Orbit

Gravitational forces keep the moon in orbit and affect other planets and celestial bodies accordingly. The orbit of a satellite around our planet is easy to describe mathematically if both bodies are considered point masses, but in real life they are not. For the same reasons that the Geoid is not a simply shaped surface, the gravitation pull of the Earth that a satellite experiences in orbit is not simple either. Moreover, satellite orbits are also disturbed by solar and lunar gravitation, making flight paths slightly erratic and difficult to forecast exactly.

The parameters characterising the satellite’s orbit. For Earth Observation, the following  four orbit characteristics are relevant:

1. Orbital altitude is the distance (in km) from the satellite to the surface of the Earth. It influences to a large extent the area that can be viewed (i.e. the spatial coverage) and the details that can be observed (i.e. the spatial resolution). In general, the higher the altitude, the larger the spatial coverage but the lower the spatial resolution.
2. Orbital inclination angle is the angle (in degrees) between the orbital plane and the equatorial plane. The inclination angle of the orbit determines, together with the field of view (FOV) of the sensor, the latitudes up to which the Earth can be observed. If the inclination is 60°, then the satellite orbits the Earth between the latitudes 60° N and 60° S. If the satellite is in a low-Earth orbit with an inclination of 60°, then it cannot observe parts of the Earth at latitudes above 60° North and below 60° South, which means it cannot be used for observations of the Earth’s polar regions.
3. Orbital period is the time (in minutes) required to complete one full orbit. For instance, if a polar satellite orbits at 806 km mean altitude, then it has an orbital period of 101 minutes. The Moon has an orbital period of 27.3 days. The speed of the platform has implications for the type of images that can be acquired. A camera on a low-Earth orbit satellite would need a very short exposure time to avoid motion blur resulting from the high speed. Short exposure times, how ever, require high intensities of incident radiation, which is a problem in space because of atmospheric absorption. It should be obvious that the contradictory demands of high spatial resolution, no motion blur, high temporal resolution, long satellite lifetime (thus lower cost) represent a serious challenge for satellite sensor designers.
4. Repeat cycle is the time (in days) between two successive identical orbits. The revisit time (i.e. the time between two subsequent images of the same area) is determined by the repeat cycle together with the pointing capability of the sensor. Pointing capability refers to the possibility of the sensor–platform combination to look to the side, or forward, or backward, and not only vertically downwards. Many modern satellites have such a capability. We can make use of the pointing capability to reduce the time between successive observations of the same area, to image an area that is not covered by clouds at that moment, and to produce stereo images

Satellites are launched by rocket into space, where they then circle the Earth for 5 to 12 years on a predefined orbit. The choice of orbit depends on the objectives of the sensor mission; orbit characteristics and different orbit types are explained below. A satellite must travel at high speed to orbit at a certain distance from the Earth; the closer to the Earth, the faster the speed required. A space station such as ISS has a mean orbital altitude of 400 km and travels at roughly 27,000 km/h. The Moon at a distance of 384,400 km can conveniently circle the Earth at only 3700 km/h. At altitudes of 200 km, satellites already encounter traces of the atmosphere, which causes rapid orbital and mechanical decay. The higher the  altitude, the longer is the expected lifetime of the satellite