Satellite Laser Ranging (SLR) has been performed in Potsdam since 1974. According to a widely accepted nomenclature within the scientific community, SLR stations are subdivided into different “generations” according to their ranging performance parameters. While the 1st generation systems employed ruby lasers and achieved single-shot accuracies of 1 – 2 meters, 2nd generation systems featured both Q-switched ruby lasers and frequency-doubled Nd:YAG transmitters. The single-shot accuracy was improved to few decimeters here, and the degree of automation was higher thus allowing for partly daylight ranging. The worldwide standard since the 1980ies were the 3rd generation systems with mode-locked, frequency-doubled Nd:YAG lasers and 10 Hz repetition rate, allowing for ranging both under daylight and nighttime conditions with single-shot accuracies of about 1 cm.
The first SLR system in Potsdam was based on on the modified satellite tracking camera SBG from Carl Zeiss Jena, located on top of the Helmert Tower. A Q-switched ruby laser with 20 ns pulse duration was set on top of the telescope and moved together with the SBG. The receiving electronics had to be located behind the main mirror of the telescope. Ranging accuracies in the order of 1 -2 meters made this system a typical 1st generation one for the period of 1974 – 1981. Several modifications included the PC control of the mount in 1979 which enabled ranging without visual control, especially for satellite passes within the Earth shadow. The system was capable of tracking all satellites equipped with laser retro reflectors up to the 6000 km orbit of Lageos .
The 20 ns ruby laser was replaced in 1981 by a model with 5 ns pulse width. Together with an improved ranging electronics this transformed Potsdam-1 (station designator 1181 Potsdam) into a 2nd generation SLR which was operated until 1993. Better laser beam quality and more sensitive electronic receivers extended the ranging capability until the 19000 km orbits of the Russian Etalon satellites in 1989. Further modifications towards 3rd generation performance were not considered because the 4-axis SBG telescope was not designed for a Coudé focus which allows the stationary use of mode-locked laser transmitters required for picoseconds pulses. Such lasers cannot be moved together with the telescope due to their fragile optical/mechanical setup.
The upgrade of the Potsdam SLR towards the 3rd generation international standard was started in 1986 with the development of the laser transmitter PLS-5 and high-speed ranging electronics capable of centimeter level accuracies. This system (station designator 7836 Potsdam-2) was finally integrated around the two-axis SLR telescope TPL designed by M. Abele from the University of Riga. This Coudé-telescope was purchased in 1990 and located near the Helmert Tower on a historical pillar which had been used for a photographic zenith telescope before. Potsdam-2 became operational in May 1992 and continued operation until June 2004. Special modifications in the receiving system in 1994/1995 allowed for the highly successful tracking of the first geodetic satellite of GFZ Potsdam, GFZ-1, which was the lowest SLR target at that time. The most important one was the insertion of a narrowband spectral filter which enabled continuous day- and nighttime operation. Potsdam-2 tracked all SLR targets up the 20000 km high GPS-35 satellite and the Russian GLONASS system. A detailed description of this station is found at the ILRS websites.
The planned relocation of the Potsdam SLR station to the dedicated tower within the new GFZ facilities opened the possibility of upgrading the system with special emphasis to a novel telescopic system and a state-of-the-art laser transmitter. This system 7841 Potsdam-3 has been operating with full capability since January 2003 and is described in detail here.