Modified Range Migration Algorithm Research Articles

A Modified Range Migration Algorithm for FMCW SAR Signal Processing

The range migration algorithm (RMA) is an accurate imaging method for processing synthetic aperture radar (SAR) signals. However, this algorithm requires a big amount of computation when performing Stolt mapping. In high squint and wide beamwidth imaging, this operation also requires big memory size to store the result spectrum after Stolt mapping because the spectrum will be significantly expanded. A modified Stolt mapping that does not expand the signal spectrum while still maintains the processing accuracy is proposed in this paper to improve the efficiency of the RMA when processing frequency modulated continuous wave (FMCW) SAR signals. The modified RMA has roughly the same computational load and required the same memory size as the range Doppler algorithm (RDA) when processing FMCW SAR data. In extreme cases when the original spectrum is significantly modified by the Stolt mapping, the modified RMA achieves better focusing quality than the traditional RMA. Simulation and real data is used to verify the performance of the proposed RMA.

A Modified Range Migration Algorithm for FMCW SAR Signal Processing

<p class="AbstractText"><span lang="EN-AU">The range migration algorithm (RMA) is an accurate imaging method for processing synthetic aperture radar (SAR) signals. However, this algorithm requires a big amount of computation when performing Stolt mapping. In high squint and wide beamwidth imaging, this operation also requires big memory size to store the result spectrum after Stolt mapping because the spectrum will be significantly expanded. A modified Stolt mapping that does not expand the signal spectrum while still maintains the processing accuracy is proposed in this paper to improve the efficiency of the RMA when processing frequency modulated continuous wave (FMCW) SAR signals. The modified RMA has roughly the same computational load and required memory size as the range Doppler algorithm (RDA) when processing FMCW SAR data. In extreme cases when the original spectrum is significantly modified by the Stolt mapping, the modified RMA achieves better focusing quality than the traditional RMA. Simulation and real data are used to verify the performance of the proposed RMA.</span></p>

A Modified Range Migration Algorithm for FMCW SAR Signal Processing

<p class="AbstractText"><span lang="EN-AU">The range migration algorithm (RMA) is an accurate imaging method for processing synthetic aperture radar (SAR) signals. However, this algorithm requires a big amount of computation when performing Stolt mapping. In high squint and wide beamwidth imaging, this operation also requires big memory size to store the result spectrum after Stolt mapping because the spectrum will be significantly expanded. A modified Stolt mapping that does not expand the signal spectrum while still maintains the processing accuracy is proposed in this paper to improve the efficiency of the RMA when processing frequency modulated continuous wave (FMCW) SAR signals. The modified RMA has roughly the same computational load and required memory size as the range Doppler algorithm (RDA) when processing FMCW SAR data. In extreme cases when the original spectrum is significantly modified by the Stolt mapping, the modified RMA achieves better focusing quality than the traditional RMA. Simulation and real data are used to verify the performance of the proposed RMA.</span></p>

High-Resolution Ice-Sounding Radar Measurements of Ice Thickness Over East Antarctic Ice Sheet as a Part of Chinese National Antarctic Research Expedition

This paper presents the ice thickness, fine resolution internal reflecting horizons (IRHs), and distinct bottom topography measurements of Chinese Kunlun Station and Grove Mountains, Antarctica, derived from sounding these glaciers with a high-resolution radar. To enable the development of next-generation ice-sheet models, we need information on IRHs, bottom topography, and basal conditions. To this end, we performed measurements with the progressively improved ice-sounding radar system, currently known as the high-resolution ice-sounding radar developed by the Key Laboratory of Electromagnetic Radiation and Sensing Technology of Institute of Electronics, Chinese Academy of Sciences, Beijing, China. We processed the collected data using focused synthetic aperture radar (SAR) algorithm named the modified range migration algorithm using curvelets and the modified nonlinear chirp scaling algorithm to improve radar sensitivity and reduce along-track surface clutter. Representative results from selected transects indicate that we successfully sounded 3-km-thick ice with a fine resolution of 0.75 m. In this paper, we provide a brief description of the radar system, discuss the focused SAR processing algorithms, and provide sample results to demonstrate the successful sounding of the ice sheet in Antarctica.

Comparison of digital beamforming algorithms for 3-D terahertz imaging with sparse multistatic line arrays

Abstract. In this contribution we compare the back-projection algorithm with our recently developed modified range migration algorithm for 3-D terahertz imaging using sparse multistatic line arrays. A 2-D planar sampling scheme is generated using the array's aperture in combination with an orthogonal synthetic aperture obtained through linear movement of the object under test. A stepped frequency continuous wave signal modulation is used for range focusing. Comparisons of the focusing quality show that results using the modified range migration algorithm reflect these of the back-projection algorithm except for some degradation along the array's axis due to the operation in the array's near-field. Nevertheless the highest computational efficiency is obtained from the modified range migration algorithm, which is better than the numerically optimized version of the back-projection algorithm. Measurements have been performed by using an imaging system operating in the W frequency band to verify the theoretical results.

Full aperture imaging algorithm for highly squinted TOPS SAR

Aiming at solving the azimuth signal aliasing problem in the Doppler domain, the azimuth time aliasing problem after range cell migration correction (RCMC) and the severe range and the azimuth coupling problem in both the phase and envelope for highly squinted terrain observation by progressive scans (TOPS) synthetic aperture radar (SAR), a novel full aperture imaging algorithm is presented. An unaliased two-dimensional (2-D) spectrum is first obtained by the azimuth preprocessing; the modified range migration algorithm (RMA) is then used to complete RCMC; and finally the azimuth signal is focused in the Doppler domain by spectral analysis (SPECAN) and deramping. Simulations and real data processing results validate the effectiveness of the proposed algorithm.

First Demonstration of Airborne SAR With Nonlinear FM Chirp Waveforms

For synthetic aperture radar (SAR), a system impulse response with low sidelobes is very important because sidelobes may interfere with the nearby scatterers and contribute to multiplicative noise. It is well known that a nonlinear frequency-modulation (NLFM) chirp waveform can shape the signal's power spectral density and provide a radar matched filter output with lower sidelobes without loss of the signal-to-noise ratio when compared with the linear frequency-modulation chirp. These advantages make the NLFM waveform to be a promising candidate to improve the imaging quality for SAR. However, so far, to our knowledge, there is no real application of NLFM waveforms for SAR. This letter, for the first time, demonstrates the airborne SAR experiment using an NLFM waveform. In the underlying experiment, the construction of the NLFM signal is investigated and a modified range migration algorithm (RMA) is developed to adapt it for focusing the NLFM SAR data. Both simulation and experimental results exhibit the promising power of the NLFM chirp and show the accuracy of the proposed modified RMA.

Focusing Bistatic FMCW SAR Signal by Range Migration Algorithm Based on Fresnel Approximation.

Frequency modulated continuous wave (FMCW) technique has recently been employed by synthetic aperture radar (SAR) to decrease the radar cost and volume. However, the operation range is limited by the direct energy leakage from the transmitting channel to receiving channel due to the operation principle of FMCW technique. Bistatic configuration is an efficient way to increase the isolation between the transmitter and receiver, which could significantly increase the radar standoff range. A bistatic FMCW SAR spectrum model is proposed by using the Fresnel approximation in this paper. This model is similar to that of a monostatic FMCW SAR spectrum, which allows the existing imaging algorithms to be used on bistatic image processing. Based on the new model and the characteristics of FMCW signal, a modified range migration algorithm (RMA) for FMCW SAR is proposed to focus the image, which requires less memory and computational load than the traditional RMA. Point-target simulation is used to verify the proposed spectral model and real data processing verified the effectiveness of the proposed RMA.

Focused Synthetic Aperture Radar Processing of Ice-Sounding Data Collected Over the East Antarctic Ice Sheet via the Modified Range Migration Algorithm Using Curvelets

In this paper, we propose a new algorithm to address the speckle noise problem in imaging of ice sheets. It is a wave-equation-based ice-sounding imaging method using curvelets as building blocks of ice-sounding data, which successfully images the topography of ice sheets. First, theory analysis has been carried on to the proposed algorithm. Then, we give the specific steps to implement this algorithm. Finally, we apply this algorithm to the simulation point targets and High-Resolution Ice-Sounding Radar data to prove its validity of imaging of ice sheets. Furthermore, compared with five previous methods in two major aspects—the power of clutter reduction and the equivalent number of looks—the proposed algorithm reduces the speckle noise during the imaging processing without degrading the ability in clutter reduction.

GNSS-based BiSAR imaging using modified range migration algorithm

This paper proposed a new focusing approach for the bistatic SAR systems with a GNSS (Global Navigation Satellite System) satellite as the transmitter. Because of the long integration time the inherited curvature of the transmitter's trajectory was adopted and the spatial variance of the equivalent velocity was derived under the principal of equivalent slant range model in the system signal model. The proposed focusing approach consists of two-step processing. Firstly, the general Doppler phase from constant velocity of the echo data is focused by modified Stolt mapping. Secondly, the residual nonlinear Doppler phase due to velocity variation is compensated by means of block-processing-based adaptive phase compensation to achieve large scene focusing. The simulation and experimental results validate the proposed algorithm in GNSS-based BiSAR systems.

Two-Dimensional Imaging of Ice Sheets of Airborne Radar Sounder via a Combined Modified Range Migration Algorithm Based on ISFT and Beamforming Using Curvelets

We develop and demonstrate a new method to address the surface clutter problem in imaging of ice sheets by airborne radar sounder. This new method utilizes the modified range migration algorithm based on inverse scaled Fourier transform (ISFT) to reduce along-track clutter and the beamforming using curvelets to reduce cross-track clutter. First, theory analysis has been carried on to the proposed method. Then, we give the particular realizing steps to implement this method. At last, we apply this method to the simulation point targets and MCoRDS data to prove its validity of imaging of ice sheets. Furthermore, we show the advantages of the proposed method through comparison with two methods in four major aspects—the power of along-track clutter reduction, the power of cross-track clutter reduction, the equivalent number of looks (ENLs), and the calculating time.

Spaceborne squinted multichannel synthetic aperture radar data focusing

This study analyses echo properties of spaceborne squinted multichannel along the track synthetic aperture radar (SAR) data. The highly squint angle and the large transmitted pulse bandwidth lead to Doppler spectrum back folding in the spaceborne squinted case. The extended Doppler bandwidth in the squinted mode increases the processing difficulty of azimuth multichannel data reconstruction. According to echo properties of the spaceborne squinted mode, a new imaging approach for spaceborne squinted multichannel SAR data focusing is proposed. The key point of this imaging approach is azimuth multichannel data preprocessing to resolve the aliased Doppler spectrum caused by both azimuth sub-sampling in each azimuth channel and the highly squint angle. Afterwards, a modified range migration algorithm is taken for consequent equivalent monostatic spaceborne squinted SAR data focusing. Simulation results on point targets and distributed target are given to validate the proposed imaging approach.

Squinted TOPS SAR Imaging Based on Modified Range Migration Algorithm and Spectral Analysis

For the squinted terrain observation by progressive scans (TOPS) imaging mode, three problems need to be considered: azimuth spectrum aliasing, serious range–azimuth coupling, and azimuth time aliasing after range cell migration correction (RCMC). For these problems, a subaperture imaging algorithm based on the modified range migration algorithm (RMA) combined with spectral analysis (SPECAN) is proposed in this letter. Echo data are properly divided into subapertues so that the 2-D spectrum of each subaperture without aliasing can be obtained; then, the modified RMA is used to perform RCMC; finally, the signal is focused in the Doppler domain by SPECAN and deramping after subaperture recombination. Both simulated and real SAR data in the squinted TOPS mode are used to validate the proposed algorithm.

Full-Aperture SAR Data Focusing in the Spaceborne Squinted Sliding-Spotlight Mode

This paper analyzes the signal properties of spaceborne squinted sliding-spotlight synthetic aperture radar (SAR). Both the squint angle and the azimuth beam steering during the whole acquisition interval will lead to the Doppler spectrum back-folding. According to the special signal properties of this mode, a new full-aperture focusing approach, which includes three major processing steps, is proposed. In this approach, the first processing step introduces an azimuth convolution and azimuth data mosaic to resolve the azimuth spectral folding phenomenon by generalizing the existing two-step focusing technique for conventional sliding-spotlight SAR data focusing. Afterward, the modified range migration algorithm is adopted to process the resulting raw data. As the azimuth time duration of the raw data is obviously reduced in the first processing step, the obtained SAR image may be back-folded in azimuth. The final azimuth postfiltering step is to resolve the possible aliased SAR image without any azimuth data extension. The proposed full-aperture focusing processor is efficient since only a limited azimuth data extension is required to resolve the back-folded Doppler spectrum and SAR image. Imaging results on simulated raw data validate the proposed imaging approach.

Novel approach based on deramping technique for squinted sliding spotlight SAR imaging

This paper investigates a novel approach based on the deramping technique for squinted sliding spotlight Synthetic Aperture Radar (SAR) imaging to resolve the azimuth spectrum aliasing problem. First of all, the properties of the azimuth spectrum and the squint angle impacts on the azimuth spectrum aliasing problem are analyzed. Based on the analysis result, an operation of filtering is added to the azimuth preprocessing step of traditional Two-Step Focusing Approach (TSPA) to resolve the azimuth folding problem and remove the influence of the squint angle on the azimuth spectrum aliasing problem. Then, a modified Range Migration Algorithm (RMA) is performed to obtain the precise focused image. Furthermore, the focused SAR image folding problem of traditional TSPA is illuminated in this paper. An azimuth post-processing step is proposed to unfold the aliased SAR image. Simulation experiment results prove that the proposed approach can solve the spectrum aliasing problem and process squinted sliding spotlight data efficiently.