Analysis of Signal Availability in the GPS Space Service Volume

The GPS Space Service Volume (SSV), when compared to the Terrestrial Service Volume (TSV), exhibits distinctly different characteristics for providing Position, Velocity, and Timing (PVT) services for space users. Factors such as large signal strength variation, reduced coverage, and side lobe issues must be considered when employing GPS services in the SSV regimes. Another important consideration is the fact that SSV regimes include altitudes both below and above the GPS constellation altitude. In fact, altitude in general is the dominant variable that determines GPS signal availability in the SSV. Different orbits require different antenna and GPS space vehicle selection strategies based on availability of access to the GPS main beams. For instance, as altitude increases in the SSV, GPS signals that spill over the limb of the Earth become the primary source of PVT services. In other words, as altitude increases, GPS signals above the space user become less available as GPS signals below (i.e. over the limb of the Earth) become more available. Therefore, some orbits will be well served to not only look “up” for GPS space vehicles, but also “down” for GPS space vehicles whose signals are visible across the Earth’s limb. This paper explores the relationships among altitude, GPS earth coverage beamwidth, and GPS space vehicle availability to a space user. It also examines various types of orbits that operate in the SSV, as well as those that traverse both the SSV and TSV. GPS signal availabilities are analyzed for each of the orbits, and selection of nadir versus zenith receiver antennas for mitigating coverage issues are discussed. Two altitudes of interest concerning GPS signal availability in space are defined: the intersection altitude, and the knee altitude. The analysis in this paper is not exhaustive; neither side lobes to the GPS main beam nor variations among different blocks of GPS space vehicles are covered, but the results presented offer a basis for first order extrapolation regarding these additional considerations. The primary intent is to illustrate general characteristics and trends in GPS signal availability that will hold true across a range of variation in analysis parameters.