An analysis of the Lunar Module Apollo S-Band Steerable Antenna Tracking System

The effect of incidental amplitude modulation (IAM) on the uplink carrier and the effects of LM transceiver noise are studied to determine if these effects cause Lunar Module (LM) Apollo S-Band Antenna Tracking System errors which limit the operation of the tracking system and the two-way communication link between the LM spacecraft and the Manned Space Flight Network (MSFN). A functional model of the LM S-Band Antenna Tracking System is used to enable a methodical analysis of the antenna tracking signals. The effect of IAM on the uplink carrier and LM transceiver noise are then analyzed and the resultant antenna tracking errors are calculated. Furthermore, the LM S-Band Antenna Tracking System is sensitive to incidental amplitude modulation on the uplink carrier, especially at the fundamental (50 Hz) and its harmonics of the antenna tracking lobing or sampling rate and that excessive antenna tracking errors occur for more than 1% IAM. The antenna tracking errors due to LM transceiver noise were found to be much smaller than those due to IAM for total received signal levels out to lunar distances. The antenna tracking model developed in this study can consistently predict the antenna tracking errors as a result of IAM on the uplink carrier and LM transceiver noise. This was verified by a comparison of several sets of measured antenna tracking error data provided by the National Aeronautics and Space Administration (NASA), Manned Spacecraft Center (MSC) with several sets of predicted antenna tracking error data. The techniques given in this study are general enough for determining the effect of other interfering signals on the tracking system as well as analysis of other antenna tracking systems.