David S. Early

Image reconstruction and enhanced resolution imaging from irregular samples

While high resolution, regularly gridded observations are generally preferred in remote sensing, actual observations are often not evenly sampled and have lower-than-desired resolution. Hence, there is an interest in resolution enhancement and image reconstruction. This paper discusses a general theory and techniques for image reconstruction and creating enhanced resolution images from irregularly sampled data. Using irregular sampling theory, we consider how the frequency content in aperture function-attenuated sidelobes can be recovered from oversampled data using reconstruction techniques, thus taking advantage of the high frequency content of measurements made with nonideal aperture filters. We show that with minor modification, the algebraic reconstruction technique (ART) is functionally equivalent to Grochenigs (1992) irregular sampling reconstruction algorithm. Using simple Monte Carlo simulations, we compare and contrast the performance of additive ART, multiplicative ART, and the scatterometer image reconstruction (SIR) (a derivative of multiplicative ART) algorithms with and without noise. The reconstruction theory and techniques have applications with a variety of sensors and can enable enhanced resolution image production from many nonimaging sensors. The technique is illustrated with ERS-2 and SeaWinds scatterometer data.

Azimuthal modulation of C-band scatterometer /spl sigma//sup 0/ over Southern Ocean sea ice

In a continuing evaluation of the ERS-1 C-band scatterometer as a tool for studying polar sea ice, the authors evaluate the azimuthal modulation characteristics of Antarctic sea ice. ERS-1 AMI scatterometer mode data sets from several study regions dispersed in the Antarctic seasonal sea ice pack are evaluated for azimuthal modulation. When appropriate, the incidence angle dependence is estimated and removed in a study region before determining whether azimuthal modulation is present in the data. Other comparisons are made using the fore and aft beam measurement difference. The results show that over the ice pack, azimuthal modulation is less than 1 dB at the scale of observation of the ERS-1 C-band scatterometer.

Enhanced resolution ERS-1 scatterometer imaging of Southern Hemisphere polar ice

The ERS-1 mission has generated a wealth of radar data over the Southern Hemisphere polar region. While the SAR mode of the ERS-1 Active Microwave Instrument (AMI) can provide only limited temporal and spatial coverage, the scatterometer mode provides frequent, global coverage, albeit at a much lower (50 km) resolution. By applying a recently developed algorithm for generating enhanced resolution scatterometer images, the ERS-1 scatterometer data can be used to study both sea ice and glacial ice to support SAR-based studies. This paper describes the algorithm and its application to ERS-1 scatterometer data with particular emphasis on sea ice. Selected images from a time series covering a complete annual cycle are presented.<<ETX>>

ERS-1 investigations of Southern Ocean sea ice geophysics using combined Scatterometer and SAR images

Coregistered ERS-I SAR and Scatterometer data are presented for the Weddell Sea, Antarctica. Calibrated image backscatter statistics are extracted from data acquired in regions where surface measurements were made during two extensive international Weddell Sea experiments in 1992. Changes in summer ice-surface conditions, due to temperature and wind, are shown to have a large impact on observed microwave backscatter values. Winter calibrated backscatter distributions are also investigated as a way of describing ice thickness conditions in different locations during winter. Coregistered SAR and EScat data over a manned drifting ice station as used to illustrate the seasonal signature changes occurring during the fall freeze-up transition. Combinations of Weddell Sea SAR and Scatterometer data are shown to be an extremely powerful tool for monitoring both sea-ice dynamics and the thermodynamic changes which accompany seasonal transitions in the Southern Ocean.<<ETX>>

Enhanced resolution imaging from irregular samples

This paper considers techniques for creating enhanced resolution images from irregular samples, with specific application to imaging from scatterometers. Using previously established irregular sampling theory and developing the idea of sub-band limited Banach space, the authors show that frequency content in attenuated sidelobes can be recovered using resolution enhancement techniques, thus taking advantage of the high frequency content of measurements made with imperfect low pass aperture filters. They briefly compare and contrast the performance of additive ART, multiplicative ART and the Scatterometer Image Reconstruction (SIR) (a derivative of multiplicative ART) algorithms with and without noise.

Error characteristics of the SIR resolution enhancement algorithm

To improve the utility of the scatterometer for land and ice studies, the Scatterometer Image Reconstruction (SIR) resolution enhancement algorithm has been developed to exploit the frequent overlapping coverage provided by the scatterometer. The authors examine the effects of feature motion on SIR imagery and show the similarity of SIR motion imagery to low pass filtered truth images. Also, resolution improvement is qualified by examining radiometric accuracy as a function of spatial frequency. The SIR algorithm applied to ERS-1 scatterometer data provides a 33% gain in radiometric accuracy when compared to simple image reconstruction of raw scatterometer data.

Comparison of enhanced resolution images of Greenland from the ERS-1 and Seasat scatterometers

With the increased awareness of the role of ice sheets as a sensitive indicator of global climate change, there is greater need for synoptic interannual studies of major glacial ice masses. Only limited large scale decadal studies of the ice sheets of Antarctica and Greenland have been possible with past and present satellite systems. A recently developed resolution enhancement technique facilitates the use of scatterometer data for extended studies of glacial ice at medium scale resolution with hemispheric or even global coverage possible. The authors compare previously published results for the Seasat-A scatterometer with current ERS-1 scatterometer data for Greenland. Illustrative images and data plots are provided.<<ETX>>

Comparison of variations in sea-ice formation in the Weddell Sea with seasonal bottom-water outflow data

Seasonal and interannual variability of Antarctic sea-ice formation is observed using ERS-1 satellite microwave radar in the Weddell Sea. Time-series from six Antarctic regions with recurring ice-shelf polynya systems indicate relationships between the timing of seasonal peaks in measured bottom water outflow and ice formation rates. Results provide evidence about the critical periods of high surface heat fluxes and clues to primary brine production locations.

Scatterometer resolution enhancement

Scatterometers can be effective for large scale monitoring of the Earths surface. However, their low resolution can limit the utility of the data. Thus, there is interest in resolution enhancement methods that can create high resolution imagery from scatterometer measurements. In this paper, a brief review of an iterative image reconstruction is presented, and the implications of this theory for enhanced resolution scatterometer imaging is presented. The result is that the multipass sample spacing is the single largest limiting factor in the reconstruction scheme, while the aperture function attenuation and noise level play secondary rolls in the limit of the enhanced resolution image reconstruction from scatterometer data.

Ice classification in the Southern Ocean using ERS-1 scatterometer data

A simple method for classifying Southern Ocean sea ice from enhanced resolution ERS-1 scatterometer images is presented. The enhanced resolution images are created with the Scatterometer Image Reconstruction (SIR) algorithm. This algorithm uses a dense, irregular sample grid created with multiple, overlapping passes of the ERS-1 scatterometer to achieve resolutions better than the nominal 50 km ERS-1 resolution. Because the scatterometer provides measurements over a range of incidence angles, the incidence angle dependence of the observed /spl sigma//spl deg/ can be used as part of the classification algorithm along with the incidence angle normalized /spl sigma//spl deg/ improving the accuracy of the classification. In this study, a third parameter the standard deviation of a measure of the anisotropy, is used to further help delineate sea ice types.