As can be seen from the above discussion, the topic of orbit perturbations is a fairly complex one. It has been my aim to demonstrate that perturbations need to be taken into account when doing the mission analysis for a real spacecraft project. Another aspect that is implicit in the discussion is that dealing with the perturbation effects requires a good deal of mathematical and computational expertise, which is a routine part of any spacecraft mission analysis activity.

The differences between the ideal, Keplerian orbits of Chapter 2, and the real orbits discussed in this chapter are nicely summed up by the geostationary Earth orbit (GEO). For example, if we have a communications satellite in GEO, then in the absence of perturbations we simply launch the spacecraft into a circular, equatorial orbit at the right height to give an orbit period of one day (see Chapter 2). To someone on the ground, the spacecraft then appears to remain stationary in the sky, and the various communica tions dishes on the ground that wish to use the satellite simply stare at this fixed position. However, the situation is a little more arduous for the satellite operators in the real world when the effects of perturbations have to be countered. As we have seen, there are three main perturbations that affect a GEO satellite: gravity anomalies, luni-solar perturbations, and solar radiation pressure. Each of these has a distinctive signature with respect to the motion of the satellite as seen from the ground. Gravity anomalies cause the satellite to move away slowly in an east-west (or longitudinal) direction from the point in the sky to which the ground station dish is directed, and in some cases this can cause the spacecraft to disappear over the horizon! Luni-solar perturbations cause changes in the orbital inclination, which in turn cause the satellite's position to oscillate in a north-south (or latitudinal) sense with a period of 1 day. Finally, we have seen that solar radiation pressure effects produce an eccentricity in the orbit, which leads to an east-west (or longitudinal) oscillation as well. Keeping the spacecraft in the line of sight of the ground dish is a nontrivial orbit-control exercise.

The rest of the chapters in this book are less technically challenging than this chapter. With this basic background in orbits, we now move on to Chapter 4 to look at some mission orbits that are a little more exotic than the popular operational orbits that we have already looked at.

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