X-SAR radiometric calibration and data quality

In April and October, 1994 the X-SAR was flown as part of the SIR-C/X-SAR space radar laboratory missions (SRL-1/2) on the Space Shuttle. Amongst other activities DLR is responsible for the calibration of all X-SAR data products and is running the German Processing and Archiving Facility (D-PAF). Calibration activities included three major parts. Before the first mission, the authors performed a detailed analysis of the overall system to localize the main error sources and developed algorithms and procedures to correct these errors. During the missions they concentrated their efforts on calibration campaigns at the Oberpfaffenhofen super test site. Post mission activities included the determination of the antenna pattern and the absolute calibration factor as well as detailed performance analyses. This paper describes the overall approach to radiometrically calibrate the X-SAR and provides information on system performance and data quality to users in the different application fields     

The SIR-C/X-SAR Synthetic Aperture Radar system

The Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR) was a joint United States/German/Italian space agency imaging radar system successfully flown aboard the shuttle Endeavor in April 1994 and again in September/October 1994. The multifaceted SIR-C/X-SAR represents a major technological step forward in radar remote sensing and is the first spaceborne multifrequency, polarimetric SAR. The United States developed SIR-C operated at L- and C-band, each with quad polarization. The SIR-C antenna was an active phased array, with the capability for electronic beam steering and multiple swath width illumination. The German/Italian X-SAR operated at X-band with a single polarization using a slotted waveguide antenna, mechanically steerable in elevation. SIR-C and X-SAR were designed to operate synchronously, collecting data over common sites synchronously. A total of 143 hours (93 terabits) of SAR data were recorded on tape     

Overview of results of Spaceborne Imaging Radar-C, X-Band SyntheticAperture Radar (SIR-C/X-SAR)

The Spaceborne Imaging Radar-C, X-Band Synthetic Aperture Radar (SIR-C/X-SAR) was launched on the Space Shuttle Endeavour for two ten day missions in the spring and fall of 1994. Radar data from these missions are being used to better understand the dynamic global environment. During each mission, radar images of over 300 sites around the Earth were obtained, returning over a terabit of data. SIR-C/X-SAR science investigations were focused on quantifying radar's ability to estimate surface properties of importance to understanding global change; and focused studies in geology, ecology, hydrology and oceanography, as well as radar calibration and electromagnetic theory studies. In addition, the second flight featured an interferometry experiment, where digital elevation maps were obtained by interfering data from the first and second shuttle flight, and from successive days on the second flight. SIR-C/X-SAR data have been used to validate algorithms which produce maps of vegetation type and biomass; snow, soil and vegetation moisture; and the distribution of wetlands, developed with earlier aircraft data     

SAR Polarimetry to Observe Oil Spills

A study on sea oil spill observation by means of polarimetric synthetic aperture radar (SAR) data is accomplished. It is based on the use of a polarimetric constant false alarm rate filter to detect dark patches over SAR images. Then, the target decomposition theorem is exploited to distinguish oil spills and look-alikes. Experiments are conducted on polarimetric SAR data acquired during the SIR-C/X-SAR mission on October 1994. The data were processed and calibrated at the Jet Propulsion Laboratory, National Aeronautics and Space Administration. Results show that the new polarimetric approach is able to assist classification     

Delineation of inundated area and vegetation along the Amazonfloodplain with the SIR-C synthetic aperture radar

Floodplain inundation and vegetation along the Negro and Amazon rivers near Manaus, Brazil were accurately delineated using multi-frequency, polarimetric synthetic aperture radar (SAR) data from the April and October 1994 SIR-C missions. A decision-tree model was used to formulate rules for a supervised classification into five categories: water, clearing (pasture), aquatic macrophyte (floating meadow), nonflooded forest, and flooded forest. Classified images were produced and tested within three days of SIR-C data acquisition. Both C-band (5.7 cm) and L-band (24 cm) wavelengths were necessary to distinguish the cover types. HH polarization was most useful for distinguishing flooded from nonflooded vegetation (C-HH for macrophyte versus pasture, and L-HH for flooded versus nonflooded forest), and cross-polarized L-band data provided the best separation between woody and nonwoody vegetation. Between the April and October missions, the Amazon River level fell about 3.6 m and the portion of the study area covered by flooded forest decreased from 23% to 12%. This study demonstrates the ability of multifrequency SAR to quantify in near realtime the extent of inundation on forested floodplains, and its potential application for timely monitoring of flood events     

EMISAR: an absolutely calibrated polarimetric L- and C-band SAR

EMISAR is a high-resolution (2×2 m), fully polarimetric, dual-frequency (L- and C-band) synthetic aperture radar (SAR) system designed for remote-sensing applications. The SAR is operated at high altitudes on a Gulfstream G-3 jet aircraft. The system is very well calibrated and has low sidelobes and low cross-polar contamination. Digital technology has been utilized to realize a flexible and highly stable radar with variable resolution, swath width, and imaging geometry. Thermal control and several calibration loops have been built into the system to ensure system stability and absolute calibration. Accurately measured antenna gains and radiation patterns are included in the calibration. The processing system is developed to support data calibration, which is the key to most of the current applications. Recent interferometric enhancements are important for many scientific applications     

Seasonal and short-term variability of multifrequency, polarimetricradar backscatter of Alpine terrain from SIR-C/X-SAR and AIRSARdata

Signatures of glaciated and ice free areas were analyzed from polarimetric SAR data at C-, L-, and P-band and single polarization X-band data. The data base includes an AIRSAR scene from June 25, 1991, and SIR-C/X-SAR images from April and October 1994 (SRL-1 and SRL-2), acquired over the high Alpine test site O¨tztal in Austria. The environmental conditions were different at the time of the three experiments. Ground measurements, meteorological observations, and backscattering modeling are the basis for interpreting the backscattering signatures. Seasonal differences are due mainly to the presence or absence of snow and due to changes of its properties. Short term variations of snow conditions can be monitored at C- and X-band. For unglaciated areas, the surface roughness has a dominant influence on backscattering in all seasons. The dependence of the mean backscattering and correlation coefficients on the incidence angle was analyzed. Spectral and depolarization ratios and the magnitude of the HHVV correlation coefficient were selected as components of the multidimensional feature vector for studying the target separability. Good separability was found between the accumulation and ablation areas on the glaciers, whereas on ice-free areas, the dominance of surface roughness limits the discrimination of different surface types. Short-term variations of backscattering have significant impact for the classification of accumulation and ablation areas on glaciers, as verified by comparisons with field data     

Polarimetric calibration of the SIR-C C-band channel using activeradar calibrators and polarization selective dihedrals

A polarimetric calibration experiment of Shuttle Imaging Radar C (SIR-C) is carried out using several different calibration targets. These are C-band polarimetric active radar calibrators (PARCs), polarization selective dihedrals (PSDs), 22.5° rotated dihedrals, and a trihedral. A novel polarimetric calibration algorithm is proposed that combines existing algorithms and uses one PARC and two PSDs. An error evaluation example is shown to estimate the typical hardware error value of the calibration targets allowable for a given calibration error. The novel algorithm gives polarimetric calibration results comparable to those obtained using the existing algorithm for three PARCs. Since PSDs work at frequencies lower than design frequency, and hence can be commonly used at multiple frequency bands, the simple addition of just one more frequency band PARC allows polarimetric calibration of a dual-frequency polarimetric synthetic aperture radar (SAR) by means of the proposed algorithm     

Polarimetric calibration of SIR-C using point and distributedtargets

In preparation for the Shuttle Imaging Radar-C/XSAR (SIR-C/XSAR) flights, the University of Michigan has been involved in the development of calibration procedures and precision calibration devices to quantify the complex radar images with an accuracy of 0.5 dB in magnitude and 5 degrees in phase. In this paper, the preliminary results of the SIR-C calibration and a summary of the University of Michigan's activity in the Raco calibration super-site is presented. In this calibration campaign an array of point calibration targets including trihedral corner reflectors and polarimetric active radar calibrators (PARCs) in addition to a uniform distributed target were used for characterizing the radiometric calibration constant and the distortion parameters of the C-band SAR. Two different calibration methods, one based on the application of point targets and the other based on the application of the distributed target, are used to calibrate the SIR-C data and the results are compared with calibrated images provided by JPL. The distributed target used in this experiment was a field of grass, sometimes covered with snow, whose differential Mueller matrix was measured immediately after the SIR-C overpass using The University of Michigan polarimetric scatterometer systems. The scatterometers were calibrated against a precision metallic sphere and measured 100 independent spatial samples for characterizing the differential Mueller matrix of the distributed target to achieve the desired calibration accuracy. The L-band SAR has not yet been adequately calibrated for inclusion here     

X-SAR interferometry: first results

Repeat-pass interferometry data were acquired during the first and second SIR-C/X-SAR missions in April and October 1994. This paper presents the first results from X-SAR interferometry at four different sites. The temporal separations were one day and six months. At two sites the coherence requirements were met, resulting in high quality interferograms. A digital elevation model in ground range geometry has been derived. The limitations of the X-SAR interferometry are discussed     

Knowledge-based classification of polarimetric SAR images

In preparation for the flight of the Shuttle Imaging Radar-C (SIR-C) on board the Space Shuttle in the spring of 1994, a level-1 automatic classifier was developed on the basis of polarimetric SAR images acquired by the JPL AirSAR system. The classifier uses L- and C-Band polarimetric SAR measurements of the imaged scene to classify individual pixels into one of four categories: tall vegetation (trees), short vegetation, urban, or bare surface, with the last category encompassing water surfaces, bare soil surfaces, and concrete or asphalt-covered surfaces. The classifier design uses knowledge of the nature of radar backscattering from surfaces and volumes to construct appropriate discriminators in a sequential format. The classifier, which was developed using training areas in a test site in Northern Michigan, was tested against independent test areas in the same test site and in another site imaged three months earlier. Among all cases and all categories, the classification accuracy ranged between 91% and 100%     

Stand age retrieval in production forest stands in New Zealand using C- and L-band polarimetric Radar

Regressions of single-, dual-, quad-, and full-polarization L- and C-band synthetic aperture radar (SAR) against stand age from 403 radiata pine stands in Kaingaroa Forest, New Zealand have been carried out, using the National Aeronautics and Space Administration's Airborne SAR instrument. The regressions attempted to find products suitable for the separation of young (two years or less) from old stands (25 years or older), and for the estimation of stand age. Local incidence angle had no significant effect for C-band, but was always significant for L-band, giving a standard error reduction of 13% to 32% for log stand age. Stand density was highly significant for both bands, giving standard error reductions of 7% to 47%. Single- and dual-polarization products were severely biased, and it was impossible to separate young and old stands, except L-band horizontal-(HH)-plus-horizontal-vertical (HV). C-band quad-polarization gave less bias and lower error than for that L-band, when local incidence angle and stand density were excluded. C-band full-polarization using covariance magnitudes gave no improvement over C-band quad-polarization, but L-band did give a significant improvement. The C- and L-band full-polarization products with six polarimetric indices gave significant improvements in the standard error. The results show that regressions of SAR data with stand age are possible with full- and quad-polarization L- and C-band datasets, although the prediction limits increase rapidly with stand age. The smallest error in estimated stand age, with an RMS of 3.22 years, was for L-band full-polarization with six polarimetric indices, calculated from a validation dataset. Separation of young and old forest stands was only possible for full-, quad-polarization, and the L-band HH-plus-HV products.     

SAR图像的极化干涉非监督Wishart分类方法和实验研究

该文在合成孔径雷达图像的极化非监督Wishart分类的基础上,给出了一种利用极化干涉信息对合成孔径雷达图像进行非监督分类的方法。该方法主要利用一(66)的极化干涉相关矩阵,从而可以同时考虑单幅图像的全极化信息以及两幅像对之间的互相关信息。该文详细阐述了该方法的具体实现,并利用NASA/JPL的SIR-C/X-SAR系统在中国天山地区的L波段实测数据进行了实验研究。给出了利用该方法对实验数据进行分类的结果,并与极化非监督Wishart分类的结果进行了比较。结果表明,该方法能够很好地分辨不同类型的地物,保持地物的细节,并且比极化非监督Wishart分类结果有很大改善。     

Phase calibration of polarimetric radar images

The problem of phase calibration between polarization channels of an imaging radar is studied. The causes of various types of phase errors due to the radar system architecture and system imperfections are examined. A simple model is introduced to explain the spatial variation in phase error as being due to a displacement between the phase centers of the vertical and horizontal antennas. It is also shown that channel leakage can cause a spatial variation in phase error. Phase calibration using both point and distributed ground targets is discussed and a method for calibrating phase using only distributed target is verified, subject to certain constraints. Experimental measurements using the NADC/ERIM P-3 synthetic-aperture radar (SAR) system and NASA/JPL DC-8 SAR, which operates at C-, L-, and P-bands, are presented. Both of these systems are multifrequency, polarimetric, airborne, SAR systems.<>     

INSAR imagery of surface currents, wave fields, and fronts

The authors demonstrate the ability of interferometric radar imagery to determine both relative and absolute surface velocities in the open ocean. Absolute phase calibration is accomplished by noting the azimuthal displacement of range-travelling targets-demonstrating for the first time that under favourable circumstances phase calibration can be achieved in open-ocean in the absence of ground truth. The high resolution of radar imagery permits observation of sharp velocity discontinuities, e.g. the Gulf Stream boundary and the wave field. The recent SIR-C/X-SAR shuttle missions dramatically emphasize the experimental and observational aspects of space-based radar. The combination of absolute velocities, high spatial resolution, and wide-area coverage suggest that interferometric radar imagery can provide a unique and powerful aid both for studies of global circulation patterns and detailed analysis of slope/shelf water interactions with ocean currents. In particular, the authors employ this measurement of the surface currents and wave field near a velocity front to help refine and bound results of their modeling of calculated radar images of the front. The results of this paper are compared with available ground truth     

Preliminary observations of volcanoes with the SIR-C radar

The two flights of the Shuttle Imaging Radar (SIR-C/X-SAR) in April and October 1994 provided a wealth of new image data over many of the world's volcanoes, including 13 of the 15 International Decade Volcanoes. A first look at these data shows that mapping of remote volcanoes (e.g., Nyiragongo and Nyamuragira in Africa) will now be easier, especially when aided by the collection of multi-parameter radar data over better known analogous volcanoes such as Kilauea, HI. The utility of the radar to identify different types of lava flows is shown for Fernandina volcano, Galapagos Islands. Growth of mudflow deposits (“lahars”) that were emplaced on Mount Pinatubo, Philippines, during the six months between the two missions is also illustrated. Additionally, interferometric radar data were collected over several active volcanoes; the potential uses of these data for topographic mapping and ground deformation studies are discussed     

Inferring snow wetness using C-band data from SIR-C's polarimetricsynthetic aperture radar

In hydrological investigations, modeling and forecasting of snow melt runoff require timely information about spatial variability of snow properties, among them the liquid water content-snow wetness-in the top layer of a snow pack. The authors' polarimetric model shows that scattering mechanisms control the relationship between snow wetness and the copolarization signals in data from a multi-parameter synthetic aperture radar. Along with snow wetness, the surface roughness and local incidence angle also affect the copolarization signals, making them either larger or smaller depending on the snow parameters, surface roughness, and incidence angle. The authors base their algorithm for retrieving snow wetness from SIR-C/X-SAR on a first-order scattering model that includes both surface and volume scattering. It is applicable for incidence angles from 25°-70° and for surface roughness with rms height ⩽7 mm and correlation length ⩽25 cm. Comparison with ground measurements showed that the absolute error in snow wetness inferred from the imagery was within 2.5% at 95% confidence interval. Typically the free liquid water content of snow ranges from 0% to 15% by volume. The authors conclude that a C-band polarimetric SAR can provide useful estimates of the wetness of the top layers of seasonal snow packs     

Interferometric alignment of the X-SAR antenna system on the spaceshuttle radar topography mission

The on-orbit alignment of the antenna beams of both the X-band and C-band radar systems during operations of the shuttle radar topography mission/X-band synthetic aperture radar (SRTM/X-SAR) was a key requirement for achieving best interferometric performance. In this paper, we consider the X-SAR antenna beam alignment in azimuth. For a single-pass cross-track SAR interferometer, we establish the relation between yaw and pitch misalignment of the antenna beams and the resulting relative shift of the Doppler frequency bands. This relation is used to provide solutions for the mechanical adjustments of the outboard antenna and electronic beam steering to correct for azimuth misalignment. Furthermore, the effects of the X-SAR effective outboard antenna pattern on the azimuth beam alignment are analyzed. As a result, a so-called "relaxing" factor is derived, which increases the limit for the difference in antenna azimuth angle with respect to the requirement on spectral overlap, and hence spatial interferogram resolution. However, we also show that the alignment requirement is driven by the constraint on decreasing the azimuth ambiguity-to-signal ratio (AASR) for the effective outboard antenna pattern to reduce the resulting additional height error. The strategy for misalignment determination and correction is presented, and results of the analysis of the in-flight X-SAR antenna beam alignment are discussed     

基于独立分量分析的极化SAR图像非监督分类方法

提出了一种针对极化合成孔径雷达(SAR)图像的新的分类方法--基于独立分量分析(ICA)的非监督分类方法.该方法将ICA和基于模糊集理论的非监督分类方法结合起来.用ICA方法对原始极化SAR图像进行特征提取,并用模糊C均值(FCM)算法对提取出的独立分量图像进行分类.该算法可对极化SAR图像进行自动分类,并减少由相干斑噪声所引起的分类错误,且其收敛速度快、稳定性高.采用SIR-C/X-SAR数据的试验证明了该算法的有效性.