Footprint

Illumination zone coverage area , footprint ( Engl. , Literally footprint ) is a term used in satellite technology . This is understood to mean a projection of the reception range of a satellite signal onto the earth's surface. The signal strength at the edge of the illumination zone is constant. The illumination zone is graphically displayed as level or height lines. The signal power , the EIRP and the minimum aperture sizes that can be calculated from them are often determined with footprints . The footprint itself is defined by the antenna characteristics of the satellite antenna, the curvature of the earth and the position of the satellite in terms of shape and size.

Hypothetical example of a footprint of a geostationary satellite with the target area Germany, Austria and Switzerland. The antenna diameter required for reception is indicated in cm on the ellipses.

Illumination zone of a "global beam" of a geostationary satellite above the zero degree of longitude with elevations for visibility

Maximum irradiable latitude depending on the elevation of a satellite

Footprint cards

Footprint maps are used particularly frequently with geostationary satellites and are used to quickly visualize whether it is possible to receive signals from the targeted satellite in a region at all. The mostly specified signal strength can be used as a reference to calculate the antenna gain required to receive a useful signal that is as low-noise as possible, which is required to receive data. A minimum antenna diameter that is necessary for this can then be estimated from this gain.

With footprint maps of broadcast satellites such as the Astra 1M (see web links below ) one can see that the signal from the “widebeam” spot of the Astra 1M satellite can also be received in the Middle East with a specific output. The European Beam spot, on the other hand, is limited to Europe.

Footprints are an essential part of the frequency coordination for satellite radio services that are regulated by the ITU -R. In addition to planning a satellite service, they also serve to estimate and protect against interferers.

However, a footprint can only represent the basic possibility of reception. The reasons that reception is not possible in reality are the same interfering influences as with any radio link. They do not include the influence of the atmosphere with the local weather, the local conditions with buildings, trees or other types of shadowing or humans with artificial disturbances.

Illuminable area

Due to the spherical shape of the earth and the beam geometry, a very distant satellite could irradiate a maximum of almost half the earth's surface, a satellite in a geostationary orbit can theoretically use a "global beam", a directional antenna with a very wide opening angle (17.2 ° aperture) cover a maximum of about 42% of the earth's surface. Such footprints are only found in low frequency bands such as B. the C-band . In practice, such satellites cover a maximum of ¼ to ⅓ of the surface.

The smaller the opening angle or the lower the orbit, the smaller the maximum area that can be irradiated:

a geostationary satellite with an altitude of 36,000 km above the equator (the radius of the orbit is therefore 36,000 km + 6,300 km for half the diameter of the earth = 42,300 km) can just be seen on the horizon up to a geographical latitude of 82 ° ,

at an altitude of 1000 km, on the other hand, only at 30 ° latitude.

With an elevation of 10 °, the width is reduced to 70 ° or 20 °.

Low earth orbit satellites (MEO, LEO) only achieve between 20 and 2 percent illumination of the earth's surface.

See also: The derivation of this relationship can be found under Visibility .

Web links

Footprints of the Astra 1M broadcast satellite

satbeams.com - coverage of many satellites ( Google Maps -based)

lyngsat-maps.com - detailed maps of the coverage areas of many satellites

Various online computers on the topic on satlex.de

Individual evidence

↑ ^a ^b ^c Satellite Technologies . Orbits and Communications Section : Low Earth Orbit and Graphics. On mpoweruk.com; accessed June 1, 2019 - stated there “ 34 percent of Earth's surface ”.

↑ Hans-Dieter Naumann: Manual satellite reception . 1st edition. Siebel Verlag, Meckemheim 2003, ISBN 3-89632-060-2 , p. 23.

^ ^A ^b G. S. Rao: Global Navigation Satellite Systems. Verlag Tata McGraw-Hill Education, 2010, ISBN 978-0-07-070029-1 , p. 375 ( limited preview in the Google book search) - indication there “ almost ¼ of earth's surface ”; general derivation of the formula ibid. Derivation of Coverage Area of Satellite on Earth , p. 371 ff.

[mpoweruk-1] Satellite Technologies . Orbits and Communications Section : Low Earth Orbit and Graphics. On mpoweruk.com; accessed June 1, 2019 - stated there “ 34 percent of Earth's surface ”.

[2] Hans-Dieter Naumann: Manual satellite reception . 1st edition. Siebel Verlag, Meckemheim 2003, ISBN 3-89632-060-2 , p. 23.

[Rao_2010-3] A ^b G. S. Rao: Global Navigation Satellite Systems. Verlag Tata McGraw-Hill Education, 2010, ISBN 978-0-07-070029-1 , p. 375 ( limited preview in the Google book search) - indication there “ almost ¼ of earth's surface ”; general derivation of the formula ibid. Derivation of Coverage Area of Satellite on Earth , p. 371 ff.