Analysis of Global Positioning System (GPS) radio occultation measurement errors based on Satellite de Aplicaciones Cientificas-C (SAC-C) GPS radio occultation data recorded in open-loop and phase-locked-loop mode

摘要

The error characteristics of Global Positioning System (GPS) radio occultation (RO) measurement errors are studied based on Satellite de Aplicaciones Cientificas-C (SAC-C) GPS radio occultation data tracked in both open-loop (OL) and phase-locked-loop (PLL) mode. The error characteristics are derived by applying dynamical error estimation, i.e., without using any external data. The computed error profiles show that the mean measurement errors are the smallest in the height range between about 5–7 km and 20–25 km, about 0.2–1% for bending angles and 0.1–0.2% for refractivity at all latitudes. The largest measurement errors are found in the lower troposphere, where the mean bending angle measurement errors are within the range from 1 to 6%, whereas the mean refractivity measurement errors are within the range from 0.2% to 1%. From the error distributions, it is found that the occultation-to-occultation variability of the measurement errors generally spans one order of magnitude. The bending angle error correlation length is about 1 km and 100 m, at high and low altitudes, respectively, corresponding approximately to the cutoff frequency of the applied noise filters. The widths of the refractivity error autocorrelation functions are notably broader. The variability and the magnitude of the OL measurement errors are larger than for the PLL measurement errors. This is mainly attributed to the ability of OL tracking to track RO signals under atmospheric conditions for which PLL tracking fails.