Precision Power Measurements of Spacecraft CW Signal Level with Microwave Noise Standards

One of the important measurements required in the evaluation of a spacecraft communications system is the determination of the absolute level of the received CW power. A new, precise measurement method which compares CW signal power with microwave noise power is described. This technique, together with statistical methods of data reduction, results in significantly increased accuracy. The overall probable error of the measurement was reduced from 0.8 dB to 0.3 dB, defined at the receiver input for an antenna/receiving system of the Jet Propulsion Laboratory´s Goldstone Deep Space Communication Complex. Application of these techniques to the Mariner IV spacecraft was begun on June 29, 1965, and continued after Mars encounter. The theory, equipment, and method of data acquisition and reduction are described. Results and accuracies are discussed. The Mariner IV spacecraft received power at Mars encounter normalized for 100 percent antenna efficiency was measured to be -154.2 dBm as compared to a theoretically predicated level of -153.1 dBm.