Abstract:
A light weight decoy for deceiving radar and forward looking infrared tracking systems. The decoy provides the same radar cross-section as that of an intercontinental ballistic missile (ICBM) and is thermally massive across the entire black body spectrum. Thermal massiveness is accomplished by measuring the temperature of the decoy outer surface and the temperature of the space surrounding the decoy, obtaining the differential temperature, and radiating heat within the decoy to maintain the surface thereof at a temperature similar to that of an ICBM.

Description:
DEDICATORY CLAUSE 
     The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalties thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     When an intercontinental ballistic missile (ICBM) is launched into an orbit toward a target, it can be readily detected by radar as it approaches a target area. Several decoy missiles, which are designed to appear to a radar system as though they are ICBM&#39;s, may accompany one or more ICBM&#39;s so that a cluster of missiles will appear to a tracking radar, thereby camouflaging the identity of the true ICBM&#39;s. However, this cluster of missiles is also being observed by detection systems other than radar. For example, passive infrared detection systems such as forward looking infrared systems (FLIRS) in the 3-5 micron band and the 8-14 micron band are used. These systems sense the temperature of an object by observing the thermal energy which it radiates. Thus, when an ICBM and accompanying decoys are launched into a polar orbit they will pass from a sun lit region into a region shaded by the earth&#39;s shadow. When this happens, both the massive ICBM&#39;s and any decoys which accompany them begin to radiate energy to the cold regions of outer space and begin to cool down. Since each radiates energy at about the same rate, the ICBM&#39;s with their large thermal mass tend to change temperature very slowly. However, the light weight decoys, because of their small thermal mass, change temperature very rapidly by comparison, allowing the FLIRS to quickly discriminate between the ICBM&#39;s and the decoys. 
     SUMMARY OF THE INVENTION 
     A light weight decoy for deployment in the vicinity of an ICBM for deceiving radar and FLIR tracking systems. The decoy is thermally massive across the entire black body spectrum, providing the same radar cross-section and thermal cross-section as that of an ICBM. A thermal source within the decoy causes it to cool at the same rate as the ICBM which it represents. The temperature of the decoy&#39;s outer surface is measured and compared with the temperature of the space surrounding the decoy. A differential temperature is obtained and heat is generated within the decoy to maintain the surface thereof at a temperature similar to that of an ICBM. Two temperature sensors, and a microcomputer provide the differential signal which drives a current control device to supply electrical power to a resistance heater for generating the heat at a variable rate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified drawing of an object in a polar orbit or flight path. 
     FIG. 2 is a perspective view of a heat generator within a decoy missile for simulating an ICBM. 
     FIG. 3 is a schematic of a preferred embodiment of the heat generator system within the decoy. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein like numerals refer to like parts in each figure, FIG. 1 discloses objects in a polar flight path. When an object such as a missile  10  is in a polar orbit or flight path around the earth and passes out of sunlight  12  into the earth&#39;s shadow  14 , energy will be radiated away from the object to outer space. For a worst case treatment of the energy that is radiated away during the time (20 seconds for example) that an ICBM  10  or a decoy  16  may be observed by tracking sensors, it is assumed that one or more decoys  16  and ICBM  10  enter the shadow region of outer space  14  wherein the temperature is zero degrees Kelvin (° K) from a sunny region where the temperature is 300° K. For this case the power radiated from the surface of a missile or decoy, per unit surface area, is 
     
       
           W =σ(Δ T ) 4   (1) 
       
     
     where W is power, σ is the Stefan-Boltzmann constant −σ=5.67×10 −8  watts/(meter) 2  per degrees Kelvin and ΔT is the temperature difference. The surface area, A, of an ICBM or decoy is approximately 4 (meters) 2 . Therefore the total power (W T ) being radiated away by a black body in this worst case is 
     
       
           W   T =(Δ T ) 4   A.   (2) 
       
     
     The Heat content, Q, of an ICBM or Decoy is given by 
     
       
         Q=McΔT,  (3) 
       
     
     where M is the mass of the decoy or ICBM, c is the specific heat in units of Joules (J) per Killogram (Kg) per degrees Kelvin (° K), and ΔT is the temperature difference. The value of c for these objects is approximately 10 3  J/Kg ° K. The change in temperature with time of one of these objects is obtained by differentiating equation (3) with respect to time. Thus                       Q          t       =     M                 c                          (     Δ                 T     )            t           ;           (   4   )                                
     or, since the temperature of outer space has been taken to be zero ° K                     Q          t       =     M                 c                          T          t       .               (   5   )                                
     However,                     Q          t       =       W   T     .             (   6   )                                
     Thus, equations (2) and (4) can be combined to provide:                       (     Δ                 T     )            t       =       σ                     A        (     Δ                 T     )       4         M                 c               (   7   )                                
     Equation (7) discloses that the rate of change of temperature is proportional to the inverse of the mass. Thus, the light weight decoys cool at a rate which is more than ten times faster than the massive ICBMs. 
     The maximum energy radiated away by an ICBM during the first twenty seconds in the shadow region is given by 
     
       
         Δ Q=W   T ( t )=σ A ΔT   4    t,   (8)  
       
     
     therefore 
     
       
         Δ Q =(5.67)(10 −8 )(4)(300) 4  (20) 
       
     
     and 
     
       
         ΔQ=37 KJ. 
       
     
     Equation (8) is derived and solved by inegrating equation (6) from t=0 to t=20 seconds for the worst case (maximum energy radiated), where W T  (and therefore T) is constant in time. The decoy contains a small energy storage source that can supply several times the energy required by equation (8). This energy is used to keep the surface temperature of the decoy the same as if the decoy were a massive ICBM. This is accomplished by using two detectors. One determines the surface temperature of the decoy and the other, which is thermally insulated and is of very small thermal mass, determines the temperature of the space into which the decoy is radiating. The power being released by the device inside the decoy is proportional to this temperature difference. The sensor which measures the temperature of the space into which the decoy radiates may actually be multiple sensors if desired whose readings are averaged. 
     As shown in FIG. 1, when the ICBM orbits or passes from the sun lit region into the shadow region, cooling takes place. FIG. 2 shows a ICBM radar decoy  16  which contains the thermal generator  20  to make it also a thermal decoy. Generator  20  generates and radiates thermal energy within housing  18  of decoy  16 . FIG. 3 is a schematic illustration of the generator  20 . The decoy  16  has a sensor  22  in the outer surface of housing  18  which measures the temperature of the outer surface and is in thermal contact with this surface. A sensor  24  is disposed in radiative thermal contact with outer space. Sensor  24  is mounted in housing  18  but is insulated from the outer surface of the decoy by the thermal insulator  26 , which may be made of asbestos or other suitable material. The signals from these two sensors or sensor groups are coupled to a simple microprocessor  28  to produce an output voltage V c  that is proportional to (T 1 −T 2 ), where T 1  is the temperature of surface  18  and T 2  is the temperature of outer space. This output voltage is then coupled as a bias to regulate the flow of current through a transistor valve  30 . A direct current source  32  supplies negative voltage to the collector of transistor  30  and a positive voltage through load resistor  34  to the emitter of transistor  30 . The current from source  32  causes I 2 R losses to be developed in the load. Resistors  36  and  38  are connected in common to the base of transistor  30  and to the input voltage V c . Resistor  38  is further coupled to the positive output of source  32 , and resistor  36  is coupled to the negative output of source  32  for developing transistor bias voltages. With the exception of sensors  22  and  24  and load resistor  34  the device is contained in a small insulated container  40  so that the heat generated in load  34  is not absorbed by the generator itself. Thus, the I 2 R losses are radiated into 4π steradians within the decoy. This energy is absorbed by the outer surface  18  and is reradiated to outer space over 4π steradians. 
     In operation, without the heat generator, the temperatures T 1  and T 2  sensed respectively by sensors  22  and  24  will rapidly become closer so that the difference therebetween goes to zero as the decoy cools. With the heat generator in operation the heater  34  may be used to heat surface  18  so that temperature T 1  is controlled to remain constant at the value it had in the sun lit region before entering the earth&#39;s shadow. Alternatively, it may be controlled to change gradually as if the decoys thermal mass were exactly the same as that of an ICBM. In the event that an ICBM is shrouded in a balloon and many like balloons (decoys), which do not contain ICBM&#39;s, are used so that all of the objects appear to a radar system as decoys, a thermal source can be placed in each balloon decoy to cause the balloon to change temperature, thereby appearing to a FLIR system as if it contained a massive ICBM. 
     Obviously sensors  22  and  24  can be single sensors or multiple sensors disposed in an array. For example, if a decoy is spinning or tumbling rapidly, one sensor will average the available spatial temperature. However, if the decoy is stable (not spinning or tumbling), several sensors, four or more, can be used to assure that the temperature readings are averaged and that a large area or space is sampled for each response time. Typically, temperature sensors may be thermocouples or a distributed thermopile. 
     Although a particular embodiment and form of this invention has been illustrated, it is apparent that various modifications and embodiments of the invention may be made by those skilled in the art without departing from the scope and spirit of the foregoing disclosure. For example, as is well established in the art, switching means such as a time delay circuit or a threshold circuit may be incorporated in container  40  to prevent the power source or microprocessor from operating until a predetermined time occurs or temperature difference occurs. Accordingly the scope of the invention should be limited only by the claims appended hereto.