Band SAR Research Articles

Remote sensing of oceanic current features by synthetic aperture radar — achievements and perspectives

It is generally accepted that synthetic aperture radar (sar) images can be quite useful for a better understanding of hydrodynamic processes in the ocean, because they provide valuable information on the location and spatial scales of oceanic features such as fronts, internal waves, and eddies. However, the retrieval of actual surface current fields from the shape and modulation depth of radar signatures is a much more challenging problem, since the imaging mechanism is a complex and nonlinear two-step mechanism which cannot be inverted easily. In this article we review the state-of-the-art in modeling radar signatures of current features, and we present the concept of an iterative scheme for inverting radar images into current fields, which will be implemented within the framework of the European project marsais. We estimate the accuracy and spatial resolution of the proposed remote sensing system on the basis of findings from recent case studies and some dedicated simulations. According to the results of our analyses, it should be possible to retrieve spatial surface current variations and current gradients from a typical spaceborne C band sar image with an accuracy on the order of 20% and a spatial resolution on the order of 50 m.

Dual Frequency Polarimetric SAR Data Classification and Analysis

In this paper, we introduce a new classification scheme for dual frequency polarimetric SAR data sets. A (6×6) polarimetric coherency matrix is defined to simultaneously take into account the full polarimetric information from both images. This matrix is composed of the two coherency matrices and their cross-correlation. A decomposition theorem is applied to both images to obtain 64 initial clusters based on their scattering characteristics. The data sets are then classified by an iterative algorithm based on a complex Wishart density function of the 6 by 6 matrix. A class number reduction technique is then applied on the 64 resulting clusters to improve the efficiency of the interpretation and representation of each class characteristics. An alternative technique is also proposed which introduces the polarimetric cross-correlation information to refine the results of classification to a small number of clusters using the conditional probability of the crosscorrelation matrix. The analysis of the resulting clusters is realized by determining the rigorous change in polarimetric properties from one image to the other. The polarimetric variations are parameterized by 8 real coefficients derived from the decomposition of a special unitary operator on the Gell-Mann basis. These classification and analysis schemes are applied to full polarimetric P, L, and C bands SAR images of the Nezer forest acquired by NASA/JPL AIRSAR sensor (1989). 248 Ferro-Famil and Pottier

Estimation of terrain elevation by multifrequency interferometric wide band SAR data

We present a phase unwrapping method using a maximum likelihood estimation technique together with frequency diversity information to reconstruct highly discontinuous ground elevation profiles. Frequency diversity can be obtained by considering the interferograms obtained by different couples of subband images.

Dual Frequency Polarimetric Sar Data Classification and Analysis - Abstract *

In this paper, we introduce a new classification scheme for dual frequency polarimetric SAR data sets. A (6 x 6) polarimetric coherency matrix is defined to simultaneously take into account the full polarimetric information from both images. This matrix is composed of the two coherency matrices and their cross-correlation. A decomposition theorem is applied to both images to obtain 64 initial clusters based on their scattering characteristics. The data sets are then classified by an iterative algorithm based on a complex Wishart density function of the 6 by 6 matrix. A class number reduction technique is then applied on the 64 resulting clusters to improve the efficiency of the interpretation and representation of each class characteristics. An alternative technique is also proposed which introduces the polarimetric cross-correlation information to refine the results of classification to a small number of clusters using the conditional probability of the cross-correlation matrix. The analysis of the resulting clusters is realized by determining the rigorous change in polarimetric properties from one image to the other. The polarimetric variations are parameterized by 8 real coefficients derived from the decomposition of a special unitary operator on the Gell-Mann basis. These classification and analysis schemes are applied to full polarimetric P, L, and C bands SAR

The relationship between ERS-2 SAR backscatter and soil moisture: Generalization from a humid to semi-arid transect

A linear relationship between ERS-2 SAR backscattering coe c cient (s0) and volumetric soil moisture was determined at both ends of a rainfall gradient in Israel. A Normalized radar Backscatter soil Moisture Index (NBMI) was derived and related strongly to soil moisture concentration. This index may allow the mapping of soil moisture conditions over large areas with C band SAR data.

Radarsat data analysis for monitoring and evaluation of irrigation projects in the monsoon

The availability of multi-look angle HH polarization C band SAR data of different swaths and resolutions from Radarsat, in addition to VV polarization C band SAR data from ERS, has raised hopes for separating individual crops and for estimating soil moisture, enabling monitoring and evaluation of irrigation projects even under cloud cover conditions. Under the Radarsat ADRO Project, an attempt was made to identify irrigated crops in the Bhadra project command area, Karnataka state, India, using temporal, multi-look angle and dual polarization C-band SAR data from Radarsat and ERS. Preliminary analysis included data quality evaluation, rectification, speckle suppression and separability comparison of different land use and land cover classes. Evaluation of Radarsat data quality showed consistency in spatial resolution. Radarsat data acquired on different dates in shallow (S7 mode) and steep (S2 mode) angles and concurrent ERS-2 data were processed and analysed for possible discrimination of individual crops. It was observed that Radarsat data showed better separability of land use and land cover classes than ERS data. Principal Component Analysis was used on multi-look angle and dual polarization data from both ERS and Radarsat to reduce data dimensionality and the results were better with the first three components.

Textural Classification of Single Band SIR‐B Data over a Part of Brahmaputra Basin, India

Texture is an important spatial feature, useful for identfying objects of regions of interest in an image. There are a number of methods for identification of textural parameters e.g. edgeness, frequency domain analysis, gray tone cooccurrence approach etc. Geologic information in radar images of heavily vegetated areas is contained mostly in the depiction of topography in image texture. Single band SIR/ERS‐1 SAR data posses a problem to the analyst for classification of the various land use/geological classes and generally multidate SAR data are used due to paucity of more number of bands. However, the multidate SAR data classification is not an ideal technique. A new technique, namely, spatial frequency band pass classification technique which generates two or more bands in the Fourier domain using the single band SAR data and then classifies various features of interest using their textural properties has been described here. Result shows higher percentage of classification using Maximum‐Likelihood Classifier (MXL) with two split‐band data as compared to the unsupervised classification of all the bands.

An investigation of the selection of texture features for crop discrimination using SAR imagery

This article presents a methodology for selecting texture measures to maximize the discrimination of agricultural land use classes in SAR images. The images were acquired during the first flight of the Shuttle Imaging Radar-C (SIR-C) experiment, in April 1994. L (24 cm)- and C (5 cm)-band SAR data at HH (horizontal transmitting and receiving), HV (horizontal transmitting, vertical receiving), and VV (vertical transmitting and receiving) polarizations both in ground range and slant range and in two different passes were analyzed. The kappa statistic was used to identify meaningful texture measures to discriminate seven classes. The results show that the classifications of land use based only on tonal averages produced a kappa coefficient only slightly higher than 0.50. A kappa threshold of 0.90 was reached with the simultaneous inclusion of 15 texture measures for the six images (two bands, three polarizations). It was also found that the inclusion of texture features when only one band and one polarization was used could produce kappa values higher than 0.85.

Observations on the relationship between SIR-C radar backscatter and the biomass of regenerating tropical forests

SIR-C SAR data were related to the above ground biomass of regenerating tropical forests in Amazonia, Brazil. C- and L- band SAR data in the conventional polarization configurations showed no significant relationship with forest biomass, which were estimated in the field to range from 63.8-141.1 tha -1. However, the strength of the relationships was increased through the use of backscatter ratios and stratification of the forests by dominant species. These results support the view that backscatter ratios enhance the relationship between radar backscatter and biomass, perhaps beyond some quoted radar saturation levels, by reducing the effect of differences due to forest type. They also demonstrate that an ability to differentiate between forests of different species composition, and canopy geometry, increases the strength of the relationship between the SAR backscatter and biomass.

Evaluation of JERS-1, ERS-1 and Almaz SAR backscatter for rubber and oil palm stands in West Malaysia

Abstract While many reports have been published on radar backscatter characteristics of coniferous and deciduous forests, little work appears to have been done on investigating the backscatter properties of palm trees. In this study, Japanese JERS-1 L HH band, European ERS-1 C VV band and Russian Almaz-1B S HH band SAR data have been acquired over parts of Kedah and Penang states in West Malaysia in order to investigate the radar backscatter properties for oil palms and rubber trees for each of these sensors. Results show that the radar backscatter for the deciduous rubber trees, for both JERS-1 and ERS-1, appear to behave in accordance with what has been reported earlier for coniferous and deciduous trees, that is, scattering on trunks, branches and twigs at L-band and scattering in the canopy at C-band. The JERS-1 backscatter shows limited correlation with the rubber growth while no relation is found in the ERS-1 data. Oil palms with their characteristic structures affect the radar signal differently compared to the situation for rubber trees. Scattering in the large crown is the dominating backscatter mechanism in both the JERS-1 and ERS-1 data. Leaf area index is correlated closest to the backscatter intensity at both bands. Results from the investigation of the Almaz S-band data are rather discouraging, contradicting earlier more positive reports on the usefulness of the sensor. In this study, the forest types and their intermediate growing stages were found to be virtually indistinguishable, including the clear felled areas. These results should however not be attributed to S-band or Almaz data in general, but rather to this particular data set. It is obvious that the quality of Almaz data varies significantly.

Inflight evaluation of L band SAR of Japanese earth resources satellite-1

Initial evaluation of the JERS-1 SAR has been conducted for its characterization using inflight calibration signal, raw data from the natural targets, such as urban, mountain, forest, and ocean areas, and some correlated images for the antenna elevation pattern determination as well as the ground based measurement for its azimuth pattern. Calibration signal was used to obtain the suitable reference function for the range correlation more than ground measurement. Natural targets' signals were used to evaluate the raw data quality as well as the targets' characterization. As a result, most of the SAR characterization was conducted and recognized as same as designed except that the saturation and bit redundancy errors of several percent due to 3 bit AD converter underestimates the received power by 0.45 dB as an averaged value and chirp rate was measured lower than pre launch data by 0.3%.

Foliage transmission measurements using a ground-based ultrawide band (300-1300 MHz) SAR system

The attenuation of a forest clearly impacts the ability of airborne SAR systems to image objects within the forest. The level of this attenuation is a function of tree characteristics over the frequency band used in the radar. To experimentally measure the transmission properties of foliage, a bistatic (line-of-site) wide-band system has been built by the Environmental Research Institute of Michigan (ERIM), sponsored by the Air Force's Wright Laboratory, Avionics directorate. This system is polarimetric and can operate coherently over the band from 300 to 1300 MHz. The variation in foliage transmission over the frequency band is important because an imaging radar typically operates coherently over a bandwidth. The system can scan foliage in angle to determine spatial variations in the foliage attenuation. This angular variation in foliage attenuation is quite important because imaging radars typically synthesize an aperture by scanning over a range of angles. The ERIM Wide-Band System is ground-based, with one antenna attached to a carriage which can move 10 m horizontally along an elevated rigid track and the other antenna attached to a fixed tripod. Measurements with the system were conducted during July 1991 at the University of Michigan Biological Field Station in Pellston, MI. The measurements of the mean attenuation as a function of depression angle (15-45/spl deg/) and frequency (300-1300 MHz) of four different forest types are presented. In addition to mean attenuation, the variance in attenuation and the autocorrelation of the attenuation (in angle) are presented. These results imply that the variation of the foliage properties over the bandwidth and scan geometry (or angular variation) will degrade the ability of a radar to focus a foliage obscured object.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Modulation of waveheight spectrum and radar cross section by varying surface currents

This paper investigates the validity of the relaxation model which is generally used to describe the modulation of wind-generated ocean waves by slowly varying surface currents, and the applicability of the model to illustrating features observed in the radar images of internal wave wakes. The amplitude and phase modulations in the waveheight spectrum are examined for sinusoidal current variations; these changes are then translated into the radar cross section (RCS) modulation using the Kirchoff scattering theory. The present study concludes that: 1) the relaxation model is valid under intermediate to strong wind conditions and/or weak current signatures, and the conditions for the validity are given; 2) the relaxation model combined with the Kirchoff model could, under certain conditions, explain the differences in amplitude and phase between the P-band and L (C- and X-) band SAR images of ship-generated internal wave wakes observed during the 1989 Loch Linnhe experiment; and 3) the directional dependence of the relaxation rate enhances these differences. >

Motion errors in an airborne synthetic aperture radar system

AbstractThe Institute for Radio Frequency Technology of the German Aerospace Research Establishment (DLR) in Oberpfaffenhofen operates an experimental synthetic aperture radar, E‐SAR. The Radar configuration was developed by DLR [1, 2] and consists of a L/C/X‐band SAR System and antennas with a wide beamwidth in azimuth and elevation. The system is installed in a small Dornier DO 228 aircraft, which operates at low altitudes between 1000 m ‐ 3000 m above ground. Due to atmospheric turbulences, the aircraft is displaced from its nominal flight path, causing a variation of the phase history of the returned radar signal. This eventually results in a degraded image quality and therefore a highly accurate motion compensation becomes necessary. It is intended to integrate an inertial measurement unit (IMU) within the E‐SAR system to support the used motion compensation algorithm during data processing [3]. The required relative accuracy of the IMU can be determined by means of statistics i.e. an approach is demonstrated to estimate the power spectral density (PSD) of the tolerable uncompensated aircraft motion. The performance of an IMU motion compensation system has been evaluated by analysis, as well as by computer simulation.

The node equation approach to wide band SAR image reconstruction

As opposed to Fourier and associated integral methods for SAR processing, the paper discusses a method, based on the numerical unfolding of a differential equation along the synthetic aperture path. The method is a particular case of a general approach, which also may be applied to other synthetic aperture techniques, such as ISAR and btstatic imaging. The integration process may be termed cumulative, since a low resolution image available at a certain time along the synthetic aperture, together with the data registered at this moment of time, form the basis for constructing an image of successively improved resolution. The approach seems suitable for real time imaging, motion compensation, adaptive radar, and whenever continuous control over the imaging process is required. The paper considers application of the method to the extreme situation of ultra wide band, low frequency SAR. It is not as yet known how the method compares to the more conventional correlation methods, in the case of narrow band microwave SAR. The method is demonstrated by several numerical tests.

Ocean wave spectral distortion in airborne synthetic aperture radar imagery during the Norwegian Continental Shelf Experiment of 1988

C band radar images of ocean gravity waves off the Norwegian coast were processed into one‐dimensional azimuth spectra. These spectra were used to measure the azimuth spectral (width) cutoff on the basis of a least squares fit to a Gaussian spectral shape. The widths were calculated for a range of wave heights (2–5 m) and wind speeds (2–18 m/s) during 3 days in March, 1988. Velocity smearing (συ) estimates were extracted, independent of R/V and incidence angle, based on an imaging model and the measured azimuth cutoffs with συ values varying from 0.4 to 0.7 m/s. Quantitative velocity smearing estimates are important as input to models describing the distortion in wave imagery. We propose a first‐order model which neglects velocity bunching for ocean swell with peak wavelengths longer than about 250 m. This model is offered as a first estimate of when ocean wave swell will be detected by the C band SAR on board the ERS 1 spacecraft. The model predicts that this swell will be imaged under light winds of the order of 2–4 m/s. Higher wind speeds cause larger smearing, which may result in significant distortion of the imaged swell provided that the swell is traveling near the direction of the spacecraft ground track.

Coherent Doppler tomography-a technique for narrow band SAR

Some concerns regarding a technique of narrowband synthetic aperture radar (N-SAR) imaging called coherent Doppler tomography (CDT), which may be a good candidate for spaceborne applications, are addressed. Using a single-frequency signal, are addressed. Using a single-frequency signal, resolution of two tenths of a wavelength can be achieved in the point spread function if the radar platform circles the ground path to be imaged. However, the high sidelobe level of -8-dB in the point spread function results in an unacceptable dynamic range. To reduce the sidelobe level, two approaches are presented: coherent processing using multiple discrete frequencies and noncoherent subaperture processing. Simulation results demonstrate that the sidelobe level is substantially reduced by both methods. However, the resolution is degraded and the computational overhead is greatly increased for noncoherent subaperture processing. Also presented is a possible satellite geometry configuration that could utilize N-SAR processing to provide high-resolution global mapping capability. >

Multifocus processing of L band synthetic aperture radar images of ocean waves obtained during the Tower Ocean Wave and Radar Dependence Experiment

As part of the Tower Ocean Wave and Radar Dependence Experiment (TOWARD) objectives, the mechanisms of SAR imaging of ocean waves are investigated using L band SAR data over the Naval Ocean Systems Center tower. This paper provides experimental evidence needed to validate the differing hypotheses. Various processing methods are investigated to generate spectra with large degrees of freedom. The results show that waves traveling in the aircraft direction are most detectable at focus settings in the range 10.0–15.0 m/s, which is consistent with the Marine Remote Sensing Experiment (MARSEN) observations reported by Jain and Shemdin (1983). Waves traveling in the direction opposite to the aircraft are most detectable at settings equal to −5.0 to −15.0 m/s. The SAR imaging system acts as a low‐pass filter with the peak of the ocean wave height spectrum occurring at higher wave numbers compared with the peak in the SAR image spectrum.

Analysis of scatterer motion effects in Marsen X band SAR imagery

Synthetic aperture radar X band images collected over the North Sea during the 1979 MARSEN experiment show numerous apparent point scatterers imaged with a degraded resolution in the along‐track direction. The observed resolution of these features is consistent with a scatterer coherence time of the order of 10−2 s or a vertical acceleration of the order of 5 m/s2. Observations of the resolution as a function of the processor integration time tend to support the coherence time explanation. Similar coherence times have been measured for breaking waves by conventional high‐resolution radars, suggesting that the same phenomena may be responsible for the features observed on the synthetic aperture radar imagery.

Analysis of MARSEN X band SAR ocean wave data

Analysis of X band SAR imagery collected during the MARSEN experiment indicates that the APD‐10 SAR system imaged both range‐ and azimuth‐traveling gravity waves. However, only the near‐edge portion of the APD‐10 imagery provided reliable spectral wave estimates. Numerous motion artifacts, which manifest themselves as azimuth‐oriented streaks, are visible on the data and are believed to be caused by breaking waves. Because of the large platform velocity, the APD‐10 SAR data are relatively insensitive to wave enhancement adjustments performed during the processing of SAR signal histories. A modulation transfer function to relate SAR‐derived spectra to in situ measurements has been developed. The transfer function is smaller and falls off more rapidly with wave number for azimuth‐traveling waves than for range‐traveling waves. This is a consequence of the smaller inherent modulation for azimuth‐traveling waves and the degraded resolution in the azimuth direction as a result of motion effects and agrees, at least qualitatively, with theoretical predictions