Abstract:
A system and method for ascertaining the range of a noise-jamming target. The inventive system includes a receiver; a data processor coupled to the receiver; and software adapted for execution by the data processor for computing range to a target transmitter using first and second assumptions with respect to the output power level thereof and interpolating with respect to a error term calculated with respect thereto. The inventive method includes the steps of: making assumptions with respect to an output power level of a transmitter located at the target; measuring a level of power received from the transmitter by a receiver at first and second distances relative to the transmitter; calculating an error term with respect to the assumptions; interpolating with respect to the error term to make a range calculation; and outputting the range calculation when the error term reaches a predetermined threshold.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to detection systems. More specifically, the present invention relates to electromagnetic target detection systems such as radar, ladar, sonar and other such systems.  
           [0003]    2. Description of the Related Art  
           [0004]    Several techniques are currently employed to detect a target utilizing electromagnetic energy such as radar, ladar, sonar and other such systems. Radar involves the transmission of a radio wave pulse which, when reflected by a target provides a return pulse which is received and processed to effectuate target detection.  
           [0005]    Unfortunately, a noise jammer may frustrate a radar system. A noise jammer is a source which transmits noise near the transmit frequency of the radar. This noise source impedes the ability of the radar system to detect the type of target or the range thereof. This is problematic inasmuch as it may cause a premature launch of a weapon outside of its range of operation. This forces the use of a smaller weapon launch zone and may lead to a loss of a first shot capability, obviously highly undesirable for military applications.  
           [0006]    The current method of obtaining range, when radar range is denied, is based on a conjecture as to the target&#39;s altitude. Unfortunately, the range obtained by this method is highly inaccurate as it is based on a supposition of the target&#39;s altitude. If a target is several miles away at some unknown range, guessing a target&#39;s altitude with any accuracy can be very difficult. At normal closing speeds there is very little time to obtain more accurate estimates and the erroneous estimates can put the aircrew in jeopardy. Further, there is no way for the aircrew to know if the initial conjectures were correct.  
           [0007]    Hence, a need remains in the art for a system or method for determining the range of a noise-jamming target.  
         SUMMARY OF THE INVENTION  
         [0008]    The need in the art is addressed by the system and method for ascertaining range of a noise-jamming target of the present invention. Generally, the inventive system includes a receiver; a data processor coupled to the receiver; and software adapted for execution by the data processor for computing range to a target transmitter using first and second assumptions with respect to the output power level thereof and interpolating with respect to a error term calculated with respect thereto. The inventive method includes the steps of: making assumptions with respect to an output power level of a transmitter located at the target; measuring a level of power received from the transmitter by a receiver at first and second distances relative to the transmitter; calculating an error term with respect to the assumptions; interpolating with respect to the error term to make a range calculation; and outputting the range calculation when the error term reaches a predetermined threshold.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a block diagram of a system for determining the range of a noise-jamming target in accordance with the teachings of the present invention.  
         [0010]    [0010]FIG. 2 is a flow diagram of the method for determining the range of a noise-jamming target in accordance with the teachings of the present invention.  
         [0011]    [0011]FIG. 3 is diagram illustrative of the operation of the range determining system of the present invention.  
         [0012]    [0012]FIG. 4 is a table illustrative of data utilized in accordance with the teachings of the present invention. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0013]    Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention.  
         [0014]    While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.  
         [0015]    [0015]FIG. 1 is a block diagram of a system for determining the range of a noise-jamming target in accordance with the teachings of the present invention. The system  10  is adapted to determine the range to a noise-jamming target transmitter  20  with respect to a receiver platform  30 . In the illustrative application, the noise source  20  is a radar transmitter. However, the present teachings are applicable to ladar, sonar, and other ranging technologies utilizing electromagnetic, acoustic or other energy mediums.  
         [0016]    As shown in FIG. 1, in accordance with the present teachings, a receiver  40  is provided on the platform  30 . The receiver  40  is adapted to receive energy transmitted from the target transmitter  20  and measure the effective radiated power (ERP) thereof. As is common in the radar art, the output of the receiver  40  is provided to a signal processor  50 , which converts and reformats the received signals to a suitable, e.g., digital format. A data processor  60  receives altitude data from an altimeter, implements the method of the present teachings via software and thereby ascertains the range of target transmitter  20 . The range is provided to an input/output (I/O) device  80  which, in practice, may be weapon fire control system, a transmitter, or other suitable device.  
         [0017]    In operation in accordance with the present teachings, certain assumptions are made:  
         [0018]    1. Target is flying at a constant altitude at the time the jammer ERP is being determined.  
         [0019]    2. The power received from the jammer is fairly steady from frame to frame. (Less then 2.5 dB)  
         [0020]    3. As the receiver platform  30  and the target  20  fly toward each other, the relationship of range, elevation angle, and altitude must remain consistent. That is, if the altitude is to remain constant, the range and elevation angle must keep changing to correspond with a constant altitude. Otherwise, the altitude will develop a rate as will be more apparent from below.  
         [0021]    [0021]FIG. 2 is a flow diagram of the method for determining the range of a noise jamming target in accordance with the teachings of the present invention. The method of the present invention essentially uses a Regula-Falsi approximation to compute the distance ‘b’ between the target  20  and the platform  30 . This is illustrated in FIG. 3.  
         [0022]    [0022]FIG. 3 is diagram illustrative of the operation of the range determining system of the present invention. As shown in FIG. 3, the range to solve for ‘b’ may be computed using c=the earth radius plus the flight altitude times the cosine of the angle A. Angle A is the compliment of the target elevation angle B. The receiver  40  supplies the target elevation angle B and azimuth angle in a conventional manner.  
         [0023]    Unfortunately, the location of the target is unknown. Consequently, the right angle at C is not available for the simple computation of ‘b’ using the cosine law. Consequently, as mentioned above and discussed more fully below, in accordance with the present teachings, the method of the present invention uses the Regula-Falsi approximation to compute the distance ‘b’ between the target  20  and the platform  30 .  
         [0024]    Returning to the flow diagram  100  of FIG. 2, the inventive method includes the steps ( 102 ) of assuming a reference power level of the target  20  from a first distance relative to the target and ( 104 ) assuming a search power level of the source from the first distance. The two different target power levels of Effective Radiated Power (ERP) are assumed in Watts/MHz. The “Reference” assumption is used to compute R1 for range  1 . The “Search” assumption is used to compute R2 for range  2 . The Reference ERP value is set to a maximum expected value and never varies. The Search ERP value is set near a minimum expected value and will vary as a function of the altitude error term.  
         [0025]    Next, the power level of energy received from the target  20  (Pj) is measured at the first distance (step  106 ). Then, at step  108 , first and second range hypotheses are computed with respect to the first distance using the measured power level and the reference and search power levels, respectively. In the illustrative embodiment, the range computations are made using the radar range equation:  
           P   J =(Assumed ERP)*λ* G   A /(4π R ) 2   [1] 
         [0026]    where:  
         [0027]    ERP=P J *B W  watts/MHz  
         [0028]    P J =jammer Power  
         [0029]    B W =Bandwidth  
         [0030]    λ=Wavelength  
         [0031]    π=constant  
         [0032]    R=range  
         [0033]    G A =Antenna Gain  
         [0034]    Then, at step  110 , the altitudes H1 and H2 are computed based on the obtained ranges using the round earth quadratic equation and the law of cosines with respect to the first distance to the target:  
           HT=Sqrt ( Rng   2 +( RE+HI ) 2 −2 *Rng* ( RE+HI ) 2 *Cos(90+ EL ))− RE   [2] 
         [0035]    where:  
         [0036]    HT=Nmi  
         [0037]    HI=Nmi  
         [0038]    RE=3440 Nmi  
         [0039]    EL=elevation angle  
         [0040]    Rng=range  
         [0041]    Next, at step  112 , the receiver  40  waits until the platform  30  and the target  20  move to a second relative distance and the first and second target altitude hypotheses are recomputed using the first and second range hypotheses, respectively, with respect to a second distance to the target.  
         [0042]    At step  114 , the first and second target altitude hypotheses with respect to the second distance are subtracted from the first and second target altitude hypotheses with respect to the first distance to ascertain first and second altitude differentials, respectively.  
         [0043]    At every new interval, altitude rates ΔH1 and ΔH2 are derived from each. Because the relationship between range and altitude is non-linear, Regula-Falsi is used to determine a range 3 between the original ranges. As is well-known, the Regula-Falsi Method is a numerical method for estimating the roots of a polynomial f(x). A value x replaces the midpoint in the Bisection Method and serves as the new approximation of a root of f(x). The objective is to make convergence faster.  
         [0044]    Next, at step  116 , an altitude error term is calculated using the first and second altitude differentials.  
           Err =(0−Δ H 2)/(Δ H 2−Δ H 1)  [3] 
         [0045]    where ΔH1 is the first altitude differential and ΔH2 is the second altitude differential.  
       EXAMPLE  
       [0046]    [0046]             Err   =                  (     0   -     Δ                 H2       )     /     (       Δ                 H2     -     Δ                 H1       )                   =                  (     0   -   10.2     )     /     (     10.2   -     (     -   11.6     )       )                   =                -   .466847                                   
         [0047]    Next, at step  118 , a third range hypothesis, range 3, is computed using the following expression:  
         range 3=( R 2+( R 2− R 1)* err*lf   [4] 
         [0048]    where  
         [0049]    ‘R1’ is the first range hypothesis;  
         [0050]    ‘R2’ is the second range hypothesis;  
         [0051]    ‘err’ is the error term; and  
         [0052]    ‘lf’ is a linearizing factor=6076.115/(2*RE)=1.13 where  
         [0053]    RE=earth radius.  
         [0054]    Using the exemplary values:  
             range3   =                (     R2   +       (     R2   -   R1     )     *   err                     =                (     86.17   +       (     86.17   -   111.25     )     *     (     -   .466847     )     *   l                 f       )                 =              99.42                               
 
         [0055]    which is only 0.1 Nmi from the exact range at this power (see FIG. 4).  
         [0056]    Through the range equation [1], the system  10  computes the ERP to be used by the Search for the next interval. This process continues until range 3 of Search gets close to the true range and the altitude rate of the search approaches zero. When the altitude rate of the search reaches zero, the ERP is “locked in” and the ERP is used to compute the range for the remainder of the pass.  
         [0057]    Next, at step  120 , a new target altitude is computed based on the new range hypothesis. Then, at step  122 , the new target altitude is compared to the previous target altitude hypothesis. If, at step  124 , the third target altitude differs from the first target altitude, then, at step  126 , the platform  30  moves to a third distance relative to the target  20  and steps  106  through  122  are repeated.  
         [0058]    If, at step  124 , the third target altitude does not differ from the first target altitude, then, at step  128 , the system  10  outputs the most recent range hypothesis as the range to the target.  
         [0059]    [0059]FIG. 4 is a table illustrative of data utilized in accordance with the teachings of the present invention. For example, the reference jammer power level is assumed to be 5000 watts/MHz and the initial jammer search power level is assumed to be 3000 watts/MHz. As shown in FIG. 4, this yields range hypotheses of 111.80 and 86.60 miles respectively. Next, target altitude is calculated for each range yielding 25,574 and 24,646 feet respectively. In the next interval, this process is repeated. The difference altitude between each interval is then determined. The error term is then computed. Using the error term, a third range is computed by the regula-falsi method of interpolation. The third range is then used to compute a third altitude and a third ERP. Not shown in FIG. 4, the third ERP is used to replace the 3000 watts/MHz in the next interval. With the third ERP, the entire process is repeated. With each repetition, all parameters ERP, range, altitude become more accurate relative to the true values. When an altitude difference of “zero” is reached in the search ERP process, the process is complete and the true ERP has been found. It can now be used to track the closing range until a launch zone within the missiles capability is available. The true ERP of 4000 watts/MHz is shown to demonstrate that the difference altitude is always “zero” when the correct ERP has been determined.  
         [0060]    Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof. For example, while the illustrative embodiment interpolates with respect to power, the inventive method may be implemented using an interpolation with respect to altitude without departing from the scope of the present teachings.  
         [0061]    It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.  
         [0062]    Accordingly,