John W. Betz, Christopher J. Hegarty, Jeffry T. Ross, Gary M. Okerson, The MITRE Corporation

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GPS satellites can transmit “intentionally ‘incorrect’ versions” of the C/A code, referred to as non-standard C/A (NSC). Concerns have been raised about NSC interference to civilian GNSS receivers, including those receiving GPS C/A signals and Galileo E1 Open Service (E1 OS) signals. Claims have been made that NSC causes insignificant interference. This paper provides a more detailed assessment of NSC interference effects, providing updated results and additional insights. Calculations, simulations, and numerically evaluated analytical results are used to assess the effects on receiver performance over a range of differential Doppler shifts and received powers. It is demonstrated that NSC can cause observable, occasional, and intermittent degradation of individual receiver channels. The overall effect on receiver operation, however, is scenario-dependent since it varies with location and time, number of satellites in view, and other factors. These results contrast with previously published claims, providing guidance if additional satellites transmit NSC for prolonged times in the future. If NSC is transmitted for long times or by many satellites, reducing its transmitted power would avoid any claims of interference.