Geosynchronous satellite

A geostationary satellite is a man-made satellite, the 35,786 km is on a circular orbit about the Earth's surface directly above the equator. There is the so-called geostationary orbit ( GEO short, English:. Geostationary Earth Orbit ), that is stationed there satellites move with an angular velocity of an orbit per day and follow the Earth's rotation with an airspeed of about 3.07 km / s This geostationary satellites are ideally always have the same point of the earth's surface and the equator.

  • 5.1 difference in the longitudes
  • 5.2 Angle between antenna location and subsatellite
  • 5.3 Horizontal orientation ( azimuth)
  • 5.4 Vertical alignment (elevation)
  • 5.5 distance to the satellite
  • 5.6 Numerical simplification
  • 5.7 graphics
  • 7.1 Communication Satellite
  • 7.2 Weather Satellite

Typical examples of geostationary satellites are communications satellites, TV satellites and weather satellites.

A transmitted from the ground radio signal passes, a geostationary satellite to the earth to a receiver due to the removal of 2 x undergoes 36,000 km and the speed of light, which is also true for the radio waves, a delay ( latency ) of about 0.25 seconds can arrive and the initial sender to a response signal, thus at least 0.5 seconds. In digital transmission, the use of data compression, encryption, or data encodings, the delay times are often magnified even further.

Types

There are two different types of geostationary satellites:

  • Spin -stabilized satellites were first developed. They are drum-shaped, and its lateral surface is covered with solar cells. The spin axis is parallel to the rotation vector of the satellite orbit and therefore always perpendicular to the current path velocity vector, and because of the equatorial railway in north-south direction, ie parallel to the Earth. If the satellite is not exactly crosses the earth's shadow, then through the spin along the periphery adjacent cells, aligned one after the other towards the sun, wherein each time one half of the cells completely shaded, and the other half in a more or less favorable angles is illuminated. Communications satellites of this type usually have on their ( northern ) top a entdralltes antenna module with permanently aligned to certain areas of the globe directional antenna (s). The exhaust nozzle of the apogee motor protrudes from the ( southern ) bottom of the satellite.
  • Twist - or three -axis stabilized satellites have a cuboid- shaped main body usually. The front side of the main body is aligned with the earth. The back, however, points to the zenith, and from it extends beyond the apogee. The pointing to the north and south sides carry the solar panels that will follow the sun, while the satellite orbits the earth. The front side carries the aligned to the ground instruments - or in some communication satellites, an antenna module (sometimes fold ) directional antenna (s). These surround the foot of a tower which carries the horn, which are the focal points of the directional antenna (s). With other communication satellites, the horn at the upper outer edge of the main body are mounted, and irradiating very large directional antennas are too large to be mounted on the top. They are attached to the still free -facing east and west sides and folded after the start. There are also satellites that have both antennas on the top as on the western and eastern sides.

Bullet hole

Typically, the launcher brings the satellite on a highly elliptical geosynchronous transfer orbit ( GTO). From there, the satellite 's apogee motor transports it into geostationary orbit ( GEO). In this case, the satellite consumes the largest part of its fuel is depleted (mostly nitrogen tetroxide and monomethyl hydrazine ), so that only about half as much as he weighs upon arrival in GEO as at the start. The remaining fuel reserves are sufficient then to make any path corrections during the lifetime of satellites in GEO.

The fact that the launcher the satellite drops off directly in GEO, occurs only in Russian satellites and U.S. military satellites. It is contemplated that a new reignitable Ariane 5 upper stage to introduce that could also put satellites directly into the GEO.

The cost of transporting payloads into GTO are at 30,000 to 50,000 € / kg for the GEO at 300,000 to 400,000 Euro / kg

Satellite orbit

The influence of the moon, the sun and particularly the Erddeformationen interfere with the geostationary orbit. Only four positions keeps a satellite 's location, and only two of them are stable: 105 ° W and 75 ° E. The other two are unstable, 15 ° W and 165 ° E. Small disturbances cause a drift to the stable positions. The positioning of a satellite outside of these points therefore requires continuous path corrections. Satellites that can not be transferred to a graveyard orbit advised and out of control, accumulate at these two points. Currently (2010) it should be more than 160.

The perturbations also affect the orbital inclination. Without corrections it increases depending on the position by about 0.5 ° per year. The satellite is no longer stationary in the sky, but moves relative to the earth on a curve in the form of a figure eight. The deviation from a circular shape toward an elliptical path is expressed in an asymmetry of the curve similar to the curve of the Analemma sun. Path corrections in a north-south direction require a lot more fuel than displacements along the equator. Therefore, the operator leave old satellites with nearly exhausted fuel stocks where possible commute in so-called Inclined orbit. With a S / N by 10 °, the variation of the W / O variation is about 0.5 °.

The International Telecommunication Union allocates frequencies and satellite positions, so that satellites do not interfere with each other. Previously, the distance was 4 ° to the neighboring satellite that beamed on the same frequency. Because of the great demand for satellite positions, the distances to 2 ° were, according to 1400 km, reduced. The actual allocated satellite position is a box in which the operator must position their satellites at ± 0.14 °, equivalent to an east-west drift of less than 100 kilometers. The radial drift must not vary more than 75 kilometers.

Co - positioning

It is possible to position on a satellite position more than one satellite. In this case there are any co- positioned satellites within the assigned box. On a satellite position it is now possible to position eight satellites.

Energy supply in Eclipse, Sun Outage

A geostationary satellite derives its energy almost all year entirely from solar cells. The nodes of the geostationary orbit to lie spring and autumn beginning near the line connecting the Sun-Earth and thus in the Earth's shadow. Therefore, he is from March to mid-April and September to mid-October night for a maximum of 70 minutes in the Earth's shadow. During the time of this eclipse, the satellite draw their energy from batteries that were previously charged by the solar cells, or restrict their performance (example: TV - SAT). Are twice a year at a certain time of day satellite, earth and sun almost on a line. Then the sun is seen from the antenna for several consecutive days for a few minutes close to the satellite. The microwave radiation from the sun then overlaying the satellite and there is a brief interruption or deterioration of the satellite link (English sun outage ). The exact time of occurrence of this event depends on the position of the observed satellites and the receiver's position on the earth; continue to antenna diameter and transmission frequency impact on the duration of the interruption.

Calculation of the altitude

The following calculation results from the classical Newton 's law of gravitation, thus neglecting relativistic effects. Furthermore, it is assumed undisturbed radially symmetric gravitational field of the earth, and the mass of the satellite is to be vanishingly small compared to the mass of the earth.

For the satellite orbit that the gravitational pull of the Earth ( centripetal force ) exactly by the centrifugal force ( centrifugal force) is repealed:

Left side: weight force ( centripetal force ), right side: centrifugal force ( centrifugal force).

It can be seen that the mass of the satellite can be canceled out, that is, the orbit radius is independent of the satellite mass. This is a consequence of the assumption. With the relationship and solving the equation for the path radius is thus obtained:

For the orbital period of the satellite, the sidereal day length should be used. The product is known more accurately than the individual terms ( gravitational constant ) and ( Earth) for measurement reasons. The exact numeric values ​​are:

With the above formula, this results in an orbital radius of about 42,164 km. To calculate the altitude of which must still the radius of the earth at the equator, 6378 km, will be deducted, which is slightly larger due to the flattening of the poles than the mean radius of the Earth. This results in the air above the equator to 35,786 km.

A satellite visibility from the Earth

As a geostationary satellite is stationary seen by an observer on Earth, the horizontal and vertical viewing angle ( azimuth and elevation) remain constant. They depend only on the latitude and longitude of the observer and the longitude of the satellite. The latitude of the geostationary satellites is always 0, and its distance from the center of the earth is 42157 km, the Earth's radius is 6371 km. The viewing angle that can be used for an antenna orientation can be calculated as follows:

Difference of the longitudes

Angle between the antenna location and subsatellite

This formula calculates the central angle of the geodesic between the position of the observer and the point lying on the earth's surface directly below the satellite.

Horizontal orientation ( azimuth)

The azimuth is measured from north through east.

Here are some case distinctions are necessary:

Vertical alignment (elevation)

The elevation is the angle between the horizon and satellite. If the satellite is below the horizon, the elevation is negative.

Distance to the satellite

The distance to the satellite is at least 35786 km for a satellite in the zenith. For a satellite on the horizon, this value can go up to 41670 km.

Numeric simplification

The ratio is 0.151, so that the formulas for elevation and distance can also be written as follows:

Graphic

The chart on the right shows the geostationary satellite orbit for different positions on the northern hemisphere of the earth. Above 81 ° latitude is no longer to see a geostationary satellite from Earth. At 70 ° latitude (dashed line ), it is a maximum of 11 ° above the horizon to the south. At approximately east-southeast and west -southwest (63 °) of the orbit intersects the horizon. An antenna on 50 ° latitude (dotted line) can the area from south to ESE or WSW use for satellite reception, because the satellite is at least 10 ° elevation high enough above the horizon.

Footprint

The illumination zone of a geostationary satellite is dependent on the directivity of its antennas. Signals from antennas with low directivity can be received anywhere, where a geometrical visibility to the satellite. With high gain antennas can be the reception area to focus on selected areas on Earth. So European satellite operator often want their satellites only illuminate Europe. The chart shows, for example, information that an antenna with a diameter of 2 meters in Ku- band on earth detected an area of ​​500 kilometers in diameter. It creates an elongated irradiation area, which covers mainland Europe from the Iberian Peninsula to Poland with suitable multi- beam antennas and beam-forming elements.

Another method to influence the shape of the footprint is that the satellite uses elliptic transmitting antennas that can be mounted obliquely to the precise shape of the coverage area of the satellite. This bundle the signal to its wider side stronger so that the footprint on the earth in the corresponding direction is narrower.

Examples of geostationary satellites

Communication satellites

  • Astra
  • AsiaSat
  • Eutelsat
  • Inmarsat
  • Intelsat
  • Hotbird
  • Telesat Canada

Weather satellites

Geoid Earth mass Nadir PAS-22 Geostationary Operational Environmental Satellite
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