ISPM SPACECRAFT ANTENNAS

Abstract

International Solar Polar Mission (ISPM) is a dual-spacecraft mission sponsored jointly by NASA and European Space Agency (ESA) to gather scientific information for further understanding of the sun and predicting its influence on the Earth’s weather and climate. Jet Propulsion Laboratory of the California Institute of Technology has selected TRW to buiId U.S. spacecraft for the joint mission. The dual spacecraft will fly to Jupiter and use that planet’s greater gravitational field to achieve a near 90 degree orbit change, placing the two spacecraft on separate trajectories to the North and South poles of the Sun from a high heliographic vantage point. The antenna subsystem of the ISPM spacecraft consists of S-/X band high gain, S-band broad coverage, and X-Band medium gain antenna. Command and ranging signals are received by the S-band high-gain and broad-coverage antennas. Scientific and engineering data are transmitted by these two antennas and the X-band high-gain antenna. Conscan acquisition is by the two S-band antennas. Emergency transmissions are by the S-band broad coverage and X-band medium-gain antennas. The S-/X-band HGA is a 1.9 meter (78 inches) diameter dual reflector Cassegrain design with a dichroic subreflector. The Cassegrain mode is excited by an efficient dual mode conical horn whose dimensions have been optimized to provide high-gain performance for X-Band. The S-band feed, located directly behind the frequency selective subreflector, illuminates the parabolic reflector as a focal point feed, laterally displaced by 2.29 cm (0.9 inch) to provide conscan signals with a 1 dB crossover level. The selected configuration permits the use of a common antenna for both X- and S-band functions and utilizes previously developed TRW hardware. The selected design represents the largest non-deployable antenna that could be accommodated by shuttle/IUS and spacecraft physical interfaces. An x-ray XUV telescope (CXX) is located on the center of the spacecraf t which is despun about the spacecraft +Z axis. The ends of the coronograph cast shadows onto the edge of the reflector up to 9.72 cm (3.83 inches) inside the reflector. The shadowing effects from the coronograph have been analyzed and subsequently verified by antenna range testing. The performance of each antenna is substantiated by analyses and test data and pertinent design and analysis results are presented.