R. Kwok

Classification of multi-look polarimetric SAR imagery based on complex Wishart distribution

Abstract Multi-look polarimetric SAR (synthetic aperture radar) data can be represented either in Mueller matrix form or in complex covariance matrix form. The latter has a complex Wishart distribution. A maximum likelihood classifier to segment polarimetric SAR data according to terrain types has been developed based on the Wishart distribution. This algorithm can also be applied to multifrequency multi-look polarimetric SAR data, as well as 10 SAR data containing only intensity information. A procedure is then developed for unsupervised classification. The classification error is assessed by using Monte Carlo simulation of multilook polarimetric SAR data, owing to the lack of ground truth for each pixel. Comparisons of classification errors using the training sets and single-look data are also made. Applications of this algorithm are demonstrated with NASA/JPL P-, L- and C-band polarimetric SAR data.

Thermoelasticity of large lecithin bilayer vesicles

Micromechanical experiments on large lecithin bilayer vesicles as a function of temperature have demonstrated an essential feature of bilayer vesicles as closed systems: the bilayer can exist in a tension-free state (within the limits of experimental resolution, i.e., less than 10(-2) dyn/cm). Furthermore, because of the fixed internal volume, there is a critical temperature at which the vesicle becomes a tension-free sphere. Below this temperature, thermoelastic tension builds up in the membrane and the vesicles internal pressure increases while the surface area remains constant. Above this temperature, the vesicles surface area increases while the tension and internal pressure are negligible. Without mechanical support, the vesicles fragment into small vesicles because they have insufficient surface rigidity. In the upper temperature range we have measured the increase of surface area with temperature. These data established the thermal area expansivity to be 2.4 X 10(-3)/degrees C. At constant temperature, we used either pipet aspiration with suction pressures up to 10(4) dyn/cm2 or compression against a flat surface with forces up to 10(-2) dyn to produce area dilation of the vesicle surface on the order of 1%. The rate of increase of membrane tension with area dilation was calculated, which established the elastic area compressibility modulus to be 140 dyn/cm. The tension limit that produced lysis was observed to be 3-4 dyn/cm (equivalent to 2-3% area increase). The product of the elastic area compressibility modulus, the thermal area expansivity, and the temperature gives the reversible heat of expansion at constant temperature for the bilayer. This value is 100 ergs/cm2 at 25 degrees C, or approximately 5 kcal/mol of lecithin. Similarly, the product of the thermal area expansivity multiplied by the area compressibility modulus determines the rate of increase of thermoelastic tension with decrease in temperature when the area is held constant, i.e., -0.34 dyn/cm/degrees C.

Changing Arctic Ocean freshwater pathways

Freshening in the Canada basin of the Arctic Ocean began in the 1990s and continued to at least the end of 2008. By then, the Arctic Ocean might have gained four times as much fresh water as comprised the Great Salinity Anomalyof the 1970s, raising the spectre of slowing global ocean circulation. Freshening has been attributed to increased sea ice melting and contributions from runoff, but a leading explanation has been a strengthening of the Beaufort High—a characteristic peak in sea level atmospheric pressure—which tends to accelerate an anticyclonic (clockwise) wind pattern causing convergence of fresh surface water. Limited observations have made this explanation difficult to verify, and observations of increasing freshwater content under a weakened Beaufort High suggest that other factors must be affecting freshwater content. Here we use observations to show that during a time of record reductions in ice extent from 2005 to 2008, the dominant freshwater content changes were an increase in the Canada basin balanced by a decrease in the Eurasian basin. Observations are drawn from satellite data (sea surface height and ocean-bottom pressure) and in situ data. The freshwater changes were due to a cyclonic (anticlockwise) shift in the ocean pathway of Eurasian runoff forced by strengthening of the west-to-east Northern Hemisphere atmospheric circulation characterized by an increased Arctic Oscillation index. Our results confirm that runoff is an important influence on the Arctic Ocean and establish that the spatial and temporal manifestations of the runoff pathways are modulated by the Arctic Oscillation, rather than the strength of the wind-driven Beaufort Gyre circulation.

Sea ice motion from satellite passive microwave imagery assessed with ERS SAR and buoy motions

Observing the motion of sea ice from space is analogous to observing wind stress over the wet oceans; both provide surface forcing for modeling ocean dynamics. Ice motion also directly provides the advective component of the equations governing the mass balance of the sea ice cover. Thus its routine observation from space would be of great value to understanding ice and ocean behavior. To demonstrate the feasibility of creating a global multidecadal ice motion record from satellite passive microwave imagery and to quantitatively assess the errors in the estimated ice motions, we have tracked ice every 3 days in the Arctic Ocean and daily in the Fram Strait and Baffin Bay during the 8 winter months from October 1992 to May 1993 and daily in the Weddell Sea during the 8 winter months from March to October 1992. The method, which has been well used previously, involves finding the spatial offset that maximizes the cross correlation of the brightness temperature fields over 100-km patches in two images separated in time by from 1 to 3 days. The resulting ice motions are compared with contemporaneous buoyand SAR-derived ice motions. The uncertainties in the displacement vectors, between 5 and 12 km, are better than the spatial resolution of the data. Both 85-GHz data with 12-km spatial resolution and 37-GHz data with 25-km resolution are tracked. These trials with the 37-GHz data are new and show quite surprisingly that the error is only about 1 km larger with these data than with the 12-km 85-GHz data. Errors are typically larger than average in areas of lower ice concentration; in the most dynamic regions, particularly near the ice edge in the Barents and Greenland Seas; and in zones of high shear. These passive microwave ice motions show a large increase in spatial detail over motion fields optimally interpolated from buoy and wind observations, especially where buoy data are virtually absent such as near coasts and in some passages between the Arctic Ocean and its peripheral seas. The feasibility of obtaining ice motion from the 37-GHz data in addition to the 85-GHz data should allow an important record of ice motion to be established for the duration of the scanning multichannel microwave radiometer (SMMR), special sensor microwave/imager (SSM/I), and future microwave sensors, that is, from 1978 into the next millenium.

Greenland Ice Sheet Surface Properties and Ice Dynamics from ERS-1 SAR Imagery

C-band synthetic aperture radar (SAR) imagery from the European Space Agencys ERS-1 satellite reveals the basic zonation of the surface of the Greenland Ice Sheet. The zones have backscatter signatures related to the structure of the snowpack, which varies with the balance of accumulation and melt at various elevations. The boundaries of zones can be accurately located with the use of this high-resolution imagery. The images also reveal a large flow feature in northeast Greenland that is similar to ice streams in Antarctica and may play a major role in the discharge of ice from the ice sheet.

Symmetry properties in polarimetric remote sensing

This paper presents the relations among polarimetric backscattering coefficients from the viewpoint of symmetry groups. Symmetry of geophysical media encountered in remote sensing due to reflection, rotation, azimuthal, and centrical symmetry groups is considered for both reciprocal and nonreciprocal cases. On the basis of the invariance under symmetry transformations in the linear polarization basis, the scattering coefficients are related by a set of equations which restrict the number of independent parameters in the polarimetric covariance matrix. The properties derived under these transformations are general and apply to all scattering mechanisms in a given symmetrical configuration. The scattering coefficients calculated from theoretical models for layer random media and rough surfaces are shown to obey the derived symmetry relations. Use of symmetry properties in remote sensing of structural and environmental responses of scattering media is discussed. As a practical application, the results from this paper provide new methods for the external calibration of polarimetric radars without the deployment of man-made calibration targets.

Block adaptive quantization of Magellan SAR data

A report is presented on a data compression scheme that will be used to reduce the SAR (synthetic-aperture radar) data rate on the US NASA Magellan (MGN) mission to Venus. The MGN spacecraft has only one scientific instrument, namely, a radar system that is used for imaging the surface. for altimetric profiling of the planet topography, and for measuring radiation of the planet surface. A straightforward implementation of the scientific requirements of the mission results in a data rate higher that can be accommodated by the available system bandwidth. A data-rate-reduction scheme which includes operation of the radar in burst mode and block adaptive quantization of the SAR data is selected to satisfy the scientific requirements. >

Interferometric estimation of three-dimensional ice-flow using ascending and descending passes

Satellite radar interferometry (SRI) provides an important new tool for determining ice-flow velocity. Interferometric measurements made from a single-track direction are sensitive only to a single component of the three-component velocity vector. Observations from along three different track directions would allow the full velocity vector to be determined. A north/south-looking synthetic aperture radar (SAR) could provide these observations over large portions of the globe, but not over large areas of the polar ice sheets. The authors develop and demonstrate a technique that allows the three-component velocity vector to be estimated from data acquired along two track directions (ascending and descending) under a surface-parallel flow assumption. This technique requires that there are accurate estimates of the surface slope, which are also determined interferometrically. To demonstrate the technique, the authors estimate the three-component velocity field for the Ryder Glacier, Greenland. Their results are promising, although they do not have yet ground-truth data with which to determine the accuracy of their estimates.

Analysis of the Arctic System for Freshwater Cycle Intensification: Observations and Expectations

Abstract Hydrologic cycle intensification is an expected manifestation of a warming climate. Although positive trends in several global average quantities have been reported, no previous studies have documented broad intensification across elements of the Arctic freshwater cycle (FWC). In this study, the authors examine the character and quantitative significance of changes in annual precipitation, evapotranspiration, and river discharge across the terrestrial pan-Arctic over the past several decades from observations and a suite of coupled general circulation models (GCMs). Trends in freshwater flux and storage derived from observations across the Arctic Ocean and surrounding seas are also described. With few exceptions, precipitation, evapotranspiration, and river discharge fluxes from observations and the GCMs exhibit positive trends. Significant positive trends above the 90% confidence level, however, are not present for all of the observations. Greater confidence in the GCM trends arises through lowe...

A Mini-Surge on the Ryder Glacier, Greenland, Observed by Satellite Radar Interferometry

Satellite radar interferometry reveals that the speed of the Ryder Glacier increased roughly threefold and then returned to normal (100 to 500 meters/year) over a 7-week period near the end of the 1995 melt season. The accelerated flow represents a substantial, though short-lived, change in ice discharge. During the period of rapid motion, meltwater-filled supraglacial lakes may have drained, which could have increased basal water pressure and caused the mini-surge. There are too few velocity measurements on other large outlet glaciers to determine whether this type of event is a widespread phenomenon in Greenland, but because most other outlet glaciers are at lower latitudes, they should experience more extensive melting, making them more susceptible to meltwater-induced surges.