Rising and setting GPS occultations by use of open-loop tracking

The success of GPS occultations has been demonstrated by several missions including GPS/MET, CHAMP, SAC-C, GRACE, and COSMIC. However, tracking the GPS signal in the lower troposphere or obtaining accurate refractivity there has proven particularly challenging. The first receiver software with open-loop (OL) tracking capability was recently tested and successfully implemented on SAC-C. Besides improving the ability to probe deeper into the lower troposphere and planetary boundary layer, OL tracking also enables the acquisition of rising GPS signals for the first time, thereby doubling the number of occultations from the same instrument. In this paper, we describe the atmospheric Doppler and delay models used in the SAC-C and COSMIC OL tracking software. We discuss the testing carried out on SAC-C and present some examples of OL processing. We show that OL data give vast improvements in sampling the lowest part of the atmosphere. Over 80% of the SAC-C OL profiles now reach below 2 km altitude in the tropics, compared to only 50% achieved under closed-loop tracking.