Send article to a friendPrint

Atmospheric real-time laser tracking

The detection and continuous tracking of fast-flying distant objects such as UAVs or space debris requires highly precise and sensitive tracking systems which accuracies in the sub-μrad range and bandwidths in the kHz range. These requirements arise from the omnipresent air turbulences in the atmosphere which can lead to a destabilisation of the centroid of a laser beam and consequently to a reduced power density at the target object.

Further interference of the beam direction is caused by vibrations of the mounting
of the transmission optics. The typical time constants for changes to the atmospheric
turbulence cells lie in the range from 10 to 100 ms. Subject to existing meteorological  conditions, angles of deflection of up to 100 μrad occur. The laser tracking systems developed in the TP-AO department fulfil these requirements and, moreover, are real-time capable. The systems are based on commercially available adjustable reflectors with piezo driven actuators. Highly sensitive quadrant diodes with measurement amplifier electronics developed by the department are used as position sensors.

A compact laser tracking unit has been successfully flange-mounted to a 10 inch Schmidt-Cassegrain reflector telescope, in order to conduct tracking experiments over large distances. To review the suitability in the field, a number of tests were carried out on the Institute of Technical Physics' laser test range. In these tests, over a main length of 130 m, a beam direction instability of 6 μrad (standard deviation) that is related to turbulence was successfully reduced to less than 300 nrad. This equates to a direction fluctuation of about 1 mm over a distance of 3 km. The data obtained in atmospheric tracking experiments permits the scaling of the system requirements to relevant distances of several kilometres. Furthermore, the basis is thereby laid to track satellites and space debris in their trajectories in LEO and GEO in a highly precise manner and to enable scientists to determine the corresponding trajectory data.