Maximum likelihood estimation of navigation parameters from downlink telemetry

Deep-space navigation uses estimates of range and Doppler to update and improve spacecraft trajectory solutions.1,2 Operationally, Doppler is generally extracted directly from the ground receiver's carrier tracking loop, and range is determined primarily by the use of specially designed “ranging tones,” or more recently (e.g., on New Horizons), Pseudo-Noise (PN) sequences. Transmission of tones or PN sequences drain power and bandwidth that could be better used for transmitting additional science data from the spacecraft. Here we describe and evaluate a novel technique that extracts range and Doppler estimates directly from the decoded data, thus enabling data-transmission at the maximum rate consistent with spacecraft range, antenna gain and available signal power. In this paper, the structure of the maximum likelihood estimator for range and Doppler is derived, and its performance determined relative to Cramér-Rao bounds via simulation and analysis. Performance of conventional Doppler and delay estimators based on carrier tracking loops is also derived, and contrasted with the performance of the optimum Doppler-delay estimator over a range of symbol SNR typically encountered in deep-space applications.