The Graz SLR station is one of the leading laser stations worldwide. Its centerpiece is an Nd:Vanadate kHz laser system. It generates 2000 laser pulses per second (2 kHz repetition rate). The duration of a single laser pulse is only 10 picoseconds. For comparison: Light with a speed of 300000 km/s travels a distance of only 3 mm during this time. A single laser pulse has an energy of 400 µJ, which corresponds to one million times one billion of photons. Arriving at the satellite, a small portion of the light is reflected back to the SLR station by retro-reflectors. Few photons (in most cases only one single photon) arrive at the SLR station in Graz, are collected by a 0.5 m diameter telescope, and registered by a Single-Photon Avalanche Detector (SPAD) with a diameter of only 200 µm (approx. 4 times the thickness of a human hair). Depending on the distance, the 2-way travel time of the photons ranges from a few milliseconds up to a quarter of a second.
A real-time observation software optimized for kHz SLR was developed by the Graz SLR team. This software is very flexible, modular and can be upgraded easily. Even un-experienced observers can learn to operate the SLR station within only one night of training.
The software already has a considerable set of Real Time Automatics:
- Automatic identification of potential returns out of intensive background noise
- Only these potential returns are stored, to minimize result file sizes
- Automatic range gate setting, range gate shifting, range gate optimization
- Automatic time bias calculations and time bias settings
- Automatic optimizing of tracking
- Recognizing pre-pulses, minimizing them via offset pointing
- Automatic search modes to find/acquire the satellite
In addition to the real-time observation software a number of other software tools are in routine operation. Using a dedicated camera the backscattering of the laser beam can be made visible during daylight, which simplifies the adjustment of the laser beam direction. During night the reflected sunlight of the satellite is used to display an image of the satellite by using a sensitive astronomical camera.
Measuring with 2 kHz, using relatively weak laser pulse energy together with a single-photon-detector – specifically for satellite at larger distances – results in very low return rates. For GPS satellites during day, the return rate can be as low as 0.001. This means out of the 2000 shots fired in each second only 2 returns per second are received - hidden within 1998 noise points. The Real Time Return Detection routines have to identify these 2 returns, have to store them, and have to reject the noise reliably.