Improved frequency estimation for radio occultation through chirp Z-transform and adaptive slicing

ABSTRACT Radio occultation (RO) studies rely on the instantaneous frequency of the radio signal propagating from an orbiter to the ground station, which gets perturbed when it passes through the atmosphere of a celestial body. Frequency residuals, the difference between observed and expected Doppler frequencies, are instrumental in deriving the vertical profiles of atmospheric parameters. Thus, accurate frequency estimation is crucial for RO experiments. However, the dynamic relative motion between the orbiter and the station, combined with fluctuations in the plasma medium along the propagation path, causes the frequency to change rapidly, posing a challenge. This article presents signal processing techniques to enhance the accuracy of frequency estimates using chirp Z-transform (CZT) and adaptive slicing, which in turn will render an accurate analysis of the planetary atmosphere. The iterative application of CZT improves frequency resolution and enables working with smaller time packets. Adaptive slicing, on the other hand, varies the length of the packets according to the orbiter’s motion. Extensive experiments on the data from the Chandrayaan-2 (CH2) dual-frequency radio science (DFRS) payload corroborate that our method provides better estimates of differential Doppler (DD) with lesser fluctuations compared to the existing schemes, leading to more precise physical parameter retrievals, as exemplified by improved estimates of integrated total electron content (iTEC) of the lunar ionosphere.