On the potential of multi-static SLR. Case study: orbit determination and prediction of space debris objects

In the last decades, Satellite Laser Ranging (SLR) has been proven to be one of the most valuable techniques to improve our understanding of the Earth's shape, gravity field and rotational motion. Apart from the ''traditional'' tasks of SLR, in the recent past the portfolio has been considerably widened. Especially the SLR-based orbit determination and orbit prediction of space debris objects is gaining increasing attention. The reliable and accurate orbit determination/prediction of debris objects is of crucial importance for any effort towards debris collision avoidance. Hitherto, this task is performed by space surveillance networks using ground-based radar tracking and passive optical tracking with telescopes. These methods, however, are characterized by low accuracy, ranging from several hundreds of meters to a few kilometers for low-Earth orbiting objects. As demonstrated successfully, SLR has the potential to considerably improve these numbers, and hence to improve debris surveillance.

One of the most severe limitations of the technique is the sparse network of SLR stations, and hence the sparseness of tracking data. Against this background, we propose to extend the existing SLR network by passive telescopes in combination with multi-static observations. Multi-static observations means that an object is tracked by only one active SLR station, but the diffusely reflected photons are detected at several passive stations (in case of only one passive station the measurements are referred to as bi-static). In order to demonstrate the performance of the principle, we analyzed (i) two-way laser ranges from Graz (3 passes), and (ii) two-way laser ranges from Graz (3 passes) in combination with bi-static measurements between the SLR stations Graz and Wettzell (3 passes); our investigations focus on the defunct ENVISAT satellite. We found the orbit determination/prediction results including bi-static observations to outperform single-station results by about one order of magnitude. For scenario (ii), our orbit prediction error ranges from 40m (prediction age 1 day) to 450m (prediction age 7 days). From our first experiments we conclude that bi-/multi-static SLR is capable to meet the requirements for debris surveillance, but might also be beneficial for more ''traditional'' applications.