Postprocessing of L1 GPS radio occultation signals recorded in open-loop mode

GPS Radio Occultation (RO) profiling from low-Earth orbiting satellites is operationally used for numerical weather forecasting and is starting to be used for climate studies. Obtaining high-quality observations near the surface requires recording of RO signals in model-aided open loop (OL) mode by the GPS receiver. Postprocessing of the OL RO signals is different from that of the signals recorded in traditional phase-locked loop (PLL) mode. It requires modeling of the signal frequency for connection of the phase between samples and removal of the GPS navigation data modulation (NDM). It is important that the postprocessing does not introduce errors (biases) in the connected phase. This paper describes the postprocessing of the OL RO signals which does not depend on the receiver model. The postprocessing includes: modeling of the RO signal frequency from refractivity climatology and the subsequent adjustment of this model by use of feedback which makes the postprocessing model-independent; internal (as in PLL) removal of NDM and the use of externally recorded NDM bit sequence. Statistical comparison of the refractivity inversion results demonstrates that external demodulation of RO signals reduces the inversion bias in the tropical lower troposphere by about 0.5%.