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- Open Access

# A residual range cell migration correction algorithm for bistatic forward-looking SAR

- Wei Pu
^{1}Email author, - Yulin Huang
^{1}, - Junjie Wu
^{1}, - Jianyu Yang
^{1}and - Wenchao Li
^{1}

**2016**:110

https://doi.org/10.1186/s13634-016-0407-2

© The Author(s) 2016

**Received:**15 March 2016**Accepted:**29 September 2016**Published:**22 October 2016

## Abstract

For bistatic forward-looking synthetic aperture radar (BFSAR), images are often blurred by uncompensated radar motion errors. To get refocused images, autofocus is a useful postprocessing technique. However, a severe drawback of the autofocus algorithms is that they are only capable of removing one-dimensional azimuth phase errors. In BFSAR, motion errors and approximations of imaging algorithms introduce residual range cell migration (RCM) on BFSAR data as well. When residual RCM is within a range resolution cell, it can be neglected. However, the residual migration, which exceeds a range cell, is increasingly encountered as resolution becomes finer and finer. A novel residual RCM correction method is proposed in this paper. By fitting the low-frequency phase difference of adjacent azimuth cells, residual RCM of each azimuth cell can be corrected precisely and effectively. Simulations and real data experiments are carried out to validate the effectiveness of the proposed method.

## Keywords

- Bistatic forward-looking synthetic aperture radar (BFSAR)
- Residual range cell migration (RCM)
- Low-frequency fitting

## 1 Introduction

Forward-looking imaging is highly desirable in some applications, such as airplane navigation and landing. Due to the ability to obtain high-resolution image of the forward-looking terrain, bistatic synthetic aperture radar (SAR) is drawing more and more attention in recent years. However, the moving platforms introduce relative motion between radars and observed scene, which induces range cell migration (RCM) to bistatic forward-looking SAR data. The procedure of RCM correction is essential for the frequency domain imaging algorithms.

Usually, RCM cannot be corrected completely in practical application. The reason is that there are some residual components, namely residual RCM. And the residual RCM is introduced by motion errors [1]. In the presumption that residual RCM is within a range resolution cell, residual RCM can be neglected [2, 3], and motion errors can be compensated by autofocus methods completely. Generally speaking, residual RCM is relatively small and can be neglected in monostatic SAR, while the unique characteristics of BFSAR makes the residual RCM exceeding range resolution cell inevitable. On the one hand, the separated platforms of BFSAR induce motion errors much larger than the errors in monostatic SAR raw data. On the other hand, the higher order terms of range migration are always neglected in BFSAR imaging algorithms. However, the impacts of these higher order terms become serious when the squint angle gets larger and the resolution gets higher. In this situation, residual RCM correction becomes a necessary procedure for BFSAR imaging. In principle, it is possible to compute the residual RCM from orbit and attitude data provided by an ancillary instrument such as inertial measurement units (IMU) and global positioning system (GPS). Nevertheless, measurement uncertainties on the data would limit the accuracy, and the data remains unknown for some unmanned aerial vehicles without ancillary instrument. Thus, residual RCM correction based on SAR data is indispensable.

To correct residual RCM based on SAR data, three alternative strategies are available. (1) Estimate the azimuth phase error term firstly, and then calculate the residual RCM from the estimated azimuth phase error [4]. In [4], the range compressed data is processed to a new coarser range resolution so that the presumption of autofocus is met. Therefore, the azimuth phase errors can be obtained using autofocus method from the new range compressed data and then the residual RCM can be calculated and compensated. However, as the azimuth phase errors are estimated in coarse resolution, the estimation precision of azimuth phase errors and residual RCM cannot satisfy the demands of high-resolution BFSAR. (2) Estimate residual RCM by making use of the relationship between azimuth phase errors and residual RCM. Mao et al. [3] deduces an accurate analytical relationship between azimuth phase errors and residual RCM in spotlight SAR. Using this relationship, a 2-D autofocus method, which can compensate the azimuth phase errors and residual RCM simultaneously, is proposed. Nevertheless, this relationship is valid just in the case of spotlight SAR data, so that applicability of this method is limited. (3) Estimate the residual RCM independently. The residual RCM correction method proposed in [5] is based on range alignment algorithm which is utilized in translational motion compensation on inverse SAR (ISAR)[6, 7]. However, parametric model of the range displacement in [6] restricts the estimation accuracy, and the interpolation procedure in [7] requires exhaustive computation.

In [8], a novel residual RCM correction algorithm based on low-frequency fitting is proposed for monostatic SAR to correct the motion-induced errors. In this paper, this residual RCM correction algorithm is greatly improved to cope with BFSAR data. By estimating the residual RCM utilizing the least-squares method in the low-frequency area and compensating the displacement in frequency domain, a sub-pixel level correction result can be achieved in this proposed method. Compared with previous works on residual RCM correction of SAR data, the proposed method can realize residual RCM correction with accuracy of sub-pixel level but requiring neither interpolation nor parametric model. Compared with [8], the novelty of the proposed method in this paper includes two parts. Firstly, a phase difference denoising procedure is added to make the proposed method more robust and can be adapted to the practical condition. Secondly, the one isolated dominant target assumption is not indeed needed for the proposed method, which makes the proposed method can be widely used in most practical condition. Furthermore, the range curvature term and even higher order terms, which is the unique problem in BFSAR, also can be solved by the proposed method. Simulation and real BFSAR data-processing results are presented to verify the effectiveness of the proposed method.

## 2 Problem formulation

*ϕ*

_{ T }and

*ϕ*

_{ R }and initial ranges

*R*

_{ Tcen}and

*R*

_{ Rcen}shown in Fig. 1 are measured at the composite beam center crossing time of the reference target

*P*.

where *K*
_{
r
} is the transmitted chirp rate, *T*
_{
r
} is the timewidth of the LFM pulse, and *T*
_{
a
} is the synthetic aperture time. The range time is given by *τ*, and *t* denotes the cross-range time, *λ* is the wavelength, and *c* is the speed of propagation.

*δ*

*R*(

*t*) denotes the instantaneous range displacement induced by motion errors, and

*R*(

*t*) represents the nominal instantaneous two-way range.

*t*=0 to its Taylor series and

*R*(

*t*) can be rewritten as

*s*

^{′}(

*τ*,

*t*) is obtained.

*Δ*

*R*(

*t*) in the range profile is

*Δ*

*R*(

*t*)/

*c*in the range profile is residual RCM. To illustrate the influence of residual RCM, the simple sketch map of residual RCM is shown in Fig. 2. In Fig. 2, the solid line denotes the migration trajectory of a prominent scatter after range compression and range cell migration.

The nonlinear migration trajectory in Fig. 2 is introduced by residual RCM. When the residual RCM exceeds a range resolution, 2-D defocus will emerge in the final image. The reason is that the energy of target scatter diffuses in several range cells and induces image defocus in range direction. Besides, the general presumption of autofocus is not valid, and there is considerable challenge for autofocus methods. When the autofocus procedures are performed in the azimuth direction, the azimuth signal is assumed to be a LMF signal. While in the condition that the residual RCM exceeds a range resolution, the energy of one target scatter diffuses in several range cells. Thus, the azimuth signal in a given range bin is not a complete LFM signal any more. Autofocus methods can not be correctly utilized as for a incomplete LFM signal. Hence, autofocus techniques are not effective if the residual errors are larger than range cell. And therefore, the image results defocus in azimuth direction as well.

Therefore, in order to obtain high-resolution BFSAR image, residual RCM correction is essential.

## 3 Residual RCM correction

In this section, a novel residual RCM correction method is proposed. Residual RCM of each azimuth cell is corrected one by one. Here, the residual RCM correction method for one azimuth cell is presented in the rest of this section.

### 3.1 Row correlation analysis

Let *f*
_{
m
}[*n*] and *f*
_{
m+1}[*n*] denote the adjacent azimuth cells of BFSAR data as shown in Fig. 2, where *m*=1,2,⋯,*M* indexes the transmitted pulses and *n*=1,2,⋯,*N* stands for the samples taken from each pulses, *M* is the azimuth sample number, and *N* is the range sample number.

*f*

_{ m }[

*n*] and

*f*

_{ m+1}[

*n*] can be written as

where \({{\bar R}_{m}}\) and *Δ*
*R*
_{
m
} denote the slant range and range displacement measured at the *m*th transmitted pulse, respectively; \({\bar R}_{m + 1}\) and *Δ*
*R*
_{
m+1} denote the slant range and range displacement measured at the (*m*+1)th transmitted pulse, respectively.

*f*

_{ m }[

*n*] and

*f*

_{ m+1}[

*n*] can be deduced.

*n*

_{ m+1}denotes the displacement between |

*f*

_{ m }[

*n*]| and |

*f*

_{ m+1}[

*n*]|.

That is to say, apart from the displacement *n*
_{
m+1}, two adjacent rows of the data can be considered as approximately equal.

*r*is the correlation value of adjacent rows,

Simulation parameters

Parameter | Value |
---|---|

Carrier frequency | 10 GHz |

Band width | 400 MHz |

Synthetic aperture time | 5 s |

Nominal Radar platform velocity | 100 m/s |

Pulse repetition frequency | 600 Hz |

Coordinates of the transmitter | (1,0.6,0.8) km |

Coordinates of the receiver | (0,1.2,0.7) km |

Size of mean filter window | 3 |

Hence, it is reasonable to regard the two adjacent rows of the data as approximately equal, and this feature provides the theoretical basis for the proposed algorithm.

### 3.2 Phase difference extraction

*F*

_{ m+1}(

*ω*) and

*F*

_{ m }(

*ω*) are the frequency spectra of

*f*

_{ m+1}[

*n*] and

*f*

_{ m }[

*n*], respectively. Then, phase difference between

*F*

_{ m+1}(

*ω*) and

*F*

_{ m }(

*ω*) can be calculated by Eq. (14).

where *Φ*(*ω*)∈(−*π*,*π*] denotes the phase difference between *F*
_{
m+1}(*ω*) and *F*
_{
m
}(*ω*). As known to all, the phase is limited to the interval of (−*π*,*π*] and leads to the transition of a phase difference from −*π* to *π*. In order to get a continuous phase difference curve, it is necessary to unwrap the phase spectrum while limited SNR of a BFSAR data can result in a great difference between adjacent rows of data. Hence, there will be a large local jitter in the phase difference curve, which will lead to failure of phase unwrapping. As a result, the number of fitting points to a linear function would decrease. Therefore, the phase difference curve should be denoised and smoothed before unwrapping.

### 3.3 Phase difference denoising

*Φ*[

*n*] denote the discrete version of the

*Φ*(

*ω*). Consider the phase sequence

*Φ*[

*n*] transform into the continuous sine/cosine phase sequence

*a*[

*n*] and

*b*[

*n*], which then can be filtered by the mean filter as

*W*is the neighborhood with center

*n*and

*M*is the size of mean filter window. In the phase difference curve, the jitter at some locations is large, while other locations are relatively flat. Therefore, the size of the mean filter window M can not be a constant value, but should change with a specific parameter. When the jitter is large, a small window should be used; otherwise, it is likely to make the curve too flat. When the jitter is flat, a larger window should be used, which makes the curve as smooth as possible. Calculate the arctangent function through the filtered sin/cosine values as

By using the arctangent function as inverse mapping, the filtered sawtooth phase is limited in the interval of (−*π*/2,*π*/2]. By judging the quadrant of the phase difference sequence through the positive and negative relationship between sine and cosine values, the filtered smooth phase difference sequence can be determined. After the phase difference denoising procedure, the jitter in the phase difference decreases a lot, which makes the phase unwrapping more accurately.

In principle, the displacement *n*
_{
m+1} can be estimated by fitting the slope of the phase difference curve *Φ*
_{1}[*n*]. However, the displacement estimated at this point is inaccurate. The reason is that there exists some minute differences between two adjacent rows due to the noise and clutter in practical application. The corresponding solution will be discussed in the next subsection.

### 3.4 Low-frequency fitting

Information between *f*
_{
m
}[*n*−*n*
_{
m+1}] and *f*
_{
m+1}[*n*] can be divided into macroscopic similarity and detailed difference, which correspond to low-frequency and high-frequency components in frequency spectrum, respectively. The low-frequency part, which corresponds to the similarity of adjacent azimuth cells, is a data segment similar to the ideal phase curve, and it can be fit to a straight line. While the high-frequency part, which corresponds to the detailed difference of adjacent azimuth cells, cannot be fit correctly.

- 1.The first-order differential of the phase difference curve can be calculated as$$\begin{array}{*{20}l} \Delta \Phi[n] = \Phi_{1}[n] - \Phi_{1}[n - 1] \end{array} $$(20)
- 2.By using a mean filter, the derivative curve is smoothed.$$\begin{array}{*{20}l} \varphi [n] = \frac{1}{{{M}}}\sum\limits_{i \in {W}} {\Delta \Phi [n]}, \end{array} $$(21)
where

*W*is the neighborhood with center*n*and*M*is the size of mean filter window. - 3.
Search peaks of

*φ*[*n*] from the central zero frequency. If the peak value is larger than the threshold, set the fitting area between zero and the peak value.

The differential phase difference in step 2 presents a notch area in the low-frequency part. The adjacent average method in step 2 aims at reducing false positives, which means avoiding putting the jitter low-frequency points into the high-frequency points. Step 3 selects the low-frequency part by searching the notch area of differential phase difference.

*n*

_{ m+1}in the low-frequency part. Then, corresponding correction result \({\tilde {f}_{m + 1}}[n]\) can be achieved according to (22).

### 3.5 Discussion

As for the azimuth bins *f*
_{
m
}[*n*] and *f*
_{
m+1}[*n*], there are significant decorrelation, and the relationship in Eq. (9) cannot be satisfied. In this condition, the assumption for the proposed method is not valid, and we cannot estimate the range deviation difference between these two range bins. In order to cope with this problem, the CV between the adjacent azimuth bins should be calculated first. Then, as for the azimuth bins that the CV is smaller than the threshold, we do not estimate the range deviation difference. After the range deviation differences of most azimuth bins are estimated, the range deviations of the azimuth bin, whose CV is smaller than the threshold, are estimated based on the range deviation difference of the adjacent azimuth bins by the fitting methods.

In addition, computational complexity of the proposed method is analyzed here. Suppose the range and azimuth samples are *N*
_{
r
} and *N*
_{
a
}, respectively. The computational complexity of the proposed residual RCM correction method for a single azimuth cell is elaborated as follows. (1) The phase difference extraction procedure uses two times Fourier transform and two times phase factor multiplication. The computational complexity is *O*(*N*
_{
r
} log*N*
_{
r
}). (2) The phase difference denoising procedure uses five times phase factor multiplication and one 1-D phase unwrap processing. The phase unwrap processing in [13] is utilized in this paper, and the computation complexity of this processing is *O*(*N*
_{
r
}). As a consequence, the computation complexity of this phase difference denoising procedure is *O*(*N*
_{
r
}). (3) The major operations of low frequency fitting procedure are three times phase factor multiplication, one time linear search, one time inverse Fourier transform, and one time linear fitting. The computational complexity is *O*(*N*
_{
r
} log*N*
_{
r
}). As a consequence, the computational complexity for the whole algorithm is *O*(*N*
_{
a
}
*N*
_{
r
} log*N*
_{
r
}).

## 4 Experimental results

In this section, both of the point target simulation and the real data experiment are performed to evaluate the performance of the proposed residual RCM correction algorithm for BFSAR.

### 4.1 Simulation and analysis

*A*,

*O*, and

*B*) are assumed to be distributed in the scene. The coordinates of the targets

*A*,

*O*, and

*B*are (−200,0), 0(0,0), and(200,0)m, respectively. Motion errors are added to the raw data of BFSAR. Let

*Δ*

*x*

_{ T }and

*Δ*

*x*

_{ R }denote the deviations in

*x*direction for the transmitter and receiver,

*Δ*

*y*

_{ T }and

*Δ*

*y*

_{ R }denote the deviations in

*y*direction for the transmitter and receiver, while

*Δ*

*z*

_{ T }and

*Δ*

*z*

_{ R }denote the height deviations of the transmitter and receiver, respectively. Functions of these deviations with respect to the azimuth time

*t*are presented in (23).

Imaging quality parameters

Range | Azimuth | ||||||
---|---|---|---|---|---|---|---|

PSLR (dB) | ISLR (dB) | IRW (m) | PSLR (dB) | ISLR (dB) | IRW (m) | ||

Target A | −13.35 | −10.22 | 0.68 | −13.19 | −10.09 | 0.88 | |

By the proposed | Target O | −13.37 | −10.23 | 0.67 | −13.26 | −10.17 | 0.84 |

method | Target B | −13.34 | −10.22 | 0.68 | −13.14 | −10.11 | 0.86 |

Target A | −8.02 | −7.97 | 0.91 | −6.03 | −4.76 | 1.98 | |

By MEM | Target O | −13.37 | −8.23 | 0.87 | −6.76 | −5.68 | 1.84 |

Target B | −8.13 | −8.12 | 0.88 | −6.12 | −5.03 | 1.90 |

### 4.2 Real data results

## 5 Conclusions

In this paper, a novel residual RCM correction method based on low-frequency fitting for BFSAR has been proposed. By fitting the low-frequency phase difference between adjacent azimuth cells, residual RCM in each azimuth cell is corrected one by one. Using the least-squares method, the estimation result is generally not an integer, and a sub-pixel level correction result can be achieved. Compared with previous works on residual RCM correction of SAR data, the proposed method can realize residual RCM correction requiring neither interpolation nor parametric model. In addition, the proposed algorithm is robust to noise. Simulations and experiments have been carried out to confirm the effectiveness of the proposed algorithm.

## Declarations

### Competing interests

The authors declare that they have no competing interests.

**Open Access** This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

## Authors’ Affiliations

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