GPS AT GEO: A FIRST LOOK AT GPS FROM SBIRS GEO1

摘要

On May 7, 2011, Lockheed Martin successfully launched the first of a new series of Space-Based Infrared System (SBIRS) satellites, SBIRS GEOl. SBIRS is intended primarily to provide enhanced strategic and theater ballistic missile warning capabilities. Part of the SBIRS GEO1 design is the inclusion of a dual frequency GPS receiver to support spacecraft navigation requirements. Launch of GEOl is significant in that it makes use of GPS in the geosynchronous orbit regime, providing a unique look at the GPS environment, expectation of performance, and new challenges in GPS applications as seen from above the GPS constellation. New user navigation accuracy and robustness requirements for GEO satellites have spurred interest in developing GPS navigation systems designed to operate in the sparse measurement environment. Understanding that environment is key to designing a successful system. Several published papers have described the well understood geometric constraints of the GPS system when operating receivers above the GPS constellation. A few operators have reported on actual data collected by receivers operating at or above the GPS constellation. Finally, several papers have been authored regarding proposed systems showing simulation results for high altitude GPS based navigation systems. These simulation results are based on modeling assumptions that are critical to the analysis. SBIRS GEOl data offers the first look at actual observations from GEO, and an opportunity to validate or improve upon models. The purpose of this paper is to present and discuss observations, postulate phenomena, and identify areas where future research or work would be useful in the exploitation of GPS in the geosynchronous orbit environment. This paper will 1) briefly summarize some earlier work in the use of GPS above the terrestrial and LEO regime, 2) present and discuss analysis of observed GPS signal from the GEO regime. These observations include a look at ionosphere delay as seen from GEO, GPS LI antenna group delay as seen from large angles off the GPS satellite beam center, and GPS signal strength at GEO relative to link predictions.