Generating Climate Benchmark Atmospheric Soundings Using GPS Occultation Data And Early Results From Formosat-3/COSMIC

Atmospheric soundings derived from Global Positioning System radio occultations (GPSRO) acquired in low- Earth orbit have the potential to be global climate benchmark observations of significant value to the Global Climate Observing System (GCOS). Geophysical observables such as atmospheric pressure and temperature are derived by measuring propagation delay induced by the atmosphere, a measurement whose fundamental unit—the second—is absolutely determined by calibration against atomic clocks. We will present an analysis of systematic and random error sources affecting GPSRO and determining upper error bounds to establish absolute accuracy for the retrievals. We discuss methods that are traceable to the definition of SI units. Residual uncalibrated delay of GPSRO signals in the Earth's ionosphere is an error source deserving further attention because it varies systematically over decadal time scales, in phase with the 11-year solar cycle. Atmospheric retrieval comparisons among the COSMIC constellation satellites are presented, establishing strong upper limits on systematic error arising from the individual COSMIC instruments and orbits. These results are encouraging but do not provide direct information on possible systematic errors arising in the ionosphere. We will discuss strategies to bound ionospheric error sources, which is essential to establishing GPSRO as an SI-traceable climate benchmark. Highly precise and accurate GPSRO instruments can be deployed on future Earth observation satellites at a low per-sensor cost and minimal interference to existing and planned observational programs.