Ionospheric scintillation studies required the development of Global Positioning System (GPS) receivers that could record high-rate correlator power measurements for the Coarse-Acquisition (C/A) code signal on the L1 carrier frequency. Spectrograms of 50-Hz power data soon revealed unusual time-varying patterns of weak power fluctuations that nonetheless were significant with respect to the background noise level. These spectrogram patterns, with approximately linear and parabolic forms, do not appear to be well known in the literature, although related phenomena have been observed and reported. We present examples of the patterns and evidence for their origin in satellite-to-satellite C/A-code interference. Among other factors, we demonstrate that the maximum value of the inter-satellite correlation term is not the -21.6 dB to be expected from the direct C/A-code cross-correlation result but rather is of the order of -9 dB. This inter-satellite correlation term permits the development of radio wave interference between pairs of satellites that meet appropriate Doppler shift and amplitude criteria, ultimately leading to the observed spectrogram patterns. It is important to ionospheric scintillation studies, and other civil radio wave interference studies, to be able to separate these inter-satellite effects from daily and transient multipath effects. Therefore, we suggest some techniques to discriminate between these phenomena. The goal of this paper is to raise general awareness of these inter-satellite interference effects for GPS, although applications to other tasks such as satellite orbital parameter estimation may be possible in the long term.
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