Digital FM receiver of command destruct subsystem for launch range safety

The spacelift range system (SLRS) utilizes the command destruct subsystem (CDS) to provide operational support for launch vehicles during emergency events. The main functions in the commonly used CDS receiver are digital FM demodulation and command destruct (CD) detection. The ultimate goal of the CDS is to have a near-zero probability of mis-detection for the CD signal. In order to accomplish this goal, the FM demodulator needs to provide a high signal-to-noise ratio (SNR) to the CD detector. The actual hardware utilizes a bank of narrowband filters after the FM demodulator to filter out the noise before taking the FFT. The FFT detects the tones, then send it to the CD detector. However, due to the fact that the CD signal consists of a sequence of pulsed tones, these filters create leakage from one pulse to another. This means that the signal energy within the desired pulse is reduced and the noise level is increased (due to intersymbol and inter-tone interferences). As a result, the SNR to the CD detector degrades significantly, which in turn substantially affects the probability of CD detection. This paper presents both analysis and simulation results of the impact of FM demodulation and narrowband filters to the input SNR value to the CD detector.