A discussion on orbital analysis - Properties of the upper atmosphere determined from satellite orbits

Abstract

Irregularities in the Earth’s gravitational potential perturb the orbits of artificial satellites in a great many ways. They cannot, however, change the mean value of the major axis of an orbit, which determines the period of revolution. To change the orbital period a dissipative force is required, such as the drag exerted on the satellite by the Earth’s atmosphere. Solar radiation pressure does not affect the period of a satellite provided the satellite does not cross the shadow cone of the Earth. If the orbit is all in daylight, the effect of the force cancels out after one revolution. If, however, the satellite goes in and out of the Earth’s shadow, and the orbit is not circular, the effect does not cancel out and radiation pressure will cause a change in the period. Atmospheric drag and solar radiation pressure are the only major forces that are known to affect the period of a satellite. Other forces, such as the interaction of an electrically charged satellite with atmospheric ions or with the magnetic field of the Earth, are undoubtedly present, but they are generally quite small. For low-orbiting satellites, with perigees below 300 km, the effect of atmospheric drag on the orbital period is so much larger than that of solar radiation pressure, that the latter can be neglected for all practical purposes. Above 400 km, however, radiation pressure makes itself felt, and above 700 km it may become more important than atmospheric drag. Actually, these figures vary a great deal with the phase of the solar cycle, since the atmosphere expands or contracts with solar activity. At sunspot minimum the effect of radiation pressure becomes comparable with that of atmospheric drag at about 600 km, while at sunspot maximum it does not become so below 1100 km.