Exploring Implicit Pilots for Precise Estimation of LEO Satellite Downlink Doppler Frequency

The performance of low earth orbit (LEO) satellite Doppler positioning is fundamentally dominated by the precision with which the Doppler frequency is estimated, generally using an explicit pilot from the received signal. This letter proposes an algorithm for the precise estimation of the LEO satellite downlink Doppler frequency based on the joint use of the explicit pilot signal and an implicit pilot, which is a signal segment opportunistically available in an LEO downlink frame with all zero bits, all one bits, or interleaved zero and one bits. The theoretical performance is derived, and simulation results show that the proposed algorithm reaches the Cramer-Rao lower bound (CRLB). Verifications are conducted on signals from the Iridium mobile satellite communications system. The results show that 58.41% of Iridium frames in the ring alert and paging channels have implicit pilots and that for frames with implicit pilots, the proposed algorithm reduces the Doppler frequency estimation error by 70% compared to algorithms that use only the explicit pilot.