Highly Elliptical Orbit
Encyclopedia
A highly elliptical orbit (HEO) is an elliptic orbit
with a low-altitude perigee
and a high-altitude (over 35786 kilometres (19,322.9 nmi)) apogee. It is a type of high Earth orbit
.
Such extremely elongated orbits have the advantage of long dwell times at a point in the sky during the approach to, and descent from, apogee. Visibility near apogee can exceed twelve hours of dwell at apogee with a much shorter and faster-moving perigee phase. Bodies moving through the long apogee dwell can appear still in the sky to the ground when the orbit is at the right inclination, where the angular velocity of the orbit in the equatorial plane closely matches the rotation of the surface beneath. This makes these elliptical orbits useful for communications satellite
s.
Sirius Satellite Radio
uses HEO orbits to keep two satellites positioned above North America while another satellite quickly sweeps through the southern part of its 24-hour orbit. The longitude
above which the satellites dwell at apogee in the small loop remains relatively constant as the earth rotates. The three separate orbits are spaced equally around the Earth, but share a common ground track
.
Examples of HEO orbits offering visibility over Earth's polar regions, which most geosynchronous
satellites lack are:
Much of Russia is at high latitude, so geostationary orbit
does not provide full coverage of the region. These Soviet HEO orbits include polar coverage.
Elliptic orbit
In astrodynamics or celestial mechanics an elliptic orbit is a Kepler orbit with the eccentricity less than 1; this includes the special case of a circular orbit, with eccentricity equal to zero. In a stricter sense, it is a Kepler orbit with the eccentricity greater than 0 and less than 1 . In a...
with a low-altitude perigee
Perigee
Perigee is the point at which an object makes its closest approach to the Earth.. Often the term is used in a broader sense to define the point in an orbit where the orbiting body is closest to the body it orbits. The opposite is the apogee, the farthest or highest point.The Greek prefix "peri"...
and a high-altitude (over 35786 kilometres (19,322.9 nmi)) apogee. It is a type of high Earth orbit
High Earth orbit
A High Earth Orbit is a geocentric orbit whose apogee lies above that of a geosynchronous orbit .Highly Elliptical Orbits are a subset of High Earth Orbits.-Examples of satellites in High Earth Orbit:...
.
Such extremely elongated orbits have the advantage of long dwell times at a point in the sky during the approach to, and descent from, apogee. Visibility near apogee can exceed twelve hours of dwell at apogee with a much shorter and faster-moving perigee phase. Bodies moving through the long apogee dwell can appear still in the sky to the ground when the orbit is at the right inclination, where the angular velocity of the orbit in the equatorial plane closely matches the rotation of the surface beneath. This makes these elliptical orbits useful for communications satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s.
Sirius Satellite Radio
Sirius Satellite Radio
Sirius Satellite Radio is a satellite radio service operating in North America, owned by Sirius XM Radio.Headquartered in New York City, with smaller studios in Los Angeles and Memphis, Sirius was officially launched on July 1, 2002 and currently provides 69 streams of music and 65 streams of...
uses HEO orbits to keep two satellites positioned above North America while another satellite quickly sweeps through the southern part of its 24-hour orbit. The longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
above which the satellites dwell at apogee in the small loop remains relatively constant as the earth rotates. The three separate orbits are spaced equally around the Earth, but share a common ground track
Ground track
A ground track or ground trace is the path on the surface of the Earth directly below an aircraft or satellite. In the case of a satellite, it is the projection of the satellite's orbit onto the surface of the Earth .A satellite ground track may be thought of as a path along the Earth's surface...
.
Examples of HEO orbits offering visibility over Earth's polar regions, which most geosynchronous
Geosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...
satellites lack are:
- Molniya orbitMolniya orbitMolniya orbit is a type of highly elliptical orbit with an inclination of 63.4 degrees, an argument of perigee of -90 degree and an orbital period of one half of a sidereal day...
s, named after the MolniyaMolniya (satellite)Molniya was a military communications satellite system used by the Soviet Union. The satellites were placed into highly eccentric elliptical orbits known as Molniya orbits, characterised by an inclination of +63.4 degrees and a period of around 12 hours...
Soviet communication satellites which used them. - Tundra orbitTundra orbitTundra orbit is a type of highly elliptical geosynchronous orbit with a high inclination and an orbital period of one sidereal day . A satellite placed in this orbit spends most of its time over a chosen area of the Earth, a phenomenon known as apogee dwell...
s, also developed for Soviet use, but only used by US Sirius Satellite RadioSirius Satellite RadioSirius Satellite Radio is a satellite radio service operating in North America, owned by Sirius XM Radio.Headquartered in New York City, with smaller studios in Los Angeles and Memphis, Sirius was officially launched on July 1, 2002 and currently provides 69 streams of music and 65 streams of...
.
Much of Russia is at high latitude, so geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
does not provide full coverage of the region. These Soviet HEO orbits include polar coverage.