Frequency Synthesis Approach to Determine Spacecraft Angular Position with Sub-nanoradian Accuracy

This paper describes a possible approach to measuring angular position of a spacecraft with reference to a nearby calibration source (quasar) with an accuracy of a few tenths of a nanoradian using a very long baseline interferometer that measures the interferometer phase with a modest accuracy. It employs (1) radio frequency (RF) phase delay to determine the spacecraft position with a high precision, and (2) multiple group delay measurements using either frequency tones or telemetry signals at different frequency spacing to resolve ambiguity of the fringe (cycle) containing the direction of the spacecraft.