Tracking tropospheric radio occultation signals from low Earth orbit

Propagation of radio occultation signals through the tropical lower troposphere with severe refractivity gradients results in significant spreading of the signal spectrum. Under such conditions a signal acquisition technique which tracks large random troposphere-induced phase accelerations more reliably than a generic phase-locked loop has to be applied. This paper discusses the results of simulations of open loop tracking of radio occultation signals that were generated with data from high-resolution tropical radiosondes. The signal has to be down-converted in real time in the receiver on orbit to a low mean residual frequency by use of a phase (Doppler) model based on predicted orbits and refractivity climatology. The down-converted complex signal is then low-pass filtered and sampled. The phase in excess of the phase model must be reconstructed from the sampled and down-linked signal in postprocessing. This may require an additional down-conversion to eliminate (minimize) aliasing of harmonics in the spectrum. Then the accumulated phase can be reconstructed by resampling the signal at a higher rate to resolve the cycle ambiguities. A fast algorithm for prediction of the Doppler based on the refractivity climatology and an algorithm for the detection of Doppler mismodeling based on sliding window spectral analysis of the down-converted signal are developed and tested. The accuracy of the Doppler modeling, ±(15-20) Hz, the required filter bandwidth, 100 Hz, and the sampling rate, 50-100 Hz, are estimated.