Abstract:
A dual mode LADAR/SAL apparatus is disclosed. In one aspect, the apparatus includes a gimbal capable of scanning in azimuth and in elevation and a sensor mounted on the gimbal. The sensor includes a telescope, a LADAR optical path, and a SAL optical path. The telescope defines an exit pupil through which the sensor collects reflected light. The sensor further includes means for directing light received through the telescope to the LADAR optical path in a first position and to the SAL optical path in a second position such that the SAL optical path receives the collected light from the exit pupil of the telescope. The means may be, for example, a mirror. In a second aspect, an apparatus includes a gimbal capable of scanning in azimuth and in elevation, and a sensor mounted on the gimbal. The sensor includes a telescope defining an exit pupil through which the sensor collects reflected light. The sensor is capable of operating in a LADAR mode and in a SAL mode and receiving collected light in the SAL mode through a limited portion of the exit pupil.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention pertains to a laser-based system and, more particularly, to a laser-based system with laser detection and ranging (“LADAR”) and semi-active laser (“SAL”) system capabilities. 
   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 and identify objects, frequently referred to as “targets,” in a “field of regard.” A common problem in military operations, for example, is to detect and identify targets, such as tanks, vehicles, guns, and similar items, which have been camouflaged or which are operating at night or in foggy weather. It is important in many instances to be able to distinguish reliably between enemy and friendly forces. As the pace of battlefield operations increases, so does the need for quick and accurate identification of potential targets as friend or foe and as a target or not. 
   Remote sensing techniques for identifying targets have existed for many years. For instance, in World War II, the British developed and utilized radio detection and ranging (“RADAR”) systems for identifying the incoming planes of the German Luftwaffe. RADAR uses radio waves to locate objects at great distances even in bad weather or in total darkness. 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. 
   One such alternative technology is laser detection and ranging (“LADAR”). Similar to RADAR systems, which transmit and receive radio waves to and reflected from objects, LADAR systems transmit laser beams and receive reflections from targets. Because of the short wavelengths associated with laser beam transmissions, LADAR data exhibits much greater resolution than RADAR data. Typically, a LADAR system creates a three-dimensional (“3-D”) image in which each datum, or “pixel”, comprises an (x,y) coordinate and associated range for the point of reflection. 
   Laser energy also finds application in these kinds of environments in what is known as a semi-active laser (“SAL”) system. 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. 
   However, LADAR and SAL technologies typically are not deployed together. For one thing, the LADAR signal, its generation, and its transmission usually are not suitable for target designation, or “spotting.” U.S. Pat. No. 6,262,800, entitled “Dual mode semi-active laser/laser radar seeker”, issued Jul. 17, 2001, to Lockheed Martin Corporation as assignee of the inventor Lewis G. Minor documents one effort at combining the two technologies. In this patent, the LADAR transceiver is modified to be used as a SAL receiver as well as a LADAR receiver. However, the sensor disclosed and claimed therein still includes no on-board designator such that it must rely on a third party designator in the same manner as conventional SAL systems. Furthermore, this design alters the LADAR optical design and structure. It also has the warm telescope supports in its field of view (“FOV”) and it requires precise positioning of the scan mirror when used in the SAL mode. Thus, there exists a need for a sensor which can function as a LADAR and also home on a designator. 
   The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above. 
   SUMMARY OF THE INVENTION 
   The invention, in its various aspects and embodiments, includes a dual mode LADAR/SAL apparatus. In one aspect, the apparatus comprises a gimbal capable of scanning in azimuth and in elevation and a sensor mounted on the gimbal. The sensor includes a telescope, a LADAR optical path, and a SAL optical path. The telescope defines an exit pupil through which the sensor collects reflected light. The sensor further includes means for directing light received through the telescope to the LADAR optical path in a first position and to the SAL optical path in a second position such that the SAL optical path receives the collected light from the exit pupil of the telescope. The means may be, for example, a mirror. In a second aspect, an apparatus comprises a gimbal capable of scanning in azimuth and in elevation, and a sensor mounted on the gimbal. The sensor includes a telescope defining an exit pupil through which the sensor collects reflected light. The sensor is capable of operating in a LADAR mode and in a SAL mode and receiving collected light in the SAL mode through a limited portion of the exit pupil. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
       FIG. 1  illustrates a laser-based system in one particular embodiment constructed and operated in accordance with the present invention in an assembled view; 
       FIG. 2  illustrates the laser-based system of  FIG. 1  in an exploded view; 
       FIG. 3  shows the laser-based sensor of  FIG. 1-FIG .  2  in greater detail; 
       FIG. 4  is an exploded view of several components of an optical train of one particular embodiment of the sensor in the LADAR system of  FIG. 1-FIG .  2 ; 
       FIG. 5A-FIG .  5 C illustrates the on-gimbal laser designator of the laser-based system of  FIG. 1-FIG .  2 , first shown in  FIG. 4 , from different perspectives; 
       FIG. 6A-FIG .  6 C depict the LADAR system of  FIG. 1-FIG .  2  in operation in a lookdown and loitering scenario; 
       FIG. 7A-FIG .  7 B illustrate in a cross section and a plan view, respectively, the sensor of  FIG. 1  with a scan mirror in the LADAR position for LADAR operations; and 
       FIG. 8A-FIG .  8 B illustrate in a cross section and a plan view, respectively, the sensor of  FIG. 1  with the scan mirror in the SAL position for SAL operations. 
   

   While the invention is susceptible to various modifications and alternative forms, the drawings illustrate specific embodiments herein described in detail by way of example. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort, even if complex and time-consuming, would be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     FIG. 1  and  FIG. 2  illustrate a laser based system  100  in one particular embodiment constructed and operated in accordance with the present invention in assembled and exploded views, respectively. In general, the laser based system  100  includes a sensor  103  mounted in a gimbal ring  106 . The assembled sensor  103  and gimbal ring  106  are housed in a chamber  109 , as shown in  FIG. 1 , defined by a forward end  112  of a platform  115 . In the illustrated embodiment, the platform  115  is an aerial vehicle, and more particularly a missile or an airborne guided submunition, but this is not necessary to the practice of the invention. 
   The platform  115  includes a faceted window  118  that closes the chamber  109 , as will be discussed further below. The faceted window  118  provides a wide Field of Regard (“FOR”). It also protects the sensor  103  and gimbal ring  106  from environmental conditions and, in this particular embodiment, aerodynamic forces. The faceted window  118  also contributes to the aerodynamic performance of the platform  115  as a whole, as will be recognized by those skilled in the art having the benefit of this disclosure. Note that the fuselage of the forward end  112  is shaped to match the faceting of the window  118 . This also is not necessary to the practice of the invention, but enhances the aerodynamic performance of the platform  115  in this particular embodiment. 
   Still referring to  FIG. 1-FIG .  2 , the flat window segments  121  (six of which are shown in  FIG. 2 , but only one of which is indicated) of the faceted window  118  provide a wide FOR. The window segments  121  are fabricated from a material that transmits the LADAR signal but can also withstand applicable environmental conditions. In the illustrated embodiment, one important environmental condition is aerodynamic heating due to the velocity of the platform  115 . Another important environmental condition for the illustrated embodiment is abrasion, such as that caused by dust or sand impacting the window  118  at a high velocity. Thus, for the illustrated embodiment, BK-7 glass is a highly desirable material, but alternative embodiments may employ fused silica. ZnSe, Al 2 O 3 , Ge, and Pyrex. 
   Using the flat window segments  121  rather than a spherical dome (not shown) also reduces the cost of the window  118 , allows wide azimuth angles, and allows more freedom in the placement of the gimbal trunions  200 . There is no significant degradation on image quality provided the window facets  121  do not have any wedge angle between their surfaces. However, the faceted window  118  increases the overall length of the front end  112 , has more aerodynamic drag and flow asymmetry, and requires seams. It also has the potential for reflection losses if the output beam meets any window surface at near grazing incidence. 
   Note, however, that the faceted window  118  is not necessary to the practice of the invention in all embodiments. Alternative embodiments may instead employ, for instance, a single conventional, spherical hypersphere (not shown) or spherical segments (also not shown) if the aerodynamic requirements for a given application are sufficiently important. Alternatively, one compromise uses a spherical segment in front and one or two others out at right angles to the missile axis. If tone one side is domed, loitering must be down in the direction that places that segment towards the ground. Thus, the window  118  may also be spherical or spherically segmented in alternative embodiments. 
     FIG. 3  illustrates the gimbaled sensor  103  in greater detail. The sensor  103  implements both a LADAR capability and a SAL capability. The LADAR side of the sensor  103  is a variation on the LADAR sensor disclosed and claimed in U.S. Pat. No. 5,224,109, entitled “Laser Radar Transceiver,” on Apr. Jun. 29, 1993, to LTV Missiles and Electronics Group as assignee of the inventors Nicholas J. Krasutsky et al. (‘the &#39;109 patent). LADAR sensors similar to that in the &#39;109 patent are also disclosed in:
         (i) U.S. Pat. No. 5,200,606, entitled “Laser Radar Scanning System,” on Apr. 6, 1993, to LTV Missiles and Electronics Group as assignee of the inventors Nicholas J. Krasutsky et al. (“the &#39;606 patent); and   (ii) U.S. Pat. No. 5,285,461, entitled “Improved Laser Radar Transceiver,” on Feb. 8, 1994, to Loral Vought Systems Corporation as assignee of the inventors Nicholas J. Krasutsky et al.
 
These patents are now commonly assigned herewith. However, as will be described more fully below, the laser for the LADAR functionality of the sensor  103  has been moved off the gimbal and the optical train on the receive side has been adapted for use with the SAL capability.
       
   Also, the SAL designator  309  has been added on-gimbal. Some embodiments may locate the lasers for both the LADAR side and the SAL side on-gimbal, but moving one off-gimbal simplifies the packaging. Off-gimbal laser configurations have been used in gimbaled system in the past but they generally used complicated mirror configurations to maintain alignment between the transmit and receive paths. See, e.g., the &#39;109 patent and other patents cited above. However, recent developments in Large Mode Area (“LMA”) optical fibers have allowed high peak powers to be transmitted while maintaining good beam optical quality. These fibers can emit directly as part of a fiber laser or amplifier, alternatively, they can be used to transmit the output from any laser up to the gimbaled platform. 
     FIG. 3  shows the sensor  103  mounted to the gimbal ring  106 . As is best shown in  FIG. 2 , the sensor  103  includes a pair of trunions  200  (only one shown) that are rotatably mounted within a pair of bores  203  (only one shown) in the gimbal ring  106 . The bores  203  include mechanical assemblies such as bearings, bushing, etc. (not shown) to facilitate rotation of the trunions  200  in the bores  203  in a manner known to the art. The LADAR sensor  103  is a variant of the sensor described in the &#39;109 patent referenced above and employs an optical train similar to that described above relative to  FIG. 3 . A servo-drive motor  204  drives the sensor  103  through the trunions  200  to scan the sensor in elevation. In the illustrated embodiment, the sensor  103  is scanned in elevation approximately ±30° relative to the axis  206  defined by the trunions  200  and shown in  FIG. 2  in broken lines. However, the amount of elevational scan is implementation specific and may differ in alternative embodiments. 
   Returning to  FIG. 3 , the sensor  103  is mounted through the gimbal ring  106  from the top  301  and bottom  302  so that extended travel and scanning in azimuth is possible. Note that “top” and “bottom” are defined relative to the nominal orientation of the platform  115  relative to the Earth&#39;s field of gravity or the ground surface. As the platform  115  changes this orientation, so, too, will the orientation of the “top”  301  and “bottom”  302  relative to these references. The sensor  103  is mounted through the gimbal ring  106  using a trunion/bore approach and bearing/bushing approach similar to that described immediately above and as is conventional in the art. The sensor  103  and gimbal ring  106  are driven in azimuth by servo-motors  305  about an axis  303  shown in  FIG. 3  in broken lines. The sensor  103  is driven in elevation by the servo motor  204  about an axis  206  shown in  FIG. 2  in broken lines. 
   The position of the gimbal in elevation and azimuth is measured by position sensing devices located on the opposite sides of the gimbal ring across from each of the servomotors  204  and  305 . The azimuthal position sensor  301  is shown in  FIG. 3  along with the corresponding azimuthal gimbal servo-motor  305 . Position can be sensed by a number of devices including potentiometers, electrical encoders and optical encoders, or other techniques known to the art, with the preferred method being optical encoders. 
   In the illustrated embodiment, the gimbaled sensor  103  is capable of scanning in azimuth substantially past 180°. In the illustrated embodiment, the goal is a full 210° scan and the term “substantially” is a recognition that sometimes manufacturing variances or tolerances or sometimes operational conditions impair achievement of a full 210° azimuthal scan. The illustrated embodiment achieves the 210° scan by scanning ±105° from the boresight  306 , or longitudinal axis of the platform  115 , shown in broken lines in  FIG. 3 . Note, however, that alternative embodiments might employ alternative gimbaling techniques and any suitable gimbaling technique known to the art may be employed. 
   Turning now to  FIG. 4A , selected portions of the optics  400  in one particular embodiment of the sensor  103  are shown in an exploded view. A gallium aluminum arsenide (“GaAlAs”) laser  430  pumps a solid state laser  432 . The solid state laser  432  emits the laser light energy employed for illuminating the target. The GaAlAs pumping laser  430  produces a continuous signal of wavelengths suitable for pumping the solid state laser  432 , e.g., in the crystal absorption bandwidth. Pumping laser  430  has an output power, suitably in the 10-20 watt range, sufficient to actuate the solid state laser  432 . 
   The pumping laser  430  and the solid state laser  432  are fixedly mounted on the housing of the forward end  112 . The output of the solid state laser  432  is transported to the gimbal by means of a high power optical fiber  433 . Since the solid state laser  432  is fiber-coupled to the gimbal, many laser types can be used, e.g., side pumped lasers and fiber lasers, provided they can be coupled into the fiber. In the case of fiber lasers it is also possible to use the lasing fiber directly to connect to the sensor head. Thus, alternative embodiments may use lasers other than solid state lasers. Output signals from the high power optical fiber  433  are transmitted through a beam input lens  431  and a fiber optic bundle  434 . The fiber optic bundle  434  has sufficient flexibility to permit scanning movement of the laser based system  100  during operation as described below. 
   Still referring to  FIG. 4 , the solid state laser  432  is suitably a Neodymium (“Nd”) doped yttrium aluminum garnet (“YAG”), a yttrium lithium fluoride (“YLF”), or a Nd:YVO 4  laser. The solid state laser  432  is operable to produce, in this particular embodiment, pulses with widths of 10 to 20 nanoseconds, peak power levels of approximately 10 kilowatts, at repetition rates of 10-120 kHz. The equivalent average power is in the range of 1 to 4 watts. The preferred range of wavelengths of the output radiation is in the near infrared range, e.g., 1.047 or 1.064 microns. 
   The output generated by solid state laser  432 , in the present embodiment, is carried to the gimbaled head by the high power fiber  433 , as mentioned above. The high power fiber  433  has sufficient flexibility to permit scanning movement of the laser based system  100  during operation as described below. The output end of the high power fiber  433  is mounted on the gibaled head so that the laser beam emerging from it passes through the beam expander  440 . The beam expander  440  comprises a series of (negative and positive) lenses which are adapted to expand the diameter of the beam to provide an expanded beam  442 , suitably by an 8:1 ratio, while decreasing the divergence of the beam. 
   The expanded beam  442  is next passed through a beam segmenter  444  for dividing the beam into a plurality of beam segments  446  arrayed on a common plane, initially overlapping, and diverging in a fan shaped array. The divergence of the segmented beams  446  is not so great as to produce separation of the beams within the laser based system  100 , but preferably is sufficiently great to provide a small degree of separation at the target, as the fan-shaped beam array is scanned back and forth over the target (as will be described below with reference to output beam segments  448 ). Beam segmentation can be accomplished by using a series of calcite wedges, a holographic diffraction grating or a phased diffraction grating. The preferred method is using a phased diffraction grating because of its predictable performance and power handling capability. 
   As shown in  FIG. 4 , the resultant segmented beams  446  are then reflected from a third turning mirror  454 , passed through an aperture  456  of an apertured mirror  458 , and subsequently reflected from a scanning mirror  460  in a forward direction relative to the platform  115 . The aperture  456  is located off the center of the aperture mirror  458 . The scanning mirror  460  is pivotally driven by a scanning drive motor  462 , which is operable to cyclically scan the beam segments  446  for scanning the target area. In a preferred embodiment, the beam segments  446  are preferably scanned at a rate of approximately 100 Hz. The turning axis of the scanning drive motor  462  is aligned in parallel with the segmenter  444  axis whereby the resultant beam array  446  is scanned perpendicularly to the plane in which the beams are aligned. 
   An afocal, Cassegrainian telescope  462  is provided for further expanding an emitted beam  464  and reducing its divergence. The telescope  462  includes a forward-facing primary mirror  466  and a rear-facing secondary mirror  468 . A lens structure  472  is mounted in coaxial alignment between the primary mirror  466  and the scanning mirror  460 , and an aperture  474  is formed centrally through the primary mirror in alignment with the lens structure. 
   The transmitted beams which are reflected from the scanning mirror are directed through the lens structure  472  for beam shaping, subsequently directed through the aperture  474  formed centrally through the primary mirror, and subsequently reflected from the secondary mirror  468  spaced forwardly of the primary mirror and is then reflected from the front surface of the primary mirror  466 . The resultant transmitted beam  476 , is a fan shaped array which is scanned about an axis parallel to its plane. The beam array  478  illustrates the diverged spacing of the beam segments as they reach the target, wherein the beams are in side-by-side orientation, mutually spaced by a center-to-center distance of twice their diameters. 
   The telescope  462  receives laser energy reflected from a target that has been illuminated by the array of transmitted beams. This received energy is then reflected successively through the primary mirror  466  and the secondary mirror  468 , the lens assembly  472 , and the scanning mirror  460 , toward the apertured mirror  458 . Because the reflected beam is of substantially larger cross-sectional area than the transmitted beam, it is incident upon the entire reflecting surface of the apertured mirror  458 , and substantially all of its energy is thus reflected laterally by the apertured mirror  458  toward collection optics  480 . 
   The collection optics  480  includes a narrow band filter  482 , for filtering out wavelengths of light above and below a desired laser wavelength to reduce background interference from ambient light. The beam then passes through condensing optics  484  to focus the beam. The beam next strikes a fourth turning mirror  86  toward a focusing lens structure  488  adopted to focus the beam upon the receiving ends  490  of a light collection fiber optic bundle  492 . The opposite ends of each optical fiber  492  are connected to illuminate a set of diodes  494  in a detector array, whereby the laser light signals are converted to electrical signals which are conducted to a processing and control circuit (not shown). 
   The fiber optic bundle  492  preferably includes nine fibers  493  (only one indicated), eight of which are used for respectively receiving laser light corresponding to respective transmitted beam segments and one of which views scattered light from the transmitted pulse to provide a timing start pulse. Accordingly, the input ends  490  of the fibers  492  are mounted in linear alignment along an axis which is perpendicular to the optical axis. The respective voltage outputs of the detectors  494  thus correspond to the intensity of the laser radiation reflected from mutually parallel linear segments of the target area which is parallel to the direction of scan. 
   However, this is not necessary to the practice of the invention in all embodiments. One intended purpose of the present invention is application in a lookdown and loitering mode, as is discussed further below relative to  FIG. 6A-FIG .  6 C. Thus, all that is required is that the gimbaled receiver  103  be able to scan sufficiently far in azimuth to one side of the platform  115  so as to enable this functionality. An embodiment capable of scanning a full 210° by scanning ±105° off boresight is more versatile. However, this functionality can be achieved by scanning off to only one side 90° off boresight. In general, any given embodiment should be able to scan at least 90° off boresight to at least one side of the platform  115 . 
   Referring again to  FIG. 4 , in the illustrated embodiment, the LADAR transmitter has been moved off the gimbal and its output is coupled to the sensor head  103  by means of an optical fiber  433 . This simplifies the packaging of the sensor  103 . Off-gimbal laser configurations have been used in gimbaled systems in the past but they generally used complicated mirror configurations to maintain alignment between the transmit and receive paths. Recent developments in Large Mode Area (“LMA”) optical fibers have allowed high peak powers to be transmitted while maintaining good beam optical quality. These fibers can emit directly as part of a fiber laser or amplifier, alternatively, they can be used to transmit the output from any laser up to the gimbaled platform. 
   The laser based system  100  will also include electronic circuitry (not shown) for generating the scan signals that drive the servo-motors, laser, detectors, and scanning drive motor and to capture the information in the detected signals. Scan signal generation can be performed by first using the scanning drive motor  462  to drive the scan mirror  360  in elevation. This produces multiple rows of pulses as shown in  FIG. 6B . Scanning the entire sensor in azimuth using the servo motor  305 , shown in  FIG. 3 , then produces a scan of the target area. Suitable information capture and processing techniques are disclosed in:
         (i) U.S. Pat. No. 6,115,113, entitled “Method for Increasing Single-Pulse Range Resolution,” on Sep. 5, 2000, to Lockheed Martin Corporation as assignee of the inventor Stuart W. Flockencier;   (ii) U.S. Pat. No. 5,243,553, entitled “Gate Array Pulse Capture Device,” on Sep. 7, 1993, to Loral Vought Systems Corporation as assignee of the inventor Stuart W. Flockencier.
 
Both of these patents are commonly assigned herewith. Note, however, that any suitable technique known to the art may be employed.
       

   The electronic circuitry and detection electronics are fixedly mounted relative to the housing or other suitable supporting structure aboard the platform  115 . The scanning and azimuth translations of the laser based system  100  therefore do not affect corresponding movement of the detection system. Accordingly, the mass of the components which are translated during scanning is substantially lower than would be the case if all components were gimbal-mounted. These benefits are amplified in the case of the embodiment shown in  FIG. 3  since the laser is also off-gimbal. 
   Since the laser based system  100  is capable of looking out at over ±90° to both sides of the platform  115 , it can be used over a wide swath as the platform  115  moves through its environment. Consider  FIG. 6A , which shows the potential for target examination out to the range  600  of the laser based system  100  on both sides of the flight path  603 , shown in broken lines. The surveillance area  606  includes the area  609  that has already been reconnoitered and the area  612  currently under surveillance. The area  612  currently under surveillance is determined by the position of the platform  115 , the range  600  of the laser based system  100 , and the extent of the azimuthal scan of the laser based system  100 . 
   The operation of the gimbaled LADAR sensor  100  in scanning is conceptually illustrated in  FIG. 6B . The gimbaled LADAR sensor  100  transmits the LADAR signal  605  to scan the area  612 . Each scan is generated by scanning elevationally, or vertically, several times while scanning azimuthally, or horizontally, once within the FOR.  FIG. 6B  illustrates a single elevational scan  607  during the azimuthal scan  608 . Thus, each scan is defined by a plurality of elevational scans such as the elevational scan  607  and the azimuthal scan  608 . The velocity, depression angle of the sensor  103  with respect to the horizon, and total azimuth scan angle of the LADAR platform  115  determine the extent of the scan. 
   The LADAR signal  605  is typically a pulsed signal and may be either a single beam or a split beam. Because of many inherent performance advantages, split beam laser signals are typically employed by most LADAR systems. A single beam may be split into several beamlets spaced apart from one another by an amount determined by the optics package (not shown) aboard the platform  115  transmitting the LADAR signal  605 . Each pulse of the single beam is split, and so the LADAR signal  605  transmitted during the elevational scan  607  in  FIG. 6B  is actually, in the illustrated embodiment, a series  611  of grouped beamlets  613  (only one indicated). The gimbaled LADAR sensor  103  transmits the LADAR signal  605  while scanning elevationally  607  and azimuthally  608 . The LADAR signal  605  is continuously reflected back to the platform  115 , where it is detected and captured. 
   The characteristics of the LADAR signal  605  will be a function of the LADAR sensor  103 , which will, in turn, be a function of the mission in a manner known to the art. The LADAR sensor  300 , shown in  FIG. 3A-FIG .  3 B, splits a single 0.2 mRad l/e 2  laser pulse into septets with a laser beam divergence for each spot of 0.2 mRad with beam separations of 0.4 mRad. The optics package includes fiber optical array (not shown) having a row of seven fibers spaced apart to collect the return light. The fibers have an acceptance angle of 0.3 mRad and a spacing between fibers that matches the 0.4 mRad far field beam separation. An elevation scanner (not shown) spreads the septets vertically by 0.4 mRad as it produces the vertical scan angle. The optical transceiver including the scanner is then scanned azimuthally to create a full scan raster. 
   Assume the laser based system  100  identifies the target  610  as an object of interest, and wishes to continue observing the object. As is shown in  FIG. 6C , the platform  115  flies a circular loiter pattern  617  over the target area  615 , including the current surveillance area  612 . In the illustrated embodiment, the loiter pattern  617  is in a clockwise direction, but could alternatively be counterclockwise. The laser based system  100  can then look out to the side and examine a portion  618 , the constant track and surveillance area, of the area  612  being circled. If the platform  115  flew level, the loitering radius for the loiter pattern  617  would need to be large enough to allow the laser based system  100  look down to see the ground at the maximum gimbal lookdown angle. If, however, bank-to-turn guidance is used, the platform  115  will bank into the turn, providing the sensor with additional lookdown capability. 
   The bank angle θ of the platform  115 , shown in  FIG. 6C , is a function of the turn radius and the velocity of the platform  115 . For highly maneuverable platforms, the bank angle Θ can exceed 60°. The banking of the platform  115  rotates the laser based system  100  and provides additional down-look capability for the seeker relative to the ground. Depending on the bank angle θ, the laser based system  100  could look straight down or even past vertical. This is evident from the indicated coverage cone  621  in  FIG. 6C . 
   More particularly,  FIG. 6C  shows two areas  612 ,  618  on the ground below the flight path  603 . The area  618  shows the portion of the ground which is always visible to the laser based system  100 , regardless of the position of the platform  115  along its flight path  603 . The area  612  is the additional area which can be seen by the laser based system  100 , depending on the position of the platform  115  along its flight path  603 . The circle  621  drawn on the ground below the loiter pattern  617  of the flight path  603  shows the line where the laser based system  100  is looking straight down. If the radius of the loiter pattern  617  is comparable to or smaller than the altitude  624  of the platform  115  much of the area  618  is viewed at steep angles to the ground. This facilitates use in urban or forested target areas where terrain masking is a problem for sensors working at shallow depression angles. 
   In the illustrated embodiment, the altitude  624  is approximately 300 m, the diameter of the loiter pattern  617  is approximately 2 km, the diameter of the area  618  is 1.2 km, and the track window of the target  609  is 200 m×200 m. Note, however, that these dimensions are implementation specific, and that other embodiments might operate with different dimensions. Thus, these dimensions are not material to the practice of the invention. 
   Returning now to  FIG. 3 , the illustrated embodiment also includes an on-gimbal laser designator  309  that provides a laser designator mode of operation. The laser designator  309  and associated turning prism  310  are better illustrated in  FIG. 5A-FIG .  5 C. More particularly, the laser designator  309  produces a pulsed laser beam  500  that may be used for target designation.  FIG. 5A-FIG .  5 C illustrate the emission of the pulsed beam  500  from the laser designator  309  through the turning prism  310  within the chamber  109  and behind the window  118 . Note that it is possible to use the LADAR transmitter for designation but, since the power and beam characteristics normally required for designation are different from those required for LADAR operation, the laser design will be an undesirable compromise between the two requirements. The designator optics can be strap-down, as in the illustrated embodiment, or equipped with scanning mechanisms (not shown). 
   More particularly, the laser designator  309  is located on the sensor  103  and generates a laser beam  500 . The laser beam  500  is directed off the sensor  103  by the turning prism  310  in a direction parallel to the optical axis  306  of the telescope  503 . Referring now to  FIG. 7A  and  FIG. 7B , the sensor  103  includes a scan mirror  703  that may be moved between two positions, one for use in LADAR operation and one for use in SAL, or designation, operations. The scanning mirror  703  is shown in the LADAR position in  FIG. 7A . The scanning mirror  703  is mounted to and moved by elevation scanner motor  806 , shown in  FIG. 8B . 
   When the sensor  103  is being used in the LADAR mode, light from the LADAR laser is directed into the far field and falls on the target area as discussed relative to  FIG. 6A-FIG .  6 C. Scattered light  701  from the target area is collected by the telescope  503  which directs it onto the elevation scan mirror  703 . The scattered light  701  is then directed upward through the optical train  704  by the elevation scan mirror  703 , and is focused onto the LADAR detector fiber array  705 . The high speed scanner rotates the elevation scan mirror  703  through a small angle center around 45°. This provides the fiber array  705  with a view of the target scene at different elevations. The external gimbal  106  is used to provide stabilization and to scan the sensor  103  in azimuth so the entire target area can be examined by the LADAR and a three dimensional scene image can be formed. 
     FIG. 8A  and  FIG. 8B  show the sensor  103  when it is being used in the SAL mode. Moving from the LADAR mode to the SAL mode is accomplished by flipping the elevation scan mirror  703  of the telescope optical path. The final position of the scanning mirror  703 , as shown in  FIG. 8A  and  FIG. 8B , is not critical as long as it is out of the way of the SAL detector optical aperture  804  so that the SAL detector  802  has a clear view through the telescope  503 . It is assumed that the target is being designated by a source external to the platform  115  in this particular embodiment. Scattered light  701  coming from the target falls on the telescope  503 . The SAL detector  802  does not utilize all of the light falling on the telescope  503 , but rather, only light  801  which falls on the shaded area  803  shown in  FIG. 8A . The SAL detector input aperture  804  is placed at the exit pupil of the telescope  503  and the shaded area  803  represents the portion if the telescope  503  input aperture subtended by the SAL detector aperture  804  at the entrance to the telescope  503 . 
   Since SAL mode detector and optics are located at the exit pupil of the sensor telescope  503 , the SAL optics have access to the entire angular field of regard of the telescope  503  but utilize only a specific, unmasked portion  803  of the telescope  503  input aperture for light collection. This allows the SAL mode to use the optical magnification of the telescope  503  while having an optical path which is unobstructed by the telescope  503  secondary supports  709 . The tradeoff is that only a portion of the entire telescope  503  aperture is used by the SAL detector. This limits the effective range of that mode but it preserves linearity and limits noise induced by the telescope  503  supports  709 . The SAL sensor range should still be adequate for most missile applications, especially where lock-on before launch capability is not required. The small SAL mode optics make packaging easier and lower system cost. Both of these benefits are significant in small missile applications. 
   The scanning mirrors currently used in most LADARs are driven by placing them on a motor shaft. The motor controller then moves the mirror through the desired pattern needed for LADAR operation. These are usually high torque motors and moving them through large angles can be difficult because it involves moving across different motor windings where the available torque is limited. While the mirror  703  is being flipped from the LADAR position to the SAL position and back, neither mode is operational so the mirror can be driven open loop through the low torque region using the rotor and mirror inertia. Alternatively, a small set of secondary windings can be used to aid in the transition. Scanning mirrors can be controlled in a number of ways the specific method is not important, only the fact that it is used as part of the optical train in the LADAR mode and is moved out of the way for the SAL mode. 
   Moving back to the LADAR mode is accomplished in a similar fashion by flipping the scanning mirror  703  back to the position shown in  FIG. 7A  and  FIG. 7B  so that it can be used to direct the light into the LADAR detectors. Moving back and forth between the two modes can be done as often as the operational scenario requires, but the two modes cannot be used simultaneously. 
   The laser based system  100  can be used to locate and track the targets, e.g. the target  610  in  FIG. 6A , and the coordinate information passed to the laser designator  309  in a number of ways. For instance, coordinate information may be passed as the coordinates of the target  610 , derived from Global Positioning System (“GPS”) coordinates platform  115  or as a targeting direction using an inertial measurement unit (“IMU”) aboard the platform  115 . In the illustrated embodiment, the LADAR sensor  103  and the designator  609  are aligned to allow pointing and targeting information to be shared directly between the two. Thus, the laser based system  100  can be operated in LADAR mode as illustrated in  FIG. 6A  to locate the target  610 . The laser based system  100  will yield three-dimensional data describing the location of the target  610 , which can then be passed to the control of the laser designator  309 . This information can then be used to designate the target  610 . 
   If the laser designator  309  and sensor  103  wavelengths are different, both can be operated simultaneously. If the laser designator  309  and sensor  103  are at the same wavelength, then the laser designator  309  might interfere with the LADAR operation of the sensor  103  when it is actively pulsing. This can be easily addressed because the duty cycle of the laser designator  309  is very low so the LADAR detectors (not shown) can be turned off during the designation pulse without significant loss in imaging capability. 
   The LADAR detectors can even be gated to pick up the return from the designation beam  500  so that the position of the designation beam  500  relative to the LADAR target image can be determined. This is an accurate way to maintain alignment between the two modes if the laser designator  309  has its own on-board steering mechanism. As the laser based system  100  loiters, the laser designator  309  can maintain a spot on the target  610  as long as the target  610  remains in area  618  of  FIG. 6A . Illuminating the top of the target  610  would prevent masking of the designator spot as the platform  115  executes its flight pattern. Alternatively, a nearby spot could be designated and the relative coordinates passed on for further use. 
   This concludes the detailed description. The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.