Stellar triangulation

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The Stellartriangulation is a method of Geodesy , in particular the national survey , wherein the target points are not of direct angle measurement , but by photographic record against the background of the night sky are measured.

The high-level targets used  - balloons , ballistic missiles (e.g. sounding rockets ) or artificial earth satellites  - do not initially have to be known with their coordinates. If they are recorded by several ground stations at the same time , their position can either be determined using cutting methods ( forward cut or cutting planes ) or completely eliminated from the calculation.

The method was developed as stellar triangulation (triangulation using stars ) by the Finnish geodesist Yrjö Väisälä in the 1950s and successfully tested using high-flying balloon probes . Väisälä was able to determine the connecting line Helsinki - Turku , which because of its 150 km length is much too large for measurements along the earth's surface , with a directional accuracy of less than one arc second (1 ″). At that time this was about twice better than the excellent triangulation network of Finland and also enabled completely independent control of the national survey due to the new methodology .

This methodology is based on the use of the starry sky as a reference surface . The missiles (from 1959 also satellites) are measured photographically by two satellite stations at the same time by “chopping up” their tracks in the sky into points and displaying them against the background of the stars.

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 . Due to the absolute orientation , the result is free of the vertical deviation , which until 1960 restricted terrestrial surveying between continents to an accuracy of about 100 meters.

Satellite triangulations

According to the principle of stellar triangulation, several intercontinental satellite triangulations were carried out around the world in the 1960s and 1970s , which was previously technically impossible due to the curvature of the earth . In the SAO (USA) network over four continents, it was possible for the first time to determine measuring distances over 5000 km, for which the balloon satellites Echo 1 and Echo 2 served as high targets ( stations between 1000 and 1500 km). The accuracy was a few meters, which exceeded the previous data by a factor of 10–20. Something similar was achieved with the very dense Western Europe network called WEST ( West European Satellite Triangulation ).

The world network of satellite triangulation , completed in 1973–1974, was also remarkable , for which several dozen international observation teams exposed thousands of photo plates at a total of 46 stations around the world. Only the Soviet Union and China avoided this hitherto unique global cooperation . About half of the plates obtained with ballistic cameras of the type BC-4 were recorded correctly (with stars and the precalculated satellite), but failed on the 1–2 respective opposite stations (mostly due to surprising cloud cover or too strong wind ). These calculated failures were compensated for by the large over-determination in the network balancing .

Worldwide reference system

The 46 ground stations (at distances of 3,000 to 5,000 km) were now known for the first time at ± 5 m in a global reference system , and after combination with a Doppler satellite network even at ± 3 m. Reaching all continents, this quality status could only be increased to the decimeter range with the usability of the GPS (from around 1990) (today already in the cm range, see ETRS and ITRF ). Further of these early research projects in satellite geodesy ( adjustment , better refraction models , time system , etc.) continue to benefit modern methods of satellite positioning and navigation.

advantages

The principle of stellar triangulation is impressively simple and has two major advantages:

Practical disadvantage

  • Stellar triangulation requires the simultaneous visibility of satellites at at least two (better three) distant ground stations or observatories . The fact that this has been successful around a thousand times in the world network has contributed significantly to the success of the first world network . Today's global geodesy avoids this disadvantage of weather dependency through the transition from light to microwaves (GPS, Galileo) and also has much simpler options for communication with measuring teams that are so far away.

literature

See also