DETERMINATION OF ORBITING SPACECRAFT ANTENNA DISTORTION BY GROUND-BASED MEASUREMENTS

WE PROPOSE TO STUDY A GROUND-BASED SYSTEM FOR MEASURING THE DISTORTIONS AND POWER DISTRIBUTIONS OF IN-ORBIT REFLECTOR ANTENNAS AND PHASED ARRAYS. THE SYSTEM WILL MEASURE THE FAR-FIELD AMPLITUDES AND PHASES IN THE MAIN BEAM AND SIDELOBES OF THE SATELLITE ANTENNA. THE FOURIER TRANSFORM OF THESE DATA GIVES THE COMPLEX APERTURE DISTRIBUTION. PHASE DEVIATIONS IN THE APERTURE DISTRIBUTION ARE RELATED TO DISTORTIONS IN A REFLECTOR ANTENNA AND PHASE ERRORS IN A PHASED ARRAY. DISTORTIONS OR PHASE ERRORS AS SMALL AS 1% OF THE OPERATING WAVELENGTH CAN BE MEASURED. VARIATIONS IN THE APERTURE AMPLITUDE DISTRIBUTION ARE RELATED TO AREAS OF LOW REFLECTIVITY OR FAILED ARRAY ELEMENTS. OBSERVATIONS AND ANALYSIS USE THE TECHNIQUES OF VERY LONG BASELINE INTERFEROMETRY (VLBI), WHICH IS ROUTINELY USED BY ASTRONOMERS TO MAP THE EMISSION OF CELESTIAL OBJECTS. THE PROPOSED SYSTEM MAKES ALMOST NO DEMAND ON SPACECRAFT RESOURCES. IN PARTICULAR, NO SPECIAL EQUIPMENT NEEDS TO BE CARRIED INTO ORBIT. A SYSTEMS ANALYSIS WILL DEFINE THE EQUIPMENT NECESSARY FOR AN OPERATIONAL NETWORK TO PERFORM NEAR REAL TIME ANTENNA EVALUATION.