Abstract:
To make small UAVs capable of geolocation of emitters, a low cost, low power, small weight and power radio receiver receives and tracks Doppler frequency at a minimum. In order to minimize the size, weight and power (SWAP), a single receiving element array is utilized. The analysis of geolocation performance with single and multiple UAV receiving platforms is considered. With a single UAV platform measuring Doppler frequency with unknown center frequency, a localization accuracy on the order of ten to 100 meters is possible within a couple of minutes, or about one to five percent of the target range.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    This invention relates generally to vehicle-mounted geolocation system. More particularly, this invention relates to a light size and weight system that consumes little power when it locates the position of emitters of electromagnetic radiation. 
         [0003]    2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
         [0004]    In the field, troops do not have an effective tactical asset under troop control that is capable of locating hostile emitters that emit signals to communicate and/or control equipment under the control of a hostile entity. Geolocation using time difference of arrival (TDOA) or frequency difference of arrival (FDOA) techniques typically require multiple platforms that are synchronized in time or frequency so that differences between platforms can be calculated. Usually, this synchronization is done with atomic clocks or synchronized stable local oscillators. Synchronization also requires electronics that consume more power or weigh more than can be carried by a small unmanned air vehicle (UAV) while maintaining persistence requirements and maintaining flight control stability. Another alternative for geolocation from a single platform require multiple element antennas to determine angles of arrival of the signals in order to determine a target angular location. These solutions may provide simple azimuth information, but fails to provide any information regarding range. More complex arrays could provide azimuth and elevation that could be used to determine range and azimuth. However, complex arrays require calibration and consume power. Additionally, complex arrays weigh more and potentially affect aerodynamics, diminishing the flight control system performance of a small tactical UAV. As such, these solutions can only be incorporated into larger platforms not under the control of the end user (troops in the field) and can only be taken advantage of using multiple airborne platforms, if available, even though they may not be tightly synchronized in time down the carrier phase level. 
       SUMMARY 
       [0005]    A geolocation system for identifying a location of an emitting source is disclosed wherein the geolocation system is hosted by a moving craft. The geolocation system includes an omnidirectional antenna used to collect source signals emitted by the emitting source. A signal processor is an electrical communication with the antenna and receives the source signals collected by the antenna. The signal processor extracts frequency data from the source signals. A frequency estimator is electrically connected to the signal processor. The frequency estimator estimates a frequency of the source signals independent of a center frequency or a frequency drift rate of the source signals. A controller calculates the location of the emitter source based upon the frequency estimator output. 
     
    
     
       DRAWING DESCRIPTIONS 
         [0006]      FIG. 1  is a perspective environmental view of a geolocation system of the prior art; 
           [0007]      FIG. 2  is a perspective environmental view of a geolocation system according to one embodiment of the invention hosted by an aircraft; 
           [0008]      FIG. 2  is a block diagram of one embodiment of the invention; 
           [0009]      FIG. 3  is a block diagram of one embodiment of the inventive system; 
           [0010]      FIG. 4  is a block diagram of a frequency estimator according to one embodiment of the invention; 
           [0011]      FIG. 5  is a block diagram of the inventive method; and 
           [0012]      FIG. 6  is a logic chart of one embodiment of the inventive method. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Aircraft have been used for tactical reconnaissance for almost as long as aircraft have been in existence. As technology changed, so too did the type of information gathered as well as how it was gathered. With the advent of UAVs, targets that are less permanent in nature have been easier to locate. This is because the UAV may be able to get closer to the target without being discovered. 
         [0014]    Referring to  FIG. 1 , a graphic representation of how UAVs were used prior to the invention is shown. In this situation, a target  10  is graphically represented as a satellite antenna and is a surrogate for any type of emitter even as simple as a handheld radio transceiver. It should be appreciated by those skilled in the art that the unknown emitter may be attached to a permanent structure or it may be an emitting device that is mobile. Signals transmitted by the target antenna are graphically represented by arrows  12 ,  14 . The signals  12 ,  14  are received by antenna (not shown) hosted by a plurality of UAVs  16 . In addition, a land-based receiving station  18  may also receive a signal  20  emitted by the target at  10 . Information from the plurality of UAVs  16  is transmitted (graphically represented by lightning symbols  22 ,  24 ) to the land-based receiving station  18 . With the information transmitted by the plurality of UAVs  16  and in addition to the signal  20  received by the land-based receiving station  18 , the land-based receiving station  18  may calculate the location of the target antenna  10 . This system is cumbersome in that it requires the synchronization of all the plurality of UAVs  16  as well as having the personnel required to control and operate the UAVs  16 . 
         [0015]    Referring to  FIG. 2 , one embodiment of the inventive assembly is generally indicated at  26 . Like the plurality of UAVs  16  in the prior art shown in  FIG. 1 , a UAV  28  receives a signal  30  of electromagnetic radiation from the target antenna  10 . After the UAV  28  receives the signal  30 , it transmits the signal (graphically represented by lightning symbol  32 ) to a land-based receiving station  34 , which then calculates the location of the target antenna  10 . The UAV  28  includes a single monopole antenna  36  consisting of a simple omnidirectional element array designed to receive the signal  30  from the target antenna  10 . An omnidirectional element array is an antenna that receives signals uniformly in all directions in one plane. These omnidirectional element arrays may be monopole or dipole antennas. Use of the simple omnidirectional element array reduces the size, weight and power (SWAP) of the geolocation system  26 . The design of the single, monopole antenna  36  will hereinafter be referred to as an omnidirectional antenna  36 . The operation of the UAV  28  will be discussed in greater detail subsequently. It should be appreciated by those skilled in the art that the craft disclosed as UAV  28  may be any type of craft or vehicle as the invention can be utilized with any moving platform. 
         [0016]    Referring to  FIG. 3 , a block diagram of the inventive assembly  26  is generally shown. The geolocation system includes an airborne sensor  38 , which is hosted by the UAV  28  in  FIG. 2 , and a computerized ground processing station  40 , which is graphically represented by the land-based receiving station  34  in  FIG. 2 . The airborne sensor  38  receives the signal  30  using a digital receiver  42 . The signal may be analog or digital, consistent or intermittent. The communication rate may be low and the geolocation system  26  will account for low communication rate. The signal received by the digital receiver  28  from the omnidirectional antenna  36  is sent to both a noise density estimator  44  and a frequency estimator  46 . The noise density estimator  44  measures the signal-to-noise ratio (SNR) and sends the measured SNR to both the frequency estimator  46  and the ground processing station  40 . The airborne sensor  38  also includes a navigation system  48 . The output of the navigation system  48  is also sent to the ground processing station  40 , which hosts at least one computer that will process the outputs received. 
         [0017]    Referring to  FIG. 4 , a more detailed representation of the computerized frequency estimator  46  is shown. As stated above, the frequency estimator  46  receives inputs from the digital receiver  42  and the noise density estimator  44 . A quality estimator  50  receives the output of the noise density estimator  44 . The output of the quality estimator  50  is received by a signal data buffer  52 . The signal data buffer  52  also receives the output of the digital receiver  42 . A modulation detector  54  detects how the signal received by the digital receiver  42  is modulated. This is required because the geolocation system  26  is going to be required to detect signals of unknown frequency. Based on the output of the modulation detector  54 , an estimator selector  56  selects, as is graphically represented by a switch  58  between a plurality of estimators  60  to select the proper estimator for the signal  30  received by the digital receiver  42 . While three estimators  60  are shown in  FIG. 4 , it should be appreciated by those skilled in the art that any number of estimators may be used to estimate the frequency of the signal  30  received by the digital receiver  42 . The output of the frequency estimator  46  is sent to the computerized ground processing station  40 , identified in  FIG. 4  as the geolocation processing. 
         [0018]    Returning attention to  FIG. 3 , the ground processing station  40  includes an emitter location processor  62  (this processor may be part of the airborne platform or the ground station processing as depicted) that receives all of the outputs of the airborne sensor  38 . The emitter location processor  62  may receive outputs from a plurality of airborne sensors  38  (one shown) and does not require multiple platform synchronization (timing on the order of 0.1 second is all that is required). 
         [0019]    The location processor  62  receives outputs from the noise density estimator  44 , the navigation system  48 , and the frequency estimator  46 . Together with a database incorporating the digital terrain elevation data  64 , the ground processing station  40  can identify the location of the target  10 . The digital terrain elevation data  64 , is not absolutely necessary, but may improve the geolocation height estimate. 
         [0020]    Referring to  FIG. 5 , a graphic representation of a data flow for a method utilized by the geolocation system  26  is generally shown at  66 . Signal processing occurs at  68  to extract the frequency of arrival for a particular signal  30 . The signal processing includes information received from the navigation system  48 . The frequency of arrival information and the platform position and velocity information from the navigation system  48  are incorporated as inputs into a geolocation algorithm  70 , which then identifies the location of the target emitter  10 . 
         [0021]    Referring to  FIG. 6 , one embodiment of the inventive method is graphically shown in a flow chart, generally indicated at  100 . The method begins at  102 . The first step in the method  100  is to move the omnidirectional antenna  36  through a pattern at  104 . The pattern is graphically shown in  FIG. 2  as a circle  106 . Depending on the conditions or the type of signal to be collected, the pattern  106  may be something other than a circle pattern. Regardless of the shape of the pattern, the pattern  106  may be repeated or only a portion of the pattern may be utilized. Performance is dependent on the specific platform-emitter geometry over the time interval of data collection. 
         [0022]    As the omnidirectional antenna  36  is moved through a pattern, a source signal is received from the emitting source or target  10  at  108 . The system also receives location data from a navigation system at  110 . Noise density is calculated from the source signal as it is received from the emitting source  10  at  112 . 
         [0023]    The frequency of the source signal is estimated at  114 . Because the omnidirectional antenna  36  is used to identify the geolocation of the emitting source or target antenna  10 , estimating the frequency of the signal at  114  requires identifying the frequency of the signal source that is affected by the Doppler frequency shift based on the location and movement of the omnidirectional antenna  36 . To do this, a calculation of time dilation must be made since the Doppler shift is itself time varying. Ignoring amplitude changes, the relationship between transmitted and received signals is: 
         [0000]        c τ( t )=| {right arrow over (r)}   R ( t +τ( t ))− {right arrow over (r)}   T ( t )|  (1)
 
         [0024]    where letter c is the speed of wave propagation, τ(t) denotes the value of travel time and  {right arrow over (r)}   T (t) and  {right arrow over (r)}   R (t) are position vectors of the transmitter and receiver, respectively. In addition to time dilation, the average Doppler frequency shift over the same period of time must be calculated. This is done using the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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         [0025]    where r RT (t) is the distance between the platform receiver and the unknown transmitter (emitter), λ is the wavelength and Δf(t) is the instantaneous Doppler frequency shift at time t. 
         [0026]    When considering the case of a stationary emitter  10 , the average Doppler shifts correspond to scaled range difference measurements (or TDOA) for positions of the receiver at the beginning and end of the time interval for the average. The equivalent time differences are: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0027]    This observation is important since Doppler emitter localization performed here is based on range difference processing over a synthetic aperture. The approach used here in one implementation is a completely linear TDOA or range difference solution, even for a single platform. This linear formulation can be used as a starting point for iterative refinement by including additional non-linear equations. Nevertheless, using the average Doppler shifts, emitter locations can be computed using a standard TDOA overdetermined set of linear equations. In this simple formulation, the use of range differences assumes f 0  or λ are known. This is not essential and the method is modified to estimate both an unknown center frequency and unknown frequency drift rate or alternatively to reformulate the equation set to eliminate them as nuisance parameters. 
         [0028]    When neither the center frequency nor the frequency drift rate are known, a few iterations near the correct solution reduce the error. To refine the solution, the Jacobian of the nonlinear equations must be calculated. The frequency model with an unknown frequency and drift rate is: 
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         [0029]    The Jacobian of h(t,θ) with respect to θ is given by: 
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         [0030]    and details of the Jacobian calculation can be found Sampling at time instants t i  the vector equation for the frequency measurement is 
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         [0031]    The Taylor series in θ is about θ 0  for h(θ) is 
         [0000]        h (θ)= h (θ 0 )+∇ h (θ 0 )(θ−θ 0 )+ . . .   (10)
 
         [0032]    so that the approximate linear equation can be written as 
         [0000]        f   m   ≈h (θ 0 )+∇ h (θ 0 )(θ−θ 0 )+ n    (11)
 
         [0033]    The covariance of n is denoted R and n has independent identically distributed components so that 
         [0000]      R=σ n   2 l   (12)
 
         [0034]    The standard least squares solution to Equation 11, above, leads to a nonlinear Newton type of iteration for θ given by 
         [0000]      θ k+1 =θ k   +[∇h (θ k )] # ( f   m   −h (θ k ))   (13)
 
         [0035]    where A #  denotes the pseudoinverse of A. The initial θ 0  is provided by the linear geolocation algorithms as a starting point to refine or improve. Equation 13 is a Gauss-Newton solution for θ. By modifying Equation 13, a robust convergence is achieved. More specifically, the step size (from θ k  to θ k+1 ) in Equation 13 is modified to explicitly put a limit or maximum step size for testing based on a particular application and field of view. This modification is built into the geolocation system  26  allowing for automatic convergence metrics. As such, convergence is achieved without the need for multiple coordinated sources, an antenna array, tight receiver synchronization, or pulsed signals. 
         [0036]    With the frequency of the signal estimated, the location of the unknown source is calculated at  116  based on the estimated frequency and as it is measured over time. 
         [0037]    This description, rather than describing limitations of an invention, only illustrates an embodiment of the invention recited in the claims. The language of this description is therefore exclusively descriptive and is non-limiting. Obviously, it&#39;s possible to modify this invention from what the description teaches. Within the scope of the claims, one may practice the invention other than as described above.