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Ground track

A ground track or ground trace is the projection of a satellite's orbit onto the surface of the Earth (or whatever body the satellite is orbiting). It is an imaginary path along the Earth's surface which traces the movement of an imaginary line between the satellite and the center of the Earth. In other words, the ground track is the set of points at which the satellite will pass directly overhead, or cross the zenith, in the frame of reference of a ground observer.

Variations

The ground track of a satellite can take a number of different forms, depending on several of the orbital elements which define the orbit of the satellite, and other orbital parameters which can be derived from the orbital elements. This article discusses closed orbits, or orbits with eccentricity less than 1, thus excluding parabolic and hyperbolic trajectories.

Direct and retrograde motion

Typically, satellites have a roughly sinusoidal ground track. A satellite with an inclination between zero and ninety degrees is said to be in what is called a direct or prograde orbit, meaning it orbits in the same direction as the Earth's rotation. A satellite with an inclination between 90 and 180 degrees is said to be in a retrograde orbit.

A satellite in a direct orbit with an orbital period less than one day will move from west to east. This is called apparent direct motion. A satellite in a direct orbit with an orbital period greater than one day will move from east to west, in what is called "apparent retrograde" motion. This is because the satellite is orbiting in the same direction as the Earth's rotation, but more slowly than the Earth. Any satellite in a retrograde orbit will move from east to west.

Orbital period

A satellite whose orbital period is an integer fraction of a day (i.e., 24 hours, 12 hours, 8 hours, etc.) will follow roughly the same ground track every day. This ground track is shifted east or west depending on the longitude of the ascending node, which can vary over time due to perturbations of the orbit. If the period of the satellite is slightly longer than an integer fraction of a day, the ground track will shift west each day; if it is slightly shorter, the ground track will shift east.

As the orbital period of a satellite increases, approaching the rotational period of the Earth, its sinusoidal ground track will become compressed laterally, meaning that the points at which it crosses the equator will become closer together.

A satellite whose orbital period is exactly the same as that of the Earth is said to be in a geosynchronous orbit. Its ground track will have a "figure eight" shape in a fixed location on the Earth, crossing the equator twice each day. It will move in the prograde direction when it is on the part of its orbit closest to perigee, and in the retrograde direction when it is closest to apogee.

A special case of the geosynchronous orbit, the geostationary orbit, has an eccentrity of zero (meaning the orbit is circular), and an inclination of zero in the Earth-Centered, Earth-Fixed coordinate system (meaning the orbital plane is not tilted relative to the Earth's equator). The "ground track" in this case consists of a single point on the Earth's equator, above which the satellite sits at all times. Note that the satellite is still orbiting the Earth — its apparent lack of motion is because the Earth is rotating about its own center of mass at the same rate as the satellite.

Inclination

The greater the inclination of a satellite's orbit, the further north and south its ground track will pass. The geographic latitudes covered by the ground track will range from –i to i, where i is the orbital inclination. A satellite with an inclination of exactly 90° is said to be in a polar orbit, meaning it passes over the Earth's north and south poles.

Argument of perigee

If the argument of perigee is zero, meaning that perigee occurs on the equatorial plane, the ground track of the satellite will appear the same above and below the equator. However, if the argument of perigee is non-zero, the satellite will behave differently in the northern and southern hemispheres. The Molniya orbit, with an argument of perigee near 90°, is an example of such a case. In a Molniya orbit, apogee occurs at a high latitude (63°), and the orbit is highly eccentric (e = 0.72). This causes the satellite to "hover" over a region of the northern hemisphere for a long time, while spending very little time over the southern hemisphere. This phenomenon is known as "apogee dwell", and is desirable for communications for high latitude regions.