Satellite triangulation

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The satellite triangulation is a method of Satellitengeodäsie that for calibration of ground stations is simultaneously photographed by means of satellite positions. It can artificial earth satellites are used by train km altitudes from 500 to 5000, where it only as high objectives serve and their tracks themselves are not considered.

history

The method was developed by the Finnish geodesist Yrjö Väisälä under the name “ Stellar triangulation ” in the 1950s - before the start of space travel - and successfully tested using high-flying balloon probes . Väisälä was able to determine the connecting line Helsinki - Turku , which is much too long for measurements on the earth's surface because of its length of 150 km , with a directional accuracy of less than one arc minute. At that time, this was about twice as good as the excellent triangulation network of Finland and, thanks to the innovative method, also enabled completely independent control of the national survey .

principle

This methodology is based on the use of the starry sky as a reference surface . The missiles (from 1959 also satellites) are measured photographically at the same time by two satellite stations by "chopping up" the tracks they have drawn in the sky into points and depicting them against the star background .

After measuring the photo plates with the help of astrometric evaluation devices (or alternatively: by means of photogrammetry ), the "observation vectors " of each track point are used to define a plane that is determined from the beginning in the coordinate system of the stars , i.e. absolutely oriented. This fact is one of the two accomplishments of the method.

The combined levels of both ground stations are then cut again, which results in the exact connection vector between the satellite cameras . As a result of the absolute orientation , the result is free from the vertical deviation , which until 1960 limited terrestrial surveying between continents to accuracies of around 100 meters.

Applications

According to the principle of stellar triangulation, several intercontinental surveying networks were observed around the world in the 1960s and 1970s , which was previously technically impracticable due to the curvature of the earth . One of these networks ( SAO project in Massachusetts) was able to implement intercontinental measuring sections with accuracies of a few meters for the first time , which exceeded the previous data by a factor of 10-20. The very dense Western Europe network WEST was of a similar quality .

In the years up to 1974 the world network of satellite triangulation was measured, in which several dozen international observation teams took part. Ballistic cameras of the type BC-4 from the Swiss company Wild Heerbrugg (today Leica) were used as measuring cameras , but those with a longer focal length of around 45 cm. The accuracy of the total of 46 ground stations (average distance 3,000 to 5,000 km) was ± 5 m after combination with a Doppler satellite network ± 3 m. This globally almost uniform accuracy status could only be increased to the decimeter range *) with the development of GPS (practically applicable from around 1990). The research of various geodesics for the combined adjustment of the satellite triangulation with Doppler and terrestrial surveying networks (e.g. Helmut Wolf 1980–1989) further improved the national surveying and is still benefiting modern satellite positioning today .

*) Today the international reference stations of the ITRF already have cm accuracy.

The method of stellar or satellite triangulation stands out methodologically for its simplicity, but in addition to its two main advantages (absolute orientation in the star system, continent-spanning range) it has a practical disadvantage: it requires the simultaneous visibility of satellites on at least two (if possible 3–4) distant ground stations or observatories . The fact that this has been successful around 1,000 times in the "world network" was the largest joint action by the " geodetic community" until 1974 . Today, special, permanent groups of scientists and international services of the IUGG and IAG ensure similar periodic measurement campaigns , which are, however, freed from the problem of weather and cloudiness because of the microwaves now used .

See also

literature

  • A. Berroth and Walter Hofmann : Cosmic Geodesy . Verlag G.Braun , Karlsruhe 1960 (especially Chapters 1, 5, 13-15)
  • DGK : Reports on the full session of the German Geodetic Commission . Institute for Theoretical Geodesy of the Univ. Bonn, DGFI-Verlag Munich 1963–1990
  • H. Deker: The application of photogrammetry in satellite geodesy. DGK booklet, Munich 1967
  • G. Hayes ( SAO ): Trackers of the Sky. Cambridge, Mass. circa 1975
  • Gottfried Gerstbach : Satellite Geodesy . Script for the lecture of the same name, Vienna University of Technology 1990
  • Karl Ledersteger : Astronomical and Physical Geodesy (Earth Measurement) , Handbook of Surveying Volume V (especially Chapters 2, 5 and 13), JB Metzler-Verlag, Stuttgart 1968.
  • Hellmut Schmid : The world network of satellite triangulation . Knowledge Communications from ETH Zurich and Journal of Geophysical Research, 1974.
  • Klaus Schnädelbach et al .: Western European Satellite Triangulation Program (WEST), 2nd Experimental Computation . Communications geodesic. Inst. Graz, Volume 11/1, Graz 1972
  • Günter Seeber : Satellite Geodesy. de Gruyter, Berlin 1989, ISBN 3-11-010082-7 .
  • The history of the geodetic VLBI in Germany (Nothnagel-Schlüter-Seeger, Bonn 2000)