1. Field of the Invention
The present invention pertains to semi-active laser systems and, more particularly, to an imaging semi-active laser system.
2. Description of the Related Art
A need of great importance in military and some civilian remote sensing operations is the ability to quickly detect, locate, and/or identify objects, frequently referred to as “targets,” in a “field of view” or in an “instantaneous field of view” within a “field of regard.” In this sense, the field of view is the portion of the environment being remotely sensed at a particular moment. A field of regard is the total area being remotely sensed. A field of regard may comprise several fields of view.
Remote sensing techniques for the tasks mentioned above have existed for many years. For instance, in World War II, the British developed and utilized radio detection and ranging (“RADAR”) systems for detecting and tracking the incoming planes of the German Luftwaffe. Sound navigation and ranging (“SONAR”) has found similar utility and application in environments where signals propagate through water, as opposed to the atmosphere. While RADAR and SONAR have proven quite effective in many areas, they are inherently limited by a number of factors. For instance, RADAR is limited because of its use of radio frequency signals and the size of the resultant antennas used to transmit and receive such signals. Sonar suffers similar types of limitations. Thus, alternative technologies have been developed and deployed.
Some of these alternative technologies are optical in nature. One such alternative technology is laser detection and ranging (“LADAR”). Instead of radio or sound waves, LADAR systems transmit laser beams and receive reflections from targets. Because of the short wavelengths associated with laser beam transmissions, LADAR data can exhibit much greater resolution than RADAR data in the right contexts. LADAR systems have exhibited significant versatility, and can be used for automatic target recognition (“ATR”), targeting, direction finding, and other, similar tasks. Thus, there are many kinds of LADAR systems, and they may be categorized in a number of ways.
One useful categorization is whether the system is “active” or “semi-active”, which centers on the point at which the detected laser signal is introduced into the field of regard. For instance, the laser signal may be introduced into the field of regard from on board the same platform from which its reflection is detected. Such systems are known as “active” systems. The laser signal may alternatively be introduced from a third party source off board the platform from which the reflection is detected. Such systems are known as “semi-active” laser (“SAL”) systems.
Another useful categorization in active systems is whether the system is a “scanned” system or a “flash” system. In some systems, the laser is mounted on a gimbal, which is then used to scan the LADAR signal in azimuth and in elevation into the field of regard, which will comprise many instantaneous fields of view. The receipt of the reflection is synchronized with the transmission of the LADAR signal. This permits determination of the “flight time” for the LADAR signal, from which the range to the point of reflection can be determined. The reflected LADAR signal is also received as a function of the angle at which it impinges upon the platform. Thus, the received reflection can be mapped into a three-dimensional image of the field of view. A flash system works in much the same fashion, except there is no scanning, and so the image is of a field of view.
Note that SAL systems are distinguishable from active systems in at least one important respect. In both scanned and flash active systems, receipt can be synchronized with transmission since transmission occurs on-board. However, in SAL systems, there can be no synchronization since the laser signal originates off-board. An absolute range to the target therefore cannot be determined. Consequently, SAL systems are not used in imaging, which permits simplification of the receiver. SAL systems typically employ what are known as “quad cell detectors,” which are optical detectors comprised of four cells. The apparatus including the SAL system is guided to maintain the receipt of the reflected laser signal in the center of the four cells.
These types of technological distinctions strongly influence the end use of the LADAR system. For instance, active systems can image, and so are used in conjunction with automatic target recognition systems such that they can automatically locate, identify, and home on a target without direct human intervention. This is not true of SAL systems, generally, because they cannot image. Typically, SAL systems are used in contexts where a target is spotted with a target designation for an off-board source and the apparatus including the SAL system follows the reflection to the target.
Each type of system has relative advantages and disadvantages. SAL systems require highly accurate pointing of the designator laser spot on the target. The weapon will hit the spot, but atmospherics and instability of the spot's position will vary the aim point for the weapon on the target. If too large of a portion of ground in front of the target is illuminated, and that ground has green foliage, then the SAL system will point low, such that the weapon might impact the ground in front of the target. Scanning LADAR systems require a high repetition rate laser that provides very accurate target information and an impressive ATR capability, but the entire system is destroyed when the weapon is destroyed—including the laser and ATR system. Flash LADAR provides a non-scanning system with a simpler optical path with the same ATR capability. However, flash LADAR is very limited because of the need from a very high power laser for scene illumination that is subsequently destroyed when the weapon hits the target.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.