The principle is actually very simple: a series of laser impulses are directed to satellites through the telescope. The satellite itself or prisms on the satellite reflect the light back to the telescope. The distance from the telescope to the satellite can be calculated with the time difference between the emission and reception of the laser signals. The impulses are sent at an interval of 10 per second and the resulting distance measurements are at an accuracy of under a centimeter. This accuracy is all the more impressive since about 20 different satellites are observed at distances of up to 20,000 km.
In the meantime observations can also be carried out during the daytime hours, so that the SLR operation runs 24 hours per day. The only disadvantage of this technique as compared to the GPS technique lies in the fact that observations are not possible when there is a cloud cover. More information can be obtained at the web site of the University of Bern.
Zimmerwald is a significant station of the International Laser Service ILRS. The same telescope is also used for optical observations such as the automatic detection of space junk.
