Acquisition of GPS Signals for High Altitude Spacecraft Navigation:Exploring Issues in Visibility and Link Analysis

The Global Positioning System has far exceeded expectations in its short lifetime, and the scientific and engineering community has churned up application after application for expanding and exploiting the system. Satellite navigation using GPS has become a hot new area of GPS applications, with several satellites making use of new commercially available GPS spaceborne receivers. It appears that GPS, originally intended for terrestrial and near-terrestrial users, is well suited to providing accurate coverage for low earth orbiting satellites below the UPS constellation. Using UPS to navigate satellites above the UPS constellation altitude poses many problems. Coverage is far more limited and signal strength drops sharply for high altitude users. Spacecraft above the UPS satellite have limited availability of the UPS signal. Possibilities to still exploit the signal do exist, including making use of the UPS cross-link ranging signal, capturing UPS signals from across the globe that are not blocked by the Earth, and using mathematical filters to determine a navigation solution when visibility is limited. Signal strength is also a major area of concern. Higher gain antennas will be required for high altitude spacecraft in order to achieve an acceptable carrier to noise density ratio required to receive the UPS signal. This paper will explore these options and describe the potential benefits of exploiting this capability decreased costs of ground operations, increased accuracy, as well as increased satellite autonomy. I will also present the possible application of UPS time tags to missile warning data from Defense Support Program satellites as well.