Satellite Capabilities and Orbits

A mission of a satellite determines the type of orbit in which it is placed. Figure 1 shows satellites with different orbits. Usually, earth observation satellites, the focus of this chapter, are placed in near earth orbits at heights between 200km and 700km. Spacecraft closest to the surface of the earth are used for defence purposes as sensors on board such satellites could achieve maximum details. Civil remote sensing satellites listed in Figure 2 are orbited at altitudes between 400km and 700km. One of the most important applications of such satellites is for improving national and international security by monitoring bilateral and multilateral agreements, such as arms control treaties, that affect security.

It is often argued that in order to carry out the monitoring of arms control treaties, peace keeping operations, early warning of conflicts and even ongoing conflicts that often result in large movements of refugees (who also have to be monitored in order to deliver, for example, aid to them), many satellites would be needed. However, it can be seen from Figure 2 that there are alreadyseveral spacecraft in orbit with optical sensors that have useful ground resolution of one metre and better. As these are launched and operated by different States, the satellites appear to be scattered and their orbits are such that observations of the earth cannot be made frequently enough and in any logical fashion. An examination of the orbital parameters of these spacecraft indicates that some are in one orbital plane and others move in orbits that are spaced out widely. The satellites are also widely dispersed relative to one another in their respective orbital planes. This means that a more frequent and wider coverage is possible but often randomly targeted.