An Assessment of the Precision and Accuracy of Altimetry Retrievals for a Monterey Bay GNSS-R Experiment

Abstract

Global navigation satellite systems (GNSS) provide signals of opportunity for bistatic radar remote sensing, called GNSS reflectometry (GNSS-R), that have potential to be used for ocean altimetry. These signals have advantages over traditional mono-static radar that include reduced cost, high density of measurements in time and space, and an inherent reference to a highly accurate time-space frame. Here, we examine GNSS-R data collected from an aircraft flying over Monterey Bay, California. A downward-looking dual-frequency left-hand circularly polarized patch antenna recorded reflected signals. An upward-looking commercial antenna recorded the direct signals. Dual-frequency carrier phase data from this antenna were also used to produce precise coordinates for the aircraft. The L1 P-code GPS data were collected over four days with two flights per day and consist of nine transects covering a 1 deg× 1 deg grid. The performance of three timing retrieval algorithms has been evaluated based on measurement precision. From the observed cross-correlation waveform, the specular reflection timing was derived from the delay of the 70% peak correlation power (HALF method), the waveform leading edge peak first derivative (DER method), or the delay associated with a best fit function approximating the nominal waveform shape (PARA3 method). It was found that the HALF method produced the most precise measurements for a 5 s integration time with a standard deviation of σ = 0.6 m. The measurement accuracy is characterized by comparison with well-established models including neutral atmospheric delay, mean sea surface height, and ocean and solid Earth tides. Biases on the order of 1-4 m are observed with respect to a modeled mean sea surface and between each flight. However, the measurements are shown to track changes in sea surface height along the ground track to within 0.6 m.