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Scientific Utility of the Signal-to-Noise Ratio (SNR) Reported by Geodetic GPS Receivers

Although GPS signal-to-noise ratio (SNR) measurements have conventionally been used only as a measure of receiver tracking or signal strength comparison between channels, new scientific applications of SNR are evolving. These scientific applications exploit the link between oscillations in SNR and carrier phase multipath under the simplified model of the receiver response to tracking a composite signal of direct plus reflected signals. This paper establishes several principles linking dual-frequency SNR data, pseudorange multipath, and phase multipath, and uses these principles to test the dependency of SNR data on the simplified multipath model for several common models of geodetic-quality receivers. SNR data from AOA ACT receivers pass these tests and demonstrate a strong link to the model, and we provide several examples of SNR-multipath science applications that can be done when SNR data are deemed to be reliable and accurate. Examples are drawn from permanent stations operating ACT receivers and include multipath map formation, construction of carrier phase multipath corrections, and soil moisture monitoring examples. However, perhaps because of the lack of requirements established for SNR measurements, we find considerable inconsistency among manufacturers and models and show that most geodetic receivers report SNR data in a way that obscures or misrepresents the multipath signature. Problematic characteristics observed in some receivers’ SNR data include strong coupling of L1 and L2 tracking errors, apparent oscillations unrelated to the multipath environment, coarse values, or data correlations inconsistent with the multipath model. Our findings suggest that changes to SNR reporting to establish a consistent and reliable measurement could greatly enhance the scientific utility of existing GPS networks.