Abstract:
In radar transponder operation, a variably delayed gating signal is used to gate a received radar pulse and thereby produce a corresponding gated radar pulse for transmission back to the source of the received radar pulse. This compensates for signal distortion due to amplitude modulation on the retransmitted pulse.

Description:
This invention was developed under Contract DE-AC04-94AL85000 between Sandia Corporation and the U.S. Department of Energy. The U.S. Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to radar and, more particularly, to radar transponder operation. 
     BACKGROUND OF THE INVENTION  
     U.S. Pat. No. 5,486,830 (incorporated by reference herein) describes a system wherein a synthetic aperture radar (SAR) apparatus carried on an airborne platform transmits a set of pulses for reception at a coherent gain-block tag. The tag processes each received pulse, first by amplitude-modulating the pulse (denoted as “chopping”), and then by applying bi-phase modulation. The tag then transmits the result (i.e., the processed pulse) to the SAR apparatus. The tag thus functions as a transponder apparatus. The processed pulse that is transmitted by the tag is also referred to as a retransmitted pulse or tag response. 
     The SAR apparatus collects a series of the retransmitted pulses and performs coherent SAR-tag processing. A tag image is produced that can be used to provide relative location of the tag within a normal SAR image. The SAR apparatus also forms its normal SAR context image using reflections of the same set of radar pulses received by the tag. In addition, the radar can process the retransmitted pulses to extract data from the tag. 
     In the system of U.S. Pat. No. 5,486,830, the amplitude modulation applied by the tag causes a significant distortion in the retransmitted pulses. This distortion manifests itself as unwanted side lobes that appear when the SAR apparatus applies range-compression to the retransmitted pulses. The first harmonic of these side lobes contains half the energy of the desired, range-compressed signal. When several tags are being illuminated by the SAR apparatus, it becomes difficult to distinguish the unwanted side lobes associated with any given tag from the main lobes (primary response lobes) of other tags. This effect makes it difficult for the SAR apparatus simultaneously to process several tags within a common set of radar pulses. 
     It is desirable in view of the foregoing to provide for reduction of distortion in the retransmitted pulses produced by transponders in systems of the type described above. Exemplary embodiments of the present invention reduce distortion in retransmitted pulses by applying a random delay to a chopping signal used for modulation in the transponder. This permits the SAR apparatus to process simultaneously many transponders within a common area of illumination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a timing diagram of selected prior art signals. 
         FIG. 2  is a frequency versus magnitude plot of a prior art signal. 
         FIG. 3  diagrammatically illustrates a SAR and tag system according to exemplary embodiments of the invention. 
         FIG. 4  is a timing diagram of selected signals associated with the tag of  FIG. 3 . 
         FIG. 5  is a timing diagram of a conventional SAR transmission signal that can be utilized by exemplary embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION  
       FIG. 1  shows at  11  a deramped radar pulse that the tag receives from the SAR apparatus in U.S. Pat. No. 5,486,830 (before chopping is applied). The chopping signal of U.S. Pat. No. 5,486,830 (provided by logic 28 to gate 23 in FIG. 2 of U.S. Pat. No. 5,486,830) is shown at  12  in  FIG. 1 . Note that the rising edge of the first pulse of the chopping signal  12  is not synchronized with the beginning of the received SAR pulse  11 . This is because the tag&#39;s internal clock is not time-synchronized with the received SAR pulse  11 . The deramped, amplitude-modulated (i.e., with chopping) pulse is shown at  13  in  FIG. 1 . This pulse  13  corresponds to the output of gate 23 in U.S. Pat. No. 5,486,830. 
     The tag return signal received at the SAR apparatus of U.S. Pat. No. 5,486,830 is given as follows (after deramping and phase stabilization):
 
 v   r ( t )= p ( t−Θ )exp{ j (2π f   r   t+φ ( t   s ))}  (1)
 
where p(t) is the chopping signal (i.e., amplitude modulation) that is applied by the tag, Θ is a time delay due to the phase difference between the SAR apparatus clock and the tag&#39;s internal clock, f r  is the residual carrier frequency that is dependent on the tag&#39;s range offset with respect to the Scene Reference Point (SRP), φ(t s ) contains the Doppler frequency term that is dependent on the tag&#39;s azimuth position with respect to the Scene Reference Point (SRP), and t s  is slow time. In equation (1), t is defined for the time that the signal is available at the receiver of the SAR apparatus.
 
     Range compression is the first step for the SAR apparatus of U.S. Pat. No. 5,486,830 when processing the tag return signal of equation (1). Assuming that no focusing is required in range or azimuth, then range compression can be performed by applying the Fourier Transform (FT) to the received signal as follows
 
 FT{v   r ( t )} =V   r ( f )= FT{p ( t−Θ )}             FT{ exp{ j (2π f   r   t+φ ( t   s ))}}  (2)

     The transform of the first term in equation (2) can be written as 
                     FT   ⁢     {     p   ⁡     (     t   -   Θ     )       }       =       P   ⁡     (   f   )       =       T   g     ⁢       sin   ⁡     (     π   ⁢           ⁢     fT   g       )         π   ⁢           ⁢     fT   g         ⁢     comb   ⁡     (     f   ;     1     2   ⁢     T   g           )       ⁢     exp   ⁡     (       -   j2     ⁢           ⁢   π   ⁢           ⁢   f   ⁢           ⁢   Θ     )                   (   3   )               
where 2T g  is the period of the chopping pulse, p(t). P(f) is a sampled sinc function multiplied by a complex exponential with an unknown phase term, −2πfΘ. The phase term is due to the unknown clock error between the SAR apparatus and the tag.  FIG. 2  is a normalized magnitude plot of equation (3).
 
     The transform of the second term in equation (2) can be approximated as follows
 
 FT{ exp{ j (2π f   r   t+φ ( t   s ))}}≈δ( f   r )exp{ jφ ( t   s )}  (4)
 
where φ(t s ) is assumed constant over a single radar pulse. For simplicity, the range sinc function has been replaced with the ideal delta function, and the signal amplitude has been ignored.
 
     Combining equations (3) and (4), the range-compressed signal can be written as 
     
       
         
           
             
               
                 
                   
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     Several observations can be made by examining equation (6). First, it does not represent the typical impulse response (IPR) of a point target. Instead, it is a sampled sinc function having a main lobe positioned where the normal point return would be expected, and having many other side-lobes separated by an amount proportional to the frequency of the chopping signal. Second, because of the convolution operation, each side-lobe is multiplied by the two phase terms of equation (6). The first phase term, φ(t s ), allows the SAR apparatus to apply conventional azimuth compression with respect to the tag response. The second phase term, 2π(f−f r )Θ, is a constant phase that is zero for the main lobe where f=f r . Also, note that in equation (5), the phase term φ(t s ) is assumed to be constant over the extent of a single radar pulse, but to vary slowly from pulse to pulse. Finally,  FIG. 2  shows that the first side-lobe is down only 3 dB with respect to the main lobe response. 
     In conventional azimuth compression processing of the received tag response, the SAR apparatus applies an azimuth Fourier Transform with respect to slow time, t s , as follows 
     
       
         
           
             
               
                 
                   
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     In equation (8), the Fourier Transform is with respect to the slow time variable, t s . The main lobe response is defined as occurring at f=f r  and is given as
 
 X ( f   r   , f   az )= T   g   FT{ exp( jφ ( t   s ))}  (9)
 
The range side-lobe can be expressed as
 
                       X   ⁡     (     f   ,     f   az       )       =       K   ·   FT     ⁢     {     exp   ⁡     (       j   ⁢           ⁢     φ   ⁡     (     t   s     )         -     j   ⁢           ⁢   2   ⁢     π   ⁡     (     f   -     f   r       )       ⁢   Θ       )       }         ,     f   =       f   r     ±     n     2   ⁢     T   g             ,     n   =   1     ,   2   ,   …           (   10   )               
where K is a constant whose value depends on the particular side-lobe. Notice that the phase term, 2π(f−f r )Θ, in the expression above is just a constant with respect to the Fourier Transform. The magnitude response is given as
 
| K|·|FT{ exp( jφ ( t   s )− j 2π( f−f   r )Θ)}|=| K|·|FT {exp( jφ ( t   s ))}|  (11)
 
     Note that the constant phase term does not contribute to the magnitude response of a side-lobe. If it is the case that the phase term 2π(f−f r )Θ is a function of slow time, then the magnitude response of a side-lobe is given as
 
| K|·|FT {exp( jφ ( t   s ))exp(− j 2π( f−f   r )Θ( t   s ))}|  (12)
 
     Recall that the side-lobes are not desired, so the goal is to suppress or eliminate them in the SAR-tag image. The question arises, what form can Θ(t s ) take in order to reduce the magnitude response given by equation (10)? An obvious choice is to set 2π(f−f r )Θ(t s )=φ(t s ). However, to do this one must know φ(t s ), which depends on the unknown azimuthal position of the tag. Another approach is to make Θ(t s ) a time-varying nonlinear function. Here, the goal is to use the integration operation of the Fourier Transform to spread the side-lobe energy over the azimuth dimension. Recall that Θ is due to the clock error or difference between the SAR and tag clocks. In particular, Θ is due to the difference between the first rising edge of the chopping signal  12  and the beginning of the incoming SAR pulse  11  (see  FIG. 1 ). Therefore, the phase term 2π(f−f r )Θ can be controlled by controlling the starting time (i.e., the time of the first rising edge) of the chopping signal  12 . 
       FIG. 3  diagrammatically illustrates a tag generally similar to that of U.S. Pat. No. 5,486,830, but suitably modified to introduce a time-varying, nonlinear characteristic to the phase term 2π(f−f r )Θ(t s ) according to exemplary embodiments of the invention. The chopping signal (also referred to herein as the gating signal)  37  from logic  28  is fed to a random time delay block  39 , which adds a random time delay to the signal  37 , such that a resulting delayed gating signal  38  is applied to the gate block  23 . The gate block  23  produces a gated pulse  35  in response to the received SAR pulse  11  and the delayed gating signal  38 . The timing diagram of  FIG. 4  shows the gated pulse  35  and a corresponding delayed gated pulse  36  (produced in  FIG. 3  by delay block  25 ). 
     The time delay Θ(t s ) is chosen, randomly, between each received SAR pulse, such that 2π(f−f r )Θ(t s ) is, in equation (10), a uniformly distributed random phase between ±π. The random phase term is then integrated, via the Fourier Transform operation performed by the SAR apparatus. This technique reduces the overall magnitude of the side-lobe impulse response in  FIG. 2 . In order to obtain maximum side-lobe reduction, the time delay should be random over the entire slow-time processing interval of the tag. To achieve this goal, the time delay is defined in some embodiments as
 
Θ( t   s   =n( 2 T   g ) /N,  0≦ n=N− 1   (13)
 
where N is the number of SAR pulses transmitted and received during the tag&#39;s slow-time processing interval, 2T g  is the period of the chopping clock in the transponder, and the index n is chosen from a uniform random distribution of integers. Thus, every pulse of the gating signal  37 , at its particular position in slow time t s , has applied thereto a randomly chosen time delay value. The equivalent random phase delay, evaluated at the frequency offset f−f r =1/(2T g ) (the first range sidelobe) is
 
2 πn (2 T   g )/(2 T   g   N )=2 πn/N    (14)
 
     The random phase delay at the second sidelobe will be 6πn/N, at the third sidelobe will be 10πn/N, and so on. The number of pulses, N, in equations (13) and (14) is determined by the known length of time, T, that the SAR apparatus illuminates the tag (see  FIG. 5 ), and the known PRF (pulse repetition frequency) of the SAR:
 
 N=PRF×T    (15)
 
     Although some embodiments are described above in the context of use with airborne SAR platforms, it will be evident to workers in the art that the techniques of the invention are applicable to radar transponders in general, and are not limited to use with SAR or with airborne radar. 
     Although exemplary embodiments of the invention have been described above in detail, this does not limit the scope of the invention, which can be practiced in a variety of embodiments.