A Laser Spot Tracker (LST) for an airborne or a ground system usually employs a proportional quad cell detection mode where all four quadrants of the detection cell have continuous illumination. Some types of laser spot trackers using a four-quadrant detection cell require a relatively large image spot diameter in order to operate in the proportional mode—that is in order to determine an energy centroid in the laser spot and orient themselves accordingly.
When a large spot diameter is achieved with a beam spoiler or similar de-focusing device located in the laser spot tracker, the energy centroid of the detected signal changes position on the quad detector cell as the signal varies over the sensor aperture. An image which is defocused to create a larger spot is sensitive to pointing error due to atmospheric intensity variations or turbulence. These errors may result from atmospheric temperature or pressure gradients between the laser source and the tracking device, or from intervening noise sources such as rain or dust.
The atmospheric turbulence can spread non-uniform reflected laser illumination over the aperture of the sensor. This effect causes the centroid of the laser spot to be incorrectly ascertained on the detector, leading to potential errors in tracking or targeting of munitions and other moving objects. For long range, high-resolution laser spot trackers on a ground vehicle, something as unavoidable as contamination on an external window of the tracker may cause boresight errors that lead to costly and potentially deadly errors in tracking and targeting.