An Updated Assessment of the GNSS L1 Radio Frequency Interference Environment

Robert Erlandson and Robert Frazier

Abstract:	In late 1996 RTCA Special Committee 159 Working Group 6 (SC-159 WG-6) completed its first assessment (RTCA/DO-235) of the RFI environment in and near the GNSS L1 frequency band (1559 – 1610 MHz). The original assessment work yielded proposed susceptibility requirements for RTCA/DO-229 WAAS MOPS, two proposals for “Big-LEO” Mobile Satellite Service transmitter emission requirements, and several other important findings. Several changes have occurred since 1996 to motivate an updated RFI assessment. More is known about Category II and III aircraft precision approach operational requirements, but the means by which DO-235 Category II/III RFI link budget was to be closed had yet to be resolved. GPS satellite system modernization and Galileo system development projects are underway and potential compatibility issues with legacy equipment have arisen. New RF systems, most notably ultra-wideband (UWB) transmission systems, have arisen with potential interference consequences for GPS. Given this latter development, the US Department of Transportation (DOT) tasked RTCA with the development of new aviation and non-aviation operational scenarios involving UWB sources and analyses to support a compatibility assessment between UWB technologies and transportation safety systems. Since mid-2000, SC-159 WG-6 has been working on an update to the DO-235 L1 assessment. Two new operational RFI encounter scenarios were developed for aviation: Category II/III aircraft precision approach and GNSS-guided airport surface movement. With outside support three non-aviation scenarios were also developed (maritime constricted waterway, airport survey, and GPS- based E911 in cell phones). Three scenarios, Non- Precision Approach, Category I Precision Approach, and Enroute/Terminal Area Navigation, developed for the earlier DO-235 assessment were reviewed and updated. SC-159 WG-6 reviewed seven major activities relating to UWB interference to aviation systems, in general, and to GPS in particular. To better understand and help generalize the measured RFI effects from specific UWB waveforms, SC-159 WG-6 did its own modeling of UWB effects on GPS receivers based primarily on the Stanford tests. That UWB susceptibility model was used in obtaining a worst-case susceptibility factor for UWB- related RFI link analyses. SC-159 WG-6 also developed an analysis approach for computing intra-system interference between several GPS C/A codes and between other GPS signals and C/A code. SC-159 WG-6 performed several RFI link analyses on key RFI sources in the operational encounter scenarios it developed. UWB RFI was assessed in all the aviation and non-aviation scenarios. Based on those analyses, UWB emission limits were proposed. Mobile Satellite Service (MSS) interference, both Earth-space and space-Earth, was assessed in precision and non-precision approaches. Limits for MSS space-Earth unwanted emissions were proposed. Intra-system interference to C/A code reception was also assessed and found to be essentially negligible under present assumptions. Other potential RFI sources were reviewed and a set of RFI mitigations discussed. As requested, RTCA prepared several interim reports for DOT in 2000 and 2001. This paper summarizes the final RTCA report for the current DOT tasking.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	591 - 599
Cite this article:	Erlandson, Robert, Frazier, Robert, "An Updated Assessment of the GNSS L1 Radio Frequency Interference Environment," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 591-599.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In