Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1999-03-29
2001-07-24
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S203600, C356S004010, C089S041060
Reexamination Certificate
active
06265704
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates generally to systems for tracking a moving object and, more particularly, to a non-imaging laser-based system and method of tracking and engaging targeted objects and a system and method of identifying target hits.
2. Discussion
Tracking systems have been developed and deployed for tracking a targeted moving object such as a rocket or missile generally for purposes of engaging the object in an attempt to destroy or disable the object for military purposes. Generally speaking, the conventional tracking system is referred to as an imaging tracker and employs an imaging device such as an electronic camera that captures an image and selects a portion of that image and then attempts to lock on to it with respect to the frame of the image. According to one tracking system, the process of acquisition of a target generally consists of finding the target, fixing the target in a large field of view tracker and then transferring the instantaneous measurements of the target's position to a narrow field of view tracker. In turn, the narrow field of view tracker fixes the picture of the target with respect to the borders of the frame, generally known as the track gate. With the image fixed in the frame, electronic circuitry configured as a servo loop generates an error signal which causes a high power laser beam or other weaponry to track and engage the target in an attempt to destroy the target.
Accordingly, the conventional imaging tracker generally requires and relies on continuous use of one or more imaging devices such as electronic cameras to first determine the approximate location and then the current instantaneous position of the targeted object. In addition, some conventional imaging trackers try to determine target velocity information and attempt to predict the anticipated projection of the target. In an optically targeted weapon, known conventional imaging trackers are associated with a separate high power laser beam or other weaponry engagement means for attempting to destroy or disable the detected object that is being targeted. The high power laser beam weaponry is independent of the imaging device and is typically steered in response to the calculated optimum position and velocity vector as determined by the processed product of the imaging device, with a targeting offset angle to engage the optimum vulnerable zone on the target.
While the conventional imaging tracker systems may effectively locate the target, a number of limitations exist with respect to the ability to continuously track a moving target and effectively engage it. First, with the conventional imaging approach, it has been found that the high power laser beam used for engagement may interfere with the imaging device and therefore obliterate the track point, thereby causing it to lose track of the targeted object. To reduce interference of the laser beam with the imaging device, the high power laser beam can be offset from the imaging device. However, the offset laser beam may introduce additional error to the overall tracking scheme and does not always effectively reduce or eliminate the interference problems. Second, the ability to use imaging sensor data to effectively point the laser depends on an accurate boresight of the sensor line of sight with the laser line of site. Historically, maintaining an adequate boresight under stressing environmental conditions has been difficult. Third, the conventional imaging tracker is generally limited to the resolution of the imaging tracker itself. That is, when the target is very small and below the resolution limit of the imaging tracker, the ability to get a fix on the target may be limited by the resolution of the tracker. Accordingly, the conventional imaging tracker generally cannot track targets smaller than the tracker's resolution limit without exhibiting unacceptably large track errors. Even with resolvable targets, track errors proportional to the second and third derivative of the track angle with respect to time and to the derivative of the aimpoint offset angle may become unacceptably large, especially for close-in fly-by encounters. This results in a limitation on the size of a target that may effectively be tracked. Without remedy, the effectiveness of the conventional high energy laser weapon is limited. Consequentially, for a given power, longer dwell times are generally required to achieve adequate killing fluence, thereby shortening keep out ranges and further draining the laser fuel system.
Additionally, disturbances, either small disturbances of the pointing or tracking systems, or propagation of the high power laser beam, may cause the laser beam to miss the target. Similarly, laser beam propagation through an optically inhomogeneous atmosphere may also cause the laser beam to be diverted away from the target. Such disturbances to the laser beam tend to go undetected since the conventional imaging device that is tracking the target is independent of the engaging high power laser beam weaponry.
Furthermore, there is a need to effectively determine the accuracy of a laser weapon tracking system by determining the effective kill engagement potential of a laser beam on the target. That is, in order to determine the effectiveness of a given laser weapon tracking system, it is desirable to be able to score laser beam “hits” on a moving target, especially in preliminary tracking tests using a low power laser. Presently, there is a need to independently assess the instantaneous location of a laser beam impinging on a moving missile target for purposes of testing its effectiveness without actually requiring destruction of the target.
Accordingly, it is therefore desirable to provide for a system and method of tracking moving objects with a laser beam that is less susceptible to error and interference problems.
It is further desirable to provide for a laser beam tracking system that tracks and locks onto a moving target with a high energy laser beam that is independent of a well boresighted imaging tracker device.
Yet, it is also desirable to provide for such a laser tracker that is capable of tracking small targets, including unresolvable targets, irrespective of imaging tracker resolution.
Also, it is desirable to provide for a system and method of independently accessing laser beam “hits” on a moving target for purposes of testing the kill effectiveness of laser weapon systems.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, a system and method for tracking a moving object with laser energy is provided. The system includes a laser generator for generating a beam of laser energy and a beam steerer for steering the beam of laser energy so as to track a targeted moving object. The beam steerer steers the beam of laser energy in an oscillatory fashion, preferably in two orthogonal directions. The high power laser beam oscillates in a first direction at a first dither frequency and in a second direction at a second dither frequency which is distinguishable from the first dither frequency. A telescope gathers laser energy that is reflected off of the targeted object and a detector detects the amount of reflected energy received. The detected energy is filtered to separate the first and second dither frequencies for each channel. The filtered signals are synchronously detected by multiplying each channel by a sinusoidal function derived from the laser mirror generator for that channel. It is an observed fact that when the beam centroid is exactly centered on, for example, a cylindrical target midline, the reflected power contains only harmonics of the dither frequency. If displaced to either side, the dither frequency component magnitude increases proportional to the displacement with a sign indicating which side. A bias signal is generated from the received reflected synchronously detected power proportional to the beam centroid displacement from the target midline, having a sign that is either plus or minus, depending on which side th
Lee John R.
TRW Inc.
Yatsko Michael S.
LandOfFree
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