Seasat Synthetic Aperture Radar Research Articles

The Newest Oldest Data From Seasat's Synthetic Aperture Radar

A new suite of digital synthetic aperture radar (SAR) imagery, featuring historic views of Earth's oceans, sea ice, volcanoes, forests, glaciers, and more, was made available in its entirety for the first time by the Alaska Satellite Facility (ASF) in midsummer 2013.

Deep‐water seamount wakes on SEASAT SAR image in the Gulf Stream region

A SEASAT synthetic aperture radar (SAR) image taken over the Gulf Stream region shows streak‐like patterns. The physics of their generation and interaction with the Gulf Stream are disputed. This study seeks a convincing interpretation for the SAR imagery patterns. Bathymetric maps show that the sea floor area beneath the streaks is the northeast Hoyt Hills, where isolated seamounts with the heights of 20 to 140 m above the background sea floor are distributed. All the SAR imagery streaks originate from these seamounts and extend downstream. Thus the SAR imagery streaks are interpreted as surface roughness imprints of the seamount wakes. Hydrostatic flow dynamics of the generation of wakes on the lee side of a solid obstacle is used to explain the generation mechanism and internal structure of the seamount wakes. The analysis indicates that boundary conditions and hydrodynamic conditions are favorable for the generation and vertical propagation of the seamount wakes to the upper layer.

Estimation of tree growth in a conifer plantation over 19 years from multi-satellite L-band SAR

The general capability of synthetic aperture radar (SAR) for monitoring forest ecosystems is well documented. However, the majority of SAR studies of forest dynamics use only imagery acquired by one SAR system and are thus limited to the lifecycle of a particular satellite. The synergistic analysis of SAR data from one of the earliest spaceborne SAR missions, the SEASAT mission, with the Japanese JERS-1 satellite-borne SAR is presented. Biophysical parameters frequently retrieved from SAR are tree biomass using backscatter and tree height from the interferometric phase. One potential application that has not been thoroughly examined is mapping of incremental tree growth from SAR backscatter changes. Tree growth measures biomass changes over time, and is correlated to the amount of carbon sequestered by the trees. This paper examines the retrieval of tree growth from multitemporal spaceborne L-band SAR. A SEASAT SAR image from 1978 and a JERS-1 SAR image from 1997 over Thetford forest, UK are used to retrieve tree growth of Corsican Pine stands. Incremental growth was estimated from the changes in backscatter coefficient, and compared to the expected tree growth from general yield class models used by the UK Forestry Commission. The accuracy of the retrieval algorithm depends on the minimum forest stand size included in the analysis. For managed forest plantations, multitemporal L-band SAR has some potential for detecting incremental biomass to support sustainable forest management.

An algorithm for ship wake detection from the synthetic aperture radar images using the Radon transform and morphological image processing

Using the Radon transform and morphological image processing, an algorithm for ship wake detection in the synthetic aperture radar (SAR) image is developed. Being manipulated in the Radon space to invert the grey-level and binary images, the linear texture of a ship wake in oceanic clutter can be easily detected. It is applied to automatic detection for moving ship from the SEASAT SAR image. The results show that this algorithm is very robust in a strong noisy background and is not very sensitive to the threshold parameter and the working window size.

The Evolution of Convective Storms from Their Footprints on the Sea as Viewed by Synthetic Aperture Radar from Space

SEASAT synthetic aperture radar (SAR) echoes from the sea have previously been shown to be the result of rain and winds produced by convective stroms; rain damps the surface waves and causes ech-free holes, while the diverging winds associated with downdraft generate waves and associated echoes surrounding the holes. Gust fronts are also evident. Such a snapshot from 8 July 1978 has been examined in conjunction with ground-based radar. This leads to the conclusion that the SAR storm footprints resulted from storm processes that occurred up to an hour or more prior to the snapshot. A sequence of events is discerned from the SAR imagery in which new cell growth is triggered in between the converging outflows of two preexisting cells. In turn, the new cell generates a mini-squall line along its expanding gust front. While such phenomena are well known over land, the spaceborne SAR now allows important inferences to be made about the nature and frequency of convective storms over the oceans. The storm effects on the sea have significant implications for spaceborne wind scatterometry and rainfall measurements. Some of the findings herein remain speculative because of the great distance to the Miami weather radar-the only source of corroborative data.

Footprints of storms on the sea: A view from spaceborne synthetic aperture radar

Synthetic aperture radar (SAR) on board Seasat observed images of stormlike echoes on the sea in 1978. The core of these images is usually an echo‐free hole which is attributed to the damping of the short (30‐cm) radar detectable gravity waves by the intense rain in the storm core. Although “the beating down of waves by rain” is consistent with observations by seafarers and with the first scientific explanation of the phenomenon by Reynolds (1875), neither theory nor experiment has provided definitive support. One experiment appears to provide the key; it shows that the kinetic energy of the rain produces sufficient turbulence in a thin fresh water layer to damp 30‐cm waves in 10–20 s, thus producing the echo‐free hole. A sequence of positive feedbacks then serves to damp the longer waves. The angular dependence of the sea surface echo cross sections seen by Seasat SAR outside the echo‐free hole indicates winds diverging from the downdraft induced by the intense rain core. The wind‐generated waves and associated echoes extend out to a sharply defined gust front. The sea surface footprint thus mimics the features of a storm microburst. The variations in surface radar cross section due to a combination of rain and wind effects impacts spaceborne measurements of surface winds by scatterometry and rainfall measurements by radar. Portions of this synthesis remain speculative but serve as hypotheses for further research.

The shuttle imaging radar‐C and X‐SAR mission

The Shuttle Imaging Radar‐C and X‐Band Synthetic Aperture Radar (SIR‐C/X‐SAR) (Figure 1) is a cooperative space shuttle experiment between NASA, the German Space Agency, and the Italian Space Agency. The experiment is the next evolutionary step in NASA's Spaceborne Imaging Radar (SIR) program that began with the Seasat Synthetic Aperture Radar (SAR) in 1978 and continued with SIR‐A in 1981 and SIR‐B in 1984. It also represents a continuation of Germany's imaging radar program, which started with the Microwave Remote Sensing Experiment flown aboard the Shuttle on the first SPACELAB mission in 1983. The SIR‐C/X‐SAR mission benefits from synergism with the Magellan mission to Venus, other international spaceborne radar programs, and prototype aircraft sensors such as the Jet Propulsion Laboratory's Airborne SAR (AIRSAR) and the German Aerospace Establishment (DLR) E‐SAR.

Analysis of synthetic aperture radar Data collected over the southwestern Greenland ice sheet

AbstractAnalyses of the first aircraft multi-frequency, Polarimetric synthetic aperture radar (SAR) data acquired over the southwestern Greenland ice sheet are presented. Data were collected on 31 August 1989 by the Jet Propulsion Laboratory SAR using the NASA DC-8 aircraft. Along with curvilinear patterns associated with large-scale morphologic features such as crevasses, lakes and streams, frequency and polarization dependencies are observed in the P-, L-and C-band image products. Model calculations that include firn grain-size and volumetric water content suggest that tonal variations in and between the images are attributable to large-scale variations in the snow-and ice-surface characteristics, especially snow wetness. In particular, systematic trends in back-scatter strength observed at C-band across regions of changing snow wetness are suggestive of a capability to delineate boundaries between snow facies. Ice lenses and ice pipes are the speculated cause for similar trends in P-band back-scatter. Finally, comparison between SEASAT SAR data collected in 1978 and these airborne data collected in 1989 indicate a remarkable stability of surface patterns associated with the locations of supraglacial lake and stream systems.

Analysis of synthetic aperture radar Data collected over the southwestern Greenland ice sheet

AbstractAnalyses of the first aircraft multi-frequency, Polarimetric synthetic aperture radar (SAR) data acquired over the southwestern Greenland ice sheet are presented. Data were collected on 31 August 1989 by the Jet Propulsion Laboratory SAR using the NASA DC-8 aircraft. Along with curvilinear patterns associated with large-scale morphologic features such as crevasses, lakes and streams, frequency and polarization dependencies are observed in the P-, L-and C-band image products. Model calculations that include firn grain-size and volumetric water content suggest that tonal variations in and between the images are attributable to large-scale variations in the snow-and ice-surface characteristics, especially snow wetness. In particular, systematic trends in back-scatter strength observed at C-band across regions of changing snow wetness are suggestive of a capability to delineate boundaries between snow facies. Ice lenses and ice pipes are the speculated cause for similar trends in P-band back-scatter. Finally, comparison between SEASAT SAR data collected in 1978 and these airborne data collected in 1989 indicate a remarkable stability of surface patterns associated with the locations of supraglacial lake and stream systems.

Interpretation of sar imagery of the greenland ice sheet using coregistered TM imagery

Landsat Thematic Mapper (TM) images are coregistered with a Seasat Synthetic Aperture Radar (SAR) image of the margin of the Greenland ice sheet and used in the interpretation of features in the SAR data. A winter TM image is used to provide topographic information while a summer TM image provides information on the pattern of melt and the location of lakes. Many previous hypotheses are confirmed, and some are rejected. It is shown that: 1) The snow line is easily d detectable in SAR imagery; 2) areas of higher radar backscatter in the bare-ice zone most often correspond to areas full of crevasses too small to resolve in the SAR image; 3) subsurface scattering causes significantlblurring of features in SAR imagery; 4) the band of surface lakes occurring above the snow line corresponds to at least a portion of the wet-snow zone; 5) water is the cause of reduced radar backscatter above the snow line; and 6) the fact that water causes reduced radar backscatter above the snow line permits the use of backscatter variations as a proxy indicator of surface topography in snow-covered regions experiencing melt.

Remote Sensing of Surface Ship Wakes

ABSTRACTRecently, there has been much interest in the remote sensing of surface ship wakes using synthetic aperture radar (SAR). Ship wake signatures obtained using a SAR have several prominent features such as a dark trailing centerline region, bright‐line images aligned at some angle to the ship's path, and sometimes the distinctive Kelvin ship wave pattern. The origins of these features are not well understood. Many different theories are available to explain one or more of the features. These theories are often in conflict and no definitive answers are yet available. Much analytical and numerical research, and both laboratory and full‐scale experiments are presently underway in order to develop proper explanations for these features.This paper first describes in some detail the distinctive features of SAR images. Then actual examples from early SEASAT SAR images, the JOWIP/Georgia Straight and SIR‐B experiments, and the recent ONR remote sensing experiments are given. The observed radar image morphology includes dark centerline wake features, narrow‐V bright line features, and Kelvin wave manifestations.

New approaches in interferometric SAR data processing

It is known that interferometric synthetic-aperture radar (SAR) images can be inverted to perform surface elevation mapping. Among the factors critical to the mapping accuracy are registration of the interfering SAR images and phase unwrapping. A registration algorithm is presented that determines the registration parameters through optimization. A figure of merit is proposed that evaluates the registration result during the optimization. The phase unwrapping problem is approached through a new method involving fringe line detection. The algorithms are tested with two SEASAT SAR images of terrain near Yellowstone National Park. These images were collected on SEASAT orbits 1334 and 1420, which were very close together in space, i.e. less than 100 m. >

Evaluation of terrain models for the geocoding and terrain correction, of synthetic aperture radar (SAR) images

The variability of the resolutions and the presence of artifacts cause inaccurate correction of the terrain-induced geometric distortions in synthetic-aperture radar (SAR) images. To quantify the effects of these inaccuracies on SAR terrain correction, corrections of a Seasat SAR image were performed using a 1 degrees US Geological Survey (USGS) terrain model, a 7.5-min USGS terrain model, and a terrain model derived from stereoimagery acquired from SPOT. Geometric verifications of the corrected imagery showed that the resolution of the 1 degrees terrain model is not adequate to resolve many features in the Seasat image. Geometric verifications of images corrected with the two higher resolution terrain models showed localized errors as large as 52 m for mountain peaks. However, comparison of data corrected with those models shows that both produce results that differ by less than the resolution of either of them. Periodic artifacts observed in the terrain models translated to ground range differences of 18 m, which are well below the resolution of the SAR imagery.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The contribution of Seasat to ice sheet glaciology

Abstract The suite of sensors flown onboard Seasat during 1978 has provided glaciologists with valuable tools for the study of ice masses, particularly in the polar regions. Of the sensor package, the most useful instruments for glaciology have been the radar altimeter and the synthetic aperture radar. The former has demonstrated the ability to map the surface of ice sheets in considerable detail (possibly to better than 50 mm over ice shelves) and over a very short period of time. Such maps provide the first step towards evaluating the long term mass balance of these ice masses. Such studies are of central importance to global climate modelling, investigation of the ‘greenhouse effect’ and prediction of world sea levels. Radar altimeter mapping has also provided unparalleled detail on surface topography relevant to ice dynamics investigations. The small dataset of Seasat Synthetic Aperture Radar (SAR) imagery gathered over ice masses, principally in Iceland and Greenland (there was no coverage of Antarct...

Waves in frazil and pancake ice and their detection in Seasat synthetic aperture radar imagery

A theoretical model of waves propagating into an ice cover composed of frazil and pancake ice is developed and compared with measurements of wavelength and direction derived from synthetic aperture radar (SAR) imagery obtained from Seasat in October 1978. The theoretical model is based on the concept that ice of these types, which consists of small crystals or cakes, has only a mass‐loading effect on the water surface. We derive the dispersion relation for phase and group velocities, finding that there is an upper frequency limit for propagation into the ice. From the reflection coefficient at the ice edge we derive the wave radiation pressure exerted on the ice, showing that it will cause a slick of frazil ice backed by thicker floes to become more dense or thick with increasing penetration. The implications for radar scattering enabling detection on SAR are that the Bragg resonant wavelength corresponds to waves above the frequency limit for propagation, so that a frazil slick appears dark on an SAR image. When the frazil ice becomes transformed into pancake ice, through slick compression or other means, the raised edges of the pancakes cause the ice to appear bright despite the fact that there are no waves present at the Bragg wavelength. These results are applied to a Seasat SAR image obtained from the Chukchi Sea. The appearance of the ice in the image corresponds to what we expect for frazil ice gradually transforming itself into pancake ice, backed by thicker floes. We derive directional wave number spectra outside and inside the ice cover by digital Fourier analysis of image subscenes, and we find that the change of wavelength and angle of refraction of the dominant wave entering the ice field are both characteristic of the dispersion relation derived theoretically. Mean ice thicknesses extracted from the theory correspond to thicknesses expected for such slicks. The technique offers a possible means of extracting the thickness of fields of frazil and pancake ice from SAR imagery; this may be of considerable utility when ERS 1 SAR is used to study the advancing winter ice edge in the Antarctic, which consists of vast areas of these ice types.

Ocean surface remote‐sensing program planned

In order to better understand the physics responsible for radar imaging of oceanographic submesoscale features, the Office of the Chief of Naval Research is sponsoring the High‐Resolution Remote Sensing Program, an accelerated research initiative managed by Frank Herr and Chuck Luther at the Office of Naval Research and Richard Mied at the Naval Research Laboratory. This program includes the disciplines of remote sensing, oceanography, meteorology, and microlayer physics. The approach will be to conduct a field program of simultaneous radar and in‐situ measurements and to conduct theoretical/modeling work to understand the feature generation, variability, and imaging physics.Since the launch of SEASAT in 1978, there has been great interest in synthetic aperture radar (SAR) imaging of oceanographic features. For example, SEASAT SAR images clearly showed evidence of internal waves, current boundaries, fronts, eddies, and tidal flow over variable bathymetry (see SEASAT Special Issue I, J. Geophys. Res., 87, C5, 1982). Unfortunately, there was little or no in‐situ data associated with most of these images, so there has been little validation of models that describe the imaging mechanism.

Full resolution focusing of Seasat SAR images in the frequency-wave number domain

Abstract Synthetic aperture radar (SAR) data focusing has been traditionally performed using matched filter techniques. However, downward continuation techniques can produce, with a great computational efficiency, results that only the most sophisticated conventional techniques can achieve. The basic idea is to decompose the measured field into monochromatic plane waves of appropriate amplitudes, wave lengths and propagation directions. The backpropagation of these plane waves, and their recombination at the time when the sounding pulse was emitted, can produce a map of the backscattered electromagnetic field. In order to obtain correct focusing of synthetic aperture radar raw data, both the geometrical and the transmission parameters of the system should be known as precisely as possible. The transmission parameters are generally known very precisely, whereas the geometrical ones (i.e. sensor-target relative position, satellite velocity and attitude, etc.) can be derived from the ephemerides of the satel...

Speckle statistics in four-look synthetic aperture radar imagery

Speckle noise in high-resolution synthetic aperture radar (SAR) images is generated by the coherent processing of radar signals and exists in all types of coherent imaging systems. We study the statistical properties of speckle noise in a four-look SEASAT SAR image concentrating on homogeneous areas of the image. Experimental determination of the signal-to-noise ratio and ofthe probability density of the recorded intensity is performed showing evidence of correlation between underlying looks. Based on a model taking into account the weighting and possible correlation between the underlying looks, a theoretical expression for the effective number of independent looks is proposed and the corresponding probability density function of the intensity is compared to the experimentally extracted histograms of intensity fluctuations.

Psi -s correlation and dynamic time warping: two methods for tracking ice floes in SAR images

The authors present two algorithms for performing shape matching on ice floe boundaries in SAR (synthetic aperture radar) images. These algorithms quickly produce a set of ice motion and rotation vectors that can be used to guide a pixel value correlator. The algorithms match a shape descriptor known as the psi -s curve. The first algorithm uses normalized correlation to match the psi -s curves, while the second uses dynamic programming to compute an elastic match that better accommodates ice floe deformation. Some empirical data on the performance of the algorithms on Seasat SAR images are presented. >

Statistical Edge Detection Operators for Linear Feature Extraction in SAR Images

Edge detection in synthetic aperture radar (SAR) images is rendered difficult by the presence of speckle. The data are often filtered using adaptive filters independently of the edge detection process when, in fact, the two steps should be coupled (i.e., the local homogeneity criterion employed by an adaptive filter should be consistent with the edge detector criterion). Five different edge detection algorithms for SAR images are evaluated and compared. The detection algorithms are comprised of two operators based on non-parametric statistical tests, the Ratio of Averages test, difference of averages (essentially a gradient method), and a test based on the mean squared to variance ratio. Two edge thinning and thresholding operations are also compared: an algorithm proposed by Nevatia and Babu (1980), and one based on mathematical morphology (Serra, 1980). Initial testing is carried out on simulated imagery for accurate control of the signal being masked by speckle noise. We obtain the best results using the ratio operator in combination with the morphological thinning operations. High real edge recovery rates are required for segmentation (Fmn > 0.95), and these levels of Fmn are only produced by high-contrast, low mean, local grey level boundaries. We show that a complete boundary delineation for segmentation purposes cannot be expected from a typical Seasat SAR agricultural scene due to the large number of low contrast edges. These methods are applicable in situations where there is a greater contrast between the targets to be discriminated. A suitable application of edge extraction from SAR, for example, is lake boundary extraction; a sample image is presented.