Dual mode semi-active laser/laser radar seeker

Abstract: The invention provides a method and apparatus for guiding a weapon to a target using an optical seeker having dual semi-active laser (SAL) and laser radar (LADAR) modes of operation. The dual mode seeker incorporates a laser light source, an optical package including a quadrant detector for operating in SAL mode and a LADAR receiver for operating in LADAR mode. The seeker further includes a high speed scanning minor for switching between modes to guide the weapon to the target. The method for guiding a weapon to a target includes receiving radiation from the target and tracking the radiation to guide the weapon; monitoring the detected radiation such that if the radiation falls below a predetermined level, a laser system on-board the weapon continues guiding the weapon by generating a laser beam; reflecting the laser beam off the target so that the reflected laser radiation is received from the target to track the radiation and guide the weapon to the target.

Description: 1.1 Field of the Invention The present invention relates to an on-board weapon guidance system and more particularly to a seeker system having dual semi-active laser and laser radar modes of operation. This invention also relates to a method for guiding a weapon to a target. 1.2 Description of Related Art Laser energy is uniquely suited to perform many specialized functions because of its coherent, extremely stable, frequency characteristics, thus making possible the generation and transmission of very well defined and characterized beams of energy. Since the development of practical laser apparatus, such apparatus are finding many applications for locating and identifying remote objects including, in military operations, target marking and guidance systems. One of the present marking and guidance systems is the semi-active laser (SAL) system. SAL systems have been used by military aircraft to support ground operations. With the SAL system, a narrow laser beam is produced and transmitted toward a target. The laser radiation is typically generated and transmitted from a laser designator aircraft manned by a forward operator. The operator directs the laser radiation to a selected target, thereby designating the target. The laser radiation reflected from the target can then be detected by the laser seeker head of a missile or other weapon located remote from both the target and the laser energy transmitter. The SAL system includes processing equipment for generating guidance commands to the missile derived from the sensed laser radiation as it is reflected from the target. Such a system can be used by pilots or other users to identify a target and guide the missile or weapon to the target. Although these systems have proven effective, the next generation missiles are expected to fly to ranges well beyond the range of imaging sensors on board the designator platform. On the other hand, there are many SAL designators already in the field with proven records of extremely high weapon accuracy and positive control. Another known seeker guidance system is the laser detection and radar (LADAR) system. Unlike its SAL cousin, the LADAR system incorporates its own laser source, thus eliminating the need for an external designator. Typical LADAR systems are adapted to scan a target area with laser energy, detect the reflected radiation, and compute range and intensity values, permitting the processing of guidance and control signals for the weapon as it approaches the target With its specialized data processing capabilities, the LADAR system provides superior ability to acquire targets autonomously. U.S. Letters Patent 4,085,910 ('910) discloses a dual mode optical seeker device having an infra-red and visible light sensor. The seeker of the '910 functions as a SAL seeker by sensing infra-red radiation transmitted from a designator platform and reflected from a target. The '910 seeker also includes a visible light sensor for determining the orientation of the missile relative to a visible target. Since the '910 seeker requires an external designator and relies on visible light to mark and track a target, it is limited to certain range and environmental conditions. The '910 seeker is also not adapted to rapid scanning possible with LADAR devices. In DE-A-36 15 266 there is described an on-board weapon guidance according to the preamble of claim 1. More specifically, DE-A-36 15 266 discloses an on-board weapon guidance system comprising a laser light source, and means for detecting radiation proceeding from a target to guide the weapon on the target. A method for guiding a weapon to a target according to the preamble of claim 20 is also known from DE-A-36 15 266. Advancement in enemy air defense systems drives the need for enhanced weapon guidance capability. It is desirable to further increase the range of modem weapons while still maintaining high accuracy and positive control. Still further, it is desired to implement these capabilities without great alteration or cost to existing weapon systems. Therefore, it is a primary objective of the present invention to provide a new, low cost, on-board seeker system that offers advanced capabilities as discussed above. To achieve this, the on-board weapon guidance system of the invention is characterized by the features claimed in the characterizing part of claim 1 and the invention provides a method according to the characterizing part of claim 20. Basically, the method of the invention includes monitoring the radiation to guide the weapon to the target such that if the radiation falls below a predetermined level a laser system on-board the weapon continues guiding the weapon by generating a laser beam; reflecting the laser beam off the target; receiving laser radiation reflected from the target; and tracking the radiation to guide the weapon to the target. In a second aspect, the on-board weapon guidance system of the invention comprises means for switching between the detection of radiation originating from a source independent of the weapon and proceeding from the target, and the detection of laser radiation originating from the laser light source and reflected from the target. 3. BRIEF DESCRIPTION OF THE DRAWINGS Other aspects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: Figure 1 is a perspective view, partially broken away and partially in section, of the dual mode seeker of the invention; Figure 2 is an elevation, sectional view of the seeker of Figure 1; Figure 3 is an illustration of the mirror assembly of the seeker of Figures 1 and 2; Figure 4 is a flow chart describing operations performed in accordance with the invention; and Figure 5 is a second flow chart describing operations performed in accordance with the present invention. 4. DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS In the interest of clarity, not all features of actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual implementation, as in any such project, numerous engineering decisions must be made to achieve the developer's specific goals and subgoals (e.g., compliance with existing systems- and-cost related constraints), which will vary from one implementation to another. Moreover, attention will necessarily be paid to proper engineering and implementation practices for the environment in question. It will be appreciated that such a development might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the field having the benefit of this disclosure. Figure 1 illustrates a particular embodiment of the dual mode seeker 10 of the invention. The manner in which the seeker generates, transmits, and receives a LADAR scan pattern is fully disclosed and claimed in U.S. Letters Patent 5,200,606; 5,224,109; and 5,285,461. The SAL mode of the present invention is implemented with minimal addition of components to the LADAR seeker referenced above, thereby significantly reducing the cost of the dual mode system. The seeker 10 of the invention includes a first optical assembly 12 configured to receive and detect electromagnetic radiation originating or proceeding from a source (not shown) independent and external of the seeker 10. For example, the first optical assembly 12 may receive light radiation emitted from a beacon or proceeding from a reflective surface of a vehicle or building. As discussed above, in typical SAL mode, the first optical assembly 12 generally receives laser radiation transmitted from an independent designator (not shown) and reflected from the target. The first optical assembly 12 includes a receiver lens 14, a narrow band filter 16, for filtering out wavelengths of undesired light to reduce background interference, and a silicon p-intrinsic (PIN) quadrant detector 18. The seeker 10 of the invention includes a moveable high-speed scanning mirror 20 connected to a mirror shaft 21 (shown in Figure 3). The mirror shaft 21 is pivotally driven by a torque motor 22. An angle position sensing device 24 is also included to determine the angular position of the scanning mirror 20 as it pivots about the mirror shaft 21 axis. Figure 2 illustrates a second optical assembly 26 housed in the seeker 10. The second optical assembly 26 includes a LADAR receiver 28 for receiving and detecting laser radiation. The present invention also includes its own laser light source 30 (shown in Figure 1), which emits the laser light energy employed for illuminating the target in the LADAR mode of operation. The seeker 10 of the present invention may be used in the SAL or LADAR mode without compromising the performance of either mode. Turning to Figure 2, the seeker 10 is shown in the LADAR mode of operation. The dashed lines in Figure 2 represent the redirected laser radiation detected during the LADAR mode of operation. In the SAL mode of operation, the torque motor 22 applies a rotating force to the mirror shaft 21, which rotates the scanning mirror 20 until the mirror 20 is held to a precise and fixed position by a mechanical stop 32 and a lever arm 33 affixed to the mirror shaft 21 (shown in Figure 3). Electromagnetic radiation received by the seeker 10 in SAL mode is redirected and focused unto the PIN quadrant detector 18 of the first optical assembly 12. The radiation detected by the PIN quadrant detector 18 is then converted to electrical signals and processed by a control circuit (not shown) using standard quadrant detector algorithms. Additional electronics (not shown) in the seeker 10 then use the processed signals to guide the weapon to the target. The PIN quadrant detector 18 response can also be compensated for obscurations, including linearity, by implementing a table lookup procedure in the algorithm. As long as the electromagnetic radiation detected by the PIN quadrant detector 18 remains above a predetermined level established in the control circuit, the scanning mirror 20 continues to redirect all of the received radiation to the first optical assembly 12. If the radiation detected by the PIN quadrant detector 18 falls below the predetermined level, the control circuit automatically switches to the LADAR mode of operation to provide autonomous target acquisition as described in the referenced Letters Patent above. In LADAR mode, the laser light source 30 emits a laser beam programmed to illuminate and scan a field for target acquisition, as described in the referenced Letters Patent. When the control circuit switches operation of the seeker 10 to LADAR mode, the torque motor 22 rotates the scanning mirror 20, through the mirror shaft 21, by 90° about the mirror shaft 21 axis (as shown by dashed lines in Figures 2 and 3). As the scanning mirror 20 is rotated away from the mechanical stop 32, the angle position sensing device 24 determines the mirror's 20 angle about the mirror shaft 21 in order to slow the mirror 20 to a stop without damaging the mirror 20. The angle position sensing device 24 may be one of many commercially available sensors offering various ranges of degree measurement. The angle position sensing device 24 is configured such that the sensor has sufficient range to scan to the mechanical stop 32 and also aid in performing linear, high speed scans when the seeker 10 is operating in the LADAR mode. The angle position sensing device 24 can be incorporated in various locations, including on the mirror shaft 21 or integrated into the torque motor 22, depending on space constraints or other limitations as recognized by those skilled in the art having the benefit of this disclosure. In the LADAR mode, the laser beam emitted by the laser light source 30 is scanned by the scanning mirror 20 through an angular range of approximately 20° about the mirror shaft 21 axis to generate a high-speed scan of the target scene. As the target scene is being scanned, all the reflected laser radiation received by the seeker 10 is redirected and focused unto the LADAR receiver 28 of the second optical assembly 26, where it is processed by the control circuit to form and track a three dimensional image of the target to guide the weapon to the target. Although the seeker 10 of the present invention cannot operate in both modes simultaneously, the switch between modes occurs nearly instantaneously, facilitating a nearly simultaneous dual mode capability. If the PIN quadrant detector 18 never receives a valid radiation pulse from the designator or independent source while in the SAL mode, the seeker 10 by default will switch to the LADAR mode and use automatic target recognition to acquire the target. Alternatively, the seeker 10 may be utilized strictly in the LADAR mode by deactivating the SAL mode before launching the weapon from the platform. In that case, the weapon would be launched toward a predetermined coordinate such that the seeker 10 autonomously acquires the target. In an alternative implementation, the seeker 10 can be used in the LADAR mode to form and process a three dimensional image of the target which can be used to identify the target class. Thus providing a means of preventing the unintentional attack of friendly forces in military operations and enhancing the performance of the seeker 10. In hindsight, it will be appreciated by those of ordinary skill having the benefit of this disclosure that the seeker 10 system disclosed herein can be used in a variety of situations apart from military implementations. Figure 4 depicts a flow chart of a method 100 for guiding a weapon to a target in accordance with the present invention. The method, executed with the seeker 10 of the present invention (or other dual mode seeker), comprises receiving radiation from the target 105 and tracking the radiation to guide the weapon to the target 110; the radiation is monitored 115 such that if the radiation falls below a predetermined level, a laser system on-board the weapon continues guiding the weapon by generating a laser beam 120 and reflecting the laser beam off the target 125 so that the reflected laser radiation is received from the target 130 to track the radiation and guide the weapon to the target 135. Figure 5 depicts a flow chart of another method 200 for guiding a weapon to a target in accordance with the present invention. This method may also be executed with the present dual mode seeker 10 (or other dual mode seeker). The method comprises generating a first laser beam from a first source 205 and reflecting the first laser beam off the target 210; the reflected laser radiation is detected on-board the weapon to guide the weapon to the target 215; a determination is made such that if the detected laser radiation originating from the first source falls below a predetermined level 220, a second laser beam from a second source is generated 225; the second laser beam is reflected off the target 230; and the reflected laser radiation originating from the second laser beam is detected on-board the weapon to guide the weapon to the target 235. All of the methods and apparatus disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure.

Claims: An on-board weapon guidance system (10) comprising : a laser light source (30),and means for detecting radiation (12, 26) proceeding from a target to guide the weapon to the target, characterizing in further comprising: means for switching (20) between : i) the detection of radiation originating from a source independent of the weapon and proceeding from the target, and ii) the detection of laser radiation originating from the laser light source (30) and reflected from the target; wherein the switching means (20) switches to the detection of reflected laser radiation originating from the laser light source (30) if the detected radiation falls below a predetermined level. The system (10) of claim 1, characterized in that the means for detecting (12, 26) comprises an optical assemply (12, 26) adapted to redirect the radiation to one of a quadrant detector (18) or a LADAR receiver (28). The system (10) of claim 1, characterized in that the switching means (20) switches from the detection of radiation originating from the independent source to the detection of reflected laser radiation originating from the laser light source (30) if the detected radiation originating from the independent source falls below a predetermined level. The system (10) of claim 1, characterized in further comprising means (10) for scanning a field with at least one laser beam originating from the laser light source (30). The system (10) of claim 4, characterized in that the scanning means (10) is adapted to generate an image from the scanned field. The system (10) of claim 5, characterized in that the generated image is a three dimensional image. The system (10) of claim 6, characterized in that the generated image is used to identify a target within the field. The system (10) of claim 1, characterized in that the means for switching (20) comprises a moveable mirror (20). The system (10) of claim 8, characterized in further comprising a sensor (24) adapted to determine the angular position of the mirror (20) about an axis. The system (10) of claim 1, characterized in that the switching means (20) comprises a mirror (20) adapted to redirect the receive radiation to different portions (12, 26) of the detecting means (12, 26). The system (10) of claim 1, characterized in that the detecting means (18, 26) includes: a first optical assembly (12) adapted to receive radiation generated by a source independent of the seeker; and a second optical assembly (26) adapted to receive laser radiation generated by the laser light source (30) and reflected from the target. The system (10) of claim 1, characterized in that the radiation generated by a source independent of the seeker is laser radiation. The system (10) of claim 10, characterized in that the mirror (20) redirects all of the received radiation to the first optical assembly (12) of the second optical assembly (26). The system (20) of claim 10, characterized in that the mirror (20) is moveable from a first position to a second position. The system (10) of claim 14, characterized in further comprising a motor (22) adapted to move the mirror (20) between the first and second positions. The system (10) of claim 15, characterized in that the mirror (20) is adapted to scan a field with at least one laser beam originating from the laser light source (30) when the mirror (20) is in the second position. The system (10) of claim 15, characterized in that the mirror (20) is held in the first position by a mechanical stop (32) and a force applied by the motor (22). The system (10) of claim 14, characterized in that the mirror (20) redirects the received radiation to the first optical assembly (12) when the mirror (20) is in the first position. The system (10) of claim 14, characterized in that the mirror (20) redirects the received radiation to the second optical assembly (26) when the mirror (20) is in the second postion. A method (100) for guiding a weapon to a target comprising: receiving (105) radiation from the target, and tracking (110) the radiation to guide the weapon to the target, characterized in that the method further includes monitoring (115) the radiation such that if the radiation falls below a predetermined level a laser system (10) on-board the weapon continues guiding the weapon comprising: (i) generating (120) a laser beam; (ii) reflecting (125) the laser beam off the target; (iii) receiving (130) laser radiation reflected from the target; and (iv) tracking (135) the radiation to guide the weapon to the target. The method (100) as set forth in claim 20, characterized in that receiving (105) radiation from the target (110) includes receiving (105) laser radiation from the target (110). The method (100) as set forth in claim 20, characterized in that tracking (110) the radiation comprises tracking an image of the target generated from the received laser radiation. The method (100) as set forth in claim 22, characterized in further comprising identifying the target class from the generated image. The method (100) as set forth in claim 20, characterized in further comprising generating an image of the target from the received laser radiation. The method (100) as set forth in claim 20, characterized in that receiving (105) radiation from the target comprises receiving (105) radiation from the target generated from a source independent of the weapon. The method (100) as set forth in claim 20, characterized in that generating (120) the laser beam comprises generating the laser beam from a source (30) on-board the weapon. The method (100) as set forth in claim 20, characterized in that receiving (105) radiation from the target comprises receiving (105) radiation from the target generated from a source on-board the weapon. The method (100) as set forth in claim 20, characterized in that generating (120) the laser beam comprises generating the laser beam from a source independent of the weapon.

Inventors: MINOR LEWIS G, MINOR, LEWIS G.
Applicants: LOCKHEED CORP, LOCKHEED MARTIN CORPORATION
Application year: 2000
Main classification: F41G7
IPC codes: F41G7