Software for Detecting Tones in Beacon Monitoring

A software package for use in a ground receiving station detects a beacon-monitor tone transmitted by a spacecraft. To recapitulate from previous NASA Tech Briefs articles on beacon monitoring: An onboard computer processes data from onboard sensors to summarize the condition of the spacecraft in terms of one of four standard statuses, then a message is transmitted on a radio-frequency carrier as phase modulation at one of four subcarrier frequencies that correspond to the four statuses. The present software implements a weak-signal-detection algorithm. The received signal is heterodyned at nine frequencies and recorded into nine corresponding frequency bands. Nominally, there is a center carrier and four pairs of symmetrical status bands. After recording, each frequency band is subdivided into subbands with a fixed temporal duration. Within each frequency subband, there is a bank of time-sequenced subband processors. The optimal linear path in the corresponding time-frequency band space is obtained by maximizing the summed squared signal amplitudes. The maximum summed subband-processor output is selected and compared to a predetermined threshold to determine whether the corresponding status message should be deemed to have been transmitted. The frequency drift of the status tone signal must be small enough to allow the search algorithm to detect the signal over a time long enough to provide a low signal-detection threshold.