136 MHz Interferometer Error Due to Galactic Nucleus

Abstract

Spacecraft orbits are determined by means of direction cosines generated from electrical phase data provided by the 136 MHz Minitrack radio-interferometer system. The phase data thus determine the direction cosines of the position vector as the spacecraft passes across Minitrack's fan-shaped antenna beam. Uncertainties introduced into the output electrical phase due to undesirable interfering sources limit the basic accuracy of the Minitrack system. Although a knowledge of the spacecraft orbital dynamics may be used to improve the accuracy of the system, nevertheless there remains a fundamental error due to interference caused by the passage of the galactic nucleus. This paper determines the error due to a distributed noise source. Furthermore, it develops an expression for the lower bound of the phase error when the noise source is not uniformly distributed across a zenith-pointed fan beam. In addition, it determines the threshold of the Minitrack input power levels below which the electrical phase is no longer determined unambiguously. The effect of the passage of the galactic nucleus coincident with the presence of a space craft has been analyzed, and the corresponding phase error determined.