Nonlinear Chirp Scaling Research Articles

Simultaneous multi-beam SAR imaging mode using phased array radar

Phased array radar based simultaneous multi-beams can be used in many fields for activities for change detection and platform location. This paper proposes a method for multi-beam antenna pattern design based on the phased array radar principle. In the proposed method, the relationships between the array structure, the direction and the width of the beams are deduced. Based on analysis of echo characters, the limiting conditions in various applications are obtained to ensure that the spectrums from different beams are not aliasing.Using the designed antenna pattern and parameters, echoes from different beams are received and separated by filtering the signal using a bank of filters corresponding to each beam. After separation, an azimuth nonlinear chirp scaling method is applied to the echoes of each beam for imaging and the imaging performance is analyzed.Simulation results show that the mode is efficient. The results also demonstrate that(1) when the platform velocity is perpendicular to the array normal line and the squint angle is not sufficiently large, the azimuth resolution is kept constant;(2) when the platform velocity is parallel to the normal line, the azimuth resolution declines sharply if the beam direction gets close to the normal line. However, if the beam direction deviates from the line,the effects of the antenna fill time becomes serious; therefore, an appropriate beam direction should be chosen.

Improved nonlinear chirp scaling algorithm for dechirped missile-borne synthetic aperture radar

An improved nonlinear chirp scaling (NLCS) algorithm for dechirped missile-borne synthetic aperture radar was proposed. The key step is to use an improved NLCS operation to equalize azimuth frequency modulation rate after linear range walk correction, secondary range compression, and range compression. By adopting a higher-order approximation, the image has better accuracy and less distortion. Validity of the algorithm is demonstrated using simulated and real datasets. Moreover, the whole procedure only consists of fast Fourier transform and complex multiplication, which means easier implementation and higher efficiency.

Modified nonlinear chirp scaling algorithm for curvilinear trajectory synthetic aperture radar

The original nonlinear chirp scaling (NCS) algorithm was extended for high precision processing of the highly squinted curvilinear trajectory synthetic aperture radar (CTSAR). Based on the analysis of slant range model and the frequency spectrum characteristics of the echo signal, a novel nonlinear chirp scaling function and more complex phase compensation factors with both velocity and acceleration parameters were proposed in the new algorithm for accommodation to curvilinear trajectory. The processing flow and computational complexity of modified NCS algorithm were fundamentally the same as the original NCS algorithm. However, the higher order phase compensation, range cell migration correction (RCMC) and range-variant secondary range compression (SRC) caused by the non-linear aperture and the severe range-azimuth coupling were accomplished accurately and efficiently without interpolation. Simulation results show that data acquired with a curvilinear aperture and a squint angle up to about 50° for X-band can be processed with no evident degradation of impulse response function.

Extended Azimuth Nonlinear Chirp Scaling Algorithm for Bistatic SAR Processing in High-Resolution Highly Squinted Mode

Accurate focusing of highly squinted azimuth-variant bistatic synthetic aperture radar data is a difficult issue due to the relatively large range migration, sensibility of the higher order terms, and the inherent geometric variance. To accommodate for this problem, extended azimuth nonlinear chirp scaling algorithm is investigated in this letter. First, range-azimuth coupling is mitigated through a linear range walk correction operation, and then, bulk secondary range compression is implemented to compensate the residual range cell migration and cross-coupling terms. Following which, the characteristics of the azimuth-dependent quadratic and cubic phase terms are analyzed, and modified scaling coefficients are derived by adopting higher order approximation and incorporating the azimuth-dependent range offset caused by the inherent geometric configuration. Compared with traditional nonlinear chirp scaling method, large azimuth depth of focusing can be realized without changing the overall procedure. Simulation results validate the effectiveness of the proposed algorithm.

A Modified Nonlinear Chirp Scaling Algorithm for Air-borne Ice Radar

In this paper, a modified nonlinear chirp scaling (MNLCS) algorithm for air-borne ice radar data is proposed. The spatial geometry based on the two-layer medium is analyzed for the studying of MNLCS. The algorithm can compensate the refraction effects and the propagation velocity changes in different mediums automatically, and can improve the focusing feature of the scatters. According to the model of echo signal, the phase compensation factors and realized steps of the algorithm are given. Numerical experiments are executed. The experimental results show the effectiveness of the proposed algorithm.

A Highly Squinted-spotlight Imaging Algorithm Based on Spectral Analysis and Extended Non-linear Chirp Scaling

针对大斜视聚束SAR成像中方位频谱混叠，且距离向与方位向严重耦合的问题，该文提出一种利用谱分析(SPECtral ANalysis, SPECAN)解方位频谱混叠、距离向采用调频变标(Chirp Scaling, CS)、方位向采用改进非线性变标(Extended Non-linear Chirp Scaling, ENCS)的大斜视聚束成像算法，首先将回波信号走动校正，然后利用SPECAN消除方位频谱混叠。进行距离徙动校正(Range Cell Migration Correction, RCMC)之后，利用CS消除距离弯曲的空变性，方位向采用ENCS补偿沿方位向变化的多普勒调频率，从而有效地提高了方位向的聚焦深度。仿真结果和分析表明，该方法能够在较大斜视角的聚束SAR模式下得到聚焦良好的高分辨率场景。

Performance Evaluation of Frequency-Domain Algorithms for Chirped Low Frequency UWB SAR Data Processing

This paper studies the performance of the frequency-domain algorithms (FDAs) for low-frequency ultra-wideband synthetic aperture radar (UWB SAR) data processing. First, a generalized theoretical derivation of the FDAs is presented from the viewpoint of SAR signal processing. The derivation not only provides a deeper understanding to the imaging principle of the extended Omega-K algorithm (EOKA), but also makes it compatible and comparable with the other FDAs. Second, the performance comparison on different FDAs is made based on theoretical analysis, simulation and experimental data. The comparison results show that the Omega-K algorithm (ωKA) has the highest imaging precision in the ideal case (i.e, no motion error), but its application is limited by the poor ability of compensating motion errors. In contrast, the EOKA and nonlinear chirp scaling algorithm (NCSA) have excellent performance on dealing with the motion error, but they can only be applied under specific preconditions. Besides, as cetner frequency gets lower, the fractional bandwidth and integration angle get larger, the imaging precision of NCSA greatly decreases, while the ωKA and EOKA still keep high precision.

Focusing Bistatic Forward-Looking SAR With Stationary Transmitter Based on Keystone Transform and Nonlinear Chirp Scaling

With appropriate geometry configurations, bistatic synthetic aperture radar (SAR) can break through the limitations of monostatic SAR on forward-looking imaging. Thanks to such a capability, bistatic forward-looking SAR (BFSAR) has extensive potential applications, such as self-navigation and self-landing. In the mode of BFSAR with a stationary transmitter (ST-BFSAR), the two-dimensional spatial variation makes it difficult to use traditional data focusing algorithms. In this letter, an imaging algorithm based on keystone transform and nonlinear chirp scaling (NLCS) is proposed to deal with this problem. Keystone transform is used to remove the spatial variation of range cell migration. NLCS can eliminate the variation of azimuth reference function. Numerical simulations show that by combining first-order keystone transform and azimuth NLCS operation, the raw data of ST-BFSAR can be well imaged.

An imaging algorithm for missile-borne SAR with downward movement based on variable decoupling

The ordinary SAR imaging algorithms are inapplicable to missile-bo-rne SAR due to the high speed dive and high-squint of missile. Aiming at this problem, this paper firstly sets up the model of echo and analyses of two-dimensional spectrum. By using azimuth nonlinear chirp scaling (NLCS) based on variable decoupling, an imaging algorithm for missile-borne SAR is proposed.It can effectively compensate the scene with longitudinal and transverse Doppler shift and improve the focusing quality while it can also simplify the geometric image correction operation. Simulation results are provided to confirm the effectiveness of the proposed algorithm.

An imaging algorithm for missile-borne spotlight SAR based on subband compensation

The missile-borne synthetic aperture radar (SAR) often use nonlinear scaling algorithm to improve the focusing effect. However, the nonlinear scaling algorithm cannot be used in missile-borne spotlight SAR because of its own limitations. Aiming at this problem, this paper first puts forward a new method named “sub-band compensation” and combineds it to the missile-borne SAR spotlight imaging algorithm. This new algorithm does not introduce the three phases of operation, so the Doppler frequency rate can be effectively compensated under low PRF. Simulation results and analysis show that compared with the nonlinear chirp scaling algorithm, the proposed one has advantages in computation, results obtained, and applicability.

An Azimuth Frequency Non-Linear Chirp Scaling (FNCS) Algorithm for TOPS SAR Imaging With High Squint Angle

During the data acquisition of a squint terrain observation by progressive scan (TOPS) synthetic aperture radar (SAR), the steering of the antenna main beam increases the azimuth bandwidth and results in the azimuth signal aliasing in the Doppler domain. Besides, the range curvature and the Doppler frequency modulation (FM) rates after linear range walk correction are azimuth-variant for squint TOPS SAR. These problems may cause some difficulties for the SAR data processing. To deal with the problems, a new imaging algorithm for the squint TOPS SAR is introduced in this paper. After linear range walk correction, the azimuth preprocessing is implemented to achieve the two-dimensional frequency spectrum without aliasing. Then, utilizing a modified chirp scaling algorithm, we complete the range cell migration correction (RCMC) and range compression to the TOPS SAR data without subaperture. Finally, the frequency nonlinear chirp scaling (FNCS) is proposed to correct the variation of the FM rates and the azimuth signal is focused in the Doppler domain via the spectral analysis (SPECAN) method. Both simulation and real data results show the effectiveness of the proposed algorithm.

Focus Improvement of Squint Bistatic SAR Data Using Azimuth Nonlinear Chirp Scaling

High-resolution imaging for squint azimuth-variant bistatic synthetic aperture radar system is a challenging task due to the existence of the spatial variance of range cell migration (RCM) and Doppler frequency modulation (FM) rate. To address this problem, azimuth nonlinear chirp scaling (ANLCS) is investigated in this letter. First, linear range walk is removed and then ANLCS is applied in the range frequency azimuth time domain to correct the azimuth-variant RCMs and to equalize the different FM rates. Taking the 2-D variance caused by the azimuth-variant configuration into consideration, a new perturbation function is derived based on the bistatic geometry. Using method of series reversion, a close form of range-azimuth coupling is obtained and corrected in the range Doppler domain by an interpolation-free operation. Incorporated with the secondary range compression, this method leads to a more accurate focusing for azimuth-variant bistatic configurations, even with high squints. Simulation results validate the effectiveness of the method.

An extended nonlinear chirp scaling algorithm for missile borne SAR imaging

In this paper, an extended nonlinear chirp scaling algorithm for focusing missile borne synthetic aperture radar data is proposed. Firstly, a missile borne SAR signal model is built. Then, LRWC and SRC operations are applied to mitigate the echo signal cross coupling and focus the range signal. Finally, a modified azimuth NLCS operation is used to equalize the azimuth frequency modulation rate for azimuth compression. By compensating the azimuth dependent characteristic of the azimuth FM rate and adopting higher order approximation processing, the image acquired has better accuracy and little distortion. The whole imaging procedure only consists of fast Fourier transform and complex multiplication, without interpolation, which means easier implementation and higher efficiency. The simulation results prove the accuracy and efficiency of the proposed algorithm.

Extended NLCS Algorithm of BiSAR Systems With a Squinted Transmitter and a Fixed Receiver: Theory and Experimental Confirmation

This paper proposes an extended nonlinear chirp scaling (CS) image formation algorithm for the bistatic synthetic aperture radar systems with the squinted transmitter and a fixed receiver. Since the transmitter with the squint mode was adopted in the system, two main problems, i.e., the spatial variance of the frequency-modulation rate and cubic phase terms, were introduced in the image formation algorithm. The former problem was solved by the linearity approximation of parameter <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$p$</tex></formula> and deduced <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$q$</tex></formula> (the second- and third-order coefficients of CS factors in range, which could be used to remove the spatial variation and high-order phase in the range direction) along the range domain while the latter one was compensated by a cubic analytical phase term in the frequency domain. A corresponding experimental hardware system and the bistatic experiments were also described in this paper. Both the simulation and experimental results validated the proposed algorithm.

General Bistatic SAR Data Processing Based on Extended Nonlinear Chirp Scaling

General bistatic SAR (BiSAR) data are more challenging to process than their translational invariant counterparts because the transmitter and the receiver have unequal velocities or/and nonparallel tracks. Although several algorithms have been used to process the data in general cases, these can work only in small invariant regions of the azimuth-variant problem because of the complicated configurations. In this letter, an extended nonlinear chirp scaling (NLCS) is proposed to address this problem. The key is to use the vector analysis of the instantaneous slant ranges to extract the azimuth-variant components. Furthermore, a new perturbation function based on the principle of NLCS used to precompensate the data. The simulation results show that our algorithm can handle data with more complicated bistatic cases than the existing algorithms.

Exact spectrum of non‐linear chirp scaling and its application in geosynchronous synthetic aperture radar imaging

Non-linear chirp scaling (NLCS) is a feasible method to deal with time-variant frequency modulation (FM) rate problem in synthetic aperture radar (SAR) imaging. However, approximations in derivation of NLCS spectrum lead to performance decline in some cases. Presented is the exact spectrum of the NLCS function. Simulation with a geosynchronous synthetic aperture radar (GEO-SAR) configuration is implemented. The results show that using the presented spectrum can significantly improve imaging performance, and the NLCS algorithm is suitable for GEO-SAR imaging after modification.

A Bistatic Synthetic Aperture Radar Imagery Simulation of Maritime Scene Using the Extended Nonlinear Chirp Scaling Algorithm

A bistatic synthetic aperture radar (Bis-SAR) imagery simulation of an extended maritime scene is presented. An efficient facet scattering model is developed to collect the raw coupling scattering data from a ship located on sea surface. Then, to avoid the inefficiency of time-domain algorithms and the azimuth-invariant limit of conventional frequency-domain (FD) methods in the bistatic case, an extended nonlinear chirp scaling (NLCS) algorithm is proposed to handle the bistatic data collected from this facet scattering model. The perfect Bis-SAR simulation results of the ship located on the sea surface could prove the validity of the proposed algorithms as well as the practicability of the model for the Bis-SAR imagery with large squint angle in the FD approach.

Focusing highly squinted data with motion errors based on modified non‐linear chirp scaling

The motion compensation (MOCO) plays a significant role to accommodate the motion errors caused by atmosphere turbulence and/or aircraft maneuvers in airborne synthetic aperture radar configurations. In broadside or small squint cases, the well-focused image can be obtained by the typical two-step MOCO to have the quadratic motion errors compensated. However, with the increasing of squint angle, not only the high-order motion errors, but also the azimuth-variant errors must be taken into account. In this study, a modified non-linear chirp scaling (MNLCS) algorithm is proposed to handle this problem in highly-squinted case. The key is to use the method of series reversion to cover the high-order motion errors, the MNLCS to precondition the data to process the azimuth-variant components and a series expansion to obtain an accurate form of the signal spectrum. The skewed spectrum in highly-squinted case is reduced through the use of a linear range cell migration correction. The simulated results have shown the MNLCS algorithm can handle data with more complicated geometries than the previous algorithm.

A Modified Nonlinear Chirp Scaling Algorithm for Spaceborne/Stationary Bistatic SAR Based on Series Reversion

This paper proposes a method of focusing the bistatic synthetic aperture radar (SAR) (BiSAR) data in spaceborne/stationary configuration. The key problem for imaging is the space variance of Doppler phase. The stationary platform induces additional and different range offsets to the range migration of targets. It causes targets with the same Doppler history, which are determined only by the moving platform, to shift into different bistatic range cells in the echo data. Therefore, the processing is not the same as monostatic SAR imaging which can be fast performed by the uniform matched-filter function in the frequency domain. In this paper, a modified nonlinear chirp scaling (NLCS) algorithm based on series reversion is formulated, which could achieve different range cell migration correction and the equalization of effective range and azimuth frequency modulation rates. The proposed algorithm is validated by simulated and real BiSAR data. In the spaceborne/stationary BiSAR experiment, the YaoGan-1 (an L-band spaceborne SAR system launched by China) is selected as the transmitter, and the stationary receiver is mounted on top of a tall building. The results show that modified NLCS algorithm can effectively focus BiSAR data with serious space variance in spaceborne/stationary configuration.

Azimuth-Variant Signal Processing in High-Altitude Platform Passive SAR with Spaceborne/Airborne Transmitter

High-altitude platforms (HAP) or near-space vehicle offers several advantages over current low earth orbit (LEO) satellite and airplane, because HAP is not constrained by orbital mechanics and fuel consumption. These advantages provide potential for some specific remote sensing applications that require persistent monitoring or fast-revisiting frequency. This paper investigates the azimuth-variant signal processing in HAP-borne bistatic synthetic aperture radar (BiSAR) with spaceborne or airborne transmitter for high-resolution remote sensing. The system configuration, azimuth-variant Doppler characteristics and two-dimensional echo spectrum are analyzed. Conceptual system simulation results are also provided. Since the azimuth-variant BiSAR geometry brings a challenge for developing high precision data processing algorithms, we propose an image formation algorithm using equivalent velocity and nonlinear chirp scaling (NCS) to address the azimuth-variant signal processing problem. The proposed algorithm is verified by numerical simulation results.