Abstract:
A robot sentry with a scanning laser observes the sky just above the geographic skyline looking for a vertical airflow pattern characteristic of the rotor inflow to a helicopter rotor. The presence of this vertical airflow pattern indicates the probable presence of a reconnaissance helicopter that is using terrain masking.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     Federally Sponsored Research or Development 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to the field of surveillance and more specifically to the detection of helicopters that potentially represent a threat. 
     2. Background Art 
     In the field of electronic surveillance, particularly on the modern battle field, helicopters such as the American Apache helicopter, the European Tiger helicopter as well as undoubtedly Russian and other countries&#39; helicopters use mast-mounted sights and terrain masking as a way of acquiring a target while remaining undetected. A typical flight scenario would be for a reconnaissance helicopter to fly very low to the ground while approaching a potential target. The helicopter would then expose a minimal portion of itself, such as a mast-mounted sight, which is analogous to observing a surface ship from a submarine. In the case of the helicopter, terrain between the helicopter and the intended target ‘masks’ the helicopter&#39;s approach. 
     In the unrelated field of aerodynamics, the operation of a helicopter is fairly well understood. It is an immutable principle of physics that helicopters—indeed any ‘heavier-than-air-craft’—can only fly because the airfoils, at any given instant, accelerate a mass of air downward that is at least equal to the mass of the aircraft. 
     On airplanes, the airfoils (called ‘wings’) are bolted firmly to the fuselage at a fixed angle and the entire craft is accelerated along the runway until sufficient ‘relative airflow’ is generated over the wings that they can deflect a sufficient mass of air to take off. “Lift” is the equal and opposite reaction to that downward deflection of the air. 
     Helicopter airfoils (called ‘main rotors’) are rotated about a hub with a feathering hinge at the root, which allows the ‘angle of attack’ to be increased or decreased, both ‘cyclically’ and ‘collectively’. Because these rotating wings are capable of generating ‘relative airflow’ solely due to the speed of rotation, it is not necessary for helicopters to have forward speed in order to fly. 
     But whether we talk about ‘rotary-wing’ or ‘fixed-wing’ aircraft, the greater the forward speed with which the aircraft flies through the air, the greater the volume of air per unit of time that the lifting airfoils will act upon. The greater the mass of air deflected, the less vertical acceleration must be imparted to that air mass in order to provide the ‘lift’ necessary to fly. For example, a crop-spraying airplane flying over a field at only one or two meters above the vegetation will barely rustle the leaves. 
     On the other hand, slow flying aircraft interact with a smaller volume of air per unit of time and therefore it is necessary to accelerate that air to a greater downward velocity in order to sustain lift. This is the case with a hovering helicopter—particularly a helicopter hovering well clear of the ground—where there is invariably a column of descending air beneath the craft. Hovering a helicopter ‘out-of-ground-effect’ requires more power than is required for forward flight or hover ‘in-ground-effect’ and is akin to trying to swim up a waterfall. 
     Referring to FIG. 1, the vertical velocity of the column of air, also known as the ‘rotor intake’ region  15 , above a hovering or slow moving helicopter  10  depends upon several factors including surface wind, main rotor radius, and ‘disc loading’ (that is—the weight of the helicopter divided by the ‘swept’ area of the rotor disc). The mass of air entering the rotor intake region is necessarily equal to the mass of air exiting the rotor ‘down wash’ region  16  from the helicopter  10 , where helicopter rotor down wash is a fairly well understood phenomenon. Larger helicopters not only have greater mass, but they generally have a higher ‘disc loading’ when compared to smaller helicopters. This is because other design influences limit the practical main rotor radius on large helicopters. 
     We have discovered a means of protecting a potential target by detecting helicopters that are using terrain masking to approach the target. Our invention uses the aerodynamic principles of helicopter flight to detect these helicopters before they have observed the target. Advantageously, our invention reveals the position of the helicopter to the potential target before the helicopter is aware that it has been detected. This invention addresses a long-felt need by ground troops for protection from approaching low flying helicopters. 
     SUMMARY OF THE INVENTION 
     A robot sentry with a scanning laser observes the sky just above the geographic skyline looking for a vertical airflow pattern characteristic of the rotor inflow to a helicopter rotor. The presence of this vertical airflow pattern indicates the probable presence of a reconnaissance helicopter that is using terrain masking. The robot sentry can be set up to survey the surrounding terrain, using for example a video camera to detect the contrast difference between a darker terrain and lighter sky. The robot sentry can automatically establish an ‘observation line’ by laser ranging to the geographic skyline or an operator can set the observation line based on local terrain features should as can be determined from a topographic map. 
     The helicopter is detected by drawing an imaginary line in space, aiming very short duration and small diameter laser pulses at various points along that line, detecting return signals from individual aerosol particles on that line, and correlating an area of vertically descending particles with the area of a helicopter rotor. Once the helicopter is detected, personnel in the area are alerted to the potential threat. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     Brief Description of the Several Views of the Drawing 
     A more complete appreciation of this invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein: 
     FIG. 1 illustrates the airflow around a hovering or slow flying helicopter as taught in the prior art; 
     FIG. 2 illustrates a robot sentry monitoring an observation line located just above a geographic skyline, in accordance with one illustrative embodiment of my invention; and 
     FIG. 3 depicts the positions of the robot sentry, geographic skyline, and observation line as shown in FIG. 1, as they would be seen on a topographical map. 
     FIG. 4 depicts the angular relationship of positions of the robot sentry, and observation line as shown in FIG. 1, with respect to an individual laser beam. 
     FIG. 5 illustrates a functional block diagram of the robot sentry according to one illustrative embodiment of our invention. 
     FIG. 6 is a flow chart showing the method steps performed by the robot sentry in detecting a helicopter. 
     FIGS. 7 and 8 illustrate specific details of the procedure of FIG. 6, in accordance with the present invention. 
     FIG. 9 illustrates an ‘edge-detected’ video image of the geographic skyline shown in FIG.  2 . 
     FIG. 10 illustrates the geometric relationship between the vertical moving column of air and the individual laser pulses, where this geometric relationship is used within the method steps of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Mode(s) for Carrying Out the Invention 
     Referring to FIG. 2, which shows one embodiment of our invention, a robot sentry  40  is scanning just above the surrounding terrain  25 . The scanning process consists of transmitting beams  45  of laser pulses and receiving back scatter returns. These back scatter returns result when the laser pulses reflect from individual aerosol particles contained in the air. In order to detect the individual aerosol particles, the laser pulses are both very narrow in diameter, such as 2 microns, and very short in duration, such as &lt;10 nanoseconds. The laser beams  45  are scanned along an observation line  30  above the geographic skyline  20  below which may be flying a helicopter. 
     Referring next to FIG. 3, the observation line  30  is depicted on a topographical map  50 . The observation line  30  is located a varying radial distance  35 , that varies with azimuth angle, from the robot sentry  40 . The radial (r) distance  35  from each individual point on the observation line  30  is determined by the underlying terrain  25 , where the distance (r), for example, is the distance from the robot sentry  40  to the nearest available terrain suitable for masking a helicopter  10 . In some embodiments of our invention, the observation line will not fully surround the robot sentry  40 , but will instead only cover an angular sector, such as an anticipated attack direction. In other embodiments of our invention, there are concentric observation lines  30 , such as would be used in mountain foothill terrain. 
     Referring next to FIG. 4 as well as the preceding FIGS., the robot sentry  40  detects the presence of a helicopter  10  by detecting a vertical column of air at the observation line  30 . The characteristics of this vertical column of air are that it has a diameter equal to the diameter of the rotor intake region, for example 30 feet (10 meters) and the air in the vertical column is moving downward at a relatively high vertical velocity, for example 50 fps (15 m/s). The diameter of the rotor intake region  15  is approximately the same as the helicopter  10  rotor diameter (R). 
     The air velocity (V) along the propagation direction of an individual laser beam  46  can be determined by a Doppler shift in the laser beam frequency. The laser return can be measured at a specific distance, such as the radial distance  35  to the observation line  30  by setting a ‘range gate’ on the laser receiver. The vertical speed component of the air velocity is given by the following equation. 
     
       
           V   v   =V * sin(θ); 
       
     
     where V is the velocity along the propagation direction of the individual laser beam  46 , V v  is the corresponding vertical velocity component, and θ is the elevation angle between the individual laser beam  46  and the local horizontal reference  55 . 
     A relatively large number of laser ‘shots’ may be required to detect a vertical column of air, substantially equal vertical wind velocities, over a horizontal distance that is equal to the rotor diameter (R) of the helicopter  10 . 
     Referring next to FIG. 5, a functional block diagram of the robot sentry  40  is shown. The robot sentry includes a laser transmitter  61 , laser receiver  62 , scanner  63  and processor  65 . In one embodiment, the laser scanner  63  includes galvanometer driven horizontal and vertical scanning mirrors  67 , similar to those that may be known in the art of laser printers. 
     The laser transmitter  61  produces the individual laser pulses  46  described previously. The laser pulses  46  are directed to the scanner  63  by a mirror  68  and a prism  69 . Each individual laser pulse  46  advantageously has certain characteristics that make it more suitable for detecting the velocities of airborne aerosol particles. These characteristics include, for example, pulse amplitude, pulse length and pulse modulation such as a frequency modulated (FM) ‘chirp. 
     The laser receiver  62  receives the back scatter laser returns  47  from individual aerosol particles through the scanner  63  and the prism  69  and demodulates them. The laser receiver  62  advantageously provides certain functions that make it more suitable for detecting the velocities of airborne aerosol particles at specific predetermined ranges from the robot sentry  40 , such as the varying range of the observation line previously described. These functions can include; for example, a selectable (time) range gate and FM demodulation for detecting Doppler frequency shift of the individual back scatter laser returns  47 . 
     Referring now to both FIGS. 4 and 5, the processor  65  first determines (step  81 ) the location of an observation line  30 , where the observation line can be expressed in the radial format of elevation angle versus azimuth direction with respect to the robot sentry  40 . 
     Referring now to FIG.  7  and describing a first embodiment of our invention, embodiment, a video camera  64  included in the robot sentry  40  is used to capture a terrain video image (step  811 ). The video image is processed using ‘edge detection’ techniques (step  812 ), whereby the geographic skyline  20  is represented by a series of elevation versus azimuth points. As an example, FIG. 9 shows an edge-detected video image of the terrain shown in FIG.  2 . Referring to both FIGS. 2 and 9, the sky typically will be considerably brighter during the daytime hours than surrounding terrain  25 . A geographic skyline  20  is established based on luminance values and contrast ratios, using one of several edge-detection algorithms that may be available in the art of image processing. 
     In an alternate embodiment of our robot sentry, the processor  65  issues elevation versus azimuth commands  71  to the scanner  63  and uses the laser receiver  62  to detect brightness values directly, in a manner similar to that of a video camera. The resulting brightness data is processed using techniques similar to the edge detection techniques discussed previously. 
     As previously discussed in conjunction with FIG. 3, the observation line  30  includes a third dimension that can be expressed as range versus azimuth. The observation line  30  is set at a predetermined angular offset, such as for example 0.10°, just above the geographic skyline  20 . The ranges of various points along the geographic skyline  20  are actively measured (step  813 ) by using the laser transmitter  61  in combination with the laser receiver as a laser range finder. The processor  65  issues elevation versus azimuth commands  71  to the scanner  63  that correspond to the geographic skyline  20  that has been previously determined. Typically, the processor  65  will search a predetermined angular offset; for example, ±2° from the detected edge, looking for a hard laser return from a terrain feature by varying the elevation commands  71  to the scanner  63 . Accordingly, the processor  65  calculates (step  814 ) elevation and range versus azimuth for the observation line  30 . 
     In other embodiments, a human operator may manually input the range versus azimuth coordinates of the elevation line, based for example on topographical map data. A computer console may be attached to the robot sentry  40  allowing a human operator to calibrate the equipment, by for example manually operated scanning routines for determining the geographic skyline  20 . 
     Once the observation line  30  has been established, the processor  65  issues commands  71  to the laser scanner  63  to scan (step  82 ) along the observation line. During the scanning step (step  83 ), the laser transmitter  61  transmits the laser pulses  46 , where the characteristics, such as pulse duration, of each laser pulse  46  are in response to commands issued from the processor  65 . Also during the scanning step, the laser receiver  62  receives the laser back scatter returns  47 , demodulates these returns, and passes ‘raw’ data back to the processor  65  for further processing. The processor  65  issues commands to the laser receiver  47 , to determine its sensitivity and range gating. 
     The processor  65  processes (step  84 ) the laser return raw data. As shown in FIG. 8, this processing includes eliminating (step  841 ) extraneous return signals, correlating (step  842 ) the returns to the geographic location of the observation line  30 , and detecting (step  843 ) the Doppler frequency shift of the individual return pulses  47 . The velocity (V) of individual aerosol particles, in the direction of the laser pulse  46  propagation is calculated (step  844 ) in accordance with the geometry of FIG. 10, which is described, below. 
     Referring back to FIG. 6, the processor  65  compensates (step  85 ) for wind gusts by averaging the aerosol particle velocities over areas that are much larger than that of a helicopter rotor disc. For example, wind gust velocity over a region 100 meters (300 feet) may be averaged out where this is 10 times the area associated with a helicopter rotor disc. In addition, the local wind behavior, due to cliffs, etc, may be observed during the time period when a helicopter is known not to be in the vicinity. This step (step  85 ) establishes the ‘background’ wind conditions against which variances can be observed. 
     Next, the processor  65  looks for vertical air velocity variances against the background wind conditions established in step  85 . The individual vertical air velocities along the observation line  30 , corresponding to the vertical movement of individual aerosol particles, are correlated (step  86 ) to a potential vertical column of air with the area of a helicopter rotor disc, for example 12 meters across. If such a vertical column of air is detected (step  87 ), the processor issues a threat warning (step  88 ). 
     Referring now to FIG. 10, there is shown the geometry associated with detecting the vertical velocity of an individual aerosol particle  100 . The individual aerosol particle  100  is moving downward at a vertical velocity  105  and is impinged upon by laser pulse  46 . Laser pulse  46  travels in a straight-line propagation direction, defining a first coordinate axis  91 . A second coordinate axis  92  is defined perpendicular to the laser pulse  46  propagation direction. The vertical velocity  105  of the aerosol particle  100  can be resolved into this ‘propagation direction’ coordinate system using basic geometry. 
     Consider only the vertical velocity of the aerosol particle and disregard horizontal motion, which is compensated for step  85 . The vertical velocity component along the propagation axis (V)  101  equals the vertical velocity (V v ) of the aerosol particle times the sine of the elevation angle θ. The table below shows some vertical air column speeds with associated elevation angles and the resultant vertical velocity component along the propagation axis (V). 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Vertical Velocity versus Elevation Angle 
               
             
          
           
               
                 Vertical Velocity (V v ) 
                 Elevation Angle (θ) 
                 Velocity (V) 
               
               
                 (meters/second) 
                 (degrees) 
                 (meters/second) 
               
               
                   
               
             
          
           
               
                 5 
                 2.5 
                 0.22 
               
               
                 5 
                 5.0 
                 0.44 
               
               
                 5 
                 7.5 
                 0.65 
               
               
                 5 
                 10.0 
                 0.87 
               
               
                 10 
                 2.5 
                 0.44 
               
               
                 10 
                 5.0 
                 0.87 
               
               
                 10 
                 7.5 
                 1.31 
               
               
                 10 
                 10.0 
                 1.74 
               
               
                 10 
                 2.5 
                 0.44 
               
               
                 10 
                 5.0 
                 0.87 
               
               
                 10 
                 7.5 
                 1.31 
               
               
                 10 
                 10.0 
                 1.74 
               
               
                 20 
                 2.5 
                 0.87 
               
               
                 20 
                 5.0 
                 1.74 
               
               
                 20 
                 7.5 
                 2.61 
               
               
                 20 
                 10.0 
                 3.47 
               
               
                   
               
             
          
         
       
     
     Accordingly, as described above we have invented both an apparatus and a method to protect ground troops from the threat posed by a potential attacking helicopter that that is using terrain-masking to conceal its presence. Our invention takes advantage of fundamental aerodynamics which dictate that a large mass of air must move essentially in a vertical column through the rotor blades of a helicopter in order for that helicopter to remain airborne. Our invention further takes advantage of modern laser technology that can detect the movement of individual aerosol particles within a moving air stream such as this vertical column of air. Advantageously, ground troops can use our invention for an extra degree of protection from attack helicopters. 
     Alternate embodiments may be devised without departing from the spirit or the scope of the invention.