ScanSAR Mode Research Articles

Double Structure of the Wind Jet through the Tsushima Strait

We have investigated the northeasterly/southwesterly wind jets blowing through the Tsushima Strait from two case studies. Using high-resolution winds derived from SAR, we have presented the detailed structure of wind in the strait. The general ∞ow is generally close to geostrophic wind in the strait, inferring from the seal level pressure flelds. Wind jets are induced by two channels and by the terrestrial gap at the center of the Tsushima Island. The wind jets are clearly separated from the coastlines. The Tsushima Strait is a generic term indicating the sea area between the Korean Peninsula and the Japanese archipelago, with the Japanese islands of Honshu to the east, Kyushu to the south and the Goto Islands to the southeast (Figure 1). In the center of the strait, a slender island, the Tsushima Island is located along the strait. This island principally consists of two mountainous islands. The Tsushima Island divide the strait into eastern and western channels; the width and maximum depth are 140km and 110m in the eastern channel, and 40km and 200m in the western channel, respectively. Both channels connect the East China Sea and the Japan Sea. From these geographical conditions, the Tsushima Strait is one of the most important sea areas where the continuous and intense monitoring researches are required from oceanographic and meteorological point of views as well as marine security point of view. Extensive previous studies have investigated the Tsushima Strait hydrography by considering wind efiects on it. However, higher-resolution and wider-area wind observations are required for better understanding of the oceanographic and meteorological environment because of the complex coastal topography of the region. This is because few studies pay attention to the formation of wind jets in the strait, wind jet structures, and the role of the Tsushima Island on the formation of the wind jets. These are open issues at the present stage. The purpose of this paper is to investigate the wind blowing through the Tsushima Strait by using high-resolution satellite wind measurements. We look into the evolution and distribution of the wind jets during representative time periods and illustrate the contribution of the Tsushima Island to the wind jet formation. Speciflc questions we would like to address are: First question is typical structures of the wind jets found in the Tsushima Strait. When the wind blows through the Tsushima Strait in southwest-northeast direction, strong winds are formed. Second, what are the roles of the Tsushima Island on the formation of the wind jets? 2. DATA We analyze wind flelds derived from two SARs. They are the European Space Agency (ESA) Environmental Satellite (ENVISAT) Advanced Synthetic Aperture Radar (ASAR) and the Cana- dian RADARSAT-1. Both SARs operate at C-band (5.3GHz). While RADARSAT operates in horizontal (HH) polarization, ENVISAT/ASAR operates in HH or vertical (VV) polarization. In this study, we use wide-swath or ScanSAR mode products. That is, the ENVISAT/ASAR Wide- swath mode images covers an area of approximately 500 £ 500km, with a pixel size of 75m. The RADARSAT ScanSAR Narrow mode image covers an area of approximately 300 £ 300km, with a pixel size of 25m. Wind speed maps are derived from the ENVISAT/ASAR and RADARSAT images by applying SAR wind retrieval by using CMOD IFR2 scatterometer model function (1) and wind direction from objective analysis data (GPV: Grid Point Data). In cases of HH polarization images, a polarization ratio conversion factor (2) is applied. While the C-band geophysical model function is originally developed for VV polarized radar cross section, the polarization ratio conversion factor enables to convert it to HH polarized radar cross section. It has been shown by numerous studies that wind speeds can be retrieved from ENVISAT/ASAR (3) and RADARSAT-1 (4) with a root mean square error of less than 2m/s and with negligible bias.

ScanSAR-to-Stripmap Mode Interferometry Processing Using ENVISAT/ASAR Data

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Interferometric synthetic aperture radar (InSAR) images of geophysical events such as preeruptive volcano deformation or interseismic strain accumulation are often limited by phase distortions from the superimposed atmospheric signature. Additionally, the approximate monthly repeat cycle of many radar satellites cannot accurately capture rapidly time-varying processes. The Scanning Synthetic Aperture Radar (ScanSAR) mode of the ENVISAT/ASAR instrument permits more frequent revisits of a given area, potentially overcoming both of these limitations. In particular, stripmap mode-to-ScanSAR images provide a denser time series of interferograms than is possible with conventional stripmap-to-stripmap mode InSAR. We present images of ENVISAT/ASAR data acquired over Hawaii in which data acquired roughly weekly in ScanSAR mode are combined with ENVISAT/ASAR conventional stripmap mode data to form interferograms at a much denser temporal spacing. The burst nature of ScanSAR data requires a new processing method to form the interferograms. We use traditional matched filtering for the range compression. For the azimuth processing, we compute the stripmap mode data on the ScanSAR sampling grid using a variation, consisting of different reference functions, of Lanari's modified SPECAN algorithm that is itself an adaptation of the chirp <formula formulatype="inline"><tex>$z$</tex></formula>-transform to readjust the azimuth pulse spacing. The resulting interferograms faithfully reflect the phase of conventional interferograms, but exhibit fewer looks and coarser resolution than those produced by fully stripmap mode data. For many problems, temporal density of the deformation observations is paramount, and the time series analysis and temporal averaging that were made possible using ScanSAR interferograms far outweigh the loss in looks and resolution. </para>

Radar Signatures of Sahelian Surfaces in Mali Using ENVISAT-ASAR Data

This paper presents an analysis of ENSIVAT advanced synthetic aperture radar data acquired over a Sahelian region located in Mali, West Africa. The considered period is 2004-2005 and includes two rainy seasons. Emphasis is put on two ScanSAR modes, namely, the global monitoring (GM) and the wide swath (WS) modes characterized by spatial resolutions of about 1 km and 150 m, respectively. Results show that the WS mode offers better performance in terms of radiometric resolution, radiometric stability, and speckle reduction than the GM mode. The latter is more appropriate for studies at large scale (> 10 times 10 km). In both modes, pronounced angular and temporal signatures are observed for most soil surfaces, and azimuthal effects are observed on markedly orientated rocky surfaces. In contrast, polarization differences (VV/HH) are small during the dry season except on flat loamy soil surfaces. Finally, a relationship is observed between the normalized WS backscattering signal at HH polarization and the surface soil moisture of sandy soils.

Interferometry With ENVISAT Wide Swath ScanSAR Data

The possibility to get efficient topographic mapping and monitoring of large-scale motions with ScanSAR interferometry has been demonstrated with the Shuttle Radar Topography Mission and RADARSAT mission. The Environmental Satellite Advanced Synthetic Aperture Radar (ASAR) sensor has been designed to provide enhanced capabilities for interferometric applications. Different types of interferometric products can be obtained by combining the various ASAR modes as stripmap synthetic aperture radar [image mode (IM)] and ScanSAR [wide swath (WS) mode]. This letter deals with the possibility to use WS data to get either mixed-mode (IM/WS) or ScanSAR mode (WS/WS) differential interferograms. The impact of digital elevation model localization errors on IM/WS interferograms and of scan pattern synchronization on WS/WS interferograms is investigated. Experimental results are encouraging and show that ASAR ScanSAR data can be routinely used for interferometric applications in both cases

The calibration concept of TerraSAR-X: a multiple-mode, high-resolution SAR

TerraSAR-X is a high-resolution X-band synthetic aperture radar (SAR) satellite due for launch in 2006. The sensor has a spatial resolution down to 1 m and operates in Stripmap, Spotlight, and ScanSAR modes with selectable or dual polarization. It can image on the left or right side of the subsatellite track, which is achieved by rolling the satellite. There are also experimental modes for wide bandwidth, providing even higher resolution, and for full polarization and along-track interferometry (ATI), the latter two being achieved by splitting the receive antenna into two halves. Owing to this high degree of flexibility of the instrument and the tight performance requirements, the calibration of the sensor is a major challenge, and new concepts are needed to keep the costs affordable. For this purpose a novel internal calibration concept was developed, the so-called PN-gating method. In using this method, individual transmit and receive modules can be characterized under the most realistic conditions. Furthermore, cost-effect concepts for external calibration are mandatory because of the large number of modes and possible antenna patterns. The characterization of the antenna is based on a precise antenna pattern model, which provides reference patterns required for the relative radiometric correction of the SAR data. The paper describes the calibration measures planned for the TerraSAR-X instrument and discusses their implementation. The concept is applicable to other advanced SAR sensors.

Combination of low- and high-resolution SAR images for differential interferometry

The authors propose a combination of low-resolution (LR) and full-resolution (FR) SAR images for differential SAR interferometry. The principal problem in such a combination is the volume scattering decorrelation that limits the useful baseline of the interferometric pair to very small, impractical values. A technique has been introduced to remove this decorrelation by exploiting a digital elevation model (DEM) of the area imaged. The resulting interferogram has the quality (in terms of coherence or phase noise standard deviation) of a conventional FR interferogram, but reduced geometric resolution. The technique is suited to those differential interferometry applications where the resolution of the "differential" phase field to be monitored is much coarser than that of local topography. As a possible application, they propose a system that exploits the wide swath imaged by low-resolution ScanSAR modes for frequent monitoring of hazardous areas.

Future NASA spaceborne SAR missions

Two Earth-orbiting radar missions are planned for the near future by NASA-Shuttle Radar Topography Mission (SRTM) and LightSAR. The SRTM will fly aboard the Shuttle using interferometric synthetic aperture radar (IFSAR) to provide a global digital elevation map. SRTM is jointly sponsored by NASA and the National Imagery and Mapping Agency (NIMA). The LightSAR will utilize emerging technology to reduce mass and life-cycle costs for a mission to acquire SAR data for Earth science and civilian applications and to establish commercial utility. LightSAR is sponsored by NASA and industry partners. The use of IFSAR to measure elevation is one of the most powerful and practical applications of radar. A properly equipped spaceborne IFSAR system can produce a highly accurate global digital elevation map, including cloud-covered areas, in significantly less time and at significantly lower cost than other systems. For accurate topography over a large area, the interferometric measurements can be performed simultaneously in physically separate receive systems. Since LightSAR offers important benefits to both the science community and US industry, an innovative government-industry teaming approach is being explored, with industry sharing the cost of developing LightSAR in return for commercial rights to its data and operational responsibility. LightSAR will enable mapping of surface change. The instrument's high-resolution mapping, along with its quad polarization, dual polarization, interferometric and ScanSAR modes will enable continuous monitoring of natural hazards, Earth's surface deformation, surface vegetation change, and ocean mesoscale features to provide commercially viable and scientifically valuable data products. Advanced microelectronics and lightweight materials will increase LightSAR's functionality without increasing the mass. Dual frequency L/X-band designs have been examined.

Calibration and image quality performance results of RADARSAT

The Canadian earth observation satellite, RADARSAT, was launched in late 1995. The aim of this space borne Synthetic Aperture Radar (SAR) program is to provide C-band HH polarized SAR data products for five years at varying incidence angles, resolutions and area coverages. The system has been designed to produce imagery using single beams in elevation and also to operate in the ScanSAR mode by combining different elevation beams. SAR images have been obtained and were used to measure system performance during the Beam Qualification Phase. The measured image quality parameters for single beam products dealing with all aspects of image quality except radiometric calibration have been found to meet and exceed the system specifications. Based on the system performance and on completion of the commissioning activities, the Initial Operational Capability for the system was declared in April 1996.

Plans for RADARSAT image data calibration

RADARSAT, the first Canadian earth observation satellite, is scheduled for launch in 1995. The mission is to provide to users calibrated C-band HH polarized Synthetic Aperture Radar (SAR) data products for five years, at different incidence angles, resolution and area coverage. The SAR is also designed to operate in the ScanSAR mode by combining different elevation beams. Emphasis in the RADARSAT Program is provision of operationally calibrated products which insures targets can be compared radiometrically within each scence and between scences. In order to maintain end-to-end radiometric quality for the mission life, plans are to perform external calibration routinely using precision transponders and homogeneous targets. The calibration data will be analyzed off-line in the calibration workstation and resulting calibration parameters update will be provided to the ground processor to generate calibrated products. The short term variations in the gain of the sensor electronics will be tracked by the on board internal calibration system and these calibration data will be transmitted to the ground with the radar signal data. This allows the processor to automatically compensate for radar gain variations in between the external calibration periods and thus maintain the required radiometric accuracy. This paper describes the calibration requirements for the mission and plans for internal and external calibrations. An outline of the image quality management is also presented.

First SIR-C ScanSAR results

During the two space shuttle missions in 1994, the Space-borne Imaging Radar-C (SIR-C) acquired several experimental datatakes: using the ScanSAR mode. Same of these ScanSAR data have been processed into images of swath wider than 200 km. Example images are shown. These early results demonstrate that ScanSAR operation is not only feasible, but also represents a potentially useful tool for regional and global scale imaging by the future SAR missions.

Innovative Operating Modes And Techniques For The Spaceborne Imaging Radar-C Instrument

The operation of the spaceborne imaging radar-C (SIR-C) is discussed. The SIR-C instrument has been designed to obtain simultaneous multifrequency and simultaneous multipolarization radar images from a low earth orbit. It is a multiparameter imaging radar which will be flown during at least two different seasons. The instrument has been designed to operate in innovative modes such as the squint alignment mode, the extended aperture mode, the scansar mode, and the interferometry mode. The instrument has been designed to demonstrate innovative engineering techniques such as beam nulling for echo tracking, pulse-repetition frquency hopping for Doppler centroid tracking, generating the frequency step chirp for radar parameter flexibility, block floating point quantizing for data rate compression, and elevation beamwidth broadening for increasing the swath illumination.

Spaceborne imaging radar-C instrument

The Shuttle Imaging Radar (SIR)-C instrument has been designed to obtain simultaneous multifrequency and simultaneous multipolarization radar images from a low Earth orbit. It is a multiparameter imaging radar that will be flown during two different seasons. The instrument has been designed to operate in innovative modes such as the squint mode, the extended aperture mode, and the scansar mode, and to demonstrate innovative engineering techniques such as beam nulling for echo tracking, pulse repetition frequency-hopping for Doppler centroid tracking, frequency step chirp generating, for polarization differentiation, and block floating-point quantizing for data compression. The instrument has also been designed to allow flexibility in selection of radar parameters such as pulsewidth and beamwidth in the tradeoff of image quality parameters. These SIR-C capabilities are to be directly transferred to the proposed Earth Observing System (Eos) synthetic aperture radar.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>