Characterization and Simulation of SVN49 (PRN01) Elevation Dependent Measurement Biases

Swen D. Ericson, Karl W. Shallberg, Clyde E. Edgar

Abstract:	IIR-20M, also designated SVN49 or PRN01, underwent a critical modification in order to use an L5 signal in space (SIS) from a GPS satellite by August 26, 2009, satisfying the requirement of an International Telecommunications Union (ITU) filing to secure primary status of GPS in the L5 radiofrequency band. While the L5 Demo transmission from SVN49 was successful, soon after launch the U.S. Air Force reported “out-of-family” performance that prevented the satellite from being set healthy. Users worldwide with receivers capable of tracking the unhealthy satellite provided observations of L1 and ionosphere-free pseudorange showing varying elevation-dependent measurement biases. The U.S. Air Force and its contractors systematically evaluated potential sources of the anomaly and determined the root cause was leakage of the primary L1/L2 signal through the power divider of the antenna array, reflection due to impedance mismatch at the L5 bandpass filter with the return signal then broadcast through the transmit antenna. Such performance manifests much like signal multipath and, as such, receivers with different tracking technology are affected by different elevation-dependent measurement errors. Concerned about the effects of this “out-of-family” SIS on WAAS processing, the FAA supported an effort to characterize the anomalous signal. At the time, the WAAS reference station at Cold Bay, Alaska (CDB) was near the sub-satellite ground trace of SVN49 (PRN01) and therefore was used as a site for collecting measurements up to approximately 89 degrees elevation angle. NovAtel receivers with a variety of tracking capabilities were used to capture PRN01 measurements from CDB, enabling detailed signal characterization. It was found that although the most noticeable effect is on L1 there also is a smaller, but still measurable, effect on L2. Concurrently, an effort to simulate SVN49 performance was initiated at SPAWAR Systems Center (SSC) Pacific using the Advanced Global Navigation Simulator (AGNS). Nominal Block IIR-M antenna gain and phase patterns were measured and reflected signal delay was estimated and input into the AGNS. The CDB live signal measurements indicated that a second, weaker reflection is present in the SIS. For AGNS modeling purposes, the second reflection was presumed to have twice the delay of the first, conceptually a double reflection path off the satellite power divider connector. Using a general model of weak multipath along with the CDB field results, the unknown phase shifts of the reflected signals were estimated and fine-tuned during dry runs of the simulation with the NovAtel receivers that were used at CDB. Later, a formal test was conducted with the AGNS including several keyed precise positioning service (PPS) receivers along with a commercial off-the-shelf Trimble NetR5 and some of the NovAtel receivers tested during the dry run. As with the NovAtel receiver results, the Trimble NetR5 results were compared with field data, both with and without Trimble’s Everest multipath mitigation technology enabled, and were found to be consistent. To determine the uniqueness of the AGNS SVN49 simulation solution, signal tracking theory with different multipath models was investigated and then verified using a Nortel (Spirent) STR2760 at Zeta Associates. This paper documents the CDB observations and signal simulations conducted at SSC Pacific and this signal tracking theory analysis. The signal characterization and simulation lessons learned in this paper are expected to be helpful in configuring GNSS simulators to test the response of Earth-based GPS user equipment to the SVN49 anomaly.
Published in:	Proceedings of the 2010 International Technical Meeting of The Institute of Navigation January 25 - 27, 2010 Catamaran Resort Hotel San Diego, CA
Pages:	963 - 974
Cite this article:	Ericson, Swen D., Shallberg, Karl W., Edgar, Clyde E., "Characterization and Simulation of SVN49 (PRN01) Elevation Dependent Measurement Biases," Proceedings of the 2010 International Technical Meeting of The Institute of Navigation, San Diego, CA, January 2010, pp. 963-974.
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