WAAS Reference Station Performance with Pulsed Interference: Current Environment and Additional Margin

Karl Shallberg and Swen Ericson

Abstract:	The FAA Wide Area Augmentation System (WAAS) has focused on interference robustness and mitigation measures since its inception. Pulsed interference recently has been revisited due to the expectation of planned satellite systems using some portion of the L2 frequency band for radar operations. The current WAAS ground system uses both L1 and L2 GPS signals to conduct signal processing for correction and integrity bounds generation. Until the GPS constellation provides an L5 capability the L2 signal is mandatory for this processing so naturally any new sources of interference must be examined to ensure system performance is not degraded. This paper will investigate the current pulsed RF operating environment at the WAAS reference stations, evaluate receiver performance in the presence of pulsed interference in laboratory testing, and characterize existing margin that could potentially be used in compatibility assessments for these emerging satellite radar systems. Several WAAS reference stations are located adjacent to military surveillance radars and these stations establish the baseline for the current WAAS pulsed interference environment. Analysis conducted to characterize the impact of these current radar systems on L2 GPS signal processing includes examination of both the receiver front end response and signal processing impacts as deduced from C/No observations. Laboratory tests were conducted to extend the field observations and demonstrate the limits of L2 signal processing for various pulse widths and duty cycles. This testing demonstrated the receiver could operate successfully with pulsed waveforms well in excess of both the baseline experienced in the field and current system requirements. A second set of tests was conducted to characterize reference receiver acquisition margin by determining the level of increased pulsed interference that could be tolerated before L2 signal acquisition was impacted. The results of the acquisition tests were especially interesting since the aiding of L2 by L1 required for semi-codeless processing actually makes the receiver quite robust to additional RF interference. The other area of laboratory investigation was examining pulsed interference mitigation performance with suppression and blanking techniques. The current WAAS reference receiver uses pulse suppression and the next generation WAAS reference receiver uses pulse blanking. Both receivers were tested to contrast pulse mitigation performance. Finally, the field observations and laboratory test results were combined to demonstrate the amount of signal processing margin that exists in the WAAS receiver for potential use with new aperiodic interference sources such as satellite based radar systems. The key aspect with this compatibility discussion is the expectation that the satellite system would cause degradation for relatively short durations and only a few times a day.
Published in:	Proceedings of the 2013 International Technical Meeting of The Institute of Navigation January 29 - 27, 2013 Catamaran Resort Hotel San Diego, California
Pages:	873 - 883
Cite this article:	Shallberg, Karl, Ericson, Swen, "WAAS Reference Station Performance with Pulsed Interference: Current Environment and Additional Margin," Proceedings of the 2013 International Technical Meeting of The Institute of Navigation, San Diego, California, January 2013, pp. 873-883.
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