1. Orbital Inclination (i)

A satellite rarely has an orbit which is aligned so that it orbits exactly in the plane of the Earth's equator. Generally the satellite's orbit is inclined at an angle ( i ) to a plane defined by the Earth's equator as illustrated in the diagram.

Such an orbit has interesting consequences for Earth observations. By giving the satellite the appropriate orbital inclination, combined with the Earth's rotation, it is possible to survey a significant fraction of the Earth's surface.

2. Ground track

It is interesting to note that if a ground observer at the Equator visually tracks a satellite as it crosses the Equator through the zenith, (the point directly overhead), and notes the angle its track makes with respect to the East-West direction, the observer can tell instantly the maximum latitude that the satellite's ground track will reach during its orbit.

For example, if a spacecraft crosses over the Earth's equator, heading North at an angle of 55 degrees, the spacecraft's track will carry it northward to 55oN latitude, and then it will begin to head southward along its track.

Orbit 1

Looking down on the Earth's North Pole from space, the Earth rotates in an eastward (counter-clockwise) direction.

In the absence of any external force, the plane of a satellite's orbit is fixed with respect to a frame of reference defined by the stars. This is called an inertial frame of reference. As a consequence, the satellite's orbital motion is independant of the Earth's rotation.

Consider the satellite orbit in the example shown to the left. On this orbit, the satellite's orbit carries it over central Mexico and northward, passing just West of the Great Lakes.

Orbit 2

As the satellite orbits the Earth, the Earth rotates (eastward), on its axis and causes an apparent westward drift of the satellite's orbit. Keep in mind that the plane of the satellite's orbit is fixed with respect to the stars and is not linked to the diurnal rotation of the Earth.

During the time it takes the satellite to complete its orbit the Earth has been constantly rotating eastward.

The next orbit of the satellite takes it over southern California and then northward towards western Hudson's Bay.

Orbit 3

By the time the satellite begins its third orbit, the Earth has rotated still further eastward. The orbital track now carries the spacecraft over Vancouver and the Canadian High Arctic.

Satellites are usually placed in orbits which are high enough so that the widths of their visual "footprint" (the distance from horizon to horizon on the Earth's surface as seen from the spacecraft) overlap on each successive orbit.

Satellites placed in such orbits and which have high angles of inclination are capable of surveying almost the entire surface of the Earth each day.

Orbital Tracks 1, 2, and 3.

Plotted on a map (Mercator Projection), the successive orbits of the spacecraft produce a plot similar to the one shown to the left.

Orbital tracks move successively to the left (westward) on this diagram as a result of the Earth's rotation eastward (towards the right).

Orbital Footprint

Since the spacecraft is in low Earth orbit (less than about 500 kilometres altitude), it can only "see" a small patch of the Earth's surface at any one moment.

This small visible patch of the Earth's surface is called the spacecraft's "footprint" (yellow disks in the illustration to the left).

For Earth surveillance spacecraft the orbital path and altitude are selected so that the satellite's footprint overlaps slightly on successive orbital paths. In this way the entire planet can be surveyed each day.

The image to the left is an actual ground track of the International Space Station showing its present track and the next two tracks (westward).

The red circle denotes the "footprint" of the International Space Station. Note that the footprint of each orbit overlaps the footprint of the previous orbit so that the ISS passes directly over the western horizon of the previous orbit.