  In order to avoid collisions with space debris, the near Earth orbit must be
continuously scanned by either ground- or spaced-based facilities. For the low
Earth orbit, radar telescopes are the workhorse for this task, especially due
to their continuous availability. However, optical observation methods can
deliver complementary information, especially towards high accuracy
measurements. Passive-optical observations are inexpensive and can yield very
precise information about the apparent position of the object in the sky via
comparison with background stars. However, the object's distance from the
observer is not readily accessible, which constitutes a major drawback of this
approach for the precise calculation of the orbital elements. Two experimental
methods have been devised to overcome this problem: Using two observatories a
few kilometres apart, strictly simultaneous observations of the same object
yield an accurate, instantaneous 3D position determination through measurement
of the parallax. If only one observatory is available, a pulsed laser can be
used in addition to the passive-optical channel to measure the distance to the
object, in a similar fashion as used by the satellite laser ranging community.
However, compared to conventional laser ranging, a stronger laser and more
elaborate tracking algorithms are necessary. The two approaches can also be
combined by illuminating the object with a pulsed laser from one observatory
and measuring the return times at both observatories. These techniques are
explored by German Aerospace Center in Stuttgart using its orbital debris
research observatory, in cooperation with the Satellite Laser Ranging station
in Graz and the Geodetic Observatory in Wettzell. This contribution will
present some of the results and plans for further measurement campaigns.
