The increasing amount of space debris poses a threat for active satellites in space. Besides currently approx. 7,000 active and more than 3,000 inactive/defunct satellites, roughly 30,000 space debris objects larger than 10 centimeter orbit our planet. The rapid increase of active satellites is mainly driven by commercial operators launching communication satellites into space. According to statistical models by ESA an approximated number of 1 million objects larger than 1 centimeter and 130 million space debris particles larger than 1 millimeter exist.
These space debris parts are mainly upper stages of old rockets, parts from explosions (e.g. aging batteries or remaining fuel), satellite collisions or fragments of military anti-satellite missions. Depending on their orbital height and area to mass ratio, space debris parts can remain in orbit for a long time. Objects at 500 km orbit will stay in orbit for up to several years, at orbital heights larger than 1,000 km the lifetime increases to more than 1,000 years. Without proper removal strategies, Earth’s orbit will become more and more congested. During reentry, driven by atmospheric drag, smaller space debris objects burns up completely in the denser parts of Earth’s atmosphere. Larger objects like rocket bodies or defunct satellites however can reach the surface of our planet posing a potential hazard to society.
The Graz SLR station has an international leading position concerning space debris research and development performing space debris laser ranging measurements to objects sized down to less than 1 meter and single photon based light curve measurements to satellites or space debris up to the geostationary orbit.
Using a higher powered laser beam allows to measure the diffuse reflection reflected by the whole satellite or space debris body – no retroreflectors are required. As the reflection is distributed over a larger area, only single photons are returning from each sent laser pulse. Due to the large repetition rate of the laser (200 Hz) the reflected photons hence provide time depth information of the target. Photons are statistically reflected from the parts of the space object being closer to or further away of to the observing station. These range measurements can be used to get information on the size of the object along the line of sight. Rotating targets show reoccurring alternating range variations which can be analyzed to measure the apparent rotation period and the orientation of the spin axis.
In 2020 Graz was the first SLR station to show that space debris laser ranging is possible during daylight. Due to the inaccurate orbit predictions of space debris it is necessary to visualize the reflected sunlight. The displayed “star-like” image of the objected is centered in the field of view of the receive telescope to reduce the search area in sky. Hence, space debris observations were only done during the terminator period where satellites are illuminated and not in Earth shadow while it is sufficiently dark at the SLR station. This leads to a few hours of observation time after sunset and before sunrise. To advance the technology towards daylight operation it is hence necessary to visualize satellites/space debris during daylight. A dedicated filter, camera and telescope system was mounted on top of the existing receive telescope. After first tests imaging stars up to magnitude 8 during daylight, upper stage rocket bodies or defunct satellites were visualized and the tracking parameters could be corrected during tracking via real-time image processing techniques. This way several passes of space debris objects during daylight conditions were successfully measured up to a sun elevation of a maximum of 39° over the horizon.
In this experiment Graz sends photons to a space debris target, the light is diffusely reflected on the object’s surface and the Graz photons are spread over central Europe. These reflected photons can then be detected by other stations across the continent. In a unique experiment, Graz sent photons with a green laser and Wettzell sent photons with an infrared laser, simultaneously. Graz’ photons were detected by Graz SLR station, Wettzell photons by Wettzell, Graz, and Stuttgart. Data analysis proved a significant increase in orbital prediction accuracy of space debris targets observed simultaneously by multiple stations.
A low cost camera system with attached objective with a field of view of approx. 10° "stares" in an arbitrary direction of the sky and records the stellar background. From the position of stars in the background the accurate pointing direction of the camera is calculated. As soon as a sunlit space debris object passes through this field of view, its celestial coordinates as referenced to the background stars are determined and stored. Using only the pointing information – without a-priori orbital information – an orbit is calculated and immediately used to track ("chase") the target with laser-based distance measurements. The whole process from the first detection of the target in the camera's field of view to successful space debris laser ranging can be completed within a few minutes.
Simultaneously to satellite or space debris laser ranging, single photon light curves are recorded, measuring the reflected sunlight from space objects. Satellite laser ranging only utilizes a single wavelength (532 nm) other parts of the spectrum are redirected to another Single Photon Avalanche Diode (SPAD) detector. Such a detector with an attached FPGA has the advantage of being able to operate with very fast temporal resolution showing fine details of the different reflecting surfaces. It was shown that even the individual mirrors of satellites can be characterized with this technique w.r.t. deformations. Due to its mostly autonomous operation, a large database of light curves was acquired in the recent years during the standard laser ranging operation of the station. Analyzing satellite laser ranging, space debris laser ranging and light curve data simultaneously (data fusion) allows to draw conclusion on the rotation period, orientation and the spin axis of the object in space.
Even to Galileo satellites in orbital heights of 23,000 km the attitude of the satellite can be determined. By analyzing the reflections of the different retroreflectors of the panel in certain alignment conditions, the incident angle of the laser beam can be determined with an accuracy of approx. 0.1°. The detailed identification of return patterns also has implications on a potential further improvement of orbit predictions of the Galileo constellation.