1. Field of the Invention
The present invention relates generally to systems for tracking a moving object and, more particularly, to a laser-based system for tracking a moving object that employs two lasers; a reference and slave laser. Each laser is a weapon, and when locked together on a target, single laser effectiveness may be doubled without a thermal blooming performance loss associated with a single laser operating at twice the power. The slave laser is dithered relative to the reference laser in a direction along a longitudinal axis of the target. The system includes an optical receiver for repetitively scanning the irradiance profile reflected by the target.
2. Description of the Prior Art
Various types of systems are known for tracking moving objects, such as rockets and missiles. Such systems can be categorized as either imaging or non-imaging systems. Conventional imaging type systems normally utilize an imaging device, such as an electronic camera, for detecting and tracking the position of a targeted moving object. While such imaging systems are effective in tracking targeted moving objects, such imaging systems; are known to have limitations when used in combination with high power laser beam weaponry. For example, in such systems, the high power laser beam is known to interfere with the imaging system potentially causing loss of the track of the targeted moving object. Although various systems are known for compensating for such an interference problem, such systems do not effectively eliminate the interference.
As such, non-imaging laser type tracking systems have been developed. An example of such a system is disclosed in copending U.S. patent application Ser. No. 08/631,645, filed on Apr. 2, 1996, now U.S. Pat. No. 5,780,838, entitled LASER CROSS BODY TRACKER (LACROSST), assigned to the same assignee as the assignee in the present invention. The system disclosed in the '645 patent application includes a laser generator for generating a single beam of laser energy and a beam steerer for steering the beam of laser energy to track a targeted moving object. The beam steerer steers the beam of laser energy in a oscillatory fashion in two orthogonal directions at a first dither frequency and a second dither frequency, respectively. The system also includes a telescope for receiving reflected laser energy from the targeted object and detecting the amount of reflected energy received. The detected energy is filtered to form 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. A bias signal is generated from the received reflected synchronously detected power proportional to the beam centroid displacement from the target midline which allows the beam steerer to steer the laser beam to center it on the target, thereby tracking the targeted object.
Unfortunately, laser based tracking systems are subject to what is known as thermal blooming. Thermal blooming results in a change in the refractive index of the beam path as a result of heating the beam path temperature by the laser. A change of the refractive index creates a lens effect that causes the laser radiation to spread relative its original direction. As such, thermal blooming increases the diameter of the laser beam as it moves away from the laser source. A detailed explanation of the thermal blooming is disclosed in U.S. Pat. No. 5,198,607, hereby incorporated by reference.
The problem of thermal blooming also reduces the effectiveness of high power laser weaponry. In order to overcome the thermal blooming problem for high power laser weaponry, the '607 patent discloses the use of two independent lasers separated by a sufficient distance to prevent interference therebetween, focused onto a single moving object, such as a missile. The '607 patent discloses the use of a known imaging type system for tracking the location of the targeted moving object. However, the two laser beams are not under closed loop control in the system disclosed in the '607 patent, thereby making it relatively difficult to direct both laser beams to a single spat on the target.