-Lien Yang, G. T. Tseng, Norman Keegan, and David Villalpando

Abstract:	GPS applications have grown tremendously in recent years and the growth is expected to be even more explosive in the future. The scope of the applications, however, has been primarily limited to relatively benign vehicles on or near the earth’s surface. For highly agile vehicles such as launch vehicles and missiles, maintaining continuous coverage and sufficient link margins with GPS satellites poses special challenges, especially for time critical range safety operations. Because of the current tracking system’s aging equipment, high maintenance and operations costs, and inability to meet the stringent new requirements, the Air Force is in the process of developing a new system architecture for range safety operations. A GPS-based architecture has emerged as the most promising candidate for implementation. Some early efforts were devoted to exploring top level concerns. A most significant finding was that a multi-patch antenna configuration should be adopted because of its universal applicability. However, further investigations are still warranted before making the final deployment decision. One major issue is the ability to maintain continuous tracking of the GPS signal in the presence of structural blockage (e.g., payload fairing separation) and plume interference. Another important question is the adequacy of link margins between a user vehicle and GPS satellites in a highly dynamic environment (e.g., errant flights). Addressing these issues is the focus of the investigation summarized in this paper. Quarter-wave antenna patches are commercially available. Their wide field of view is vulnerable to multipath interference from structural elements in the low elevation region as is the case for the payload fairing bulkhead of a launch vehicle. To alleviate this problem, The Aerospace Corporation has developed a half-wave antenna patch design that provides good low elevation multipath rejection capability at a reduced field of view. It was determined that either a three quarter-wave or a four half-wave patch antenna configuration would provide adequate GPS coverage, Both antenna patch configurations were modeled in our simulations using the Titan IV and Delta II as sample user vehicles. For a three quarter-wave patch configuration on Titan IV, there is a short time period of less than one second during the fairing separation that the number of visible GPS satellites drops below four. An adequate number of GPS satellites are visible for the four half-wave patch configuration at all times including the fairing separation event. The antenna azimuth and elevation angles of the satellites relative to the user vehicle vary wildly during errant flights causing very large and rapid fluctuations in the C/No values. However, our results indicate that they remain within acceptable ranges.
Published in:	Proceedings of the 53rd Annual Meeting of The Institute of Navigation (1997) June 30 - 2, 1997 Albuquerque, NM
Pages:	369 - 376
Cite this article:	Yang, Jo-Lien, Tseng, G. T., Keegan, Norman, Villalpando, David, "GPS Coverage Issues for Highly Agile Vehicles," Proceedings of the 53rd Annual Meeting of The Institute of Navigation (1997), Albuquerque, NM, June 1997, pp. 369-376.
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