Measurement principle and facts

Satellite laser ranging (SLR) allows to measure the range to satellites equipped with retroreflectors orbiting our planet. Short laser pulses with a wavelength of 532 nm and with pulse durations of 10 picoseconds are sent to the target and the transmitted light is reflected back to the SLR station. Before being released into space, the laser beam is guided from the laser room to the observatory dome using mirrors and then further expanded to a final output diameter of 7 cm. This allows to reduce the beam divergence, leading to a smaller laser beam diameter arriving at the satellite. Even with optimized divergence, beam diameters at larger orbital heights can reach up to 100 meters. Depending on the orbit of the target, travel times vary between 0.002 and 0.25 seconds.

The Graz SLR station currently measures with a laser repetition rate of 2 kHz, ideally allowing up to 2,000 range measurements per second. A highly accurate time measurement of the moment the laser photons leave and arrive back at the SLR station allows measurement precisions down to 3 millimeters. An individual laser pulse approximately contains 1015 (1,000,000,000,000,000) photons from which typically only a single photon arrives back at the SLR station. The rest of the photons are scattered in the atmosphere or missing the retroreflector due to spreading of the beam on its way through space. Finally, the photons are distributed on Earth’s surface within a radius of up to 1 kilometer. This radius again depends on the orbital height of the target, but also on the diameter and characteristics of the retroreflector itself. Relativistic effects slightly shift the center of the reflection pattern arriving on Earth forward in the direction of movement. This “velocity aberration” results from the relative movement between the satellite and observing station and is the reason why retroreflectors are carefully selected depending on the orbit. The receiving telescope can only cover an aperture of 0.5 meters, hence only very few photons reach our detector. These photon events can however be detected by using highly efficient Single Photon Avalanche Diode (SPAD) detectors. The returning photons are gathered by the receive telescope and precisely focused on the SPAD detector with a diameter of only 200 micrometers. Orbit predictions are not only necessary for tracking the satellite; the sensor also needs to be activated close to the predicted arrival time to reduce incoming noise. Analyzing the travel time (range) of the measured photons in real-time “true” returning photons from the retroreflector of the satellite can then be statistically identified and distinguished from the sky or detector intrinsic noise.

Observed targets and performance

During day and night, at clear sky, IWF’s SLR station routinely measures to more than 150 satellites. In the year 2022 on 262 days Graz SLR station measured more than 2,000 hours acquiring over 13,000 satellite passes. Statistics show that Graz is one of the most accurate and reliable stations worldwide

Satellites can be divided into to three different target classes:

  1. Passive/geodetic satellites
  2. Low Earth Orbit satellites
  3. Navigation satellites

Passive or geodetic satellites are of spherical shape and constructed in a way not to be influenced by external forces except gravitation. Typically, a large amount of retro-reflectors leads to response signals which can be easily identified by all SLR stations. Orbital heights range from 800 to 20,000 km and their main area of use is the highly precise measurement of the Earth's gravitational field and the contribution to the International Terrestrial Reference Frame (ITRF), for which SLR is the main contributor to the Earth’s center of mass determination. The precise knowledge of the orbit of geodetic satellites can also be utilized for validation relativistic effects like the Lense-Thirring precession.

Satellites in Low Earth orbit (LEO) can be found in orbital heights between 450 and 1,300 km. Their field of use is versatile and ranges from the measurement of Earth's ice mass, ocean currents, sea level rise etc. up to high resolution radar images. Accurate orbital data for those scientific missions is generated from SLR measurements.

Besides the European (Galileo) and American (GPS) satellite navigation systems, several other countries such as China (Beidou), Russia (Glonass) and India (IRNSS) sent their own navigation satellites into space. Orbital heights vary between 20,000 and 36,000 km, with total masses of 600 to 1,400 kg. Their field of use is the exact positioning and navigation on Earth.

In addition to active satellites equipped with retroreflectors IWF also observes space debris objects utilizing various techniques.

Megahertz laser ranging

Graz SLR Graz station is actively working on technical improvements providing a foundation for progress of the whole laser ranging community. Various technologies pioneered in Graz are e.g. high-repetition rate satellite laser ranging, time-walk compensated detectors, (daylight) space debris laser ranging or single photon light curve detection.

The most recent technological upgrade is the transition from satellite laser ranging with kHz repetition to MHz repetition rate. First measurements were already performed showing a potential increase to up to 250,000 valid measurements per second for Low Earth Orbit satellites.

The increased demand on software and hardware resulting from large repetition rates leads to an ongoing process refurbishing the key components of the SLR station. A central claim of the SLR station is to develop as many components of the stations as possible by ourselves. This allows to repair the station in case of a defect avoiding longer outage times while giving the team members a deep insight in the related technology. The interfaces controlling the telescope, the real-time ranging software, FPGA-interfaces, programs doing image analysis and the post-processing software are hence all own-developed tools. The technological knowledge of Graz SLR station led to the participation in several international projects related to building new SLR stations or upgrading existing stations towards space debris laser ranging. IWF designs, simulates and fabricates laser and detection packages in a modular setup for these stations.

Laser and detection packages

An increasing demand of facilities capable of doing satellite laser ranging, space debris laser ranging or optical communication pushes the demand to create accurate, cost-efficient, reliable and yet simple components to be integrated. IWF Graz creates new concepts for modular piggyback laser and detection packages for multiple SLR stations and space agencies worldwide (e.g. Tenerife, Japan, Spain). In 2022 ESA SLR station Izana-1 on Teide Tenerife was officially handed over and is now part of the ILRS station network, continuously delivering accurate data.

Optical components are mostly based on commercial off-the-shelf (COTS) components and simulated with raytracing software. Laser packages consist of the laser with two separate beam expansion telescopes with a collimated part in between, which can be used for imaging of e.g. the backscattered laser beam or the visualization of stars for alignment purposes. A combination of wave plates and polarizing beam splitters allow for power adjustment and measurement. One of the lenses is mounted on an electronically movable lens holder which gives the possibility of flexible variation of the beam divergence and a tip-tilt mirror enables direction control of the laser beam. Furthermore, start pulse detection is integrated.

Detection packages are mounted in one of the Nasmyth foci of COTS type astronomical telescopes. In the beam path direction, the field of view iris is followed by optics to collimate the entering photons to approx. 1 cm with some flexibility to tune the field of view of telescope. Dichroic mirrors separate the incoming light w.r.t. wavelength and distribute it to various sensor modules (e.g. single photon avalanche detectors for green and infra-red, single photon light detection, optical guiding cameras and beam adjustment cameras).

Laser and detection packages operate a temperature control system and the necessary interfaces to connect to e.g. event timers, power supply or control PC.