A More Complete and Updated Methodology for Assessing Intrasystem and Intersystem Interference for GPS and Galileo

Intersystem interference is defined as interference from signals from one satellite navigation system to/from another such as between Galileo and GPS, though also extended to Satellite Based Augmentation Systems (SBAS). To date, the assessment of these interference effects has generally been overly conservative for the sake of simplicity in performing the analyses. This situation is further compounded by the fact that some of the signals, such as the GPS C/A code signals, are coded with relatively short repeating codes. Over the course of the last few years, a more complete interference analysis methodology has been developed to support RTCA SC159 Working Group 6, the working group that evaluates and defines interference to satellite navigation signals used for aviation navigation equipment requirements. This methodology evaluates the effects of signal Doppler on short code spectral lines, data and symbol modulation effects, signal power versus elevation angle, and, more importantly, critical signals. The methodology accounts for the fact that the codes are modulated with navigation data and error correction codes that spread the inherent spectral line structure over the symbol bandwidth. Representative satellite and user equipment antenna patterns are used to vary received signal power as a function of elevation angle. Lastly, only interference effects on critical signals are evaluated since only critical signals are absolutely required to perform the navigation function. This is the crux of the paradigm because weak signals need not be included unless they are critical signals. In most scenarios, critical signals are those at higher elevation angles, or there are no critical weak signals at all. This methodology was applied to GPS intrasystem interference effects, including the effects to/from SBAS signals, by the RTCA Working Group 6 and reported in a recent RTCA publication (DO-235A). It is now extended to GPS/Galileo intersystem interference effects. This paper will present the results of the extended methodology.