Data Acquisition Performance for Deep Space　Communications in Solar Probe Plus Frontier Radio

摘要

Radio receivers for deep space telecommunications require tracking loops that are robust in low signal-to-noise ratio conditions for not only carrier tracking, but also subcarrier tracking and bit synchronization. However, the loop band-widths must not be too narrow so as to accommodate Doppler dynamics, oscillator drift, and requirements for expedient and reliable data acquisition. The present work describes the data acquisition performance of Frontier Radio for the NASA Solar Probe Plus mission. The data acquisition time is a statistical quantity, as it depends on the frequency and phase state of the uplink waveform which is random with respect to the receiver. In order to rigorously characterize the performance and determine a nominal worst-case acquisition time, an automated test procedure was developed to execute a large number of acquisition trials. By architecting an automated procedure for the remote control of instruments, including timing control and uplink signal phase randomization, acquisition time measurements were more precise, more accurate, more consistent between trials, and greater in number than previous attempts. Thus, through this procedure we are able to estimate the upper bound on acquisition time to the 98th percentile with high confidence. Worst-case acquisition time occurs when the uplink data phase is 180° out of phase with the receiver bit clock; to mitigate this effect a circuit has been developed to detect the 180° condition and induce a phase step in the receiver bit clock with intent to align the two phases. We confirmed that the phase injection feature improves the worst-case acquisition time by a statistically significant margin: 62% to 85% acquisition time decrease in the 90th percentile acquisition time as a function of uplink data rate, with a 98% confidence interval. We describe the acquisition time results at all supported uplink data rates, over a range of signal power and Doppler offset conditions. Also described is the automated procedure, its contributions to the experimental soundness of the measurement compared to previous methods, and an estimate for its reduction of labor costs over time.