Leveraging the L1Composite Signal to Enable Autonomous Navigation at GEO and Beyond (2013)

The Global Positioning System (GPS) was originally conceived to provide the United States military with reliable navigation and timing on and close to Earth. By design, the signals broadcast from the GPS satellites are extremely weak when received on Earth. Signal processing in the user receiver boosts the desired signal to power levels at which they can be acquired and tracked. Today, the primary use of the system is for civilian applications. 
Over time, the use of GPS for space applications has been steadily raising. Ingenuity and enhancements in computational capabilities have contributed to this increase. The US government is of the opinion that GPS has transformed the way nations operate in space; from guidance systems for crewed vehicles to the control of communication satellites to entirely new forms of Earth remote sensing. Most of these applications are Low Earth Orbit (LEO) missions wherein the GPS receiver has unobstructed line of site to multiple Medium Earth Orbit (MEO) GPS satellites. 
The use of GPS and upcoming GNSS signals for Geostationary Orbit (GEO) and Highly Elliptical Orbit (HEO) missions has special design challenges. In these missions, the GPS receiver is at an altitude above the altitude of the GNSS constellations. Consequently, the only signals reaching the receiver at these altitudes originate from satellites on the opposite side of Earth. The received signals are 10 to 100 times weaker with limited satellite spatial diversity. The Navigator GPS Receiver developed at NASA Goddard Space Flight Center is a space grade receiver with fast signal acquisition and weak signal tracking capabilities. Using the GPS constellation alone, rarely are four or more satellites visible simultaneously. This limits the possibility of autonomous navigation at GEO and beyond. 
This paper provides results from a systems engineering analysis of upgrading the Navigator to support multiple constellations. Satellite visibility is evaluated for a combined GPS + Galileo constellation as a function of receiver tracking thresholds. Availability is analyzed for three scenarios; geostationary orbits, the elliptical orbit of phase 1 of the Magnetospheric Multiscale (MMS) mission and the highly eccentric elliptical orbit during phase 2 of the MMS mission. A combined GPS + Galileo system is chosen to leverage the common L1 composite signal without changing existing rad-hard L1 RF frontends. The analysis assumes the same GPS Block III satellite transmit antenna model for the Galileo satellites as well. Starting with the Block III satellites, the mean beam of the transmitted GPS signals would be enhanced to 23.5 degrees (half angle) compared to the current 21.3 degrees. This will improve visibility of the signal main lobe at GEO and beyond. The Galileo Interface Control Document (ICD) does not specify the transmit signal beam width. A parametric study is performed to establish the effect of Galileo signal beam width on satellite visibility for the three scenarios under consideration. Dilution of Precision is computed when at least four or more satellites are simultaneous tracked. 
The L1C signal structure allows for lower thresholds compared to the L1 C/A signal. Upgrading the Navigator the support the L1C signal with lower thresholds would significantly improve autonomous navigation in GEO and HEO. At GEO, continuous autonomous navigation is realizable with the combined constellation at current Navigator thresholds. The authors do appreciate the significant implementation challenges in upgrading the receiver. Techniques to achieve lower tracking thresholds for the L1C signal and potential implementation challenges would be addressed in subsequent publications.