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Encyclopedia > Ground track
Ground track of the International Space Station for approximately two periods. The light and dark regions represent the regions of the Earth in daylight and in night.
Ground track of the International Space Station for approximately two periods. The light and dark regions represent the regions of the Earth in daylight and in night.

A ground track is an imaginary path along the surface of the Earth which traces the movement of an imaginary line between a given 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. International Space Station insignia ISS Statistics Crew: 3 As of December 19, 2006 Perigee: 352. ... An Earth observation satellite, ERS 2 For other uses, see Satellite (disambiguation). ... In broad terms, the zenith is the direction pointing directly above a particular location (perpendicular, orthogonal). ... A frame of reference is a particular perspective from which the universe is observed. ...

Contents

Appearance

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 hypherbolic trajectories. The elements of an orbit are the parameters needed to specify that orbit uniquely, given a model of two ideal masses obeying the Newtonian laws of motion and the inverse-square law of gravitational attraction. ... (This page refers to eccitricity in astrodynamics. ... In astrodynamics or celestial mechanics a parabolic trajectory is an orbit with the eccentricity equal to 1. ... In astrodynamics or celestial mechanics a hyperbolic trajectory is an orbit with the eccentricity greater than 1. ...


Direct and retrograde motion

Typically, satellites have a roughly sinusoidal ground track. A satellite with an inclination between zero and 90 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. In trigonometry, an ideal sine wave is a waveform whose graph is identical to the generalized sine function y = Asin[ω(x − α)] + C, where A is the amplitude, ω is the angular frequency (2π/P where P is the wavelength), α is the phase shift, and C... Inclination in general is the angle between a reference plane and another plane or axis of direction. ... Direct motion is the motion of a planetary body in a direction similar to that of other bodies within its system, and is sometimes called prograde motion. ...


A satellite in a direct orbit with an orbital period less than that of the Earth will appear to move from west to east. This is called apparent direct motion. A satellite with an orbital period greater than that of the Earth will appear to move east to west, in what is called "apparent retrograde" motion. This is due to the fact that the satellite is orbiting in the same direction as the Earth's rotation, but more slowly that the Earth. The orbital period is the time it takes a planet (or another object) to make one full orbit. ...


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 the same ground track every day. This ground track is shifted east or west depending on the longitude of the ascending node. 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. The integers are commonly denoted by the above symbol. ... The Longitude of the ascending node (☊, also noted Ω) is one of the orbital elements used to specify the orbit of an object in space. ...


As the orbital period of a satellite approaches 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. The Earths rotation is the rotation of the solid earth around its own axis, which is called Earths axis or rotation axis. ... World map showing the equator in red In tourist areas, the equator is often marked on the sides of roads The equator marked as it crosses Ilhéu das Rolas, in São Tomé and Príncipe. ...


A satellite whose orbital period is exactly the same as that of the Earth is said to be in a geosynchronous orbit, and its ground track will have a "figure eight" shape, 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. It has been suggested that this article or section be merged with geostationary orbit. ... Perigee is the point at which an object in orbit around the Earth makes its closest approach to the Earth. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ...


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 due to the fact that the Earth is rotating about its own center of mass at the same rate as the satellite. A geostationary orbit (GEO) is a geosynchronous orbit directly above the Earths equator (0º latitude). ... (This page refers to eccitricity in astrodynamics. ... ECEF stands for Earth-Centered, Earth-Fixed, and is a Cartesian coordinate system used for GPS. It represents positions as an X, Y, and Z coordinate in meters. ... Fig. ... In physics, the center of mass of a system of particles is a specific point at which, for many purposes, the systems mass behaves as if it were concentrated. ...


Inclination

The greater the inclination of a satellite's orbit, the further north and south its ground track will pass. A satellite with an inclination of exactly 90 degrees is said to be in a polar orbit, meaning it passes over the Earth's north and south poles. A satellite in a polar orbit passes above or nearly above both poles of the planet (or other celestial body) on each revolution. ...


Argument of perigee

The ground track of a Molniya orbit
The ground track of a Molniya orbit

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 is an example of such a case. In a Molniya orbit, apogee occurs at a high latitude, and the orbit is highly eccentric. 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-latitide regions. The argument of periapsis (ω) is the orbital element describing the angle between an orbiting bodys ascending node (the point where the body crosses the plane of reference from South to North) and its periapsis (the point of closest approach to the central body), measured in the orbital plane and... Molniya orbit is a class of a highly elliptic orbit with inclination of +/-63. ... Latitude, usually denoted symbolically by the Greek letter phi, , gives the location of a place on Earth north or south of the equator. ...


External links

  • NASA J-Track - see ground tracks for various satellites

 
 

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