Rising and Setting GPS Occultations by Use of Open Loop Tracking

The technique of GPS occultation for sensing the atmosphere, where a receiver in low-Earth orbit tracks the signal phase and amplitude of a GPS satellite setting or rising behind the Earth's atmosphere, has been successfully applied in over 7 missions over the past decade. These missions have consistently yielded temperature profiles that are accurate at the 1K level at altitudes between 5-30 km when compared to independent data sets and numerical weather prediction models. However, a persistent challenge has been the ability to accurately sense the refractivity in the lower troposphere without biases. Also, a challenge has been the detection of rising occultations. The difficulties in accurately detecting the GPS signal in the lower troposphere are due to atmospheric defocusing causing signal attenuation, atmospheric multipath causing strong signal dynamics, and atmospheric ducting causing temporary signal disappearance. These difficulties are exacerbated for rising occultations since the receiver, at least when operating in its traditional phase-lock loop mode, has the extra burden of first acquiring the signal, a process which requires sufficient signal-to-noise ratio that is not available in the lower troposphere. These challenges are mostly overcome when the data is recorded in open-loop (both for rising and setting) and post-processed to extract phase and amplitude information. Open-loop recording of setting and rising occultation was recently demonstrated on the SAC/C mission for the first time. This talk will present an overview of the challenges and difficulties encountered with traditional techniques of sensing the lower troposphere by GPS occultations and evaluate the degree to which these challenges are overcome with the recent open-loop data from SAC/C collected during rising and setting occultations.