1, Ph.D. Student, Iranian Research Organization for Science and Technology
2, Associate Professor, Iranian Research Organization for Science and Technology
One of the main challenges in tracking radio occultation (RO) signals of open loop approach is the excess Doppler estimation accuracy in lower troposphere. Propagation of RO signals through the lower troposphere with severe refractivity gradient results in high phase acceleration and low signal to noise ratio (SNR) signal. Because of the high refractivity in the lower moist troposphere, the received RO signals by global navigation satellite system (GNSS) receivers can have large excess Doppler which may vary rapidly. Due to limitation on bandwidth and transmitted information to post processing, we can't use very high sampling rate at GNSS receivers at LEO satellite. Therefore, try to enhance the accuracy of the Doppler predicted models in the satellite makes the GNSS-RO receivers by using efficiently frequency estimation method at GNSS receivers. In this regard, we investigate the event of RO signal and then the various frequency estimation methods to improve the estimates of the Doppler frequency in post processing are evaluated. The various frequency estimation methods are investigated from performance and computational complexity perspective. Via simulation, the excess Doppler estimation accuracy in post processing for different frequency estimation methods are compared by its root mean squared error. Based on simulation results, it is seen that both ESPRIT and Jacobsen with Bias schemes have the better performance and the estimation error of them are less than that of other methods.