Adriano Meta

Signal Processing for FMCW SAR

The combination of frequency-modulated continuous-wave (FMCW) technology and synthetic aperture radar (SAR) techniques leads to lightweight cost-effective imaging sensors of high resolution. One limiting factor to the use of FMCW sensors is the well-known presence of nonlinearities in the transmitted signal. This results in contrast- and range-resolution degradation, particularly when the system is intended for high-resolution long-range applications, as it is the case for SAR. This paper presents a novel processing solution, which solves the nonlinearity problem for the whole range profile. Additionally, the conventional stop-and-go approximation used in pulse-radar algorithms is not valid in FMCW SAR applications under certain circumstances. Therefore, the motion within the sweep needs to be taken into account. Analytical development of the FMCW SAR signal model, starting from the deramped signal and without using the stop-and-go approximation, is presented in this paper. The model is then applied to stripmap, spotlight, and single-transmitter/multiple-receiver digital-beamforming SAR operational mode. The proposed algorithms are verified by processing real FMCW SAR data collected with the demonstrator system built at the Delft University of Technology.

Processing of Sliding Spotlight and TOPS SAR Data Using Baseband Azimuth Scaling

This paper presents an efficient phase preserving processor for the focusing of data acquired in sliding spotlight and Terrain Observation by Progressive Scans (TOPS) imaging modes. They share in common a linear variation of the Doppler centroid along the azimuth dimension, which is due to a steering of the antenna (either mechanically or electronically) throughout the data take. Existing approaches for the azimuth processing can become inefficient due to the additional processing to overcome the folding in the focused domain. In this paper, a new azimuth scaling approach is presented to perform the azimuth processing, whose kernel is exactly the same for sliding spotlight and TOPS modes. The possibility to use the proposed approach to process data acquired in the ScanSAR mode, as well as a discussion concerning staring spotlight, is also included. Simulations with point targets and real data acquired by TerraSAR-X in sliding spotlight and TOPS modes are used to validate the developed algorithm.

Modified range-Doppler processing for FM-CW synthetic aperture radar

The combination of compact frequency-modulated continuous-wave (FM-CW) technology and high-resolution synthetic aperture radar (SAR) processing techniques should pave the way for the development of a lightweight, cost-effective, high-resolution, airborne imaging radar. Regarding FM-CW SAR signal processing, the motion during the transmission of a sweep and the reception of the corresponding echo were expected to be one of the major problems. In FM-CW SAR, the so-called stop-and-go approximation is no longer valid due to the relatively long sweeps that FM-CW radars transmit. The main effect of the continuous motion is a Doppler frequency shift throughout the SAR observation time. This Doppler frequency shift can be compensated for by modifying the range migration compensation.

TOPS Imaging With TerraSAR-X: Mode Design and Performance Analysis

This paper reports about the performed investigations for the implementation of the wide-swath TOPS (Terrain Observation by Progressive Scan) imaging mode with TerraSAR-X (TSX). The TOPS mode overcomes the limitations imposed by the ScanSAR mode by steering the antenna along track during the acquisition of a burst. In this way, all targets are illuminated with the complete azimuth antenna pattern, and, thus, scalloping is circumvented, and an azimuth dependence of signal-to-noise ratio and distributed target ambiguity ratio (DTAR) is avoided. However, the use of electronically steered antennas leads to a quantization of the steering law and a nonideal pattern for squinted angles (grating lobes and main lobe reduction). The former provokes spurious peaks, while the latter introduces slight scalloping and DTAR deterioration. These effects are analyzed and quantified for TSX, and a TOPS system design approach is presented. Next, the requirements concerning interferometry are investigated. Finally, several results are shown with the TSX data, including a comparison between the TOPS and the ScanSAR modes and the reporting of the first TOPS interferometric results.

TerraSAR-X System Performance Characterization and Verification

This paper presents results from the synthetic aperture radar (SAR) system performance characterization, optimization, and verification as carried out during the TerraSAR-X commissioning phase. Starting from the acquisition geometry and instrument performance, fundamental acquisition parameters such as elevation beam definition, range timing, receiving gain, and block adaptive quantization setting are presented. The verification of the key performance parameters-ambiguities, impulse-response function, noise, and radiometric resolution-is discussed. ScanSAR and Spotlight particularities are described.

Range Non-linearities Correction in FMCW SAR

The limiting factor to the use of Frequency Modulated Continuous Wave (FMCW) technology with Synthetic Aperture Radar (SAR) techniques to produce lightweight, cost effective, low power consuming imaging sensors with high resolution, is the well known presence of non-linearities in the transmitted signal. This results in contrast and range resolution degradation, especially when the system use is intended for long range applications, as it is the case for SAR. The paper presents a novel processing solution, which completely solves the non- linearity problem. It corrects the non-linearity effects for the whole range profile at once, differently from the algorithms described in literature so far, which work only for very short range intervals. The proposed method operates directly on the deramped data and it is very computationally efficient.

Signal processing algorithms for FMCW moving target indicator synthetic aperture radar

The combination of frequency modulated continuous wave (FMCW) technology and synthetic aperture radar (SAR) leads to lightweight, cost-effective imaging sensors of high resolution. In FMCW SAR applications the conventional stop-and-go approximation used in pulse radar algorithms cannot be considered valid anymore, so the motion within the sweep needs to be taken into account. Analytical development of an FMCW SAR algorithm starting from the deramped signal and without using the stop-and-go approximation is presented in this paper; it is then validated processing simulated and real data. Furthermore, the effects of a moving target as they appear in an FMCW SAR image are described and the results can be used to assist moving target indicator (MTI) capabilities.

Development of signal processing algorithms for high resolution airborne millimeter wave FMCW SAR

For airborne Earth observation applications, there is a special interest in lightweight, cost effective, imaging sensors of high resolution. The combination of frequency modulated continuous wave (FMCW) technology and synthetic aperture radar (SAR) techniques can lead to such a sensor. In this paper, a developed algorithm for SAR imaging that takes into account the special characteristics of FMCW signals is presented. Constrains for the validity of the stop and go approximation are shown. A demonstrator system has been built at Delft University of Technology and some results from the last airborne campaign are presented.

Correction of the effects induced by the continuous motion in airborne FMCW SAR

The combination of frequency modulated continuous wave (FMCW) technology and synthetic aperture radar (SAR) leads to lightweight, cost-effective imaging sensors of high resolution. In FMCW SAR applications the conventional stop-and-go approximation used in pulse radar algorithms can be not valid anymore, therefore the motion within the sweep needs to be taken into account. Analytical development of the FMCW SAR signal model, starting from the deramped signal and without using the stop-and-go approximation, is presented in this paper. The model is then applied to spotlight and single transmitter/multiple receiver digital beam forming (DBF) operational mode, where the effect of the motion during the transmission and reception of the pulse, if not compensated for, can become seriously degrading for the SAR image quality. The derivation is then verified by processing real FMCW SAR data collected with the demonstrator system built at the Delft University of Technology.

Non-linear Frequency Scaling Algorithm for FMCW SAR Data

This paper presents a novel approach for processing data acquired with frequency modulated continuous wave (FMCW) dechirp-on-receive systems by using a non-linear frequency scaling algorithm. The range frequency non-linearity correction, the Doppler shift induced by the continuous motion and the range migration removal are performed at the same time in the wavenumber domain with a very efficient solution. The cross-range focusing is then achieved with conventional matched filtering. Real data have been acquired with the FMCW SAR demonstrator system built at the Delft University of Technology. Stripmap images processed with the proposed method are presented in order to show the validity of the non-linear frequency scaling algorithm