Target-Tracking Camera for a Metrology System
- Created: Friday, 01 May 2009
Angular measurements are updated at a rate of hundreds of hertz.
An analog electronic camera that is part of a metrology system measures the varying direction to a light-emitting diode that serves as a bright point target. In the original application for which the camera was developed, the metrological system is used to determine the varying relative positions of radiating elements of an airborne synthetic-aperture-radar (SAR) antenna as the airplane flexes during flight; precise knowledge of the relative positions as a function of time is needed for processing SAR readings.
It has been common metrology system practice to measure the varying direction to a bright target by use of an electronic camera of the charge-coupled-device or active-pixel-sensor type. A major disadvantage of this practice arises from the necessity of reading out and digitizing the outputs from a large number of pixels and processing the resulting digital values in a computer to determine the centroid of a target: Because of the time taken by the readout, digitization, and computation, the update rate is limited to tens of hertz. In contrast, the analog nature of the present camera makes it possible to achieve an update rate of hundreds of hertz, and no computer is needed to determine the centroid.
The camera is based on a position-sensitive detector (PSD), which is a rectangular photodiode with output contacts at opposite ends. PSDs are usually used in triangulation for measuring small distances. PSDs are manufactured in both one- and two-dimensional versions.
Because it is very difficult to calibrate two-dimensional PSDs accurately, the focal-plane sensors used in this camera are two orthogonally mounted one-dimensional PSDs. The camera also includes a beam splitter and two cylindrical lenses to focus line images of the target onto the PSDs — more specifically, to form a horizontal line image on the vertically oriented PSD and a vertical line image on the horizontally oriented PSD. The outputs from both ends of each PSD are processed by analog circuitry (see figure) to obtain an analog signal proportional to the displacement of the image centroid from the midlength position along the PSD. The direction-measuring error of the readout has been found to be no more than 1/2,700 of the angular width of the field of view.
This work was done by Carl Liebe, Randall Bartman, Jacob Chapsky, Alexander Abramovici, and David Brown of Caltech for NASA’s Jet Propulsion Laboratory. NPO-41466
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