Ephemeris observability issues in the Global Positioning System (GPS) autonomous navigation (AUTONAV)

The AUTONAV feature of the Global Positioning System (GPS) Block IIR satellites (SVs) provides both improved accuracy and survivability of the navigation function for 180 days without ground contact. These improvements are accomplished by means of a UHF crosslink measurement and data communication system and an onboard computational capability. The operational control segment (OCS) uploads a long term prediction of its ephemeris to each SV. Each satellite inputs SV-to-SV pseudorange measurements and communicated data to its onboard estimator to improve the OCS ephemeris prediction and to estimate its clock. The improved ephemeris and clock estimates are then downlinked to users. Unfortunately there is insufficient information content in the crosslink measurements to fully estimate the ephemeris states. This initially cast doubt on the feasibility of the AUTONAV concept. This paper formally develops the concept of unobservable rotation error. It quantifies the system error due to rotation error in the OCS upload that cannot be corrected by the onboard estimators. It also presents an algorithm that prevents the imperfectly modeled onboard estimators from accumulating additional spurious rotational errors in their estimates. Perhaps most importantly, it provides a basis for an intuitive understanding of why AUTONAV can work.<<ETX>>