BC-4
BC-4 is the name of a satellite camera that was developed and built in the 1960s by the optical company Wild Heerbrugg for the purposes of satellite geodesy .
The name stands for Ballistic Camera (BC) and the substructure of the large universal theodolite Wild T4 , the alidade of which was converted into a very stable mount for the camera. It can therefore also be used for satellite photogrammetry purposes .
The BC-4 was built in around 100 copies and in two variants:
- short focal length ( lens focal length 30 cm) with higher light intensity
- long focal length (lens focal length 45 cm) with higher resolution.
- Both variants used photo plates of the same dimensions, 18 cm × 18 cm.
In addition to general satellite observation , the most important use of this special camera, weighing around 50 kg, took place in 1969–1974 in the global network of satellite triangulation . This zero order global surveying network, consisting of 46 ground stations , made it possible for the first time to establish a precise geodetic connection of all continents.
The camera itself is similar to the aerial cameras used in aerophotogrammetry . However, it is not directed downwards, but diagonally upwards towards the night sky , in order to photograph the traces of the orbit of artificial earth satellites .
The term ballistic means that the camera was also designed for measurement images from ballistic missiles ( sounding rockets) and for observing balloon probes . Simultaneously with the rocket or satellite track, it also records the starry sky , against the background of which the evaluation takes place in a spatial coordinate system . The method corresponds to the stellar triangulation developed in 1959 by the Finnish geodesist Yrjö Väisälä .
During the recording, the camera remains firmly aligned with the starry sky, so in contrast to astrographs (at observatories) or the Baker-Nunn camera, it does not track the stars or the satellites. The stars are therefore shown as short tracks interrupted by time stamps , whereas the satellites are long line tracks . A rotation lock attached in front of the camera lens is used to produce the time stamps .
