The Satellite Laser Ranging (SLR) Station of the GFZ Potsdam has been continuously operating within the framework of the International Laser Ranging Service (ILRS) since January 2003. The method of satellite laser ranging was established in 1964 and still serves as one of the most precise space geodetic techniques with continuously improving accuracy.
The main scientific contributions of SLR are:
- Monitoring of Earth rotation parameters (polar motion and length of day)
- Monitoring of 3-dimensional deformations of the solid Earth (station coordinates and velocities), time-varying geocenter coordinates
- Static and time-varying coefficients of the Earth’s gravity field
- Calibration / validation of microwave instruments onboard of satellites
The principle of SLR is relatively simple: short laser pulses are transmitted by a telescope towards a Laser Retro Reflector (LLR) onboard a satellite. A small fraction of the reflected signal is received by a telescope and the two-way roundtrip time of the laser signal is retrieved from the receive and fire time of the pulses at the station. Via the speed of light and some corrections for the signal path within the atmosphere, the exact distance to the satellite at the given epoch is obtained. To maximise the signal reflection over the sometimes long distances of up to 25000 km to the satellites, mirrors of special design are used. These so called Laser Retro Reflectors (LRR) redirect the light always back to that direction where it came from. This priniciple is for example also utilized with so called "cateyes" which bikers use to be seen better when thy cycle in the dark. Satellites with LRRs onboard are tracked by SLR stations on all continents (except Antarctica) under coordination of the ILRS . The accuracy of the SLR technique is mainly depebds on the technical parameters of the ground stations and the quality of the satellite LRR. The Potsdam SLR station can measure the distance to satellites in orbits between 400 and 25000 km over the Earth with an accuracy and precision around 1 cm.
The SLR station Potsdam features a bi-static system which consists of separated telescopes for transmission of reception of the laser signal. The main technical parameters are the following:
- Transmit telescope: 15 cm Coudé refractor on alt-azimuthal mount
- Receive telescope: 40 cm Coudé-Cassegrain on alt-azimuthal mount
- Laser type: Nd:YOV, frequency-doubled (532 nm)
- Output energy: 700 µJ/pulse, repetition rate 2000 Hz, pulse width 10 ps
- Detector: hybrid photomultiplier or avalanche photodiode
- Time-of-flight registration: Event Timer with 3 ps resolution
- Single-shot accuracy and precision: <1 cm
Further details of the system can be found here .
The station is highly automated. Both telescopes and the ranging electronics are controlled by PC. The highly centralized operation software was developed by DiGOS Potsdam in Linux whereby only one person is required for the system operation. Despite the small diameter of the receiving telescope of 0.4 m in comparison of diameters of up to 1 m of other systems, the SLR system Potsdam has a good productivity und night and day conditions. The ILRS criteria for high performance stations of 3500 passes per year has been met and exceeded in the last years. Furthermore the system has a good short and long term stability of only several millimieters ( see the ILRS reports).
Since September 2011, the station is equipped with a solid state laser of 2 kHz repetition rate (formerly 10 Hz) and shorter pulse width (10 ps instead of 50 ps) which leads to a considerably increased data rate per satellite pass. Such measurements are e.g. used for the determination of both the spin frequency and orientation of fast rotating spherical satellites ( Lares , Blits, Ajisai). Furthermore the laser power increased from 0.8 to 1.4 W due to hardware upgrade in September 2017. Distances from 400 to 25000 km from the ground station to the satellites in their orbits above Earth can be measured with the laser system at centimeter accuracy and precision. Together with the SLR station Graz multiple space debris objects have been observed in May 2017 during so called bistatic experiments. Thereby the Graz laser station sned laserpulses to the space debris objects for measuring the distance, which were received at Graz but also at the Potsdam laser station. With that distance meaurements to the space debris objects can be recorded from multiple stations, even though only one laser station is sending laser pulses. Such measurements allow for accurate orbit determination within a short time, whereby the additional measurements improve the accuracy even faster. Having accurate orbit predictions wihtin a short time can become relevant in case of potential collisions of space debris objects with active satellites which might incorporate collision avoidance maneuvers.
Some informations about the history of Satellite Laser Ranging in Potsdam are here.
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.