GPS World, Innovation: Autonomous Navigation at High Earth Orbits
Calculating a spacecraft's precise location at high orbital altitudes-22,000 miles (35,800 km) and beyond-is an important and challenging problem. New and exciting opportunities become possible if satellites are able to autonomously determine their own orbits. First, the repetitive task of periodically collecting range measurements from terrestrial antennas to high altitude spacecraft becomes less important-this lessens competition for control facilities and saves money by reducing operational costs. Also, autonomous navigation at high orbital altitudes introduces the possibility of autonomous station keeping. For example, if a geostationary satellite begins to drift outside of its designated slot it can make orbit adjustments without requiring commands from the ground. Finally, precise onboard orbit determination opens the door to satellites flying in formation-an emerging concept for many scientific space applications. The realization of these benefits is not a trivial task. While the navigation signals broadcast by GPS satellites are well suited for orbit and attitude determination at lower altitudes, acquiring and using these signals at geostationary (GEO) and highly elliptical orbits is much more difficult. The light blue trace describes the GPS orbit at approximately 12,550 miles (20,200 km) altitude. GPS satellites were designed to provide navigation signals to terrestrial users-consequently the antenna array points directly toward the earth. GEO and HE0 orbits, however, are well above the operational GPS constellation, making signal reception at these altitudes more challenging. The nominal beamwidth of a Block II/IIA GPS satellite antenna array is approximately 42.6 degrees. At GEO and HE0 altitudes, most of these primary beam transmissions are blocked by the Earth, leaving only a narrow region of nominal signal visibility near opposing limbs of the earth. This region is highlighted in gray. If GPS receivers at GEO and HE0 orbits were designed to use these higher power signals only, precise orbit determination would not be practical. Fortunately, the GPS satellite antenna array also produces side lobe signals at much lower power levels. NASA has designed and tested the Navigator, a new GPS receiver that can acquire and track these weaker signals, thereby dramatically increasing the signal visibility at these altitudes. While using much weaker signals is a fundamental requirement for a high orbital altitude GPS receiver, it is certainly not the only challenge. There are other unique characteristics of this application that must also be considered. For example, Position Dilution of Precision (PDOP) figures are much higher at GEO and HE0 altitudes because visible GPS satellites are concentrated in a much smaller area with respect to the spacecraft antenna. These poor PDOP values contribute considerable error to the point solutions calculated by the spacecraft GPS receiver. Finally, spacecraft GPS receivers must be designed to withstand a variety of extreme environmental conditions. Variations in acceleration between launch and booster separation are extreme. Temperature gradients in the space environment are also severe. Furthermore, radiation effects are a major concern-spacecraft-borne GPS receivers must be designed with radiation-hardened electronics to guard against this phenomenon, otherwise they simply will not work. Perhaps most importantly, there are no opportunities to repair or modify any space-borne GPS receiver after it has been launched. Great care must be taken to ensure all performance characteristics have been analyzed prior to liftoff.