Open-loop tracking of rising and setting GNSS radio-occultation signals from an Airborne Platform: Signal model and statistical analysis

Global Navigation Satellite System (GNSS) radio occultation (RO) is an atmospheric sounding technique based upon the change in propagation direction of low-elevation GNSS signals through the stratified atmosphere. Atmospheric water vapor profiles can be retrieved from inverting the bending-angle measurements. Open-loop (OL) tracking, in which the received RO signal is cross-correlated with a model signal that does not include refraction effects, is applied to estimate the small excess phase due to atmospheric bending. OL tracking is demonstrated on rising as well as setting satellites observed from an airborne receiver. Setting signals are tracked by processing the sampled signal in reverse, allowing the initialization of tracking after the satellite has reached a higher elevation. A model has been developed to relate the signal to noise ratio (SNR) of the complex correlation to the variance in the residual phase estimate and is used to set a threshold on the minimum SNR for OL tracking. This model is shown to agree well with experimental measurements.