Orbit determination of 1965-09A for analysis of the eccentricity at 15th-order resonance

Abstract

A satellite experiences 15th-order resonance with the Earth's gravitational field when its track over the Earth repeats each day after 15 revolutions : the orbit is then very sensitive to the effects of 15th-order harmonics in the geopotential. By analysis of many such orbits, 15th-order harmonics of degree 15–36 have been evaluated with an accuracy equivalent to 0.6 cm in geoid height for degree 15–23, but with much poorer accuracy for the even degrees 24, 26, … 36, because of the absence of results for even degree from satellites with orbital inclination less than 43 . The satellite Pegasus 1, 1965-09A, at inclination 31.8° offers the chance to fill the gap: it passed slowly through the 15th-order resonance between November 1973 and January 1976. Orbits have been determined at 73 epochs from 4057 observations, chiefly U.S. Navy, and the variations in orbital eccentricity have been analysed to determine values of four lumped 15th-order harmonics with an accuracy equivalent to between 2 and 6 cm in geoid height. By adding these new results, the solution for the individual harmonics is greatly improved in accuracy, by a factor of 3.0 on average for degree 24, 26, 28 and 30, and a factor of 1.4 on average for degree 32, 34 and 36.