Molniya orbit

A Molniya orbit is a hochelliptischer orbit with an inclination of 63.4 ° and a period of exactly half a sidereal day. Molniya orbits are named after the series of Soviet Molniya communications satellites that use this type orbit since the mid- 1960s.

A set in a Molniya orbit satellite spends its apogäische reverse idle most of its orbital period over a particular area of ​​the world.

Formation and properties

The majority of the territory of the former USSR and Russia in particular is quite high northern latitudes. To get to send from a geostationary, so standing on the equator, satellites, a considerable transmission power has to be applied due to the unfavorable shallow incidence angle. A satellite with a polar orbit is more appropriate, since he moved away perpendicularly about it for communication purposes in such areas. A typical circular sun- synchronous orbit, however, would leave the satellites appear only for a short time about Russia. Also, would this cause at the same time in north-south direction and, by the earth's rotation, move westward, making the track with the transmitting and receiving antennas is difficult. In contrast, an inclined equatorial web are used on the satellite is located north of the Equator in half the time and in the east-west direction, by means of an equatorial mount can be easily tracked. So it appears but only for a short time over the use area or for a continuous coverage of the use of a large number of satellites is therefore necessary.

The object is to make the smallest possible number of satellites as high as possible above the area to use available, situated in a highly elliptical orbit. As the web speed of a satellite according to Kepler's laws of its proximity to the Gravizentrum (in this case the center of the earth ) is inversely proportional, it passes through the near-Earth part of its orbit quickly and the remote slowly. By ostgerichtete orbit angular velocity is close to the rotational velocity of the earth and so the apparent position of the satellite around the highest point of its orbit, apogee, over a longer period changed only very slightly.

Due to the earth rotation, the apogee is not always on the same point and not usable apogees to avoid the period of the orbit is selected so that it amounts to an integer divider or a multiple of a day, and the apogee regularly over the same area on the earth comes to rest. The typical Molniya orbits have a period of approximately 12 hours, making the satellite appear every second round, so once a day, for about 8 hours over the use area. Thus it can be ensured for a given area with only 3 satellites, a 24 -hour cover. A concept like this orbit with a period of 24 hours is called the Tundra orbit. By Abgeflachtheit Earth perturbations caused in satellite usually that shift the argument of periapsis and thus the position of the apogee and must be compensated by position corrections. This is the Molniya orbit by choosing an inclination of 63.4 °, at which cancel the perturbations are avoided.

Use

The main application of the Molniya orbit was the use of the same number of Soviet communications satellites. After two starts in 1964 were unsuccessful, on 23 April 1965, the first satellite was launched into orbit with this Molniya 1-01. The Molniya - 1 satellite came in 1968 for military long-distance communication used, but only had a short life and had to be continually replaced. The successor system Molniya -2, allowed both military and civilian communications and served to build up the USSR -wide Orbita television transmission system. The more followers is the number Molniya - third Among various sources, there is disagreement over the designation of the series, is being suggested that all satellites in use Molniya - 3, and a distinction is made on the basis of their application between Molniya -1 through -3.

With slight modifications, the same orbits were used by Soviet spy satellites, was the apogee of the United States. Geostationary orbits provide Although the observation of the United States to, but were caused by the used sensor technology, high-contrast viewing angle required, which could be achieved only by the higher latitudes. An example is the US- KS early-warning satellite to detect U.S. missile launches, with their subsequent improvements allowed the use of geostationary orbits.

In part, the United States Molniya orbits used in turn for spy satellites, with the long residence time of the satellite, was used in the northern latitudes, which is so beneficial for the Soviet communications to listen to just this. The electronic reconnaissance satellites Jumpseat and their successors Trumpet also used Molniya orbits. Another application is the Satellite Data System, SDS, to handover the data of operating over Russia spy satellites to U.S. ground stations. The SDS allowed the real-time data transfer from the low-flying KH -11 reconnaissance satellite during its flyby on its near polar orbits below the SDS satellites.

Molniya orbits are not suitable for human space flight, as they cross the repeated high-energy Van Allen belts.

Swell

• Ernst Schmid knife, Stefanos Fasoulas: Space Systems: An Introduction with exercises and solutions, Springer, 2005, ISBN 3-540-21037-7

• Celestial mechanics
• Space Physics
• Satellite Geodesy