RANGE SAFETY RECEIVER-DECODER FOR SPACE SHUTTLE LAUNCH VEHICLE

Abstract

Flight Termination Sub-Systems for range safety purposes are a part of all spacecraft launch vehicles. The Command Receiver and Decoder portion of this sub-system receives the rf up-link signal and initiates the flight termination action. For launch vehicles for unmanned spacecraft, the range safety up-link command signal is composed of selected IRIG audio tones, in a specified sequence, frequency modulated on an rf carrier. For the Space Shuttle launches, a more sophisticated high-alphabet modulation technique is used. This provides for a better probability against an undesired output caused by interfering signals. The Shuttle system uses a complex command modulation format composed of various audio tones frequency modulated on the standard UHF carrier. The characters in the command word are made up of two simultaneously transmitted tones selected from seven possible frequencies. These seven tones, taken two at a time, provide an alphabet of twenty-one different characters from which a command word is formed. The transmitted sequence of characters is selectable from mission to mission. Approximately 1012 possible code combinations exist for any one mission. For the Space Shuttle launches, the range safety Command Receiver-Decoders will be used on the Solid Rocket Boosters and the External Tank portions of the complex launch vehicle. The receiver section of these units is a single superheterodyne design fixed tuned to the proper rf center frequency at the time of manufacture. The decoder section utilizes a microprocessor to effect the decoding function. The decoder is “programmed” prior to flight to recognize only the code of the mission. In addition, the microprocessor is used to accomplish the audio tone demodulation using a Fast Fourier Transform (FFT) algorithm. Solid state output switches provide the decoder output voltages to the flight termination destruct mechanism.