An orbit is an intangible curve. A satellite remains in perpetual motion along an invisible “path” dictated by the laws of gravity, like a captive train on a never-ending track.

Another kind of virtual track, on the ground this time, represents the regions on the globe covered by a satellite. This ground track is the series of points on the Earth’s surface directly beneath the satellite along its orbit. In other words, it projects the path of the satellite’s motion onto the Earth.

^{Ground track of a circular orbit. Points a, b and c on the ground mirror points A, B and C on the orbit. But this track—in red on both diagrams—is based on the fictional assumption that the Earth is immobile. The green track shows the real pattern traced by a satellite with the Earth rotating on its axis: because the Earth rotates eastward, the ground track shifts slightly westward on each orbital revolution.
Conception : Jean-Pierre Penot (CNES) et Bernard Nicolas, illustration : Bernard Nicolas.}

For a satellite on a circular orbit, the ground track projected onto a map is a sinusoidal curve. This may seem surprising, given the actual shape of the orbit, but there are 3 reasons for this:

• the inclination of the orbit with respect to the equatorial plane, which varies according to the satellite mission ;
• the Earth’s rotation (it completes one revolution in 23 h 56 min. 04 s);
• the principles applied when projecting a spherical object (the terrestrial globe) onto a flat surface (a map).

Closer examination of the ground track reveals that the orbit inclination corresponds to the extreme latitudes of the satellite’s coverage zone. So, for a geostationary satellite the track is in fact a dot on the equator.