A Precision Frequency Measurement Technique for a Moving Signal with a Low Signal-to-Noise Ratio

A technique for measuring the frequency of the RF carrier from a spacecraft, at a low signal to noise ratio, was developed for the Mariner V S-band occultation experiment. The technique consisted of analog recording of the noisy signal, simultaneously with a high-frequency tone. This tone was used as a clock to trigger a digitizer. After digitization, spectra were taken at several points to permit fitting a polynomial to the frequency. Digital filtering techniques, in which the signal was moved, in frequency, to stay in the filter bandpass, were then applied. The filtered data were then processed by a digital phase-locked receiver program that gave amplitude and frequency versus time as outputs. Several iterations were required to reach thelevel of -190 dBm at which point the signal was lost.