Sun-synchronous orbit
As a sun-synchronous orbit or sun-synchronous orbit (SSO, sun synchronous orbit also ) refers to an orbit around a planet whose orbital plane undergoes the same rotation change as the circled planets around the sun. Of the earth means that ( round trip time of the earth around the sun) rotates once, the orbital plane of a satellite in a year by the earth. Thus, the orbital plane has a fixed angle value to the Earth - Sun line.
Must be distinguished from a planet -synchronous orbit around the sun, for example, in a given by the Lagrange points orbit.
Properties
If left undisturbed, a satellite orbits the Earth at a constant angular momentum on a plane which is fixed in space, see purple curve in the figure above. However, the Earth flattening applies a torque, resulting in a displacement of the right ascension of ascending node. In opposite to the rotation of the earth orbits the precession acts in the same direction as the Earth's rotation. The error is greater, the smaller the inclination and the lower the flying height. With a suitable choice of height and inclination of the track shifts just so much that she once per year, the earth rotates (see green curve in the figure above).
As a new satellite orbit element the local time of the ascending node (english Local Time of Ascending Node, LTAN ) defines the overflight local time.
In SSO orbit, the satellite passes a point on the surface is always at the same local time. The observations of different days can be compared more easily, since the reflection behavior of surfaces does not change at the same angle of incidence of the sun rays. The satellite moves along the twilight zone (morning or evening hours, English Dusk - Dawn ) by 9 h LTAN, can be applied to optical images, the height of objects from the length of the shadow derived. If the satellite is in addition the earth orbits so that it does not happen the Earth's shadow ( by 6 h LTAN, inclination greater than 101.45 ° ), it can be constantly supplied with energy from solar cells. On-board batteries are only needed for the start-up phase or loss of attitude control.
Examples of use:
- Weather satellites such as TIROS, Nimbus, DMSP, MetOp
- Earth observation satellites such as Landsat, ERS etc.
- Solar observation satellites such as ACRIMSat, TRACE, Hinode
- Research satellites such as DLR TUBSAT
- Some space telescopes such as Infrared Astronomical Satellite, Wide - Field Infrared Survey Explorer
Calculation
The ground potential development coefficient describes the Massewulst the earth at the equator. It causes the precession and the displacement of the right ascension of the ascending node:
With:
Taking into account the dependence of the rotational speed of the orbital altitude, yields the relationship shown in the diagram. With an inclination of 96 ° the torque on the orbital angular momentum is very small. The satellite would have the Earth in a circle SSO lower than 100 km. Earth observation satellites flying at an inclination between 98 ° and 99 °. The height of 650 km to 900 km is a good compromise between interference caused by the Earth's atmosphere and distance to the observation of objects on Earth. SSO orbits with low inclination and an altitude of up to 6000 km have no practical significance.