Space safety is becoming an increasingly important topic for our society, in particular with respect to space debris. According to the ESA’s Space Environment Report 2025, the number of satellites, and, consequently, the amount of space debris, particularly in low Earth orbits (LEOs), is growing rapidly. This poses a significant challenge to the continued safety and functionality of space operations, which requires strategies to overcome. For collision avoidance, it is necessary to implement countermeasures such as tracking and monitoring space debris. The geodetic observation technique Satellite Laser Ranging (SLR) enables the measurement of distances to satellites and space debris with high-range resolution. However, it is essential to know the approximate positions of the space objects to be tracked via SLR in order to align the SLR station’s laser pointing accordingly. Since there are no accurate orbit predictions available for space debris, its approximate position is determined using the so-called two-line elements (TLEs). Unfortunately, TLEs have the disadvantage that they are restricted in their precision. To achieve a more precise laser alignment for space debris tracking, we present an approach to improve the accuracy of orbit predictions based on TLEs, using the SLR functionality of the Gravity Recovery Object Oriented Programming System (GROOPS) software toolkit. Validation using active satellite missions (Swarm-2 and LAGEOS-1) shows that fitting a dynamic orbit to TLE-based positions significantly stabilizes the orbit and reduces the along-track error. Furthermore, we demonstrate that incorporating a small number of near real-time measurements enhances the accuracy of orbit predictions and reduces the remaining time bias for station laser pointing alignment in subsequent passes. Final space debris measurement campaigns highlight the feasibility of using a freely available geodetic toolkit for high-precision space safety applications.

Satellite laser ranging and space debris laser ranging are two closely related range measurement techniques with slightly different setups relying on different lasers. Satellite laser ranging measures light reflections of corner cube retro reflectors at mm-level range precision. Space debris laser ranging gathers diffuse reflections from the whole space debris object and offers a precision down to the sub meter-level. Within this work we show the usage of Megahertz lasers to combine the strengths of both systems within one setup. During the regular tracking schedule to scientific satellite laser ranging targets, specific space debris objects of interest can then be tracked without the need of making adaptions to the system. Megahertz satellite laser ranging measurements to the defunct Jason-2 satellite lead to a measurement precision down to a few μm when ranging to retro reflectors. Space debris laser ranging data reveals reflections from individual surfaces of the target and allows to draw conclusions on the rotational behavior. Space debris laser ranging, and satellite laser ranging are currently performed with slightly different setups. Here, the authors show a single setup with a Megahertz laser for both high-precision satellite laser ranging and space debris laser ranging.

High-precision satellite laser ranging measurements to Galileo retroreflector panels are analyzed to determine the angle of incidence of the laser beam based on specific orientations of the panel with respect to the observing station. During the measurements, the panel aligns with respect to the observing station in such a way that multiple retroreflectors appear at the same range, forming regions of increased data density—separated by a few millimeters. First, measurements to a spare IOV-type retroreflector mounted on an astronomical mount at a remote location 32 km away from the Graz laser ranging station are performed. In addition, more than 100 symmetry passes to Galileo satellites in orbit have been measured. Two novel techniques are described to form laser ranging normal points with improved precision compared to traditional methods. An individual normal point can be formed for each set of retroreflectors at a constant range. The central normal point was shown to be up to 4 mm more accurate when compared with a precise orbit solution. Similar offsets are determined by applying a pattern correlation technique comparing simulated with measured data, and the first method is verified. Irregular reflection patterns of Galileo FOC panels indicate accumulated far-field diffraction patterns resulting from non-uniform retroreflector distributions.

Using ultra-high repetition rate lasers (≥100kHz) is one of the most promising strategies for the next generation of satellite laser ranging (SLR) systems. We present successful 1 MHz repetition rate SLR to targets up to inclined geosynchronous orbits at nighttime. Among those, a maximum return rate of up to 53% was achieved, equivalent to 265 k returns per second for the satellite Swarm-B. In addition, daytime megahertz (MHz) SLR was realized by utilizing a propagated MHz range gate to reduce the massive background noise. In the future, MHz SLR will greatly improve current technology with respect to data amount and data precision, shorter acquisition time, target signature detection, and attitude determination.

Satellite laser ranging (SLR) with higher repetition rate is the recent trend for its various advantages. Laser backscatter (coincidence between recently transmitted pulses and received pulses near the detector) is found to be a constraint for the repetition rates higher than 20 kHz, due to, overlapping with photons returning from a satellite with the present constellation of most of the SLR systems. Such an overlap occurs at every 75 km satellite distance change at 2 kHz repetition rate, and remains for about 7.5 km; for a 20 kHz system however, it will occur after every 7.5 km and remains for 7.5 km, resulting in constant backscatter overlap – leaving no chance to avoid it. The resulting noise is 5 times more than before causing a serious problem in detection and lowers the signal to noise ratio of the overall SLR system. However, decreasing energy per shot at higher repetition rates – assuming a constant power laser – the resulting backscatter may decrease fractionally.

The tumbling motion of defunct satellite TOPEX/Poseidon (T/P) is governed by the u-scale Solar Radiation Pressure (SRP) torque that spun up the satellite from the initial, nadir-pointing to the current fast spinning state [1] . In this paper, we advance the methods of Quanta Photogrammetry (QPM) in an effort to determine satellite attitude parameters - the inertial orientation of the body angular momentum vector L, the rotational phase and rate as well as the object pole coordinates with respect to L; the determined pole offset is 3.52 degrees. The presented data support the hypothesis that the full characteristics of the satellite attitude dynamics can be obtained from a single radiant photogram measured by the photon-counting system of the Graz Satellite Laser Ranging (SLR) Observatory during an overhead pass. The analysis of the observed radiant signature of T/P gives the elevation angle of the normal vector to the solar array front surface at -1.7 degrees, RMS = 0.63 degrees, in the spacecraft body-centered reference frame. This information, along with the pole position, is used to model the SRP forces and torques on the satellite and determine the T/P moment of inertia I = 69728, sigma(1) = 75.2 [kgm(2)], by best-fitting long-term spin simulation to the observed trend.

Satellite laser ranging allows to measure distances to satellites equipped with retroreflectors in orbits up to 36000 km. Utilizing a higher powered laser, space debris laser ranging detects diffuse reflections from defunct satellites or rocket bodies up to a distance of 3000 km. So far space debris laser ranging was only possible within a few hours around twilight while it is dark at the satellite laser ranging station and space debris is illuminated by the sun. Here we present space debris laser ranging results during daylight. Space debris objects are visualized against the blue sky background and biases corrected in real-time. The results are a starting point for all space debris laser ranging stations to drastically increase their output in the near future. A network of a few stations worldwide will be able to improve orbital predictions significantly as necessary for removal missions, conjunction warnings, avoidance maneuvers or attitude determination.

More than 40 Satellite Laser Ranging (SLR) stations around the world routinely measure distances to about 150 retro-reflector-equipped satellites, from Low-Earth-Orbiters (LEO), up to the satellites in geostationary orbit, and determine their orbits with an accuracy of down to few millimetres. Lasers with 10 Hz up to several kHz repetition rate are used, mostly emitting at 532 nm wavelength, and with energies from a few μJ up to several 10 mJ per shot. Most SLR stations are able to detect single photons, reflected from the targets in space, even in full daylight conditions.

The conventional detection of micrometeoroid and orbital debris (MMOD) impacts on satellites is based on insitu sensing or direct, visual inspection of the retrieved surface elements exposed to the particle flux and thus requires active in-orbit operations. We propose Quanta Photogrammetry (QPM) as an optical method for remote detection of surface structural anomalies of passive satellites by measuring solar photon flux reflected off the satellite surface towards the ground detection system.The hypertemporal light curves of the Experimental Geodetic Satellite - Ajisai (NORAD 16908, altitude of 1490 km) are collected with the state-of-the-art photon counting system designed and operated by the Graz Satellite Laser Ranging Observatory (Austria). QPM utilizes the inertial attitude model of the satellite to project the high-rate photometric samples onto the spacecraft bodyfixed frame. The single-photon light curves collected from Oct 2015 until Jan. 2018 are used to map reflectivity of 149 mirror panels on-board Ajisai (approx. size of 20 x 20 cm each) and reveal relatively small, irregularly shaped and spot-like anomalies that can indicate surface degradation due to the long-term environmental interactions and MMOD hypervelocity impacts.

Attitude is the important parameter for active debris removal and collision avoidance. This paper deduced the spin axis orientation and spin period of the rocket body, CZ-3B R/B (NORAD ID 38253), using the satellite laser ranging and light curve data measured with single-photon detector at Graz station. The epoch method and LC & SLR residuals fitting were combined to determine these values. The derived right ascension angle was around 220 degrees, the declination angle was near 64 degrees and the sidereal period was calculated to be 117.724 s, for 2017-07-03. The results derived from the two distinct methods were mutually validated. Rocket bodies are a major contributor to space debris and this work provides a reference for attitude determination and attitude modelling. (C) 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.

A Galileo retroreflector panel was mounted on a tripod 32 km outside of the satellite laser ranging station Graz. The panel was tilted to achieve laser beam incident angles between $$0^{\circ }$$ and approx. $$18^{\circ }$$ while simultaneously doing distance measurements. At incident angles larger than approx. $$8^{\circ }$$ it was possible to identify fine structures within the data corresponding to the different columns of retroreflectors within the panel. The range differences between these columns were determined via a histogram analysis. Knowing the panel geometry it was possible to recalculate the laser beam incident angle on the panel. To compare these ground-based measurements to measurements to a specific pass of Galileo 103, matching incident angle conditions were chosen. Similar structures were found within the data set and it was possible to verify the incident angle of the laser beam. Such a method provides an excellent way to validate the attitude of Galileo satellites and is possible by analyzing the fine details of mm-accuracy kHz SLR data only.

The hypertemporal light curves of the sunlit Ajisai satellite allow for reflectivity measurement of the individual on-board mirror panels. The photon counting technology developed at Graz Satellite Laser Ranging (SLR) station makes it possible to distinguish between the solar flux diffused and specularly reflected off the spinning Ajisai. The flux intensities measured at 10 kHz sampling rate during the period from Oct. 2015 until Jan. 2018 are analyzed through the spacecraft micro-model link budget equation and indicate reflectivity of the 149 mirrors of between 82.3% and 88.2% with the mean value of 85.3% and the RMS of 1.2%. It is predicted that this high specular reflectivity of the satellite will allow for the establishment of a laser link between the distant ground locations with the individual mirrors acting as a zero-latency, passive optical relay. Simulations of the laser link between the Matera (Italy) and Graz (Austria) SLR systems via spaceborne mirror reflections indicate that such a channel can be operated at mean signal strength of 3.46 photoelectrons per laser pulse. The predicted mean number of the laser link intervals per pass is 874.6 with a mean interval duration of 9.15 ms. (C) 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.

A new, mostly COTS based “Laser Ranging Station for Cooperative Targets” is being built for the European Space Agency (ESA) by a consortium of European companies and institutes (D, A, CH, LV) under the lead of DiGOS Potsdam GmbH. The objective of the ELRS is to establish a flexible and economical basis for various optical applications. Starting with laser ranging to cooperative targets and demonstrating passive-optical debris tracking, the system provides the adaptability for future applications to be integrated including laser ranging to non-cooperative targets/debris, space-to-ground laser communication, and for serving as a general test bed for optical technologies. We will present the multi-purpose concept and the flexibilities of the ELRS design with a focus on current and future debris tracking capabilities which are being strongly advanced together with the scientific partners IWF and AIUB.

Satellite Laser Ranging (SLR) is a well-established, mature technique for precise orbit determination (POD). Its overall performance however significantly relies not only on the specific hard- and software equipment of the respective SLR stations - which is relatively expensive -, but also on their global distribution. Upgrading already existing astronomical telescopes with a low-energy laser (e.g, 15 μJ / 2 kHz / 532 nm), plus single-photon detector plus a suitable event timer - all relatively low cost units - creates new SLR systems, which allows laser ranging from Low Earth Orbits (LEO) up to Geosynchronous Orbit (GSO) targets.

TechnoSat is a 20 kg in-orbit technology demonstration satellite of Technische Universität Berlin, which was launched on July 14, 2017. TechnoSat carries fourteen commercial off-the-shelf 10 mm laser retroreflectors and has been tracked successfully by several ILRS stations. Within several experiments the feasibility and the credibility of attitude determination by using KHz SLR technique was demonstrated. By design, a maximum of 24 reflectors could have been mounted on 6 outer surfaces of TechnoSat. However, to achieve attitude and rate determination unique SLR signature was realised on each side. To this end, the optimal distribution of fourteen reflectors was simulated and tested on the ground. Estimated by Degnan’s radar link equation, the expected detection probability of the implemented reflectors in a 600 km orbit and with a 45 degrees incidence angle is about two times of LAGEOS, which enables most of ILRS stations to get a reasonable amount of returns even in the worst case when only one reflector is visible to the station. In orbit, the attitude of TechnoSat varies between freely tumbling, nadir-, inertial-, or target-pointing according to the demands of different experiments. This in turn results in different scenarios for laser ranging. In this paper, we present our results in attitude analysis based on KHz SLR data from the laser ranging station in Graz, Austria and its comparison to records from the on-board attitude and rate measurements.

We perform decoy-state quantum key distribution between a low-Earth-orbit satellite and multiple ground stations located in Xinglong, Nanshan, and Graz, which establish satellite-to-ground secure keys with ∼kHz rate per passage of the satellite Micius over a ground station. The satellite thus establishes a secure key between itself and, say, Xinglong, and another key between itself and, say, Graz. Then, upon request from the ground command, Micius acts as a trusted relay. It performs bitwise exclusive or operations between the two keys and relays the result to one of the ground stations. That way, a secret key is created between China and Europe at locations separated by 7600 km on Earth. These keys are then used for intercontinental quantum-secured communication. This was, on the one hand, the transmission of images in a one-time pad configuration from China to Austria as well as from Austria to China. Also, a video conference was performed between the Austrian Academy of Sciences and the Chinese Academy of Sciences, which also included a 280 km optical ground connection between Xinglong and Beijing. Our work clearly confirms the Micius satellite as a robust platform for quantum key distribution with different ground stations on Earth, and points towards an efficient solution for an ultralong-distance global quantum network.

We are reporting on identification and calibration of one-way systematic biases in Satellite Laser Ranging systems. SLR is a standard technique to measure the distance of satellite as a function of time with millimeter precision and accuracy. For one-way laser ranging, laser time transfer ground to space and for biand multi-static laser ranging to space debris the identification and measurement of biases related separately to transmitting and receiving parts of the system are needed. The epochs of transmission and reception of optical signals have to be referred to the coordinated time scale with the accuracy reaching one nanosecond level or better. This requirement is about one hundred times more accurate than in standard SLR applications. A new procedure of calibration of one-way delays related to the SLR systems have been developed and tested. The necessary hardware components needed for calibration measurements were designed and developed. The calibration procedure and related hardware were tested in a real measurement at the SLR sites in Graz, Austria and Herstmonceux, UK. The one-way systematic biases were determined with the accuracy better than 20 ps.