Patent Publication Number: US-6670907-B2

Title: Efficient phase correction scheme for range migration algorithm

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to radar signal processing systems. More specifically, the present invention relates to phase error correction in range migration algorithm (RMA) for synthetic aperture radar (SAR) systems. 
     2. Description of the Related Art 
     In imaging applications such as ground mapping, a radar system is used to generate a two-dimensional image of a portion of a ground surface in the range and azimuth (cross-range) directions. A large antenna aperture is required in conventional imaging radar systems in order to achieve a narrow beamwidth and, consequently, fine azimuth resolution. Synthetic aperture radar (SAR) systems have been developed as an alternative means for improving azimuth resolution by synthesizing pulse-to-pulse return signals collected by a moving platform with a small antenna. The signal synthesis from many successive locations of the moving platform accomplishes what would otherwise require a larger antenna aperture. 
     Polar format algorithm (PFA) has been widely used for high-resolution SAR systems. However, PFA has the shortcomings of limited depth of focus and geometric distortion that increases with the map size. Range migration algorithm (RMA) is one of the most attractive and advanced SAR processing techniques to avoid the problems with the PFA. A difficulty with the RMA, however, is performing efficient phase correction. 
     Normal SAR data collection requires phase coherence, not only within each pulse for range resolution, but also from pulse to pulse over the collection time needed for azimuth resolution. The platform position affects the pulse-to-pulse phase coherence over the synthetic aperture. Phase error introduced by the inaccuracy of navigation data or undesirable platform motion causes smearing or duplication of the target image. Since motion compensation at the early processing stage based on the navigation data is not sufficient for producing a focused image, it is a common practice to employ data driven autofocus algorithms in high resolution SAR systems in order to maintain phase coherence and achieve good image quality. 
     Considering the computational efficiency and simplicity, it is desirable to implement the autofocus function after range compression during batch processing as is usually done for the case of PFA. However, for the case of RMA, it is difficult to implement the autofocus function during batch processing because the signal support areas from different targets are not aligned. For this reason, the autofocus function had to be implemented before batch processing starts, at the cost of increased complexity and processing time. 
     Because of the difficulty in implementing the autofocus function during batch processing, current RMA systems perform the autofocus function during the pulse-to-pulse processing phase using a separate polar format processing algorithm. This approach, however, has the disadvantage of implementation complexity and tighter processing timeline requirements. Furthermore, the increased timeline requirement makes it more difficult, if not impossible, to implement more advanced autofocus techniques. 
     Hence, a need exists in the art for an improved system or method for efficient phase error correction in range migration algorithm. 
     SUMMARY OF THE INVENTION 
     The need in the art is addressed by the present invention, a system and method for efficient phase error correction in RMA implemented by making proper shifts for each position dependent phase history so that phase correction can readily be performed using the aligned phase history data during batch processing. In its simplest form, the invention is comprised of two main parts. First, alignment of the phase error profile is achieved by proper phase adjustment in the spatial (or image) domain using a quadratic phase function. Then, the common phase error can be corrected using autofocus algorithms. 
     Two alternative embodiments of the invention are described. The first embodiment adds padded zeros to the range compressed data in order to avoid the wrap around effect introduced by the FFT (Fast Fourier Transform). This embodiment requires a third step: the target dependent signal support needs to be shifted back to the initial position after phase correction. The second embodiment uses the range compressed data without padded zeros. Instead, an aperture of greater length needs to be generated by the Stolt interpolation. In this embodiment, the third step of shifting the signal support back can be eliminated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is an illustration showing two targets at different azimuth locations in the spatial domain. 
     FIG. 1 b  is an illustration showing the corresponding signal support areas in the spatial frequency domain of the two targets in FIG. 1 a.    
     FIG. 2 is an illustration showing shifted signal support areas with common phase error. 
     FIG. 3 is an illustration showing two targets in the target image separated in azimuth in the spatial domain. 
     FIG. 4 is a simplified block diagram of an airborne SAR system using RMA with the new phase correction scheme in accordance with the teachings of the present invention. 
     FIG. 5 is a simplified block diagram showing the functions in RMA with the new phase correction scheme in accordance with the teachings of the present invention. 
     FIG. 6 a  is a simplified block diagram of a portion of RMA showing the inputs for a padded zero embodiment of the new phase correction scheme in accordance with the teachings of the present invention. 
     FIG. 6 b  is a simplified block diagram of a portion of RMA showing the inputs for an extended aperture embodiment of the new phase correction scheme in accordance with the teachings of the present invention. 
     FIG. 7 is a flow diagram of a padded zero embodiment of the new phase correction scheme in accordance with the teachings of the present invention. 
     FIG. 8 a  is an illustration showing the input data with padded zeros. 
     FIG. 8 b  is an illustration showing the phase adjustment needed to align the signal support. 
     FIG. 8 c  is a plot of phase versus K x  for three target positions prior to autofocus. 
     FIG. 8 d  is a plot of phase versus K x  for three target positions after autofocus. 
     FIG. 8 e  is an illustration showing the phase adjustment needed to remove the previous phase shift. 
     FIG. 9 is a flow diagram of an extended aperture embodiment of the new phase correction scheme in accordance with the teachings of the present invention. 
     FIG. 10 a  is an illustration showing two targets at different range locations in the spatial domain. 
     FIG. 10 b  is an illustration showing the corresponding signal support areas in the spatial frequency domain of the two targets in FIG. 10 a.    
     FIG. 11 is a block diagram showing an alternative embodiment of the present invention where the input data is partitioned into range bins. 
    
    
     DESCRIPTION OF THE INVENTION 
     Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention. 
     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. 
     Considering the computational efficiency and simplicity, it is desirable to perform phase correction in synthetic aperture radar (SAR) systems during batch processing. It is difficult to implement autofocus techniques during batch processing in range migration algorithm (RMA) because the signal support areas in the spatial frequency domain from different targets are not aligned. FIGS. 1-3 help illustrate this problem. 
     FIG. 1 a  is an illustration showing two targets, a and b, in the spatial (or image) domain at different azimuth locations, separated by an angle α. FIG. 1 b  is an illustration showing the corresponding signal support areas, A and B, in the spatial frequency domain of the two targets, a and b, respectively, shown in FIG. 1 a . The signal support areas A and B do not occupy the same area. They are shifted, or more exactly, rotated, by the angle α. Thus, the signal support in the spatial frequency domain is shifted depending on the target&#39;s azimuth location. 
     FIG. 2 is an illustration showing shifted signal support areas A and B with corresponding phase history (or phase variation) θ a  and θ b , respectively. Phase error due to uncompensated platform motion induces the same effect to phase history for all targets. The phase histories θ a  and θ b  are identical except they are shifted by the angle α. The common phase error follows the rotated signal support for a displaced target in azimuth. (Since the angle span of signal support is small in most practical SAR systems, rotation can be approximated to be a linear shift in the azimuth direction.) 
     The position dependent shifting of the signal support in the spatial frequency domain makes it difficult to implement autofocus algorithms during batch processing for the RMA. In the case of polar format algorithm (PFA), the signal support areas of all targets are identical and therefore common phase error for all targets can readily be estimated and removed. 
     Misalignment of phase error in the spatial frequency domain is due to shifting of the signal support that is determined by the relative Doppler cone angle of each target. Since the amount of shifting can be calculated for a given target location in the azimuth direction, alignment of the phase error profile can be achieved by proper phase adjustment in the spatial domain. Once the phase error profiles are aligned, then the common phase error can be corrected using autofocus techniques. 
     FIG. 3 is an illustration showing two targets, a and b, in the target image I separated in azimuth in the spatial domain. The image I is oriented in the range and azimuth directions. The image center is located a distance R from the platform (located at the origin). Targets a and b are separated by a distance X in the azimuth direction. Target a has a Doppler cone angle Φ 0  measured from the along track-direction v (the direction of platform motion). Target b has a Doppler cone angle Φ 1  measured from the along track-direction v. 
     The azimuth variable in the spatial frequency domain, K X,AT  in the along-track direction is expressed as:                K     X   ,   AT       =       -     2   λ          cos                 ϕ             [   1   ]                         
     where λ is the wavelength of the transmitted radar signal and Φ is the Doppler cone angle that is measured from the along-track direction. 
     The variation in K X,AT  between targets a and b is therefore given by:                Δ                   K     X   ,   AT         =       -     2   λ            (       cos                   ϕ   1       -     cos                   ϕ   0         )               [   2   ]                            =       4   λ          sin        (         ϕ   0     +     ϕ   1       2     )            sin        (         ϕ   0     -     ϕ   1       2     )                   [   3   ]                            ≅       -       2      X       R                 λ            sin                   ϕ   0                 [   4   ]                         
     for X&lt;&lt;R and Φ 1 ≈Φ 0 . 
     Since the image is oriented in the range and azimuth directions, then K x  and K y  should be oriented in the same manner and ΔK X  is expressed as:                Δ                   K   X       =         Δ                   K     X   ,   AT           sin                   ϕ   0         =     -       2      X       R                 λ                   [   5   ]                         
     The derived expression for shifting in K x  can be used for aligning the shifted signal support from multiple targets that are separated in azimuth. Required shifting in K x  to align common phase error history can readily be accomplished by adjusting the phase in the spatial domain by:                Δ                 θ     =       2                   π   ·   Δ                     K   K          X   /   2       =       -   2                     π   ·       X   2       R                 λ                     [   6   ]                         
     The dividing factor 2 is for the desired group delay ΔK X  when Δθ is differentiated in X. 
     The present invention provides a system and method for efficient phase error correction in RMA by making proper shifts for each position dependent phase history so that phase correction can readily be performed using the aligned phase history data during batch processing. Alignment of the phase error profile is achieved by proper phase adjustment in the spatial domain using a quadratic phase function. Then, the common phase error can be corrected using autofocus algorithms. 
     FIG. 4 is a simplified block diagram of an airborne SAR system using RMA with the new phase correction scheme in accordance with the teachings of the present invention. A SAR system  16  mounted on an aircraft  10  generates an electromagnetic wave  12  which is reflected off a ground surface  14  and received by the SAR system  16 . The SAR system  16  includes an antenna  20 , a transmitter  22 , and a receiver  24  for radiating and receiving the electromagnetic waves. The receiver  24  forms input video phase history (VPH) data  28  from the received data and sends it to an on-board signal processor  26 . Residing in the signal processor  26  is a range migration algorithm  30  with the new phase correction scheme of the present teachings which outputs a focused image  50 . 
     FIG. 5 is a simplified block diagram showing the basic components of RMA and where the new phase correction scheme should be inserted in accordance with the teachings of the present invention. The basic components of RMA are well known in the art. The input VPH data  28  is successively passed through a Motion Compensation function  32 , a Range Deskew function  34 , an Along-Track FFT  26 , a Matched Filter  38 , a Stolt Interpolation  40 , and a Range FFT  42 . The new phase correction scheme  44  is inserted after the Range FFT  42 , such that the input to the Phase Correction unit  44  is range compressed data  52 . The phase corrected data  54  output from the Phase Correction unit  44  can then be weighted and passed through an Azimuth FFT  46  to generate a final focused image  50 . 
     Two alternative embodiments of the present invention are described. The first embodiment adds padded zeros to the range compressed data in order to avoid the wrap around effect introduced by the FFT (Fast Fourier Transform). In this embodiment, the target dependent signal support is shifted back to the initial position after phase correction. The second embodiment uses the range compressed data without padded zeros. Instead, an aperture of greater length needs to be generated by the Stolt interpolation. 
     FIG. 6 a  is a simplified block diagram of a portion of RMA showing the inputs for a padded zero embodiment of the new phase correction scheme in accordance with the teachings of the present invention. In this embodiment, the Stolt Interpolation  40   a  is implemented as normal for RMA, using the aperture length required for the specified resolution. The range compressed data  52   a  generated by the following Range FFT  42  is input to the Padded Zero Phase Correction unit  44   a.    
     FIG. 6 b  is a simplified block diagram of a portion of RMA showing the inputs for an extended aperture embodiment of the new phase correction scheme in accordance with the teachings of the present invention. In this embodiment, the length of the aperture generated by the Stolt interpolation  40   b  is greater than the length required for the specified resolution. The length of padded zeros in the first embodiment and the data extension in the second embodiment should be identical, and it is determined by the mapping range and the size of the processed image in the azimuth direction. 
     Because of the increase in the usable phase information, performance in estimation and correction of phase by the second embodiment is better than that of the first. The second embodiment also does not include the last two FFT functions required in the first embodiment since the signal support is not required to shift back to the initial position. Considering the computational efficiency and performance, the second method is recommended for implementation by just increasing the aperture size in the Stolt interpolation. 
     FIG. 7 is a flow diagram of a padded zero embodiment  44   a  of the new phase correction scheme in accordance with the teachings of the present invention. In this embodiment, the invention is comprised of three major parts. The first part  60 , including Steps  2  through  4 , is for shifting the signal support through quadratic phase adjustments in the spatial domain. The second part  62 , including Steps  5  through  7 , is for phase correction using autofocus algorithms. The third part  64 , including Steps  8  through  10 , is for shifting the signal support back to its initial position. It should be noted that all FFT functions in this figure are performed in the azimuth direction. 
     In STEP  1   a , zero padding is applied to the input range compressed data  52   a.  FIG. 8 a  is an illustration showing the input data block with padded zeros. Zero padding to the range compressed data in the azimuth direction (K x ) is required to avoid a wrap around effect when the FFT function is performed. Nzero, the number of padded zeros in each side of the data, is determined by the mapping range and the size of the map, W az , in the azimuth direction:              Nzero   =         2   λ     ·         W   az     /   2     R     ·     1   dkx       =       1   λ     ·       W   az   2     R                 [   7   ]                         
     In STEP  2 , a forward FFT function is applied to the zero padded data. This converts the data to the spatial domain. This is the first step to accomplish shifting of the signal support in the spatial frequency. It is followed by phase correction (STEP  3 ) before converting back to the spatial frequency domain by the IFFT function (STEP  4 ). 
     In STEP  3 , the phase is adjusted using a quadratic phase function. For the shifting of signal support by ΔK x  as expressed in Equation 5, the phase of the data after the FFT function (STEP  2 ) needs to be adjusted by:                           θ   =       2                   π   ·   Δ                       K   X     ·   X       =       -   2                     π   ·     2     R                 λ       ·     X   2                     [   8   ]                         
     However, since a shift in K x  should be              1     2                 π            (          θ          X       )       =       2      X       R                 λ         ,                   
     the correct phase adjustment should be:                Δ                 θ     =       θ   2     =       -   2                     π   ·       X   2       R                 λ                     [   9   ]                         
     FIG. 8 b  is an illustration showing the phase adjustment Δθ needed to align the signal support for different points a, b, and c in the up-sampled image data formed after the FFT in STEP  2 . 
     The phase that needs to be applied to the pixel data at an index n from the map center can be expressed as:                Δ                   θ   n       =       -   2                     π   ·       d   pxl   2       R   ·   λ       ·     n   2                 [   10   ]                         
     where d pxl  is the pixel spacing. 
     In STEP  4 , an inverse Fast Fourier Transform (IFFT) is applied to the phase shifted data to convert the data from the spatial domain to the spatial frequency domain, completing the target dependent shifting in K x  for the alignment of common phase error history. 
     After the alignment of signal support through the previous steps, any efficient autofocus function can be performed to correct the now aligned common phase error for all targets. In the illustrative embodiment of FIG. 7, the application of a non-parametric method such as phase-gradient autofocus or multiple discrete autofocus algorithm is depicted. It includes STEP  5 , selection of strong isolated targets and shifting to DC to remove the linear phase component; STEP  6 , combining intermediate results for improved accuracy of the estimated common phase error; and STEP  7 , compensating for the estimated phase error in the data formed after STEP  4 . FIG. 8 c  is a plot of phase versus K x  for the three positions a, b, and c shown in FIG. 8 b  prior to autofocus. FIG. 8 d  is a plot of phase versus K x  for the three positions a, b, and c shown in FIG. 8 b  after autofocus. Other autofocus methods such as phase difference autofocus algorithm can also be used for the estimation and correction of phase error. 
     After autofocus is applied, Steps  8  through  10  shift back the shifted signal support that was performed earlier. 
     In STEP  8 , an FFT function is applied to the phase corrected data. The output of this FFT function is an up-sampled image. 
     In STEP  9 , the inverse function of the phase adjustment performed earlier (STEP  3 ) is applied to shift back the signal support that is dependent on the target azimuth location. FIG. 8 e  is an illustration showing the phase adjustment −Δθ needed to remove the previous phase shift. 
     In STEP  10 , an inverse FFT function is applied to convert the data back to the spatial frequency domain. The shifting back of the signal support is completed by this function. 
     Finally, in STEP  11 , the extra data corresponding to the padded zeros inserted in STEP  1  are removed, and the phase corrected output  54  is then sent to the Weighting and Azimuth FFT function  46  to generate a final focused image. 
     FIG. 9 is a flow diagram of an extended aperture embodiment of the new phase correction scheme in accordance with the teachings of the present invention. This embodiment is based on the fact that the last four steps in the first embodiment can be eliminated if data corresponding to the padded zeros is available. Therefore, the aperture in K x  for this method is longer than that of the first embodiment by the number of the padded zeros. With the extended output aperture from the Stolt interpolation, this method has less processing steps and provides better performance than the first method. 
     This embodiment is comprised of two main parts, identical to the first two parts of the padded zero embodiment. The first part  60 , including Steps  2  through  4  in FIG. 9, is for the shifting of signal support through quadratic phase adjustments in the spatial frequency domain. The second part  62 , including Steps  5  through  7 , is for phase correction using autofocus algorithms. 
     In STEP  1   b , input data with an extended aperture is generated (by the Stolt interpolation  40   b ). This embodiment takes as input the range compressed data  52   b  after the Range FFT function  42 . It is important to note that the length of the aperture in K x  should be extended as much as the maximum shifting that occurs with the image edges. K x  the required aperture length for the resolution δ az  with the broadening factor k a  is: 
       K   X   =k   a /δ az   [11] 
     Maximum shifting K shift  is equal to Nzero in the padded zero embodiment. Therefore, the required aperture length is: 
     
       
         ( k   a /δ az )+2 ·Nzero   [12] 
       
     
     Steps  2  through  7  are identical to those in the padded zero embodiment. The output of the autofocus function  62  is the phase corrected output  54  which is then sent to the Weighting and Azimuth FFT function  46  to generate a final focused image. 
     The phase that was applied in the phase adjustment function was given as a function of n 2 , where n is the azimuth pixel index. This was obtained from the approximated expression in Equation 4. Approximation is valid when the variation of Doppler cone angle for all targets in an image is small, which is true in most SAR systems. When the variation of Doppler cone angle is not small enough, a more accurate expression derived as follows can be used for the required phase adjustment. 
     Since Φ 1 =Φ 0 −X/R in FIG. 3, phase due to shifting of ΔK X,AT  can be expressed as: 
     
       
         Δθ=2 π·A·ΔK   X,AT   ·X /sin Φ 0   [13] 
       
     
     
       
         
           
             
               
                 
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     Here A is the variable that needs to be determined to provide exact shifting ΔK X . 
     By taking the derivative of θ in X for ΔK X,AT , that is,            Δ                   K     X   ,   AT         =       1     2      π                 θ          X           ,                   
     A is determined to be:                    A   =         cos        (       ϕ   0     -     X   /   R       )       -     cos                   ϕ   0           (       cos        (       ϕ   0     -     X   /   R       )       -     cos                   ϕ   0       +       X   /   R     ·     sin        (       ϕ   0     -     X   /   R       )                           =     1     1   +       X   R     ·       sin        (       ϕ   0     -     X   /   R       )         2        sin        (       ϕ   0     -       X   /   2        R       )            sin        (       X   /   2        R     )                               [   15   ]                         
     This expression for A needs to be inserted into Equation 14 for the calculation of Δθ. For X&lt;&lt;R, A becomes 0.5, and Equation 14 is approximated to Equation 6. 
     Signal support span is determined by the SAR angle. As illustrated in FIG. 10, signal support for a target at the near range is greater than that at the longer range. FIG. 10 a  is an illustration showing two targets, a and b, at different range locations in the spatial domain. FIG. 10 b  is an illustration showing the corresponding signal support areas, A and B, in the spatial frequency domain of the two targets, a and b, in FIG. 10 a.  As shown in this figure, common phase error can be stretched or expanded depending on the range to the targets. Since the target position variation in the azimuth direction is generally very small compared with the mapping range in most SAR applications, the effect of expanding or stretching is negligible. When the map size gets big enough compared with the mapping range, phase error estimation and correction can be done separately for each range block as shown in FIG.  11 . 
     FIG. 11 is a block diagram showing an alternative embodiment of the present invention where the input data is partitioned into range bins. The range compressed data  52  is partitioned into different range bins  70 . Phase correction  44  (shifting and focusing as described above) is then performed on each range block separately. The combined output data is then sent to the Azimuth FFT  46  to produce a final focused image  50 . 
     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. 
     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. 
     Accordingly,