The last 14 days of Skylab 1: orbit determination and analysis

The orbit of Skylab 1 (1973-27A) has been determined using some 1400 NORAD observations during the 14 days prior to decay on 1979 July 11. There are 14 daily orbits, with standard deviations corresponding to average accuracies of 40 m cross track and 30 m radial. A 15th orbit, only slightly less accurate, was determined from observations on July 11 between the manoeuvre at 07.45 UT and decay at 16.37 UT. The variations in inclination due to atmospheric rotation and 16th order resonance with the geopotential have been successfully analysed, to give the first values of 16th-order geopotential coefficients determined from resonance, and a value of 1.10 ± 0.07 rev/day for the atmospheric rotation rate at a height of 210-220 km. The daily changes in semi-major axis have been used to determine 13 daily values of air density, at heights from 252 km down to 179 km. All agree weIl with the CIRA 1972 model, and indicate a smaller semi-annual variation than in the early 1970s. The variations of eccentricity and argument of perigee take unusual forms, but detailed analysis shows that the variations are in full accord with the theory for an atmosphere with day-to-night variation in density, with the perigee progressing towards the point of minimum density.