Apparatus and methods for covertly determining the rate of motion between
a pixel-based imaging device and a target being viewed by the pixel-based
imaging device. The method involves using the pixel-based imaging device
to acquire a first and a second digital image of the target. The second
digital image is acquired a time interval after the first digital image.
The time interval and pixel counts for the pixel set within the first and
the second digital image representing a particular portion of the target
are then used to determine a rate of relative motion between the
pixel-based imaging device and the target. Advantageously, the rate of
relative motion is determined covertly such that the locations of the
pixel-based imaging device and the target are not disclosed or broadcast
to third parties.