GPS signals are sparse and weak at high altitudes above the GPS constellation; this includes GEO and HEO orbit regimes. Spacecraft operating here need a high-sensitivity receiver capable of acquiring and tracking these weak signals throughout their orbits. Most space GPS receivers are designed to operate in the LEO regime below the GPS constellation and will perform poorly at high altitude. The Goddard Space Flight Center (GSFC) Navigator GPS receiver is designed for above-the-constellation applications and can provide adequate performance for many missions. The innovation described here provides large sensitivity increase through improvement of the receiver’s antenna gain. The technology can be applied to a standard LEO GPS receiver to enable adequate high-altitude performance, or to a specialized high-altitude receiver, like the GSFC Navigator, to further improve its high-altitude performance. The innovation can also be used to mitigate interference, including multipath and RF jamming signals for operation in any regime.
The innovation comprises a digitally steered phased antenna array for GPS applications that can be steered toward signals of interest and away from interferers. This includes the development of antenna array front-end electronics, realtime software signal processing, and beam-forming methods to coherently combine the signals from the antenna array. Demonstration hardware and software was developed to control a 16-element antenna array, which provides better than l0dB of additional antenna gain over a single element.
The additional 10dB of gain provided by the technology would allow a standard LEO receiver to take advantage of signals transmitted from the side lobes of the GPS satellites to greatly improve signal availability and increase the usability of GPS for spacecraft in high-altitude orbit regimes. Importantly, multiple beams can be synthesized to provide this peak gain to each tracked transmitter, and each synthesized beam can have its gain optimally distributed to simultaneously attenuate interferers.
As part of the development, different array signal processing methods were tested in the GSFC NavSDR — a purely software defined radio platform designed for rapid integration of new technologies into the GSFC Navigator GPS receiver. In each method, the combined signal is obtained by summing the product of complex array weights and the input signals from each antenna element. By controlling the array weights, the array’s beam pattern can be optimized to achieve certain desirable characteristics including directivity, signal-to-noise ratio gain, and interference mitigation. In the first method, the deterministic beam former, a fixed array weighting system is used to maximize gain in a particular direction. Additional methods based on adaptive algorithms were also developed. These attempt online optimization of some performance index evolving with the signal and interference environment. Importantly, the adaptive algorithms do not require full attitude information of the GPS receiver platform.
The digitally steered antenna array can be used with the Navigator GPS receiver to produce the next-generation high-altitude space-GPS receiver. The 16-element antenna array will provide more than 10dB improvement in sensitivity over the current Navigator GPS receiver, which will significantly improve performance and extend its range of operation. Additionally, the use of the digitally steered antenna array will improve performance in the presence of interference.
This work was done by Luke B. Winternitz, Heitor D. Pinto, Jennifer Valdez, Samuel R. Price, Lawrence L. Han, Monther A. Hasouneh, and Harry Stello of Goddard Space Flight Center. GSC-16889-1