Communication Link Prediction and Assessment for LCROSS Mission Operations

Abstract

The Lunar Crater Observation and Sensing Satellite (LCROSS) spacecraft orbited the earth in an inclined orbit that intersected the moon's south pole on October 9, 2009, resulting in an impact that allowed measurement of the water content in the debris cloud. The main communications requirement was providing robust command and telemetry links for spacecraft operations, including trajectory correction maneuvers and housekeeping. The payload consisted of cameras and spectrometers capable of imaging the visible, near and mid-IR spectrums, requiring 1 Mbps downlink for full science capability at lunar range. This communications performance was delivered by a pair of omni directional antennas coupled with medium gain antennas used for the high-speed science rate. Over the complex flight path that lasted over 3 months, there were many intervals where communications rates were much reduced due to interference between the two omni-directional antennas. Analytical Graphics' Satellite Toolkit was used for modeling the LCROSS spacecraft orbit, its antennas and its attitude with respect to the relevant Deep Space Network ground stations. Link predictions were used to confirm adequate uplink margin for commanding the spacecraft and to recommend downlink data rates supported at the current attitude and range. After initiation of flight operations, a detailed assessment of link performance was performed in order to validate the modeling and to make any necessary improvements. The primary goal was to confirm the accuracy of the modeled omni-directional antenna radiation patterns, particularly the interferometry region. Also of interest was the effect of small attitude motion within the attitude control system dead-band. The capability of predicting variations in link margin based on actual antenna radiation characteristics and dead-band motion greatly improved the accuracy of link margin prediction. Link performance data from actual LCROSS maneuvers are presented to illustrate the methods. Nomenclature dB = decibel IR = infrared Kbps = kilo-bits per second Km = kilometer Mbps = mega-bits per second RF = radio frequency