A highly elliptical orbit (HEO) is an

elliptic orbit In astrodynamics or celestial mechanics, an elliptic orbit or elliptical orbit is a Kepler orbit with an eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. In a stricter sense, it ...

with high

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...

, usually referring to one around Earth. Examples of inclined HEO orbits include

Molniya orbit A Molniya orbit ( rus, Молния, p=ˈmolnʲɪjə, a=Ru-молния.ogg, "Lightning") is a type of satellite orbit designed to provide communications and remote sensing coverage over high latitudes. It is a highly elliptical orbit with an ...

s, named after the

Molniya Molniya (Russian for ''lightning'') may refer to: * Molniya (satellite), a Soviet military communications satellite ** Molniya orbit * Molniya (explosive trap), a KGB explosive device * Molniya (rocket), a variation of the Soyuz launch vehicle * OKB ...

Soviet communication satellites which used them, and

Tundra orbit A Tundra orbit (russian: орбита «Тундра») is a highly elliptical geosynchronous orbit with a high inclination (approximately 63.4°), an orbital period of one sidereal day, and a typical eccentricity between 0.2 and 0.3. A satelli ...

s. 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 An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...

. Bodies moving through the long apogee dwell appear to move slowly, and remain at high

altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...

over high-latitude ground sites for long periods of time. This makes these elliptical orbits useful for

communications satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. C ...

Geostationary orbit A geostationary orbit, also referred to as a geosynchronous equatorial orbit''Geostationary orbit'' and ''Geosynchronous (equatorial) orbit'' are used somewhat interchangeably in sources. (GEO), is a circular geosynchronous orbit in altitud ...

s cannot serve high latitudes because their elevation above the horizon from these ground sites is too low. molniya

Sirius Satellite Radio Sirius Satellite Radio was a satellite radio (SDARS) and online radio service operating in North America, owned by Sirius XM Holdings. Headquartered in New York City, with smaller studios in Los Angeles and Memphis, Tennessee, Memphis, Sirius ...

used inclined HEO orbits, specifically the Tundra orbits, to keep two satellites positioned above

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

while another satellite quickly sweeps through the southern part of its 24-hour orbit. The

longitude Longitude (, ) is a geographic coordinate that specifies the east–west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter l ...

above which the satellites dwell at apogee in the small loop remains relatively constant as Earth rotates. The three separate orbits are spaced equally around the Earth, but share a common

ground track A ground track or ground trace is the path on the surface of a planet directly below an aircraft's or satellite's trajectory. In the case of satellites, it is also known as a suborbital track, and is the vertical projection of the satellite's or ...

References

{{DEFAULTSORT:Highly Elliptical Orbit Earth orbits Satellite broadcasting