Stellar triangulation

Stellar triangulation is a method of geodesy and of its subdiscipline space geodesy used to measure Earth's geometric shape. Stars were first used for this purpose by the Finnish astronomer Yrjö Väisälä in 1959, who made astrometric photographs of the night sky at two stations together with a lighted balloon probe between them. Even this first step showed the potential of the method, as Väisälä got the azimuth between Helsinki and Turku (a distance of 150 km) with an accuracy of 1′′. Soon the method was successfully tested by ballistic rockets and for some special satellites. Adequate computer programs were written for the astrometric reduction of the photographic plates, the intersection of the "observation planes" containing the stations and the targets, and the least-squares adjustment of stellar-terrestrial networks with redundancy. The advantages of stellar triangulation were the possibility to cross far distances (terrestrial observations are restricted to approx. 30 km, and even in high mountains to 60 km), and the independency of the Earth's gravity field. The results are azimuths between the stations in the stellar-inertial navigation system, despite of no direct line of sight. In 1960, the first appropriate space probe was launched: Project Echo, a 30 m diameter balloon satellite. By then the whole of Western Europe could be linked together geodetically with accuracies 2–10 times better than by classical triangulation. During the late 1960s, a global project was begun by H.H. Schmid (Switzerland) to connect 45 stations all over the continents, with distances of 3000–5000 km. It was finished in 1974 by precise reduction of some 3000 stellar plates and network adjustment of 46 stations (2 additional ones in Germany and the Pacific, but without the areas of Russia and China). The mean accuracy was between ±5 m (Europe, USA) and 7–10 m (Africa, Antarctica), depending on weather and infrastructure conditions. Combined with Doppler measurements (such as from Transit) the global accuracy was even 3 m.