Abstract:
A position-adaptive radar method and device for small UAV platforms capable of detecting “leakage signals” that, for example, propagate between two buildings or “leak through” penetrable surfaces such as walls or layers of the ground. The position-adaptive radar comprises a monostatic radar receiver that measures and processes leakage signals and then “self adapts” in position to establish line-of-sight between a mini-UAV platform and an obscuration channel that propagates the leakage signal. This allows a mini-UAV platform to process signals in real-time while gathering intelligence information and locating objects-of-interest that may be embedded within an obscuration channel.

Description:
The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
   BACKGROUND OF THE INVENTION 
   The invention relates to on-board radar for mini unmanned air vehicles and more specifically to position-adaptive on-board radar for mini unmanned air vehicles in urban environments. 
   Low-altitude “robotic” mini unmanned air vehicle technology has important applications for emerging and future threat scenarios in hard urban environments where, for example, “objects of interest” may be concealed between buildings. Other challenging environments and scenarios include surveillance and interrogation of regions-of-interest that may be embedded within mountainous terrain. 
   The current state-of-the-art for sensors on mini-UAV helicopter platforms are generally video cameras. While video technologies for remote sensing and surveillance have many useful applications, they are not useful for embedded or obscured objects. The present invention is a low-cost lightweight radar system that allows for information gathering for special applications such as in embedded environments (i.e. inside buildings), detection and characterization of hidden and obscured targets (i.e. between buildings), as well as potential for detection and pre-characterization of very small and slow-moving man-made and biological threats that, to date, have not been detected with currently available prior-art sensor technologies. 
   SUMMARY OF THE INVENTION 
   A position-adaptive radar method and device for small UAV platforms capable of detecting “leakage signals” that, for example, propagate between two buildings or “leak through” penetrable surfaces such as walls or layers of the ground. The position-adaptive radar comprises a monostatic radar receiver that measures and processes leakage signals and then “self adapts” in position to establish line-of-sight between a mini-UAV platform and an obscuration channel that propagates the leakage signal. This allows a mini-UAV platform to process signals in real-time while gathering intelligence information and locating objects-of-interest that may be embedded within an obscuration channel. 
   These and other objects of the invention are achieved by the description, claims and accompanying drawings and by an on-board based instrumentation system for position-adaptive convergence of a mini-UAV helicopter between two structures comprising: 
   a mini-UAV helicopter; 
   a positional control system; 
   a close-range monostatic radar system for transmitting, receiving and processing a radar signal (in real time); and 
   a system controls processor for interfacing said radar system with said positional control system and wherein said controls processor iteratively controls the location of the helicopter in real-time; 
   wherein large range values from said close-range monostatic radar system correspond to leakage points and wherein said leakage points correspond to lower values of signal attenuation and higher values of medium penetration, and 
   whereby the mini-UAV helicopter converges and hovers at said leakage point. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a position-adaptive radar concept for mini-UAV helicopter according to the invention. 
       FIG. 2  is a block diagram for impulse radar design according to the invention. 
       FIG. 3  is is a conceptual circuit model for impulse radar transmitter according to the invention. 
       FIG. 4  is a retractable dual spiral antenna concept for mini-UAV helicopter according to the invention. 
       FIG. 5  is a basic block diagram of position-adaptive radar system for mini-UAV helicopter platform according to the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates a position-adaptive concept for a mini-UAV helicopter according to the invention. The preferred arrangement of the invention is a close-range monostatic radar design concept where the radar is interfaced to the positional control system on the mini-UAV via a system controls processor.  FIG. 1  depicts a situation where a helicopter  100  laterally hovers back and forth, illustrated by the arrow at  103 , in front of the two buildings,  101  and  102 , while transmitting, receiving, and processing (in real-time) a radar signal. Depending on the characteristics of the radar signal, the on-board control processor iteratively controls the location of the helicopter in real-time. This “smart sensor-based” approach to real-time controls allow the helicopter to isolate “signal leakage points” within the desired region-of-interest. For illustrative purposes, four notional leakage points are shown in  FIG. 1 ; two signal leakage points,  104  and  105 , in front of building  101 , one signal leakage point,  106 , between buildings  101  and  102 , and one signal leakage point,  107 , in front of building  102 . 
   The illustration in  FIG. 1  can also be interpreted as a model for measurements and testing of the radar-based sensor controls. For example, the sensor design can be prototyped and tested by constructing a scaled-model of the environmental configuration in  FIG. 1  and by implementing a few simple approaches to on-board and real-time sensor control. Additionally, a control algorithm that iteratively measures and compares radar range values can be used to position-adaptively control the helicopter platform. With approach, relatively large values of range measurements from the on-board radar (as a function of lateral helicopter trajectory) correspond to “leakage points-of-interest” since these locations correspond to lower values of signal attenuation and higher levels of penetration through the medium. Under this concept, real-time onboard iterative comparison of the range values allows the helicopter to “converge” or hover in front of the leakage points and, for example, form an image of potential objects-of-interest by exploiting the “line-of-sight” signal propagation between the helicopter platform through the leakage points. 
   A preferred arrangement of the invention can be demonstrated by implementing a system that position-adaptively converges to leakage points by processing onboard radar signals and then using a small onboard camera to generate a line-of-sight photographic image of potential objects-of-interest. The system can be re-programmed and upgraded with more sophisticated processing techniques that apply under more severe conditions by, for example, deploying a tier of mini-UAV&#39;s that each process additional signal information such as phase discontinuities in the signal. A tier of mini-UAV&#39;s operate to converge to leakage points from a variety of different aspect angles. This multi-aspect information can then be combined via communications links between the platforms to arrive at a multi-dimensional approach to collecting intelligence information in a number of challenging environments. 
   For the position-adaptive radar method and device of the invention, the radar analysis and design is based on state-of-the-art ultra-wideband (UWB) impulse radar technology. A frequency range for a preferred arrangement of the invention is selected from about 100 MHz to 2 GHz and is derived from recent advances in MIR (Micro Impulse Radar) COTS (Commercial-Off-The-Shelf) technologies. Many existing MIR implementations have range limits of a few feet. Some potential future application areas of this promising “tiny low-cost radar-on-a-chip” technology can be categorized into automotive, security, home appliance, and manufacturing. A number of specific implementations have been developed including COTS modules for fluid level sensing and motion detection. 
     FIG. 2  is a block diagram of an impulse radar design for a preferred arrangement of the invention. The radar detection principle for this arrangement of the invention is based on correlating a delayed version of the transmit pulse train with the backscattered pulse train to generate a coherent radar signal output. The transmit signal  201  is coded within a relatively simple pulse generator circuit  200  that applies a “white noise” pulse-position modulation (PPM) signal to a 2 MHz pulse train. The transmitter  201  shapes the pulse from the pulse generator into a short impulse and irradiates the region-of-interest or target  203  with RF energy. The receiver  204  detects the backscattered RF energy from the region-of-interest or target  203 . The nanosecond delay line  202 , fast sampler circuit  205 , and low-pass filter  206  function in tandem as an efficient and synchronous analog cross-correlation function between the transmitted ultra-wideband signal and the received signal. Synchronous cross-correlation of ultra-wideband signals generates very precise measurements of basic radar quantities such as range and Doppler. 
   A set of design calculations can be performed via a link-budget analysis using Eqs. (1)–(4). These equations serve as radar range equation models in voltage form. The information from the results of this type of link-budget analysis can be used to specify components. 
   
     
       
         
           
             
               
                 
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                   V   n     =         (     4   ⁢   k   ⁢           ⁢   T   ⁢           ⁢     B     R   ⁢           ⁢   F       ⁢     R     d   ⁢           ⁢   e   ⁢           ⁢   t         )       1   /   2         N     1   /   2                 Eq   .           ⁢     (   3   )                   N=τ ( PRF )  Eq. (4) 
   In Equations (1)–(4), S/N denotes signal-to-noise ratio, V r  represents the received signal voltage, V n  is noise voltage, V t  is the peak transmitter voltage, G is antenna gain, λ is the wavelength at center frequency, σ is radar cross-section of detection object-of-interest, R is radar range, k is Boltzman&#39;s constant, T is temperature, B RF  is signal bandwidth, R det  represents ohmic losses in the detector, N is number of integration pulses in the receiver, τis the integration time, and PRF is the pulse repetition frequency of the impulse radar. 
     FIG. 3  shows a conceptual circuit model and is useful for purposes of visualizing the design analysis of the arrangement of the invention.  FIG. 3  illustrates the impulse generation and transmission process where a short pulse  300  is generated at the base of the microwave transistor, Q 1  illustrated at  301 . This short pulse has the effect of saturating the transistor into a conduction state and generating a fast discharge of capacitor, C 1 , illustrated at  302 , through the transistor and the Schottky {acute over (d)}iode, D 1 , illustrated at  303 . This discharge process results in the radiation of a short impulse waveform  305  from the antenna  305 . 
   Two sets of design calculations were performed using Equations (1)–(4). For the first set of calculations, a signal-to-noise ratio of 13 dB for selected for a σ=1 m 2  object at a maximum range, R, of 100 m. The antenna gain, G, was set at 5 dB and the detection resistance, R det , was approximated at 100 ohms. This leads to a pulse integration number of about 2000 and a baseband receiver signal bandwidth of about 250 Hz. These parameters correspond to a radar design in an open (“free space”) environment where the maximum detection range is 100 m. 
   A second set of design calculations were performed by using the same basic design parameters (as the first set of calculations) except by adding an additional loss term estimate due to ½ meter building penetration. These calculations were performed by using the signal voltage values from the first set of calculations, incorporating a loss term estimate, and “backsolving” Equation 2 for the range term, R. The loss term was estimated using Equation (5). 
   
     
       
         
           
             
               
                 
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                 = 
                 
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   This equation was evaluated with penetration depth d=0.5 m at a center frequency of f=1 GHz. This leads to a loss estimate of about 25 dB. The resulting range term, R, was about 25 m. 
     FIG. 4  illustrates the antenna concept for the position-adaptive mini-UAV radar device and method of the invention. This is a lightweight design where a two-arm spiral is fabricated over styrofoam-type material. This is a two-antenna design where one antenna is bracketed to the left side of the helicopter and the other antenna is bracketed to the right side of the helicopter. One antenna is a transmitting antenna and the other antenna is a receiving antenna. The position of the antennas can be controlled for landing  400  where the antennas are retracted (into a “back-looking” position) via a electronic control mechanism. In addition, the depression angle of the antennas can be adjusted between “down-looking”  401  and “forward-looking”  402 . 
   The frequency response of these antennas are approximately related to the front and back diameters of the antenna&#39;s conical section by the following expression. 
   
     
       
         
           
             
               
                 
                   
                     f 
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   where f max =2 GHz and f min =100 MHz defines the upper and lower frequency limits of this position-adaptive radar design. This 20:1 “aspect ratio” can be implemented by, for example, selecting the diameter of the large end of the conical section, D max  as 30 cm and the diameter of the small end of the conical section D min  as 1.5 cm. Then, selecting an typical cone angle of about 7.5 degrees leads to a antenna length of about 3.5 feet. 
   An approximate expression of the frequency dependent phase response of this type of conical spiral antenna is given in reference as: 
   
     
       
         
           
             
               
                 
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   where f represents frequency, θ represent cone angle, and α is denoted as the “wrap angle” for the spirals. 
   In order to minimize the distortion and ringing effects in the radiated and received impulse waveforms that propagate within these antennas, some relatively simple linear and quadratic approximations for phase inversion networks can be formulated in order to realize phase compensation functions that reduce the distortion effects from the fast impulse functions in this radar design. 
     FIG. 5  is a basic block diagram of position-adaptive radar system for mini-UAV helicopter platform according to the invention. The radar portion  502 ,  503 ,  504  of the preferred arrangement of the invention is based on ultra-wideband impulse radar technology. The lightweight and reconfigurable antenna design  503   504  allows for geometrical adjustment of the sensor look direction for a specific application. An integral part of this “smart sensor” concept is the development on a radar interface and system control module  501  for joint control and operation of the onboard radar system with the onboard autonomous control system  500 . This “smart control module”  501  is designed so that the mini-UAV helicopter platform has the capability to “positionally-adapt” to characteristics of the “leakage signal” that is measured by the radar. This approach allows the mini-UAV to locate and gather information from different types of “obscuration channels”. 
   While the apparatus and method herein described constitute a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus or method and that changes may be made therein without departing from the scope of the invention, which is defined in the appended claims.