Avalanche photo diodes operated in the Geiger mode in an laser ranging application can be considered almost as digital detectors. The output signal amplitude does not show any dependence on the input light level. On the other hand the incoming light intensity accounts for the characteristic delay in the response time of this semiconductor detector. Once a break down of the bias voltage occurs, the amplitude steeply increases to a preset voltage level until the avalanche is being quenched by the electronic circuit. However the evaluation of the signal risetime makes a correction of the observed delay caused by a variant transit time inside the avalanche diode possible. A simple simulation model was made and matched to experimentally obtained measurements in order to understand the basic mechanism of the observed timewalk. A linear relationship between the characteristic delay of the detector response and the measurable output pulse risetime can be exploited for a correction to the measured range between a laser ranging tracking station and the target satellite. An electronic circuit was designed for an automatic shot by shot correction of the SLR-measurements, where large fluctuations in the receive signal intensity are commonplace because of the turbulent atmospheric propagation path. This unit has been evaluated under laboratory conditions and in the satellite ranging application.
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