Date of Award


Document Type


Degree Name

Master of Science in Electrical Engineering


Department of Electrical and Computer Engineering

First Advisor

Stephen C. Cain, PhD


This research effort identifies and models the nonlinear time-variant behavior exhibited by an avalanche photodiode (APD) array based Laser Ranging and Detection (LADAR) system. Based on the original Linear Time-Invariant (LTI) model, the evolution of error in the LADAR signal is examined sequentially from the outgoing pulse through signal digitization. This error evolution shows that the LTI model does not contain a mechanism for causing the observed signal deviations or the failure to meet the Cramer-Rao lower bound for range accuracy. A nonlinear time-variant model is developed based on the interactions of the avalanche photodiodes in the array with the array's voltage regulator. In the refined model, the sum photo-current for the entire array loads the voltage regulator. The resulting reverse bias voltage variations cause the responsivity of each APD to vary in a nonlinear fashion. Because each APD in the array's responsivity depends upon the entire array's photonic loading, each individual APD's response is time variant.

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DTIC Accession Number