Satellite Laser Ranging Station Potsdam

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 improved 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 of 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 registered together with the epoch of transmission. Via the knowledge of 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. 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 determined by the technical parameters of the ground stations and the quality of the satellite LRR. Typical values for modern systems are single-shot accuracies of 1-2 cm for ranges up to 25000 km.

Laser station Potsdam
Laser station Potdam

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: 400 µJ/pulse, repetition rate 2000 Hz, pulse width 10 ps
  • Detector: hybrid photomultiplier or avalanche photodiode
  • Time-of-flight registration: Event Timer with 1 ps resolution
  • Single-shot accuracy: <1 cm

Further details of the system can be found here .

SLR Station Potsdam during nighttime operation
SLR Station Potsdam during nighttime operation

The station is highly automated. Both telescopes and the ranging electronics are controlled by PC. Only a single person is required for system supervision. Despite its small size with a receiving telescope diameter of only 40 cm, the SLR system Potsdam has a good productivity under both night and daylight ranging conditions (>2200 annual satellite passes) and displays both short-term and long-term stabilities in the few millimeters range according to the official ILRS analysis centre 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). Higly precise range measurements to the satellites of the European Galileo navigation system are carried out as well.

Some informations about the history of Satellite Laser Ranging in Potsdam are here.

Laser transmitter in operation (protecting cover removed)
Laser transmitter in operation (protecting cover removed)

Contact

Mr. Dr. Ludwig Grunwaldt
Global Geomonitoring and Gravity Field

Telegrafenberg
Building A 19, room 102
14473 Potsdam
tel. +49 331 288-1733