A Doppler Correcting Software Deļ¬ned Radio Receiver Design for Satellite Communications Figure 1. (a) Flow diagram of a traditional radio for satellite communications that performs Doppler correction using external range rate estimates based on accurate a-priory knowledge of the position (such as TLE). (b) Structure of the proposed SDR with active Doppler correction. Note, that no range rate or external information is required. The bottom of (b) shows the details of the Doppler search block, while the top shows the timing recovery. normally provided by a propagator, which predicts the satellites path and range rate based on a TLE. Provided that the TLE is accurate, the Doppler offset is removed sufficiently for envelope detection, differential decoding, or for a PLL to lock on the carrier. An inaccurate TLE or path propagation 40 of a satellite can result in a significant error in the Doppler estimate, which results in decreased effectiveness of the demodulation. Additionally, the presence of unknown carrier offsets due to crystal drift or atmospheric effects cannot be predicted using a TLE. IEEE A&E SYSTEMS MAGAZINE FEBRUARY 2020