Patent Publication Number: US-6665063-B2

Title: Distributed laser obstacle awareness system

Description:
This application is a continuation-in-part of the following pending patent applications which include a common specification and drawings: 
     U.S. patent application Ser. No. 09/946,057, now Pat. No. 6,556,282; entitled “Combined Loas and Lidar System”; 
     U.S. patent application Ser. No. 09/946,058; entitled “Wide Field Scanning Laser Obstacle Awareness System”; and 
     U.S. patent application Ser. No. 09/946,048, now Pat. No. 6,542,227; entitled “System and Method Of Measuring Flow Velocity In Three Axes”, all of which being filed on Sep. 4, 2001 and assigned to the same assignee as the instant application. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is directed to aircraft obstacle awareness systems, in general, and more particularly, to a distributed laser obstacle awareness system for use on an aircraft for the detection of ground and air based obstacles. 
     A common flight hazard of any aircraft operating near the Earth is the potential for collision with ground structures and obstacles. Helicopters, in particular, and now new classes of aircraft known as unmanned air vehicles (UAVs), often operate less than five hundred feet above ground level (AGL). In this environment, it is not uncommon for these aircraft to collide with electrical power lines, support wires for radio towers, or various structures and obstacles. These collisions typically result in loss of life, significant aircraft damage, damage to the structures or obstacles themselves, subsequent loss of power distribution on the electrical grid, and danger to persons and property on the ground. Aircraft, such as helicopters and UAVs, for example, typically operate in these low altitudes for take-off and landing, various low-level military maneuvers, and commercial applications, such as electrical utility inspection or emergency rescue missions. 
     Inspecting electrical power lines from an aircraft requires flying close to the Earth along high tension power lines and support structures looking for damaged equipment. Use of helicopters permit electric utility inspection crews to cover a large area of the power grids over a short period of time. Other helicopter applications which require low flying flight profiles include emergency and rescue missions, medical emergencies, border surveillance, and supply of floating oil platforms, for example. Likewise, UAV applications require autonomous control for surveillance, take-off, landing and delivery of munitions. In all of these applications, the flight crew and aircraft are at risk of colliding with obstacles like power lines, cables, towers, and other similar support structures. The risk becomes even greater with poor visibility and flights over unknown terrain. Depending on the type of aircraft canopy, the lighting, and the environmental conditions, many obstacles may become effectively invisible to the pilot and crew due to background clutter even under daylight conditions. Also, because of the narrow field of view offered the pilot by the aircraft, some obstacles may not be seen until it is too late for avoidance. Surprisingly, the highest accident rates are typically associated with clear conditions which indicates that during reduced states of pilot situational awareness, identification of hazardous ground obstacles may occur less regularly. 
     Some helicopters are equipped with structural wire strike protection kits which are fitted on the front end of the aircraft and intended to force a wire in the path of the aircraft to slide over the top or under the bottom of the aircraft. However, for this device to be effective, a contacted wire must slide across the canopy and into the wire cutters. When this occurs, the wire is likely to be severed by the wire cutter(provided it meets certain size and strength envelopes), freeing the aircraft from the hazards. It is not uncommon for electrical utility companies to identify cut wires but have no report of a wire strike accident. In some cases this indicates the flight crew did not know they hit a wire, much less cut it, or are reluctant to report the incident. However, if the wire does not slide across the canopy, and impacts other areas of the helicopter such as the rotors or landing skids, the wire cannot be severed by the wire strike protection system. As tension builds in the wire due to the forward motion, damage to the aircraft ensues with penetration into the canopy and flight crew, damage to the main rotor resulting in an imbalance, or loss of tail rotor control. In all these cases, the flight crew is in immediate life threatening danger. Depending upon the degree of interaction, fatalities can be attributed to the high-g accelerations of the rotor imbalance, blunt force trauma due to subsequent impact with the ground/aircraft, or harmful interactions with the wire resulting in significant lacerations or electrocution. Accordingly, due to the many low-level flying applications and the increasing risks posed thereby, obstacle avoidance warning systems for these aircraft have become of paramount importance for the safety of the pilot and crew of the aircraft. These devices are intended to warn the flight crew in advance of the collision with the obstacle, so that they(or an automated flight control system)can take evasive action prior to collision. 
     Thus, for manned aircraft, collisions with ground and air based obstacles result in numerous fatalities each year, while for UAVs, the aircraft can be lost, rendered uncontrollable, or unable to conduct the desired mission. Often UAV platforms are quite new with numerous sensors and signal intelligence suites. While mechanisms exist to render these devices useless to prevent a lost UAV from falling into enemy hands, lost UAVs can compromise mission capabilities and/or intent. Loss of mission control of a UAV or a precision guided munition (PGM) often results in the vehicle veering off course with unpredictable results, like hitting an unintended target, for example. Today, PGMs and UAVs are taking on a greater presence in military missions, often operating in densely packed urban environments and required to fly complex routes to avoid civilian causalities. In the future, there will be a greater reliance on PGMs and UAVs to successfully navigate a cluttered environment to find the target of interest without veering off course or colliding with unintended obstacles. 
     Amphitech International of Montreal, Canada, has developed a radar based obstacle awareness system named OASYS which was presented at the Quebec HeliExpo 2001. While it is proposed that OASYS can detect small obstacles, such as power lines, for example, up to two kilometers away even in adverse weather conditions, it is a rather heavy, bulky and costly unit, which may render it prohibitive for small aircraft usage. 
     Another obstacle awareness warning system is being developed by Dornier GmbH, in its Defense and Civil Systems Business Unit of Friedrichshafen, Germany under the tradename of HELLAS (Helicopter Laser Radar). In this unit, a laser beam is sequentially scanned through a line series of approximately one hundred optic fibers to create a raster line scan which is projected from the system. The line scan is steered vertically by a pivoted, oscillating mirror. The field-of-view is approximately plus and minus 32 degrees in azimuth and elevation with respect to a line of sight of the system. While Dornier promotes HELLAS as being an effective obstacle detection unit, it remains a relatively narrow field of view device that is rather complex and costly. In addition, the large number of optic fibers required for effective obstacle detection resolution, appears to render the device difficult to repeatedly align which may lead to manufacturing difficulties. 
     Another problem encountered in these low-level flight profile aircraft applications is the wind or air flow conditions surrounding the aircraft while it is carrying out its tasks. In some cases, an aircraft may encounter substantially different air-flow conditions from side to side. For example, when flying in a canyon, the aircraft may have a mountain wall on one side and open spaces on the other. Landing on the flight deck of an aircraft carrier poses similar risks. Such uneven air flow conditions may have an adverse affect on the responsiveness of the aircraft to the avoidance of detected obstacles. 
     Accordingly, it is desirable to have an obstacle awareness system that can be distributed through the aforementioned aircraft and that significantly reduces cost, weight, size and complexity of the obstacle sensing devices. Such a system would favor the use of a plurality of small, compact and directed scan devices. As such, a distributed system can be applied to a wide range of fixed wing, helicopter, UAVs and PGMs. The present invention is intended to provide for these desirable features through a distributed laser based obstacle awareness system as will become more evident from the description thereof found herein below. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a distributed laser based obstacle awareness system for use on-board an aircraft comprises: a plurality of obstacle detecting sensors disposable at a corresponding plurality of locations of the aircraft for emitting laser energy from the aircraft into a predetermined region of space and for receiving return laser energy from an obstacle in the predetermined region of space; a laser source for emitting a laser beam along an optical path; a plurality of bistatic optical channels, each channel comprising a plurality of transmission fiber optic cables and at least one receiver fiber optic cable, each optical channel extending from the laser source to a corresponding obstacle detecting sensor of the plurality, the transmission fiber optic cables of each optical channel operative to direct the laser beam from the optical path to its corresponding obstacle detecting sensor of the plurality for emission into the corresponding predetermined region of space; and a light detector, return laser energy from an obstacle received by any one of the obstacle detecting sensors being propagated through the receiver fiber optic cable of the corresponding optical channel to the light detector for use in detection of the obstacle in the corresponding predetermined region of space. 
     In accordance with another aspect of the present invention, a distributed laser based obstacle awareness system for use on-board an aircraft comprises: a plurality of obstacle detecting sensors disposable at a corresponding plurality of locations of the aircraft for emitting laser energy from the aircraft into a predetermined region of space and for receiving return laser energy from an obstacle in the predetermined region of space; a laser source for emitting a laser beam along an optical path; an optical switch disposed in the optical path; a plurality of bistatic optical channels, each channel comprising a plurality of transmission fiber optic cables and at least one receiver fiber optic cable, each optical channel extending from the optical switch to a corresponding obstacle detecting sensor of the plurality; the optical switch operative to redirect the laser beam in a time sequence manner from the optical path to selected optical channels of the plurality, the laser beam being propagated through the plurality of transmission fiber optic cables of the selected optical channel to the corresponding obstacle detecting sensor for emission into the corresponding predetermined region of space; and a light detector, return laser energy from an obstacle in the corresponding predetermined region of space received by the corresponding obstacle detecting sensor and therefrom propagated through the receiver fiber optic cable of the selected optical channel to the light detector for use in detection of the obstacle in the corresponding predetermined region of space. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram schematic of a wide field scanning laser obstacle awareness system (LOAS) suitable for embodying at least one aspect of the present invention. 
     FIG. 2 is a graph of an exemplary scan pattern generated from the LOAS embodiment of FIG.  1 . 
     FIG. 3 is a block diagram schematic of a light beam scan pattern generator/receiver suitable for use in the embodiment of FIG.  1 . 
     FIG. 4 is an illustration of an exemplary environment in which the LOAS embodiment of FIG. 1 may operate. 
     FIGS. 5A and 5B are time graphs exemplifying the signal processing of the LOAS embodiment of FIG.  1 . 
     FIG. 6 is a flowchart illustrating an exemplary programmed operation of a processor suitable for use in the LOAS embodiment of FIG.  1 . 
     FIGS. 7A and 7B are sketches illustrating an exemplary dithering operation of a perturbation mirror suitable for use in the embodiment of FIG.  1 . 
     FIGS. 8A and 8B are sketches illustrating the effects of a predetermined angle tilt of the perturbation mirror on an image projected in space. 
     FIG. 9 is a sketch of two rotationally operative optical elements suitable for use in embodiment of FIG. 1 for effecting a variety of beam scan patterns. 
     FIGS. 10A-10C are illustrations of exemplary beam scan patterns that may be effected by the rotationally operative optical elements of the embodiment of FIG.  9 . 
     FIG. 11 is a sketch of a light indicator display suitable for use in the embodiment of FIG.  3 . 
     FIG. 12 is a sketch of an exemplary screen of a multi-function video display (MFD) alternately suitable for use in the embodiment of FIG.  3 . 
     FIGS. 13A-13D are plan view illustrations in time progression (time slices) of an aircraft approaching obstacles near and in its flight path shown by way of example. 
     FIGS. 14A-14D are illustrations of exemplary MFD screen displays of the time slices of FIGS. 13A-13D, respectively. 
     FIG. 15 is a block diagram schematic of a combined LOAS and LIDAR system suitable for embodying another aspect of the present invention. 
     FIG. 16 is a sketch of a rotationally operative optical element suitable for use in the embodiment of FIG. 15 for directing two beams from the combined system with different predetermined scan patterns. 
     FIG. 17 is a sketch of a block arrangement of optical elements of a LIDAR system suitable for embodying another aspect of the present invention. 
     FIG. 18 is a sketch of an alternate block arrangement of optical elements of a LIDAR system. 
     FIG. 19 is a block diagram schematic of a LIDAR system for determining 3-axis flow velocity suitable for embodying yet another aspect of the present invention. 
     FIGS. 20,  20 A and  20 B illustrate functionally by way of example the processing involved in determining the 3-axis flow velocity by the embodiment of FIG.  19 . 
     FIG. 21 is an illustration of an embodiment of the present invention mounted to an aircraft with it own coordinates. 
     FIG. 21A depicts a set of three equations suitable for use in transforming a 3-axis flow velocity from one coordinate system to another. 
     FIG. 22 is an exemplary program organization for use in programming a processor for determining a 3-axis flow velocity measurement. 
     FIG. 23 is an exemplary software flow diagram of a foreground function routine suitable for use in the program organization of FIG.  22 . 
     FIG. 24 is an exemplary software flow diagram of a clock function interrupt service routine (ISR) suitable for use in the program organization of FIG.  22 . 
     FIG. 25 is an exemplary software flow diagram of a trigger function ISR suitable for use in the program organization of FIG.  22 . 
     FIG. 26 is an exemplary software flow diagram of a serial function ISR suitable for use in the program organization of FIG.  22 . 
     FIG. 27 is an exemplary software flow diagram of an evaluate function routine suitable for use in the program organization of FIG.  22 . 
     FIG. 28 is an exemplary software flow diagram of a velocity function routine suitable for use in the program organization of FIG.  22 . 
     FIG. 29 is a block diagram schematic of a combined LOAS and LIDAR system wherein the scan optical elements are embodied in a scan head in accordance with another aspect of the present invention. 
     FIG. 30 is a sketch of an embodiment of a scan head suitable for use in the embodiment of FIG.  29 . 
     FIG. 31 is an illustration of the scan optical elements disposed in the scan head embodiment of FIG.  30 . 
     FIG. 32 is an illustration of a LOAS embodying multiple scan heads in accordance with another aspect of the present invention. 
     FIG. 33 is an illustration of an exempalry optical switch suitable for use in the embodiment of FIG.  32 . 
     FIG. 34 is an illustration of a combined LOAS and LIDAR system embodying multiple scan heads in accordance with another aspect of the present invention 
     FIG. 35 is an illustration of an aircraft embodying a distributed laser based obstacle awareness systems (DLOAS) in accordance with the present invention. 
     FIG. 36 is a block diagram illustration of a DLOAS suitable for embodying the principles of the present invention. 
     FIG. 37 is an illustration of an optical switch for distributing a pulsed laser beam to a plurality of optical channels in a time sequenced manner. 
     FIG. 38 is an illustration of a bundled bistatic optical channel suitable for use in the embodiment of FIG.  36 . 
     FIG. 39 is an illustration of an optical scanner sensor suitable for use in the embodiment of FIG.  36 . 
     FIG. 40 is an illustration of a circular reconnaissance flight path of an aircraft about a target landing zone utilizing the DLOAS embodiment of FIG.  36 . 
     FIG. 41 is an illustration of an alternate embodiment of an optical scanner sensor suitable for use in the embodiment of FIG.  36 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a block diagram schematic of a wide field scanning laser obstacle awareness system (LOAS) suitable for embodying at least one aspect of the present invention. Referring to FIG. 1, a light source for generating a pulsed bean of light is comprised of a laser driver circuit  10  and a laser source  12  which is driven by the circuit  10 . In the present embodiment, the laser source  12  comprises a micro chip laser diode which may be of the type manufactured by Nanolase bearing model number NP-10320-100, for example. The laser source  12  is driven by the circuit  10  to emit a pulsed light beam with a pulse width of approximately one to two nanoseconds (1-2 nsec.) or longer, and at a pulse repetition rate on the order of ten kilohertz (10 kHz) or more. The light beam of the present embodiment is generated with a diameter of approximately three hundred micrometers (300 microns), with a wavelength of fifteen hundred and fifty nanometers (1550 nm) or one thousand sixty four nanometers (1064 nm), for example, and in a state of linear polarization. The laser source  12  may include beam conditioning optics (not shown) for collimating and enlarging the laser beam diameter from 300 microns to three millimeters (3 mm). 
     The pulsed laser beam of light is guided over an optical path  14  to a polarizing beam splitter optical element  16  which passes most of the pulsed beam  14  along an optical path  18  to the input of a laser beam expander  20 . A small amount of the pulsed beam  14  is reflected by the beam splitter  16  to a light detector  22  along an optical path  24  to act as a transmission time synchronization pulse as will become more evident from the further description found herein below. In the present embodiment, the light detector  22  comprises an avalanche photodiode (APD) which may be of the type manufactured by Analog Modules bearing model number 756, for example, and may include a variable gain circuit for increasing sensitivity to pulses of small amplitude. In addition, the beam splitter  16  may include a quarter wavelength (λ/4) plate at the output thereof which converts the linearly polarized beam passed by the beam splitter  16  to a circularly polarized beam which is passed along optical path  18  to the beam expander  20 . 
     In the present embodiment, the circuit  10 , laser source  12 , light detector  22  and beam splitter  16  are all mounted on an optical bench  26  in a proper alignment to create the optical paths  14 ,  18  and  24 , for example. The optical bench is then affixed structurally to a mounting structure  28  which supports the entire LOAS in the present embodiment. The laser beam expander  20  which may be of the type manufactured by Special Optics bearing model number 52-71-10X-905-1064, for example, is also mounted to the structure  28  such that its input or entrance aperture is aligned with the optical path  18  to receive the pulsed beam from the beam splitter  16 . The beam expander  20  enlarges the diameter of the pulsed light beam with a 10X magnification, for example, and passes the enlarged pulsed collimated light beam along an optical path  30  to at least one rotationally operated optical element  32  for directing the expanded pulsed laser beam from the LOAS along an optical path  36  with a predetermined pattern scanned azimuthally over a wide field. A conventional fold mirror optical element  34  may be mounted to the structure  28  and aligned for guiding the expanded light beam from the expander  20  to the at least one rotationally operated optical element  32  along the path  30 . It is understood that the use of the fold mirror  34  in the present embodiment is merely by design choice. 
     Pulsed light reflected along an optical path  40  from an obstacle or object  38 , like a wire, for example, along the predetermined pattern is received by the at least one rotationally operated optical element  32  and directed back to the beam expander  20  along an optical path  42  via fold mirror  34 . If there are more than one object in the path of the predetermined pattern, then the LOAS will receive pulsed light reflections from each of the detected objects. In the beam expander  20 , the pulsed light reflections are collected into a condensed collimated beam in the vicinity of its input aperture. The light reflections from the object  38  are reversed in circular polarization from the transmitted light beam. For example, if the transmitted beam was polarized with a clockwise polarization, then the light reflections would have a counter-clockwise polarization and vice versa. Aft optics in the optical bench  26  guide the light reflections from the beam expander  20  along an optical path  44  to the λ/4 plate and beam splitter  16 . The λ/4 plate reconverts the circularly polarized reflected light to linearly polarized light at right angles to the transmitted light beam which causes the polarizing beam splitter to reflect the returned light pulses to the light detector  22 , which may be coupled to signal processing circuits  50  that are also mounted to the common structure  28 . The operation of the light detector  22  and circuits  50  in connection with the detection of an obstacle will be explained in greater detail herein below. 
     More specifically, in the present embodiment, the at least one rotationally operated optical element  32  comprises a first rotationally operated optical element  52  for receiving the expanded pulsed laser beam from the expander  20  and directing it to a second rotationally operated optical element  54  along optical path  56  with the predetermined pattern as will become more evident from the following description. The second element  54  receives the pulsed light beam from the first element  52  and directs the received beam with the predetermined pattern azimuthally over a wide field which may be on the order of plus and minus ninety degrees or more with respect to a reference axis of the LOAS. Pulsed reflections from objects along the predetermined pattern are directed from the second element  54  to the first element  52  over an optical path  58 . One or both of the elements  52  and  54  may be configured as fold mirrors. 
     The optical element  52  may be a rotating optical wedge which has one surface inclined at a predetermined angle relative to an opposite surface and is rotated about an axis normal to the opposite surface, or a wobble mirror rotated about an axis at a predetermined nutation angle from its normal axis (e.g. a Palmer scan mirror), for example, wherein the expanded pulsed laser beam may be reflected from the mirrored surface of the optical element with the predetermined pattern. In either case, the element  52  is coupled to and driven by a conventional high speed circular scan module  60  which may include a drive system  62 , like an electric motor, for example, and a conventional bearing system  64 . In the present embodiment, the module  60  with its drive  62  and bearing system  64 , which may be a Palmer mirror assembly, for example, is mounted to the common structure  28  and properly aligned thereon. The drive  62  rotates the element  52  about its intended axis at an angular speed of approximately fifty (50) cycles per second, for example, which creates a cyclical conical pattern of pulsed laser beam projected from the LOAS via element  54 . 
     Element  54  may also be a mirrored optical element which is driven by an azimuth scan motor  66 , which may be a stepper motor, for example, to rotate and scan the conical pattern of the element  52  azimuthally through an arc of approximately 180°, i.e. ±90° or more with respect to the reference axis of the LOAS, over a time period of 0.5 seconds, for example. Thus, the predetermined pattern will include an elevation variation in relation to a line of sight axis of the system. An exemplary scan pattern at 500 meters from the system is illustrated in the graph of FIG.  2 . Referring to FIG. 2, the reference axis of the system is shown by the vertical axis  70  and the line of sight axis of the system is shown by the horizontal axis  72 . The helical-like line  74  represents the scan pattern as it is being rotated by the first element  52  and scanned azimuthally by the second element  54 . In this example, the first element  52  is an optical wedge mirror with a wedge angle of ten degrees and rotated at approximately 115 Hz. The graph of FIG. 2 only depicts an azimuth translation from 0° to +90°. It is understood that the azimuth translation from −90° to 0° would appear as the mirror image to what is depicted in FIG.  2 . The azimuth scan rate of the illustration of FIG. 2 is approximately 2 Hz. 
     Note that with each scan pattern cycle of the illustration of FIG. 2, the light beam pattern  74  moves in elevation in relation to the line of sight or horizontal axis  72  and in azimuth in relation to the reference or vertical axis  70 . Also, since the pattern  74  takes approximately 9 msec. to complete a cycle and since the LOAS generates light beam pulse every 0.1 msec., then there would be approximately 90 light beam pulses uniformly generated per scan pattern cycle. As will become more evident from the following description, the LOAS of the present embodiment may determine a location of each detected obstacle along the path of the predetermined scan pattern in range, azimuth, and elevation. 
     Returning to FIG. 1, in the present embodiment, the azimuth scan mirror optical element  54  is coupled to the azimuth scan motor  66  in a scan mounting assembly  68  which is also mounted to the common structure  28  via the scan module  60 , for example. Accordingly, all of the elements of the present embodiment may be mounted and fixedly aligned on the common mounting structure  28 . In some systems, an element of the LOAS may be adjustably realigned at its mounted position from time to time should the need arise. In addition, while the present embodiment is described as having two rotationally operated optical elements  52  and  54 , it is understood that it is possible to combine the scan pattern and the azimuth rotations into one optical element which is driven by two motors, one for the cyclical scan pattern and the other for the azimuth scan without deviating from the broad principles of the present invention. Also, more than two mirrors may be used as will be described in connection with an alternate embodiment herein below. In addition, a single mirror can be used to scan in elevation, using a resonant oscillating motion of the mirror in the vertical plane, for example, while simultaneously being driven in azimuth by a motor, producing a raster scan pattern. 
     A block diagram schematic of a wide field light beam scan pattern generator/receiver suitable for use in the present embodiment is shown in FIG.  3 . Like reference numerals will be maintained for those elements already described for the embodiment depicted in FIG.  1 . Referring to FIG. 3, electrical signals generated over signal line  78  by the photodiode  22  are representative of received light beam reflections from objects along the predetermined pattern of the transmitted pulsed light beam. FIG. 4 illustrates an exemplary environment in which the LOAS embodiment may operate. Referring to FIG. 4, the pulsed light beam projected from the LOAS along the path  36 , for example, may be reflected from such obstacles as a cable  80 , cable support towers or structures  82  and background clutter which may take the form of trees and bushes  84 , for example. The light reflections from the obstacles are received by the LOAS and directed to the light detector  22  as described in connection with the embodiment of FIG. 1 wherein the light reflections are converted to electrical signals representative thereof. 
     The time graph of FIG. 5A is illustrative of such electrical signals produced by the light detector  22  from the pulsed light beam reflections during an interpulse period of the transmitted pulsed light beams. FIG. 5A illustrates only the first approximately ten microseconds of a one hundred microsecond interpulse period, for example. In FIG. 5A, the first pulse  90  may be representative of the transmitted beam for time synchronization; the second pulse  92  which is close in range may be just an electrical noise pulse; the third pulse  94  may be representative of a reflection from a first obstacle, like the cable  80  or structure  82  as shown in the illustration of FIG. 4, for example; and the fourth pulse  96  may be representative of a reflection from a second obstacle further in range, like one of the trees  84 , for example. 
     Referring back to FIG. 3, the electrical signals from the photodiode  22  over signal line  78  may be coupled to one input of a circuit  86  which is configured as a comparator circuit. An electrical signal representative of a threshold level may be coupled to another input of the comparator  86  for comparison with the electrical signals from the photodiode  22 . The threshold level is shown by the dashed line  98  in FIG.  5 A. FIG. 5B is a time graph which exemplifies the operation of the comparator  86  in response to incoming electrical signals from the photodiode  22 . For example, as pulse  90 , the sync pulse, exceeds the threshold  98 , the output of the comparator  86  is caused to change state from a high potential (+5V) to a low potential (+2V). Note that in the embodiment of FIG. 3, the output of the comparator  86  is coupled to a signal processor  88  which may be part of the signal processing electronics  50 . The processor  88  may be a digital signal processor of the type manufactured by Texas Instruments bearing model number TMS-320C6711, for example. Accordingly, the processor  88  may be programmed to detect the change in state at  100  in the time graph of FIG. 5B caused by the sync pulse  90  and measure the time of all subsequent detected pulses with respect to the sync pulse or first change in state  100  of the comparator  86 . In the present embodiment, the comparator  86  may have a predetermined response time before it may return its output to a high level to detect the next pulse or detected obstacle. Thereafter, in the example of FIG. 5B, the comparator changes state at  102  in response to pulse  94  representative of the detection of one obstacle and again at  104  in response to pulse  96  representative of another obstacle. Note that no change of state occurs in response to pulse  92  which falls below the threshold level  98 , and thus, is considered electrical noise. With preprogrammed data of the speed of light, the processor  88  may be also programmed to determine the range to a detected obstacle from the time difference between the sync pulse and the pulse representative of the obstacle. The processor may also determine the azimuth and elevation location of the obstacle as well, as will be described in connection with the following paragraphs. 
     Referring back to FIG. 3, the scan pattern module  60  may be coupled to and drive the rotationally operated optical element  52  through a shaft  110  which may include an indication of its angle position with respect to a reference angle. In one embodiment for sensing the angular position of the optical element  52 , the shaft may be marked with indicia representative of its relative angle or include a wheel thereon or attached thereto with such angle markings. In either case, the indicia may be read by a conventional reader and digitally provided to the processor  88  as a measure of the angle of rotation of the scan pattern optical element. Thus, the processor will have stored at any time the measured angle of the scan pattern which it may use to calculate azimuth and elevation of a detected obstacle. In another embodiment, the shaft  110  may include markings like grooved teeth, for example, or have affixed thereto a wheel with teeth grooved therein. A conventional proximity device  112  may detect each grooved tooth and generate an electrical pulse in response. These electrical pulses may be counted in a counter  114  which count may be a measure of the current scan pattern angle of optical element  52 . The element  52  may include a mechanical, proximity or optical switch positioned to generate a reference pulse  116  each time the element  52  is rotated past the reference angle. The reference pulse  116  may be coupled to the counter  114  to reset it to zero so that it may start counting with respect to the reference angle with each rotation cycle. Accordingly, as the processor  88  detects an obstacle in time, it may read the contents of the counter  114  which is a measure of the concurrent angular position of the optical element  52  and from which the processor may determine elevation of the detected obstacle. 
     In yet another embodiment for sensing angular position of the scan pattern, the processor  88  may include a clock of a predetermined rate for counting up in a designated register thereof a count that is a time based measure of the angular position. The reference pulse  116  may be provided to the processor for resetting the count in the designated register. Each time the reference pulse  116  is received, the processor  88  saves the total count in the counting register and resets the register to start counting up from a zero count. In this embodiment, when an obstacle is detected, the processor  88  merely reads the concurrent count in the counting register and compares it to the saved total count to obtain a ratio from which it may determine the angular position of the scan pattern. The elevation of the obstacle with respect to the line of sight of the LOAS may be determined by taking, for example, the sine of the sensed scan pattern angle of the detected obstacle and multiplying it by the maximum elevation amplitude at the measured range of the detected obstacle. That is, one half of the diameter of the plane section of the conical scan pattern at the range of the detected obstacle will be the maximum elevation amplitude. This is illustrated in the scan pattern example of FIG. 2 for a range of 500 meters. 
     The embodiment of FIG. 3 also exemplifies a way for determining substantially the azimuth position of the directed pulsed laser beam for determining the location of a detected object in at least range and azimuth. Referring to FIG. 3, a conventional digital clock circuit  120  generates a clock signal  122  at a predetermined rate. Signal  122  is coupled to select logic circuitry  124  and to a rate divider circuit  126  which divides the rate of clock signal  122  by a factor N. The divided rate signal  128  from the circuit  126  is coupled to the select logic circuitry  124  and to an azimuth position counter  130  which increases its count with each received pulse. The select logic circuitry  124  generates a clockwise signal (CW) and a counter-clockwise signal (CCW) for use in controlling the electric motor  66 , which may be a stepper motor, for example. The motor  66  is coupled to the azimuth scan mirror assembly  54  by a shaft  132  for rotating the mirrored element  54  through its 180° rotation. The azimuth mirror assembly  54  may include a first switch positioned to activate and generate a START signal at substantially the 0° azimuth position, and a second switch positioned to activate and generate a STOP signal at substantially the 180° azimuth position, for example. The START and STOP signals are provided to the select logic circuitry  124 . In some applications, the signal processor  88  may be coupled to the divider circuit  126  over signal line  134  for setting the number N by which the rate of signal  122  will be divided. The signal processor  88  is also coupled to the counter  130  over signal line  136  for reading the azimuth position count thereof. 
     In operation, the signal processor  88  may set the number N of the divider  126  which ultimately sets the rate at which the laser beam scan pattern is rotated azimuthally. It is understood that this number N may be preprogrammed into the rate divider circuit  126  as well. So, the select logic  124  receives both a fast rate signal  122  and a slower rate signal  128  and selects one of the rate signals to control or step the motor  66  through its rotation. For example, when the select logic  124  receives the START signal from the scan mirror assembly  54 , it selects the slow rate signal  128  to control the motor  66  via the CW control line to rotate clockwise through its 180° rotation in a predetermined time, like 0.5 seconds, for example. When the STOP signal is generated, the select logic  126  responds by selecting the fast rate signal  122  to control the motor  66  via the CCW signal to rotate counterclockwise back to its starting position whereupon the process is repeated. It is understood that the azimuth scan may be controlled to rotate at the slower rate in a counterclockwise rotation and returned to its starting angular position at a much faster rate as well without deviating from the broad principles of the present invention. 
     Each time the select logic receives the START signal, it generates a ZERO signal to the counter  130  for resetting the count thereof to zero. The STOP signal may be also coupled to the signal processor  88  which responds to the signal by reading and storing the total count in the counter  130  which is representative of an azimuth angular position of 180°, for example. So, each time an obstacle is detected by the signal processor  88 , it may read the concurrent count in the azimuth position counter  130  and use the read count together with the total count to determine the azimuth position of the detected obstacle. In the present embodiment, the circuits  120 ,  124 ,  126  and  130  may be part of the signal processing circuitry  50 . It is understood that the functions of these circuits may also be programmed into the signal processor  88 . 
     In some applications, the azimuth scan may be controlled to rotate at the programmed rate for both of the clockwise and counterclockwise directions in which case, the counter  130  will count up from the starting position in one direction and count down from the stop position in the opposite direction. In these applications, the counter may still be reset to zero by the select logic  124  in response to the START signal and the processor  88  may read the total count of the counter  130  in response to the STOP signal. And, similarly, each time an obstacle is detected by the signal processor  88 , it may read the concurrent count in the azimuth position counter  130  and use the read count together with the total count to determine the azimuth position of the detected obstacle. 
     The flowchart of FIG. 6 illustrates a programmed operation of the signal processor  88  by way of example. Referring to FIG. 6, the diode laser source  12  may be controlled to fire periodically at a rate of 10 KHz or 10,000 pulses per second, for example, with an interpulse period of 100 μsec. autonomously by the driver circuit  10  or may be controlled to fire by the programmed processor  88  as shown by the block  140 . In either case, the processor detects the sync pulse as described supra and starts a processor range timer in block  142 . Thereafter, the processor begins searching for return pulses of reflections from the targets or obstacles along the predetermined scan pattern in block  144 . When a return pulse is received in block  146 , which is representative of a detected obstacle, the processor bins the return signal according to its time of flight in block  148 . That is, the return pulse is indexed and stored in a designated memory location of the processor along with its recorded range processor time which is the count in the timer concurrent with the time of detection. This count is representative of the range of the detected obstacle. Concurrent with the detection of the obstacle, the instantaneous positions of the Palmer scan pattern and azimuth mirrors are recorded as described supra, preferable in the designated memory location for the indexed detected obstacle, in block  150 . Each time an obstacle is detected by the processor in the interpulse period of laser firing, the blocks  146 ,  148  and  150  are repeated and the obstacle index and its range and location representative data for azimuth and elevation are recorded in a designated memory location or bin. 
     After, the initial approximately 6 μsec. of the interpulse period between laser firings or some other appropriate initial time period ends, the processor stops searching for detected obstacles in block  152 . Thereafter, the processor may use the remaining time before another laser firing to compute the range and location in azimuth and/or elevation for each obstacle detected and indexed in the current interpulse period from the recorded data thereof. In block  158 , this range and position location information for the detected obstacle(s) may be configured for display and transferred to a display  154  such as shown in the block diagram schematic of FIG. 3, for example. This information may also be provided by the processor  88  over a signal line  156  to other systems for use therein. At the end of the interpulse period, the laser source  12  may be controlled to fire again in block  140  and the process as just described is repeated. In this manner, each obstacle along the predetermined scan pattern may be detected and its location determined and the detected obstacles and their respective locations may be displayed to an operator for awareness purposes as will become more apparent from the description found herein below. 
     The wide field scanning LOAS embodiment described in connection with FIGS. 1-6 detects obstacles along a predetermined scan path using a pulsed laser beam spot size on the order of a meter in diameter at about a kilometer in range, for example. As shown by the pattern example of FIG. 2, obstacles will not be detected in the cusp areas between the scan paths of the pattern  74 . To improve the obstacle detection effectiveness of the wide field LOAS embodiment, a beam perturbation or dither mirror may be disposed in the optical path  18  between the beam splitter  16  and input or entrance aperture of the expander  20 , preferably in the aft optics of the optical bench  26 , for example. The perturbation mirror  160  as shown in FIGS. 7A and 7B, which is configured as a fold mirror, may be supported on a pivot and rotated back and forth across a center axis of the optical path  18 . In so doing, it will change the beam approach angle into the entrance aperture of the beam expander  20 . For example, in the present embodiment, a ±1° pivot or tilt of the perturbation mirror  160  with respect to the central axis of the optical path  18  is expected to move the laser beam spot ±5 meters at a kilometer in range. If the mirror is dithered in this manner at a high rate, like on the order of one to ten Kilohertz (1-10 kHz), for example, the 1 meter laser beam spot size would be smeared to become effectively 5 meters at 1 kilometer. Accordingly, a greater percentage of the scene would be observed by an effectively wider laser beam spot size. That is, the width of the path of the scan pattern would be increased effectively five fold. 
     FIGS. 7A and 7B illustrate by way of example the dithering operation of the perturbation mirror  160 . In FIG. 7A the mirror  160  is at shown configured as a fold mirror pivoted about an axis  163  looking into the drawing sheet. In FIG. 7A, the mirror  160  is shown at a zero angle tilt. Note that in this position of the mirror  160 , the rays of the beam guided through the optical path  18  are centered about a central axis  162  of the entrance aperture  164  of the beam expander  20 . In FIG. 7B, the mirror  160  is tilted or pivoted downward approximately 1° from its zero angle position of FIG. 7A causing the rays of the beam to move off the central axis  162  downward at an approach angle to the entrance aperture of approximately minus one degree. Similarly, as the mirror  160  is tilted upward 1° from the zero angle position, the rays of the beam will move off the central axis  162  upward at an approach angle to the entrance aperture of approximately plus one degree. A rapid movement of the mirror  160  rotating between the ±1° tilt positions will result in the effective spread of the laser beam spot along the scan pattern. 
     FIGS. 8A and 8B show the effect of the 1° tilt of the mirror  160  on an image projected in space. In FIG. 8A, the mirror  160  is at the zero degree tilt position. Note that the laser beam reflected along path  18  expands through the beam expander  20  as shown by the departing rays. As the beam exits the expander  20 , it becomes collimated with parallel rays at path  30 . The expanded collimated beam is reflected from mirror  52  along path  56  to the mirror  54  where it is again reflected along path  36  and directed from the system along the predetermined scan path. To better illustrate the effects of the dithering of the perturbation mirror  160  on a projected image, like the spot size, for example, a converging lens  168  is disposed at the output of the system to focus the beam to a focal point or spot  170  in space a predetermined range from the system. This converging lens  168  is used in the present example merely for image analysis purposes. In FIG. 8B, the mirror  160  is tilted downward 1° causing the collimated beam exiting the expander  20  to shift downward which results in a deflection of the focal spot to a new position  172  that is only slightly away from the original focal position  170  as shown in FIG.  8 A. In the present example, a 1° tilt resulted in only a 1.6 meter deflection of the focal spot at a range of one kilometer. Thus, a minor perturbation of the mirror  160  will not result in substantial defocusing or distortion of an obstacle image detected at substantial distances from the system. 
     A perturbation mirror  160  suitable for use in the embodiment of FIG. 1 may be any one of a variety of commercially available mirrors, like a Palmer or wobble mirror assemble or a scan mirror, for example. But to effect the speeds of pivoting or dithering desired for the present embodiment which may be on the order of 200-600 Hz, for example, a mirror assembly that has a low inertia, like a mirror assembly made using micro electro-mechanical systems (MEMS) technology, is preferred. These type of low inertia mirror assemblies may use a small piezoelectric power supply. The area of mirrored surface of the perturbation mirror  160  may be made quite small, like on the order of the width of the laser beam it is reflecting. Several commercially available “fast” dither mirrors operated by piezoelectric drivers for optical image stabilization would be suitable for this purpose. 
     In accordance with another aspect of the present invention, the rotationally operative scan optical element  52  may comprise two rotationally operative scan mirrors  174  and  176  configured as fold mirrors with respect to each other as shown in the illustration of FIG. 9 to project a plurality of different output scan patterns of the laser beam along the optical path  56  to the azimuth scan mirror  54  wherein the scan pattern is steered azimuthally through a wide field as described herein above in connection with the embodiment of FIGS. 1-6. A single scan mirror  52  generates the helical pattern  74  when steered across the wide azimuth field as illustrated in FIG.  2 . But, this pattern may not be an ideal or a preferred scan pattern for the application at hand. Therefore, it would be desirable to have the option of tailoring an appropriate scan pattern for a particular application or be able to change the pattern due to varying conditions. The dual fold mirror assembly of this aspect of the present invention permits the tailoring of a scan pattern by setting and/or varying the phase, direction and rotational speed of one mirror  174  with respect to the other mirror  176 . In the present embodiment, the mirrors  174  and  176  may comprise Palmer or wobble mirror assemblies, each rotationally operative at a predetermined nutation angle, like on the order of 5°, for example. However, it is understood that optical wedge type mirrors may be configured to function just as well without deviating from the broad principles of the present invention. 
     In the illustration of FIG. 9, the rotationally operative mirror  174  is configured for directing the laser beam which is incident to a surface  178  thereof along optical path  30 , for example, to the other rotationally operative mirror  176  along an optical path  180  with an intermediate scan pattern. The other rotationally operative mirror  176  is configured for directing the laser beam which is incident to a surface  182  thereof along path  180  to the azimuth scan mirror  54  over path  56  with the desired scan pattern. The mirrors  174  and  176  are adjustably rotationally operative about respective axes of rotation  184  and  186  in speed, direction and phase angle in relation to each other to effect the desired output scan pattern of the plurality of output scan patterns of the laser beam. In the present embodiment, an electric scanner motor may be coupled to each mirror and controlled to rotate each mirror at a predetermined nutation angle (angle  188  for mirror  174 , and angle  190  for mirror  176 ) with the desired speed, direction and phase angle in relation to the other mirror to effect the desired output scan pattern. FIGS. 10A,  10 B, and  10 C illustrate exemplary scan patterns which may be effected by the rotationally operative mirrors  174  and  176 . Other scan patterns are also possible with different combinations of rotations and speeds. 
     In FIG. 10A, a sawtooth scan pattern is shown generated by the dual mirror assembly embodiment of FIG. 9 by operating mirror  174  at a rotational speed of 50 Hz in a clockwise direction with a nutation angle of 5°, and operating mirror  176  at a rotational speed of 50 Hz in a counter-clockwise direction in relation to mirror  174 , with a nutation angle of 5°. In this example, the azimuth steering rate is approximately 360° per second. This scan pattern may be better suited for detecting vertical or horizontal shaped obstacles. In FIG. 10B, a large circular scan pattern is shown generated by the dual mirror assembly embodiment of FIG. 9 by operating mirror  174  at a rotational speed of 50 Hz in a clockwise direction with a nutation angle of 5°, and operating mirror  176  at a rotational speed of 50 Hz also in a clockwise direction, but 180° out of phase to mirror  174 , with a nutation angle of 5°. In this example, the azimuth steering rate is approximately 360° per second. Finally, in FIG. 10C, a small circular scan pattern is shown generated by the dual mirror assembly embodiment of FIG. 9 by operating mirror  174  at a rotational speed of 50 Hz in a clockwise direction with a nutation angle of 50, and operating mirror  176  at a rotational speed of 50 Hz also in a clockwise direction, but with a 22° phase difference to mirror  174 , with a nutation angle of 5°. In this example, the azimuth steering rate is also approximately 360° per second. Accordingly, the size of the pattern, as shown by FIGS. 10B and 10C, may be varied by changing the phase angle of one mirror in relation to the other while maintaining the rotational speed substantially fixed. It is also possible to change the density of the pattern in azimuth scan by altering the speed of the azimuth scan mirror. Note that the side edges of the patterns of FIGS. 10A-10C appear somewhat compressed because the pattern is projected onto a flat surface disposed directly in front of the system. The horizontal and vertical units shown in the Figures are normalized to a ±90° azimuth scan and a predetermined target range, respectively. 
     In accordance with yet another aspect of the present invention, the wide field scanning LOAS embodiment described above in connection with FIGS. 1-6 may be disposed on-board an aircraft, like a helicopter, for example, for use in alerting an operator or pilot of the aircraft of obstacles posing a risk of collision with the aircraft. The processor  88  described above in connection with the embodiment of FIG. 3 determines the location of one or more detected obstacles in range, elevation and azimuth in relation to a flight path of the aircraft and drives the display  154  which may be located in the cockpit of the aircraft, for example, to display to the pilot or an operator an indication representing the one or more obstacles or objects in range, azimuth and elevation. It is understood that the processor  88  may first determine the location of a detected obstacle in relation to the reference axes of the LOAS and then, convert the location to the reference axes of the aircraft. This conversion from one set of reference axes to another will be explained in greater detail herein below. 
     One embodiment of the display  154  comprises a panel  200  of light indicators  202  as shown by the illustration of FIG.  11 . The light indicators  202  of panel  200  may be light emitting diodes (LEDs), for example. In this embodiment, the panel  200  includes at least one row  204  and at least one column  206  of indicators  202 . The row  204  may represent a horizontal axis of the flight path of the aircraft and the column  206  may represent an elevation axis thereof. Accordingly, the indicator  208  at the intersection of the row  204  and column  206  represents the line of sight or instantaneous directional path of the aircraft. The light indicators  202  may be controlled to emit light of different colors to indicate the location of the one or more objects in elevation and azimuth in relation to the flight path of the aircraft. A color change from green to yellow to red, for example, may indicate the range of a detected object from the aircraft. In the illustration of FIG. 11, the colors are represented by gray scale. For example, a blackened indicator  210  is indicative of red and indicates that the detected object represented thereby is close in range to the aircraft, but below the aircraft. A gray indicator  212 , for example, may represent a detected object at mid range to the aircraft, but substantially off to the left thereof. Those indicators  202  which are not lit or are only slightly gray (green) represent no detected objects of detected objects far in range from the aircraft, respectively. A change in color of an indicator on the panel  200  may also indicate to the operator the risk of a collision of one or more detected obstacles with the aircraft. 
     Another embodiment of the display  154  comprises a multi-functional video display (MFD), an exemplary screen of which being illustrated in FIG.  12 . The screen of the MFD may display a forward looking view, like the view shown in FIG. 12, for example, obtained from a video or forward looking infrared (FLIR) camera or radar unit (not shown) mounted to the front of the aircraft. Generally, radar and video or FLIR cameras have a relatively narrow field of view, on the order of ±thirty degrees (±30) in azimuth from the flight path of the aircraft, for example. Accordingly, the operator may view only those obstacles in the field of view of the camera to ascertain risks from obstacles in the aircraft&#39;s path. Note that in the screen of FIG. 12, the MFD displays a wire stretching horizontally across the path of the aircraft shown by the dotted line  216  which may change in color according to the detected range thereof Note also that a variety of information obtained from sensors on the aircraft or received from uplinked transmissions to the aircraft is displayed on the screen of FIG.  12  through use of overlay or image integration technology which is well known to all those skilled in the pertinent art. An exemplary MFD for use in the present embodiment is manufactured by Goodrich Avionics Systems, Inc. under the tradename of SmartDeck™ display. These type of MFDs display such information as aircraft velocity, i.e speed and heading, altitude, above ground level (AGL) readings, aircraft power levels and the like. 
     The present invention enhances the situational awareness of the pilot or operator of the aircraft by displaying the locations of detected obstacles in relation to the aircraft outside of the azimuth field of view of the display screen of the MFD. It does this by overlaying an image in the form of at least one vertical bar  218  onto the screen image of the MFD for representing one or more detected objects and the locations thereof. In the present embodiment, one vertical image bar  218  is overlaid to the far left of the screen image and another vertical image bar  220  is overlaid to the far right of the screen image. Each bar  218  and  220  is split into two areas, one area  222  above the center line of the display screen, which is representative of the current altitude of the aircraft, and the other area  224  below the center line. Each bar  218  and  220  is controlled to light upon the detection of an object azimuthally outside of the field of view of the MFD starting at the bottom area  222  with a color indicative of the range to the detected object. In the present embodiment, the LOAS may have a field of regard of 50 meters to 1 kilometer in range, ±90° in azimuth and ±10° in elevation, for example. 
     For example, as an object is first detected at a range far from the aircraft, but azimuthally outside the field of view of the MFD, the bottom of the corresponding bar  218  or  220  becomes lit with a green color indicting.the elevation of the object is determined to be optically below the altitude of the aircraft and at a far range thereto. As the aircraft approaches the detected obstacle, the image bar will change in color, like from green to yellow, for example, to indicate a change in the range thereof and also may grow vertically in size if the elevation of the obstacle is determined to be optically closer to the altitude of the aircraft. And, as the detected obstacle becomes very close to the aircraft in range, the color of the corresponding image bar will change from yellow to red, for example, and if the obstacle is determined to be above the altitude of the aircraft, the colored portion of the image bar will extend above the center line of the display screen in the portion  224  thereof. In this manner, the pilot or operator will be alerted to detected obstacles outside of the azimuth filed of view of the MFD and their locations in range(color) and elevation (height of bar) in relation to the aircraft. 
     FIGS. 13A-13D are plan view illustrations in time progression (time slices) of a helicopter  228  containing a wide field scanning LOAS similar in type to the foregoing described embodiments and including an MFD like the type described in connection with FIG. 12, for example, approaching an electrical power line  230  supported by poles  232  and a 200 meter radio tower  234  and connecting support lines  236 . Circled lines  238 ,  240  and  242  are representative of ranges 200 meters, 400 meters and 600 meters, respectively, from the aircraft  228  which is heading in the direction of the arrow  244 . The field of view of the MFD is shown by the wedged area  246  and may be on the order of ±15° in relation to the flight heading  244  of the aircraft. Exemplary MFD screen displays of the time progression illustrations of FIGS. 13A-13D are shown in FIGS. 14A-14D, respectively. 
     Referring to FIG. 13A, which is the first illustration in time, the aircraft  228  is shown at a range of greater than 600 meters from both of the power line and tower obstacles  230  and  234 , respectively. Accordingly, since the power line  230  is partially within the field of view (FOV) of the MFD, it is displayed as an overlaid dotted line in the screen of FIG.  14 A. But, since the obstacles  230  and  234  are outside of the 600 meter range, the vertical bar images  218  and  220  are not lit. The 600 meter range is set by design choice for the present example, and it is understood that this range may vary according the specific application at hand. In the next time slice, the helicopter has moved closer to the power line  230 , poles  232  and tower  234  and a portion  248  of the power line  230  and poles  232  is within the 600 meter range of the LOAS, albeit outside of the azimuth FOV  246  of the MFD. The obstacles  248  are detectable within an azimuth sector  250  of the wide field scanning LOAS of the aircraft  228  and thus, are displayed in the vertical bar  220  with a color green, for example, and at a height indicative of the determined elevation thereof. In the present example, the color green illustrated by light gray is indicative of a range of a detected obstacle between 400 and 600 meters. The height of the lit vertical bar  220  is below the center line of the screen indicating to the operator that the obstacle is below the altitude of the aircraft  228 . 
     In the time slice of FIG. 13B, the aircraft  228  has moved closer to the obstacles to the point where a portion  254  of the power line and poles are within a range between 200 and 400 meters in azimuth sectors  250  and  256 . Upon detection by the LOAS of the aircraft  228 , the vertical image bar  220  as shown in FIG. 14B displays a green portion (white or light gray)  252  representing the portion  248  of the power line and poles falling between 400 and 600 meters in range, and a yellow portion (darker gray)  258  representing the portion  254  of the power line and poles falling between 200 and 400 meters in range. The height  260  of the vertical bar image  220  of the screen of FIG. 14B reflects the elevation of the detected obstacles in relation to the altitude of the aircraft, i.e. center line of the screen. Note that the obstacle portion  254  is outside of the azimuth FOV  246  of the MFD and would not be observed by the pilot without the aid of the LOAS and its vertical bar image overlay  220  onto the screen image of the MFD. Note also that the LOAS of the aircraft  228  detects the tower  234  in an azimuth sector  262  outside of the FOV  246  and lights the vertical bar image  218  as an indication thereof, albeit beyond the 600 meter range. 
     In the time slice of FIG. 13C, the aircraft has moved closer to the power line  230  and tower  234  and indicates this to the operator through the vertical bar images overlays  218  and  220  as shown by the screen of corresponding FIG.  14 C. Note that the vertical bar image  218  has increased to the height  264  indicating that the obstacle is at an elevation close to the altitude of the aircraft  228  although more than 600 meters in range. Also, the vertical bar image  220  has increased to a height  266  beyond the center line of the display to indicate that the detected obstacles in azimuth sectors  256  and  250  are at an elevation above the altitude of the aircraft and the risk of a collision with such obstacles has increased. In the time slice of FIG. 13D, the aircraft  228  has moved even closer to the power line  230 , a portion  268  of which now detected by the LOAS of the aircraft to be within 200 meters in range. In response, the LOAS lights the vertical bar image  220  with a red color (illustrated by dark gray) at a height  272  well beyond the center line of the screen. This indicates to the pilot that the power line is within 200 meters and at the altitude of the aircraft. In other words, collision of the aircraft  228  with the portion  268  of the power line is imminent unless immediate evasive action is taken. On the other hand, the LOAS of the aircraft also detects the tower  234  in an azimuth sector  270  within 600 meters in range of the aircraft and indicates through the lighting of the vertical bar image  218 , its range by color and elevation by height. Note that the vertical bar image  218  depicts the detected elevation of the tower  234  approximately at the altitude of the aircraft, represented by the center line of the screen. So, the pilot is also aware of the tower  234  and its range and elevation and can avoid it in the evasive action taken to avoid the power line portion  268 . 
     Therefore, the foregoing description of FIGS. 13A-13D and  14 A- 14 D illustrate by way of example the operation of the wide field scanning LOAS in use on-board an aircraft and the enhanced situational awareness it provides to the pilot and/or operator in the form of a dynamically changing display that extends beyond the visual field of view or a field of view of an MFD of the aircraft. Without the aid of the LOAS on-board the aircraft and the displayed overlaid images of detected obstacles and their locations with respect to the flight path and altitude of the aircraft, the pilot and/or operator of the aircraft may not be made aware of the risk of imminent collision of the aircraft with such obstacles and collision may not be otherwise avoided. 
     While the wide field scanning LOAS described above provides an enhanced awareness to the operator, the ability to avoid a detected obstacle in the flight path of the aircraft may be further improved knowing the wind conditions around the aircraft as well. So, combining a wide field scanning LOAS for detecting obstacles in the vicinity of the aircraft with a laser air data system, like a light detection and ranging (LIDAR) system, for example, for measuring the wind velocity at points around the aircraft and particularly, at the detected obstacle or at a launch point of a weapon for a military platform is desirable. A suitable embodiment of such a combined system is shown in the block diagram schematic of FIG.  15 . 
     Referring to FIG. 15, the pulsed laser beam transmitting and receiving optical elements of a LOAS is shown in the dashed line enclosed block  280 , the continuous wave (CW) laser beam transmitting and receiving optical elements of a LIDAR system is shown in the dashed line enclosed block  282 , optical elements common to the LOAS and LIDAR systems  280  and  282  are shown in the dashed line block  284 . Like reference numerals will be used for those elements already described in connection with the LOAS embodiment of FIGS. 1-6 herein above. For example, in block  280 , a pulsed laser source of the present embodiment may comprise the elements of the laser driver  10  and laser diode  12 . Beam conditioning optics for collimating and expanding the generated pulsed laser beam width along optical path  14  is shown by block  11 . Beam splitter  16  and the quarter wavelength plate  17  pass the pulsed laser beam along path  18  with a circular polarization. A portion of the generated pulsed laser beam is reflected by the splitter  16  over path  24  to the light detector  22  which may be an APD, for example. The electrical signals generated by the light detector  22  are provided to the threshold detector or comparator circuit  86  which is coupled to the processor  88 . Azimuth position data may be provided to the processor  88  in a similar manner as that described for the embodiment depicted by FIG. 3, for example. 
     In the LIDAR block or module of elements  282 , a laser source  286  is controlled to generate a linearly polarized CW laser beam at a wavelength substantially different from wavelength of the pulsed laser beam of the LOAS elements  280 . The LIDAR generated laser beam may be at one wavelength in the range of 850 to 1550 nanometers, for example, and the LOAS laser beam may be at a different wavelength in the range of 850-1550 nanometers, for example. However, it is understood that other wavelength ranges may work just as well and the present invention is not limited to any specific wavelength or wavelength range. The CW laser beam is generated along an optical path  288  to beam conditioning optics  290  which collimate and expand the CW beam before passing it along an optical path to a polarizing beam splitter  294 . Most of the linearly polarized light is passed by the beam splitter  294  along path  296  to a one-quarter wavelength (λ/4) plate  298  which converts the linearly polarized light to circularly polarized light before passing the beam along an optical path to beam converging optics  300 . Back at polarizing beam splitter  294 , a small portion, like on the order of 2% or so, of the generated CW beam is reflected along an optical path  302  to an acousto-optical modulator (AOM)  304  which shifts the frequency of the reflected beam by a predetermined frequency which may be on the order of 80 MHz, for example. The reason for this frequency shift is to avoid a directional measurement ambiguity as a result of the heterodyning operation which will become more evident from the following description. The frequency shifted beam exiting the AOM  304  is optically guided along an optical path  306  by one or more optical elements to another polarizing beam splitter  308 . 
     Reflected light from an aerosol particle, for example, at a predetermined distance from the combined system is returned through optics  300 , the λ/4 plate  298 , and along optical path  296  to the beam splitter  294  wherein it is reflected along an optical path  310  to the beam splitter  308 . The returned beam is combined, i.e. heterodyned, with the transmitted (shifted frequency) beam portion in the beam splitter  308  to effect a light beam with a Doppler frequency content caused by the reflection off of the particle in space. In the present embodiment, if the returned beam is unshifted in Doppler frequency, the heterodyning will result in a combined light beam signal at the center frequency for heterodyne processing which may be set at 80 MHz, for example. Thus, if the returned beam is Doppler shifted, the heterodyning process will result in a combined beam with Doppler frequency content of either greater than or less than 80 MHz. In this way, the process will not be confused by negative Doppler frequency shifts caused by receding targets, which are indistinguishable from the positive Doppler frequency shifts caused by approaching targets if the heterodyning light beam is unshifted in frequency. The combined beam with the Doppler frequency content is guided along an optical path  312  to a light detector  314  which may be a photodiode, for example. The photodiode  314  converts the combined light beam into a time varying analog electrical signal  316  which is passed on to the processor  88  via signal conditioning circuit  318 . If the processor  88  is a digital signal processor, the time varying analog signal  316  may be digitized by the signal conditioning circuit  318  according to a predetermined sampled data rate for processing by the processor  88 . 
     The beam converging optics  300  may be a variable laser air data range module which includes a group of focusing elements that permits adjustably setting the focal point for the LIDAR generated beam at a spot in space which may vary from say 5 meters to 20 meters, for example, from the system. This focal spot is space is where the beam reflections from one or more particles flowing in space are concentrated. In one embodiment, the optics  300  includes the selection of a particular focusing lens to effect the desired distance to the focal spot in space. Each different lens will provide for focusing to a spot in space a discrete predetermined distance or range from the system. This lens selection process may be performed manually by plugging in the desired focusing lens or electro-mechanically by apparatus comprising a mechanical carousel having different lens, for example, which carousel may be controlled to rotate to the selected focusing lens. In another embodiment, the optics  300  may include a lens which is electronically controlled to change its focusing characteristics to effect the desired range of the focal spot in space. 
     In the common optical elements block or module  284 , the coherent CW light beam exiting the optics  300  is guided along an optical path  319  to a dichroic filter optical element  320 . The pulsed coherent light beam along optical path  18  is also guided to the dichroic filter  320 . With proper alignment, the two coherent light beams of different wavelengths may be guided to the dichroic filter  320  such that one is reflected and the other is passed along a common optical path  322  towards the entrance aperture of the beam expander or telescope  20  which is aligned to accept and expand the two coherent beams and exit the expanded coherent beams along another common optical path  324  at an output thereof. The expanded coherent beams are guided along common path  324  to be incident upon the at least one optical element  32  as described in connection with the embodiment of FIG.  1 . The at least one optical element  32  in turn directs the two beams from the system into space. Reflections of the CW coherent beam from particles at the focal spot and reflections of the pulsed coherent beam from obstacles are all returned to the at least one optical element  32  which receives such reflections and directs them along path  324  back to the beam expander  20  wherein they are focused to a focal point of the beam expander  20  along path  322 . The dichroic filter  320  may be disposed in the vicinity of the focal point of the beam expander  20  along path  322  to receive the focused reflections and separate the focused light reflections corresponding to the pulsed coherent beam from the focused light reflections corresponding to the CW coherent beam based on the different wavelengths thereof. 
     Separated light reflections corresponding to the pulsed coherent beam are directed back to the LOAS module  280  along path  18  for use in detecting one or more objects as described in connection with the embodiments of FIGS. 1-6, for example. In addition, separated light reflections corresponding to the CW coherent beam are directed back to the LIDAR module  282  along path  319  for determining flow velocity as will be more fully described. As has been described supra, the at least one optical element  32  comprises at least one common rotationally operated optical element which may direct both of the CW and pulsed coherent beams incident thereon from the system, the CW beam being directed from the system with a first predetermined pattern and the pulsed beam being directed from the system with a second predetermined pattern. In the embodiment described above in connection with FIGS. 1-6, the at least one rotationally operative element  32  comprises optical elements  52  and  54  which together may be configured and rotationally operated to direct both of the CW and pulsed coherent beams substantially colinearly from the system along path  36  with the azimuthally steered, conical beam pattern that is depicted in FIG.  2 . In this manner, the first and second patterns will be substantially the same and directed substantially to common azimuth positions in the azimuthal scan. An embodiment for directing the two coherent beams from the system with different first and second patterns will be described herein below. 
     Separated light reflections that are guided along path  319  back to the LIDAR module  282  will pass through the beam converging optics  300  to the λ/4 plate  298  wherein the circularly polarized light is converted back to linearly polarized light and passed on to the beam splitter  294  over path  296 . However, since the circular polarization direction of the transmitted beam is reversed upon reflection from a particle, the converted linear polarization state of the reflected light will be at right angles to the linear polarization state of the transmitted beam. Accordingly, instead of being passed by the beam splitter  294 , the returned light reflections will be reflected along path  310  and heterodyned with the transmitted beam (shifted in frequency) in splitter  308  as has been described herein above. The processor  88  may compute the flow velocity in the vicinity of the aircraft at various azimuth positions from the time varying electrical burst signals converted by the light detector  314  using Doppler signal processing, like Fast Fourier Transform (FFT) processing, for example, which is well-known to all those skilled in the pertinent art. The flow velocity may be computed in one or more axes as will become more evident from the description found herein below. Azimuth position may also be determined by the processor  88  from inputs of azimuth determining apparatus as described in connection with the embodiment of FIG. 3, for example. Accordingly, flow velocity may be correlated with azimuth position in the processor  88 . And, since the light reflections of the CW beam and the pulsed beam are at common azimuth positions in the present embodiment, flow velocity may be computed at the azimuth position of a detected obstacle as well as in other azimuth positions. 
     In some applications, having the CW beam and pulsed beam directed from the system colinearly with substantially the same predetermined pattern is not desirable, particularly where single dimensional flow velocity will suffice. An exemplary embodiment for directing the two beams from the system with different predetermined patterns is shown in the illustrations of FIGS. 16 and 16A. In the embodiment exemplified in FIG. 16, the rotational operative optical element  52  comprises a dichroic wedge optical element including a wedged surface  330  and a flat surface  332 . The optical element  52  may be rotated about an axis normal to the flat surface  332  shown by the dashed line  333 . The wedged surface  330  may be coated with a dichroic coating which has the characteristics of passing light substantially at the wavelength of the CW beam and reflecting light substantially at the wavelength of the pulsed beam, for example. And, the flat surface  332  may be coated with a reflective coating, like gold or silver, for example, which reflects light substantially at the wavelength of the CW beam. Referring to FIG. 16, the pulsed beam exiting from the beam expander  20  along path  324  illustrated by the rays  334  is reflected from the wedged surface  330  of the optical element  52  with a conical pattern towards the mirrored optical element  54  which steers the conical pattern of the pulsed beam azimuthally to effect a helical-like pattern such as the pattern  336  shown in FIG.  16 A. In addition, the CW beam exiting from the beam expander  20  along path  324  illustrated by the rays  338  is passed through the wedged surface  330  of the optical element  52  to the flat surface  332  where it is reflected towards the element  54 . Note that no pattern is imparted to the CW beam because the reflective surface is flat and the optical element  52  is being rotated about an axis normal to the flat surface  332 . Therefore, the optical element  54  will reflect and steer the CW beam in a line pattern through an azimuthal scan like the pattern  340  shown in FIG. 16A, for example. In this manner, the CW beam and pulsed beam may be directed from the combined system with two different patterns steered azimuthally. 
     While the foregoing described embodiment of FIG. 16 describes the optical element  52  as including a wedged optical element, it is understood that other optical elements may be used to serve substantially the same function. For example, a dichroic wobble mirror may be used as optical element  52  for reflecting light of one wavelength from one surface thereof and directing light of another wavelength from another surface thereof. Accordingly, there are a variety of other similar optical elements or combinations of optical elements that could be used as the element  52  just as well as the ones described to impart different predetermined patterns for the CW and pulsed beams.(*) It is further understood that even a single rotationally operated optical element, wedged or otherwise, may be rotated and steered azimuthally to impart the different predetermined patterns to the CW and pulsed beams without deviating from the broad principles of the present invention. 
     In accordance with yet another aspect of the present invention, the optical elements of the LIDAR module  282  may be configured in a block arrangement  350  such as illustrated in FIG. 17, for example. Referring to the embodiment of FIG. 17, the block  350  is comprised of a plurality of glass modules, delineated by dashed lines, which are aligned together to form a plurality of optical paths in the block and secured together to maintain the alignment. The collimated light source  286 , which may comprise the laser diode  286  and beam conditioning optics  290  (see FIG.  15 ), for example, may be secured to the block  350  for generating a coherent beam of light over at least one optical path  354  in the block  350  which guides the coherent beam of light to an exit point  356  thereof. The light detector  314  is also secured to the block  350  which is operative to receive the return coherent beam of light over an optical path  360  and configured to conduct the return coherent beam to the light detector  314  over at lest one other optical path formed therein. Accordingly, the block  350  may be disposed in a LIDAR system on-board an aircraft as a whole and endure the shock and vibration environment of the aircraft without substantial loss of alignment or at least reduce the number of realignments over its lifetime. Thus, once the optical elements are secured in place, the alignment between the optical elements of block  350  should be maintained. 
     Referring to FIG. 17, two of the glass modules  362  and  364  of the plurality are secured together, preferably by cementing, to form the beam splitter  294  (see FIG. 15) that is disposed in the optical path  354  for passing light in a first polarization state along an optical path  366  to exit the block at point  356  and reflecting light in a second polarization state along an optical path  368 . The quarter wavelength plate  298  may be secured, preferably by cementing, to the block  350  at the exit point  356  for converting the polarization of the exiting beam over path  360 . The beam splitter  294  is also formed in the path  366  of the return coherent beam of light. Another pair of glass modules  370  and  372  of the plurality are secured together, preferably by cementing, to form the beam splitter  308  that is formed in an optical path  374  of the return beam. The AOM  304  is disposed in a cavity  376  and secured in place, preferably by cementing. Another module  378  of the plurality comprises a dove prism which is cemented to at least one other module  380  of the plurality to form the optical path  368  that guides the light reflected from the beam splitter  294  to the AOM. The dove prism  378  includes polished surfaces  382  and  384  for forming the optical path  368  by internal light reflections. Light exiting the AOM enters another glass module  386  which has a polished surface  388  for reflecting the light exiting the AOM along an optical path  390  to the beam splitter  308 . 
     An alternate embodiment of a block arrangement  400  for the LIDAR optical elements  282  is shown in the illustration of FIG.  18 . Referring to FIG. 18, the laser source  286  and optics  290  are secured to the block  400  at one side of a glass module  404  for generating a coherent beam of light which is guided along an optical path  402  through the module  404 . A surface  406  of module  404  is cemented to a surface of another glass module  408  to form the beam splitter  294  in the path  402  of the coherent laser beam. Light of one polarization state of the coherent beam is passed through the beam splitter  294  and exits the block  400  at point  410  where the λ/4 plate  298  is secured. Light of another polarization state of the coherent beam is reflected from the beam splitter  294  into a dove prism glass module  412  which is cemented to the glass module  404 . The dove prism  412  includes two polished surfaces  416  and  418  which reflect the reflected light from the beam splitter  294  along an optical path  414 . The AOM  304  is disposed and secured in an opening or cavity  420  which is formed by the sides of the glass blocks  404 ,  408  and a third glass block  422 . Light reflected from the polished surface  418  is passed through glass module  404  and into the AOM  304 . A beam correction optical element  424  may be affixed to the exit end of the AOM  304  to compensate for or readjust the position and angle of the light beam exiting the AOM  304 . A surface  426  of the glass module  422  is cemented to a like surface of the glass module  408  to form the beam splitter  308 . One side  428  of the module  422  is polished to reflect the beam existing the beam correction element  424  along an optical path  430  to the beam splitter  308 . The return beam along path  432  is converted to a linear polarization state by the plate  298  and passed to the beam splitter  294  wherein it is reflected along an optical path  434  through the module  408  to the beam splitter  308  to be combined with the beam from path  430 . The combined beam is directed along an optical path  436  through module  422  to the light detector  314  which is secured to module  422 . 
     Some or all of the glass modules of block  350  or block  400  may be secured together by cementing using an adhesive, preferably an ultraviolet cured optical adhesive, for example. Note that for both glass block embodiments,  350  and  400 , the collimated light source  286  is secured to one side of the block and the exit point of the transmitted collimated light beam is at another side of the block. In addition, the alignment of the glass modules of each block  350  and  400  forms a direct line optical path between the collimated light source  286  and the exit point of the block. In addition, the light detector  314  of each block embodiment  350  and  400  is secured to a side of the block other than the side to which the laser source is secured. Still further, the optical paths of the transmitted and return coherent light beams are co-linear within the block. 
     The illustrations of FIGS. 17 and 18 also depict by symbols the various polarization states of the light beams as they are guided along their respective optical paths. For example, the circled X symbol represents light in a state or plane of linear polarization going into the page parallel to the optical path along which it is guided and the directional arrow symbol represents light in a state or plane of linear polarization going into the page perpendicular to the optical path along which it is guided, that is, at right angles to the circled X polarization state. Also, light in a circularly polarized state is depicted by an arrowed rotation symbol, the direction of rotation is depicted by the arrow. Knowledge of these polarization symbols will yield a better understanding of the operation of the optical elements of the exemplary block embodiments  350  and  400 , which operation having been described in connection with the block diagram embodiment of FIG. 15 herein above. 
     In accordance with yet another aspect of the present invention, a LIDAR system having an embodiment similar to the embodiment described in connection with FIG. 15, for example, is operative to measure flow velocity in three axes of a predetermined coordinate system as will become more evident from the following description. A suitable embodiment of the 3-axis flow velocity determination elements is shown in the block diagram schematic of FIG.  19 . Reference numerals of elements previously described for azimuth determination, scan position determination, display and processing for the embodiment depicted by the block diagram embodiment of FIG. 3 will remain the same for the embodiment of FIG.  19 . Accordingly, these elements will operate structurally and functionally the same or similar to that described for the embodiment of FIG. 3 except that their use in the embodiment of FIG. 19 will be for flow velocity measurement and display. Those elements of the block diagram of FIG. 19 not previously described will now be described. 
     Referring to FIG. 19, as previously described for the LIDAR system embodiment of FIG. 15, electrical return signals which are generated by the light detector  314  in response to light reflections from a particle along the predetermined scan pattern of the transmitted CW laser beam are passed over signal line  316  to the signal conditioning circuit  318  which may comprise conventional amplification and filtering circuits appropriate for conditioning the electrical signals. These electrical signals will be burst signals of Doppler frequency content lasting as long as a particle is within the width of the transmitted laser beam which will herein after be referred to as a “hit”. After the signal conditioning of the circuitry  318 , each burst of electrical signaling is sampled and digitized in an analog-to-digital (A/D) converter  440  in accordance with a predetermined sampled data rate which may be on the order of one-hundred and seventy-five million samples per second (175 MSPS), for example. The resultant data samples of each hit are provided to a digital signal processor (DSP)  442  for processing to determine the Doppler frequency associated therewith which is stored in a memory  444  thereof in the form of a data word for retrieval by the processor  88  as will be more fully described herein below. The processing of the digitized data samples of a burst or hit may take the form of a Fast Fourier Transform (FFT) algorithm or autocorrelator algorithm, for example, programmed into the DSP  442 . Signal lines  446  coupled between processor  88  and DSP  442  provide for handshaking and data word transfers as will become evident from the following description. In the present embodiment, the processors  88  and  442  may be DSPs of the type manufactured by Texas Instruments bearing model numbers TMS320-C33 and TMS320-C6201, respectively, for example. It is understood that separating out and performing the system functions in two digital processors in the present embodiment offer design convenience and ease and that in an alternate embodiment, the functions of the DSP  442  may be programmed into a single DSP, like the processor  88 , for example, which may perform by itself the functions of both processors  88  and  442 . It is also possible that more than two processors may be used to embody the overall processing functions. Accordingly, this aspect of the present invention should not be limited to the number of processors, which will be determined based on the particular application of the invention. 
     FIGS. 20 and 20A illustrate functionally the processing involved for the determination of flow velocity in the 3-axes of the predetermined coordinate system. As has been described herein above, in one embodiment, the LIDAR system projects a laser beam  450  of a predetermined width in a conical pattern as shown in the illustration of FIG.  20 . In FIG. 20, a plane  452  which is circular in cross-section (see FIG. 20A) is taken through the conical pattern at a range R from the LIDAR system where a hit  454  occurs. This plane or slice  452  is referred to herein as a scan circle brought about by the rotation of the optical element  52 , for example. As described herein above in connection with the embodiment of FIG. 3, each time the optical element  52  is rotated past a reference point of the cyclic rotation, a trigger signal is generated. This reference point is referred to as the trigger position  456  of the scan circle. In the present embodiment, Y and Z quadrature axes of the predetermined coordinate system exist in the plane of the scan circle. More particularly, the Y-axis is along a line  458  drawn from the center  460  of the scan circle to the trigger position  456  and the Z-axis is along a line  462  drawn from the center  460  of the circle  452  90° counter-clockwise from the Y-axis. The X-axis of the coordinate system is along a line  464  drawn perpendicular to the scan circle plane  452  through the center  460  thereof. Accordingly, the X-axis is projected from the apex of the conical pattern as it exits the LIDAR system through the center  460  of the plane  452 . Now that the ground-work has been laid, the concept of determining the flow velocity in three axes, Vsx, Vsy, and Vsz, may be described. 
     Each time a hit like at point  454 , for example, is detected from the resulting electrical signal burst, a Doppler frequency is determined from the data samples of the associated burst. Knowing the wavelength of the laser beam, a one-axis flow velocity V 1  for the hit may be determined from the corresponding Doppler frequency. In addition an angle a 1  on the scan circle corresponding to the hit point  454  may be determined in relation to the Y-axis based on the elapsed time from the last trigger signal and the scan circle period, i.e. the total time to complete a scan of the circle pattern, which will become more evident from the description found herein below. The angle t that the hit makes with the X-axis remains substantially fixed for the circular scan pattern. Accordingly, a set of three equations may be established for three hits H 1 , H 2  and H 3  around the scan circle based on their single axis velocities V 1 , V 2  and V 3  and scan circle angles a 1 , a 2  and a 3  (angle t being fixed for the present embodiment) using trigonometric identities as shown by way of example in FIG.  20 B. Referring to FIG. 20B, the top, middle and bottom equations may be each solved for flow velocities Vsx, Vsy and Vsz along the X-axis, Y-axis, and Z-axis, respectively. Also, knowing the azimuth position of the scan circle pattern from which the three hits are taken will establish a reference point in azimuth of the 3-axis flow velocity. 
     One complication arises by not knowing when a hit will occur, i.e. a hit may not be forced to occur. Rather each hit occurs naturally as a particle, such as dust or gaseous or vapor condensation, for example, crosses the width of the laser beam as it is guided along its predetermined pattern. Another complication arises as a result of the large number of hits likely to occur and the burden on the processor should all of the detected hits be processed. Thus, a selection criteria is desirable to determine which of the detected hits along the path of the scan pattern should be processed and which of the processed hits should be used to determine the 3-axis flow velocity. These selection criteria will be described in greater detail in the following paragraphs. 
     In addition, the predetermined coordinate system described above for determining the 3-axis flow velocity is referenced to the LIDAR system and may not be the same as the flight coordinate system of the aircraft on-board which LIDAR system is mounted. FIG. 21 exemplifies a LIDAR system  470  mounted on-board an aircraft  472 , which, for this example, is a helicopter, with the two coordinate systems of the LIDAR and aircraft being not the same. That is, the LIDAR scanner  470  has its X, Y and Z coordinate system as described herein above and the aircraft  472  has its own X, Y and Z coordinate system. Since it may be important that the pilot or operator know the flow velocity based on the aircraft&#39;s coordinate system, the flow velocity of the LIDAR system Vsx, Vsy and Vsz may be converted to a flow velocity referenced to the aircraft&#39;s coordinate system Vax, Vay, and Vaz using a set of three equations shown by way of example in FIG.  21 A. Transformation constants a ij  may be formed into a 3×3 matrix, where i represents the column and j represents the row of the matrix. This 3×3 conversion matrix may operate on the LIDAR velocity vector which is expressed as a single column matrix comprising the velocity components of the LIDAR coordinate system to obtain the aircraft&#39;s velocity vector which is also expressed as a single column matrix comprising the velocity components of the aircraft&#39;s coordinate system. 
     An exemplary program flow organization for programming the processor  88  to determine 3-axis flow velocity measurements is shown by the block diagram of FIG.  22 . Referring to FIG. 22, upon turning on processor  88 , a main program, which will described more fully herebelow, is run to initialize the processor in block  474 . Next, the processor enters a foreground program in block  476  which will be more fully described in connection with the flow diagram of FIG.  23 . The foreground program  476  is executed continuously to call various other programs like an evaluate function program  478  (see FIG.  27 ), a velocity function program  480  (see FIG.  28 ), and an output function program  482  based on a plurality of interrupt service routines (ISRs), like a clock function ISR  484  (see FIG.  24 ), a trigger function ISR  486  (see FIG.  25 ), and a serial function ISR  488  (see FIG.  26 ). In the present program organizational example, that which triggers the clock function ISR  484  is a Timer  0  which may be a designated register of processor  88  configured to count through a total count which represents a predetermined time period. Each time Timer  0  counts through its predetermined time period, which may be 100 microseconds, for example, the function clock ISR  484  is executed. Another register of processor  88  may be designated as Timer  1  and configured to start counting from zero each time the processor  88  receives the trigger signal  116  described in connection with the embodiment of FIG. 19 through a an interrupt port INT  0 . The trigger signal  116  causes the trigger function ISR  486  to execute. Also, when a data word is received from DSP  442  via a serial Port  0 , it will be stored in a register of the processor  88  designated as a data receive register  490  as will be more fully described below. Upon completion of the transfer of the data word into processor  88 , the serial function ISR  488  is executed. 
     In an exemplary software flow of the main program  474 , the serial Port  0  is configured to be the port through which requests for data words are made to the DSP  442  in response to the generation of a Frame Sync Signal  494  by the foreground function routine  476  (see FIG.  22 ). Port  0  is also configured to receive the data word from the DSP  442  and store it into register  490  and call serial function ISR  488  upon completion of the data word transfer. Timer  0  is configured to call the clock function ISR  484  each time it counts through a count representative of 100 microseconds, for example. Timer  1  is configured to count freely until reset by the trigger function ISR  486 . The INT  0  port is configured to call the trigger function ISR  486  each time a trigger signal  116  is received over a line coupled thereto from the scan pattern scanner  52  (see FIG.  19 ). A display write function of processor  88  is initialized with certain commands well-known to all those skilled in the pertinent art to form text messages and control the screen of the display  154 . Once the initialization tasks of the main program  474  are complete, the foreground function routine  476  is called. 
     Referring to FIG. 23, in block  506 , it is determined whether or not a “Get Data Flag”  508  is set true which is effected every 100 microseconds by the clock function ISR  484 . If true, block  510  generates the Frame Sync Signal  494  to Port  0  to initiate the request for a data word from the DSP  442 , sets the Get Data Flag  508  false, and executes decisional block  512 . If the Get Data Flag  508  is determined to be false by block  506 , the execution of block  510  is bypassed and decisional block  512  is executed. In block  512 , it is determined whether or not a Data Ready Flag  514  is set true by the serial function ISR in response to the completion of the transfer of the data word into register  490 . If true, the evaluate function routine  478  is called for execution by block  516 . Upon completion of the tasks of the evaluate function  478 , program execution is returned to  516  whereupon the Data Ready Flag  514  is set false and block  518  is executed. If the Data Ready Flag  514  is determined to be false by block  512 , then block  516  is bypassed and decisional block  518  is executed. In block  518 , it is determined whether or not a Display Flag  520  is set true by the clock function ISR  484 . If true, block  522  calls the velocity function routine  480  for execution and when its tasks are complete, program execution is returned to block  522 . Block  522  next calls the output function routine  482  for execution and when its tasks are complete, program execution returns to block  522  which next sets the Display Flag  520  false. Upon completion of the execution of block  522  or if the Display Flag  520  is determined to be false by block  518 , program execution is returned to decisional block  506  and the program flow repeated. In this manner, the foreground function  476  is continuously executed. 
     Referring to the flow diagram of FIG. 24, each time the Timer  0  counts through its predetermined count, i.e. every 100 microseconds, program execution is interrupted and the clock function ISR  484  is called for execution. In block  526 , the Get Data Flag is set true and a Display counter which may be a designated register of the processor  88  is incremented by one count. Next, in block  528 , it is determined whether or not the count of the Display counter has reached a desired count which is indicative of an increment of time. For example, if the Display counter is incremented one count every 100 microseconds and the increment of time desired is 250 milliseconds, then the desired count would be 2500. Accordingly, the Display counter is a vehicle used to establish time increments of 250 milliseconds in the present embodiment. Thus, every 250 milliseconds as determined by block  528 , block  530  sets the Display Flag true and resets the Display counter to zero. Thereafter, program execution returns to where it was interrupted and the clock function ISR  484  sits idle waiting for the next internal interrupt from Timer  0 . 
     Referring to the flow diagram of FIG. 25, each time the trigger signal  116  is received by the interrupt port INT  0 , program execution is interrupted and the trigger function ISR  486  is called for execution. In block  532 , the count in Timer  1  which is representative of a period of one scan cycle is read and stored in a designated register of processor  88  and Timer  1  is reset to zero count. Thereafter, program execution continues from its interruption point and the trigger function ISR sits idle waiting for the next external interrupt signal  116 . Referring to the flow diagram of FIG. 26, each time the data word transfer is completed, the serial function ISR  488  is called for execution. In block  534 , the data word of register  490  which is indicative of the Doppler frequency of the hit and the count of Timer  1  which is indicative of the corresponding scan circle angle al of the hit are read and stored in designated registers of the processor  88  and the Data Ready Flag is set true. Thereafter, program execution continues from its interruption point and the serial function ISR sits idle waiting for reception of the next internal interrupt signal. 
     In accordance with the foregoing described embodiment, the processor  88  requests and inputs a data word from the DSP  442  every 100 microseconds. Since it is unknown whether or not a hit has occurred during the most recent 100 microsecond interval, it is not known if the received data word from the DSP  442  for the current 100 microsecond interval is the same data word received for the previous 100 microsecond interval, i.e. no hit during the current interval. Thus, some indication should be provided to the processor  88  to indicate that at least one hit occurred during the current interval. In the present embodiment, this indication is provided in the form of one of the bits of the data word designated as “New Bit” being set to a “1” to indicate that the data word is representative of the Doppler frequency of a hit during the current interval. Accordingly, with each received data word from the DSP  442 , an evaluation thereof is performed by the evaluate function  478 , a flow diagram of which being shown in FIG.  27 . 
     Referring to the flow diagram of FIG. 27, in block  540 , it is determined whether or not New Bit is set to a “1” in the received data word. If not, program execution of the evaluate function routine  478  is aborted and execution is returned to block  516  of the foreground routine  476 . Otherwise, it is next determined in block  542  if the new data word is the first hit or data point for the current evaluation period. If so, in block  544 , the data word (Doppler frequency) and angular position of the first hit or data point is stored and designated as belonging to the first data point. Also, in block  544 , and target positions for the 2nd and 3rd hits along with acceptance regions therefor are established. In the present embodiment, the target positions for the 2nd and 3rd hits may be approximately 120° and 240°, respectively, in relation to the position of the first data point and the acceptance regions of each may be on the order of ±60°, for example. Then, in block  546 , a data point counter of processor  88  having a count indicative of the number of data points received for the present evaluation period is incremented by one. Program execution is then returned to block  516 . 
     If, in block  542 , it is determined that the most recent data point is not the first, then, in block  548 , its angular position is determined from a ratio of the count of Timer  1  corresponding to the recent hit and the count representative of the period of the scan cycle. The angular position of a data point subsequent the first data point is subtracted from the angular position of the first data point. Next, in block  550 , it is determined if the difference in angular position is within the target and acceptance region for the 2nd data point or 120° ±60°, for example. If so, in block  552 , the data word (Doppler frequency) and its corresponding angular position are stored and designated as belonging to the 2nd data point. Also, in block  552 , after each 2nd data point with an acceptable target and acceptance region is determined, the acceptance region is tightened. For example, after the first 2nd data point, the acceptance region may be set to ±50°, and after the second 2nd data point, the acceptance region may be set to ±40°, and so on until no more 2nd data points fall within the region. This evaluation process ensures that only the closest 2nd data point to the target of 120°, for example, will be used in the determination of the 3-axis flow velocity. Further, in block  552 , a “Point  2  Valid Flag” is set true to indicate that a 2nd data point is found valid for processing. If it is determined that a subsequent data point to the first data point is found not to be within the target and acceptable regions set for the 2nd data point, then in blocks  554  and  556 , the same processing as for blocks  550  and  552  is repeated for the 3rd data points to establish a 3rd data point within the closest acceptable region of the set target angle or 240°, for example, in relation to the first data point. After each execution of either block  552  or block  556 , the data points counter is incremented by one in block  546  so that its total count is representative of the total number of data points evaluated for the current evaluation period which may be on the order of 250 msec., for example. In this manner, three data points are selected from all of the data points processed in each 250 msec. period and their respective angular positions are the closest to being 120° apart along the scan circle pattern. 
     An example flow diagram of the velocity function routine  480  which is run every 250 msec. in the present embodiment is shown in FIG.  28 . Referring to FIG. 28, in block  560 , the data point counter is read to determine if at least three data points were processed in the preceding evaluation period. If so, in block  562 , it is determined if the Valid Flags for the 2nd and 3rd data points are set true which is an indication that there are three data points which fall within the predetermined acceptance criteria of relative angular positions about the scan circle, i.e. the selected data points. If so, then three single axis velocities V 1 , V 2  and V 3  are determined in block  564  from the Doppler frequencies (data words) of the selected three data points. Thereafter, in block  566 , a 3-axis flow velocity measurement is determined from the three single axis velocities V 1 , V 2  and V 3  and their respective angular positions a 1 , a 2  and a 3  (t being fixed for all 3 data points) in accordance with the exemplary equations of FIG. 20B, for example. The velocity components Vsx, Vsy and Vsz based on the predetermined coordinate system of the LIDAR may be converted to velocity components Vax, Vay and Vaz of the aircraft on-board which the LIDAR system is mounted in block  568 . And, in block  570  the data used in the aforementioned calculations may be characterized in some manner. For example, a data validity flag may be set to good data, if the data point distribution in the acceptance regions is considered good, and a data rate may be calculated. Finally before returning execution to block  522  of the foreground function routine, all of the flags set by the evaluate function routine  478  in the previous evaluation period are reset in block  572  for the next evaluation period. 
     Now, if it is determined in block  560  that in the previous evaluation period less than three data points were processed, then, the data quality will be characterized by setting data validity to a low data rate, for example, and calculating the data rate in block  574 . Also, if it is determined in block  562  that there are not three valid data points for processing based on the current acceptance criteria for data point distribution, then, in block  576 , the data may be characterized by setting data validity to poor data distribution, for example, and calculating the data rate. After either block  574  or  576 , program execution is passed to block  572  for resetting the flags as previously described. 
     An exemplary flow of an output function routine  482  suitable for use in describing the programmed processing of the processor  88  will now be described. This routine  482  is also called every 250 msec., for example, after the velocity function routine  480  is executed. First it is determined if data validity was set at low data rate and if so, certain message text is selected for display on the screen of the display  154 . For example, a text message which displays an indication of Low Data Rate may be generated and sent to the display. Also, a signal which is formatted to indicate low data rate may be generated and provided to an interface to other aircraft avionics. Similarly, if it is determined that data validity was set to poor data distribution, then an appropriate text message may be generated and sent to the display and formatted for distribution to other aircraft avionics to indicate this condition. If neither determination is a true or affirmative condition, a text output or message indicative of the 3-axis flow velocity measurement is generated and sent to the display screen, and also, the velocity measurement is formatted and sent to other aircraft avionics over signal line(s) interfaced with the processor  88 , for example. After the tasks are completed, program execution is returned to block  522  of the foreground function routine  476 . 
     While an embodiment of a combined LOAS and LIDAR system has been described herein above in connection with the block diagram of FIG. 15, it is understood that from a practical perspective when applied to a moving vehicle like a helicopter or UAV, for example, the common optical elements  284  may be embodied in a scan head  600  remotely located from the optical elements of a single LOAS  280  or the combined LOAS  280  and LIDAR system  282  such as shown in the exemplary block diagram schematic of FIG.  29 . Common elements between the embodiments of FIGS. 15 and 29 will have like reference numerals. In the embodiment of FIG. 29, the optical elements of  280  and  282  may be disposed within the vehicle and well supported and protected from the environment of the vehicle. Conventional fiber optic cabling may be used for the optical paths  18  and  319  leading to and aligned with the dichroic filter optical element  320  which was previously described for the embodiment of FIG. 15. A further fiber optic cable provides for the optical path  322  from the dichroic filter  320  to the scan head  600  which includes the common optical elements  284 . The fiber optic cabling for the optical path  322  may take a circuitous route within the vehicle to reach the scan head  600  which may be mounted to the external surface of the vehicle to permit the beam scan patterns to be projected out from the vehicle. More than one scan head may be used in the present embodiment as will become more evident from the description found herein below. 
     A suitable embodiment of the scan head  600  is shown in the sketch of FIG.  30 . This scan head controls movement of the optical beam scan patterns along three axes  602 ,  604  and  606 . A top  608  of the scan head  600  may be mounted to a surface of the vehicle, like the front underbelly of a helicopter or UAV, for example, such as shown in the sketch of FIG. 21. A window area  610  of the scan head  600  through which the beam scans are emitted would be pointed in the direction of movement of the vehicle or flight path, if the vehicle is an aircraft. The fiber optic cable of the optical path  322  may be passed through a hole in the skin of the vehicle and into the scan head  600  through an opening  612  at the top  608  thereof. The optical elements within the scan head  600  which will be described in greater detail herein below cause the beams passed by the path  322  to be scanned 360° about the axis  606 . A conventional motor assembly (not shown) within the scan head  600  controls movement of a lower portion  614  thereof ±90° about the axis  602  azimuthally with respect to the flight path of the vehicle. This movement occurs along a seam  616  between the top and bottom portions,  608  and  614 , respectively, and effectively moves the axis  606  along with the lower portion  614  which projects the beam scan pattern through a helical pattern much the same as that described in connection with the example of FIG.  2 . 
     Another portion  618  of the scan head  600  which includes the window area  610  and falls within the portion  614  moves azimuthally with the portion  614 . Another conventional motor (not shown) disposed within the scan head  600  controls movement of the portion  618  about the axis  604  +30° to −90° in elevation, for example, with respect to the flight path or direction of the vehicle. This movement causes the axis  606  and scan patterns to move in elevation with the portion  618 . In the present embodiment, the window area  610  of the portion  618  may be controlled to move upward and inside the portion  614  to protect it from the environment when not in use. The corrugated skin or surface in the area  620  at the top portion  608  acts as a heat sink to improve the transfer of heat away from the scan head  600  during operation thereof. 
     A sketch exemplifying the common optical elements inside the scan head  600  is shown in FIG.  31 . Referring to FIG. 31, the fiber optic cabling of the optical path  322  is aligned with the axis of the input aperture of the beam expander  20 . The beam exiting the expander  20  may be reflected from a fold mirror  325  over an optical path  324  and passed into the rotating optical element  32 . In the present embodiment, the rotating optical element  32  comprises a rotating optical wedge element  622  centered and rotated about the axis  606  and having a flat surface  624  at its input side and a surface inclined at a predetermined angle at its output side. It is understood that other elements may be used for the rotating optical element  32 , like a transparent liquid crystal scanner, for example, without deviating from the broad principles of the present invention. 
     The beam conducted over path  324  is aligned with the axis  606  and passed from the input side to the output side of the wedge element  622 . The light beam is refracted in its path through the wedge element  622  and exits perpendicular to the inclined output surface  626  thereof. This refraction of the light beam causes it to exit the scan head  600  as beam  36  through the window area  610  at an angle  628  to the axis  606 . Accordingly, as the wedge optical element  622  is rotated 360° about the axis  606 , the beam  36  is projected conically from the scan head  600  to form the scan pattern  630 . Return beams will follow the same optical paths as their emitted beams as described herein above. The window area  610  may comprise a clear, flat, zero power optical element made of a material like glass, for example, so as not to interfere substantially with the scan pattern of the exiting beam  36 . In the present embodiment, the wedge optical element  622  and window  610  are structurally coupled to move together along the azimuth path  632  and elevation path  634  to cause the optical axis  606  to move along therewith. In this manner, the scan pattern  630  is forced to move in azimuth and elevation with the portions  614  and  618  of the scan head  600 . 
     As noted above, the present invention may be embodied to include more than one scan head mounted at different locations on the vehicle. Depending on the application, some of the scan heads may utilize fewer optical elements and less scan angle than that described for the embodiment of FIGS. 30 and 31. In one application, the scan head  600  may be mounted at the front under belly of a helicopter or UAV as described herein above to detect objects and wind conditions at the front and sides of the aircraft, for example, and a second scan head  640  may be mounted at the tail section of the helicopter, for example, to detect objects at the rear and sides of the aircraft. A system suitable for embodying this application is shown in the block diagram schematic of FIG.  32 . In this embodiment, an optical switch  642  is disposed in the output optical path  644  of the LOAS  280 . The path  644  may be formed by a fiber optic cable. The optical switch  642  may be controlled by a signal  646  to direct the beam of path  644  to one of a plurality of optical paths. For example, the optical switch  642  may be controlled to direct the LOAS beam over the fiber optic cable of path  18  to the dichroic filter  320  and on to the scan head  600  as described herein above in connection with FIG. 29, or to direct the beam over an optical path  648 , which may be formed by a fiber optic cable, to the tail scan head  640 , or to direct the beam to other scan heads (not shown) mounted elsewhere on the vehicle over other optical paths  650 . The return beam will follow substantially the same optical path as the directed beam. 
     A suitable embodiment of the high-speed optical switch  642  is shown in the sketch of FIG.  33 . In this embodiment, a flip mirrored element  652  is mounted with vertical hinges  654  and  656  to be controlled in a horizontal rotation thereabout and is mounted with horizontal hinges  658  and  660  to be controlled in a vertical rotation thereabout. The optical switch may be fabricated on a substrate using micro-electromechanical system (MEMS) techniques with miniature motors coupled to the hinged mountings for controlling the movement of the mirrored element  652  to direct the beam  644  to one of the optical paths  18 ,  648 , or  650  at any given time. Accordingly, the beam  644  and its returns may be multiplexed among the aforementioned paths by controlling the optical switch with the control signal  646  which positions the motors of the switch. It is understood that the embodiment of FIG. 33 is merely an exemplary embodiment of the optical switch  642  and that other embodiments may be used just as well. For example, a rotating disc having a portion that is substantially clear to permit passage of the beam and its returns along one of the paths  18 ,  648  or  650 , and a portion that has a reflective coating to cause the beam and its returns to be reflected along another of such paths may be positioned by a motor controlled by the control signal  646  to direct the beam  644  and its returns to a designated optical path by passage or reflection thereof. 
     In yet another embodiment as shown by the block diagram schematic of FIG. 34, multiple scan heads may be mounted at various locations on the vehicle to detect objects and determine wind conditions at predetermined regions surrounding the scan head locations. For example, one scan head  662  may be located at one wing of an aircraft or side of a vehicle and another scan head  664  located at the other wing or side. The scan head  662  which may be mounted on the right wing or side with respect to the direction vector of the vehicle may be adjusted to scan azimuthally from 0° to +90° (0° being the direction vector of the vehicle) to cover the front right side region of the vehicle. Similarly, the scan head  664  which may be mounted on the left wing or side with respect to the direction vector of the vehicle may be adjusted to scan azimuthally from 0° to −90° to cover the front left side region of the vehicle. Other scan heads may be mounted at other locations like at the tail of the aircraft or rear of the vehicle, for example. All such scan heads are processed by a single LOAS or a combined LOAS  280  and LIDAR  282  system. For this reason, a high speed optical switch  666  is utilized and controlled to multiplex the emitted beams of the single or combined system and their returns among optical paths  668 ,  670  and  672  to and from the scan heads  662 ,  664  and others, respectively. In the present embodiment, the switch  666  may be disposed in line with the optical path of the LOAS and/or LIDAR beams exiting the dichroic filter  320  and may be the same or similar to the type of optical switch used for the embodiment of FIG. 33 described herein above. 
     A common technique used by EMS and other rescue personnel during landing operations is to conduct higher level reconnaissance flight patterns in an attempt to identify obstacles, including electrical wires and support structures, for example, and avoid potential strikes with the aircraft during descent from five hundred feet and below. Typically, these EMS aircraft include searchlights, night vision cameras, and other equipment installed under the aircraft to assist in obstacle avoidance. As such, the placement of obstacle sensing devices in this area of the aircraft may have obstructed fields of view to detect obstacles due to the presence of all of the other assist devices, and the landing wheels or skids. Moreover, today&#39;s modem higher performance helicopters, such as the S-76, for example, are very aerodynamic limiting the use of bolt on protruding devices under the aircraft, preferring rather sensing devices mounted flush to the fuselage of the aircraft. 
     With respect to military applications, often UAVs and PGMs have very specific outer aerodynamic profiles developed to reduce the signature from thermal emissions, RADAR, acoustic and other aircraft detection systems. This aircraft architecture limits the ability of introducing protruding obstacle detection devices to the platform due to the size and weight thereof and signature rendered thereby. Moreover, a new class of suitcase sized mini-UAVs has caused the development of new technologies in propulsion, sensing and electronic systems. Accordingly, in order to operate small or large UAVs significant consideration to the size and weight of the obstacle sensing device, as well as the signature it renders, is important. 
     In operating aircraft, such as fixed wing aircraft, helicopters, UAVs and PGMs, for example, at low altitudes in the presence of obstacles, such as buildings, trees, structures, wires and the like, the speed of such aircraft is controlled so as to give sufficient response time to navigate around the obstacles. Generally, for commercial applications, this is acceptable in order to give the pilot and/or flight crew sufficient time to visualize and avoid the obstacles. However, despite the slower forward or descent speed, often flight crews do not see the obstacles or simply neglect the presence of the hazard. On the other hand, for military missions at low altitude flight profiles, it is often not desirable to slow the forward motion of the craft because ground based light munitions become as great or a more significant threat than surrounding obstacles. Under these circumstances, it is highly desirable to fly the nap of the Earth (NOE) type missions at higher speeds while visualizing on avoiding obstacles in the flying environment. Further, designers of such aircraft would prefer to automate the ability of UAVs, PGMs and helicopters to maneuver through these obstacle cluttered flight environments at high speeds. 
     In accordance with the present invention, a distributed laser based obstacle awareness system (DLOAS) uses a plurality of small, obstacle detection sensors mounted flush to the fuselage of an aircraft to detect range to a target obstacle in known directions or corridors. FIG. 35 illustrates such a system embodied, by way of example, on a helicopter aircraft  700 . In the present embodiment, four obstacle sensing devices are flush mounted to the lower sides of the fuselage of the aircraft  700  to scan the four quadrants about the aircraft  700 . Two such sensors are shown flush mounted at  702  and  704  on the side of the aircraft  700  shown in the illustration of FIG.  35  and two other sensors are mounted at the same or similar locations of the fuselage on the other side of the aircraft not shown in the illustration. Each of the four sensors emits a laser beam which is line scanned vertically over an elevation range of from 0° to −90° with respect to the horizontal, for example, as illustrated by the scan lines  706  and  708  for the sensors  702  and  704 , respectively. As the aircraft is flown in a circular reconnaissance flight path about a targeted landing zone, for example, the four directional laser beam sensors will paint out a scan of the complete landing zone area from 0° to −90° in elevation from the flight vector and 360° around the zone as will be described in greater detail in connection with the illustration of FIG. 40 herein below. 
     While a helicopter aircraft is used for the present embodiment, it is understood that the DLOAS may just as well be mounted on other aircraft, such as fixed wing aircraft, UAVs and PGMs, for example. In addition, it is further understood that less or more than four obstacle detection sensors may be mounted to the aircraft for scanning out predetermined corridors in space without deviating from the broad principles of the present invention. 
     A block diagram illustration of a DLOAS suitable for embodying the broad principles of the present invention is shown in FIG.  36 . Referring to FIG. 36, a laser source  710  emits pulsed laser energy over an optical path  712  to an optical switch  714  via a collimating lens  716 . The optical switch  714  which may be comprised of any one of a conventional mechanical scanner, a resonant scanner, a micro electromechanical systems (MEMS) scanner, such as that described in connection with FIG. 33 herein above, or a fiber optic switch, for example, is actuated to redirect the laser beam from path  712  to one of a plurality n fiber optic channels CH 1 , CH 2 , . . . CHn. This operation is illustrated by way of example in FIG. 37 using a resonant scanner mirror as the optical switch  714 . As the resonant scanner  714  is actuated from one position to another about an axis  718 , it directs the laser beam  712  to each input of the plurality of optical channels CH 1 , CH 2 , . . . , CHn. 
     In the present embodiment, each optical channel includes a plurality of transmission fiber optic cables designated as T 1 , T 2 , . . . , Tn and at least one receiver fiber optic cable designated as R 1 , R 2 , . . . , Rn. In each channel, the transmission fiber optic cables may be separated from the at least one receiver fiber optic cable or bundled together therewith. Each channel of the present embodiment comprises a bundle of fiber optic cables arranged in a densely packaged configuration, preferably with a plurality of the transmission cables T 1  surrounding the at least one receiver cable R 1  such as shown in the illustration of FIG. 40 so as to transmit and receive laser energy with a minimum amount of energy loss. Accordingly, laser energy is directed from the optical path  712  into each fiber optic bundle CH 1 , CH 2 , . . . , CHn in a time sequenced manner and propagated along to a corresponding optical scanner or collimating sensor SC 1 , SC 2 , . . . , SCn, respectively, which are distributed about the fuselage of the aircraft as described in connection with the embodiment of FIG. 35, for example. At each such sensor, the laser energy directed thereto from its respective fiber cable is emitted from the aircraft in a ring-like fashion within the predesignated zone or region of the sensor to a target obstacle. At the target, laser beam divergence of each transmission fiber T 1  will blend the ring-like energy emissions of the plurality of fibers into a uniform laser spot. 
     An exemplary embodiment of an optical scanner SCi, where index i may range from 1 to n, suitable for use with a bundled optical channel CHi is illustrated in FIG.  39 . Referring to FIG. 39, the obstacle detection scanner SCi is flush mounted to the fuselage  720  of the aircraft such that the pulsed laser beam may be emitted and received through a windowed area  722  which is flush to the surface of the fuselage  722 . In this embodiment, laser energy is directed to the sensor SCi over the transmission optical fibers Ti of the bundled channel CHi. As the laser beam exits the channel, it expands or diverges naturally. A collimating lens  724  located in close proximity to the beam exit point prevents further divergence of the beam and collimates and directs the beam towards an optical scanner  726  which in the present embodiment comprises a small, light weight resonant scanner, for example. The resonant scanner  726  includes a mirrored element  728  which is oscillated or pivoted back and forth by a motor  730  via a mechanical linkage  732 . The collimated laser beam is reflected by the mirror  728  and emitted from the aircraft via windowed area  722  in elevation line scans ranging from 0° to −90° with respect to the flight vector of the aircraft. For collimated sensor applications, the collimated laser beam may be emitted directly from the collimating lens  724  without the use of the scanning optics  726 . In other applications, the scanner  726  may be rotated in azimuth as well as line scan oscillated in elevation. 
     When the emitted beam makes contact with an obstacle  736  within its search region  738 , a beam of energy is reflected back to the sensor unit SCi and therein directed to and focused on the receiver fiber optic cable Ri of the bundled optical channel CHi, via optical elements  728  and  724 , wherein it is propagated back to the light detector  740 . The light detector  740  may comprise an avalanche photodiode (APD) or, in the alternative, a PIN photo diode with an Erbium doped fiber amplifier (EDFA), for example. The EDFA may be used to increase the optical system gain normally accomplished by the APD. Also, the receive optical fibers R 1 . R 2 , . . . , Rn of the present embodiment are separated from their bundled channels away from the obstacle detecting sensors and bundled together and optically combined for focusing onto the detector  740  using a conventional optical return signal combiner  742  as illustrated in FIG.  38 . The system as described is referred to a bistatic optical system because the transmission and return laser pulses are propagated along different optical paths, thus eliminating the need for a beam splitter as described herein above. Also, since each region or corridor corresponds to an assigned optical channel and obstacle detecting sensor, the region or corridor of a detected obstacle may be determined from the corresponding dwell time or switch position during which the reflected laser energy is received. Range and position of the obstacle in the known region or corridor may be determined in a similar manner as for the LOAS embodiments described herein above. 
     An alternate embodiment of an optical scanner SCi suitable for use with an unbundled bistatic optical channel CHi is illustrated in FIG.  41 . Referring to FIG. 41, a pulsed laser beam directed over the plurality of transmission fiber optic cables T i  is directed to a fiber optic collimator  760  which may expand the beam diameter by a factor of 5, for example. Laser light exiting the collimator  760  which may be on the order of four millimeters in diameter, for example, is directed to a beam expander  762  wherein the beam diameter may be expanded again by 5X, for example, to a beam diameter of approximately twenty millimeters. The expanded beam as shown by the darkened arrowed lines exiting the expander  762  is reflected from a turning mirror  764  to a turning prism or another turning mirror  766  from which it is directed to the optical scanner  726  which has been described in connection with the embodiment of FIG.  39 . In the present embodiment, the optical scanner  726  could be a resonant scanner, an optical wedge or even a Palmer mirror, for example, to create a sinusoidal pattern in the search area. As in the embodiment of FIG. 39, for collimated sensor applications, the collimated laser beam may be emitted directly from the turning prism  766  without the use of the scanning optics  726 . Laser energy reflected from an obstacle back to the scanner unit SCi as shown by the thin arrowed lines is directed to a receiver achromatic lens which focuses the returned beam to a focal point  770 . A supporting device  772  maintains the receiving end of the receiver fiber optic cable R i  at or near to the focal point  770  so that all or most of the returned laser energy enters the receiver fiber optic cable wherein it is propagated back to the light detector  740 , which may be a GaAs avalanche photodiode, for example, as described herein above in connection with FIG.  38 . 
     As the optical switch  714  is actuated to a position to direct the laser beam to an optical channel, it is held at that position for a period of time (dwell period) for the pulsed beam to exit the aircraft and return from an object in the search area. In those applications in which the DLOAS is embodied on a mini-UAV, for example, that may autonomously be guided through a building or cave, the DLOAS would have to look out less than fifty feet, for example. However, when the DLOAS is embodied onboard an a helicopter or conventional UAV, the system would have to look out further, like on the order of several hundred feet, for example. As such, the time of flight of the pulsed laser beam would govern the dwell period of the optical switch  714  on a particular optical channel. Moreover, once information is known about a given obstacle in a particular corridor or region, the optical switch  714  may be tasked to dwell on the corresponding optical path for an extended period of time. 
     Use of the DLOAS in the helicopter  700  when preparing to land in a predetermined landing zone  750  is illustrated in FIG.  40 . Referring to FIG. 40, the helicopter  700  may move in a circular flight path around the landing zone  750  as shown by the arrows  752  and  754 . In so doing, the elevation line scan  756  emitted by each of the four obstacle sensing devices will scan optically its respective quadrant corridor or region. Forward movement of the aircraft will cause the elevation line scan to scan in azimuth as well. Upon completion of the circle of flight above the landing zone  750 , the DLOAS will have scanned the complete airspace above the landing zone  750  with the four sensors and will have detected the obstacle or wire  758  therein, and thus could avoid a collision therewith. 
     The DLOAS approach has the advantage of being overall very light weight and small due to the centrally disposed laser source  710 , fiber optic channels and small, light weight optic scanners or sensors distributed about the aircraft. Accordingly, the system does not use a single large, heavy mechanical scanner. Rather, since the centrally disposed laser source  710  need only provide enough pulsed laser energy for a single bistatic optical fiber channel and region or corridor at any given dwell time and since the loss of laser energy during propagation is minimized by the use of closely packaged fiber optic cables in each channel, the laser source  710  and optical switch  714  can be made as small and light weight as possible. In addition, sensor performance is limited to short ranges in most applications, primarily looking side to side and biased downward for landing. This operation limits mounting conflicts with other sensors and assist devices, maintains aerodynamic performance, and detects obstacles primarily encountered in low altitudes or during landing in a low cost system package. 
     For other applications, where size and weight may not be as great a factor, the laser source  710  may direct the pulsed laser beam to the transmission fiber optic cables of all of the optical channels in parallel without the need of an optical switch. In this application, detection of an obstacle in a corridor or region corresponding to a receiver optic fiber would have to be distinguished by some technique other than time sequencing. For example, a separate light detector could be coupled to each receiver optic fiber and referenced accordingly. Thus, as a light detector detects an obstacle, it may be referenced to the associated corridor or region. Other such techniques may work just as well. In addition, once an obstacle is detected in a particular corridor or region, the DLOAS may be tasked to dwell on the corresponding light detector of the corresponding optical path for an extended period of time. 
     While the aspects of the present invention have been described herein above in connection with a variety of embodiments, it is understood that these embodiments were merely provided by way of example and should not be considered limiting to the present invention in any way, shape or form. Rather, the present invention and all of the inventive aspects thereof should be construed in accordance with the recitation of the appended claims hereto.