Mary R. Keller

A wave tank study of the dependence of X band cross sections on wind speed and water temperature

Measurements of normalized radar cross sections of wind-generated waves were made at X band for both vertical and horizontal polarization for incidence angles of 10°, 28°, 48°, and 68°. The study, conducted in the Naval Research Laboratory wind-wave facility, sought to measure effects on the backscatter of varying water temperature, wind speed, and wind stress. The cross-section measurements were averaged for 2.13 min simultaneously with wind speed and wind stress. Air and water temperature were measured periodically with mercury thermometers. The results were compared with the empirical model functions developed for the Seasat-A satellite scatterometer, SASS I and SASS II, and with the physically based models of Burden and Vesecky, Plant, and Donelan and Pierson. In order to use the SASS I and SASS II models for these comparisons, differences between Ku and X bands were assumed to be small; where possible, the other models were evaluated at X band. It was found that none of the models was consistently accurate at all wind speeds, incidence angles, and polarizations. Part of the inconsistency can be assigned to the fact that the models were developed for open ocean conditions with much higher sea states. However, Plants model can easily be adjusted to account for this effect by using a relationship between mean squared slope and wind stress appropriate for tank conditions. When this was done, the model did fit the data better but the improvement was not dramatic. This indicates that inaccuracies in the models are probably due to other factors as well. When plotted versus 19.5-m winds on a log-log scale, the measured cross sections do not fall on straight lines. Thus a power law dependence of cross section on wind speed is not a good representation of that relationship over our whole wind speed range. The data exhibit large variations at low wind speeds, however, which are not related to system noise. This may indicate that the statistics of backscatter depend markedly on wind speed. When these low wind speed data were omitted, a power law was found to fit the remaining data rather well. Although the water temperature was varied from 9°C to 36°C when the measurements were made at a 48° incidence angle, no temperature dependence was detectable above the low-wind-speed variability. The wave tank data compare well enough with 10 GHz, 3.0 cm (X band) aircraft measurements, and with the 14.6 GHz, 2.1 cm (Ku band) satellite data used in the SASS II model to cast doubt on the hypothesis that cross section depends on antenna altitude.

Comparison of optically‐derived spectral densities and microwave cross sections in a wind‐wave tank

The most popular model of microwave backscatter from rough water surfaces at mid-incidence angles (20° < θi < 70°) is composite surface theory. This theory holds that the backscattered return is directly proportional to the spectral density of centimetric, Bragg-resonant water waves which are tilted and advected by longer waves. A stringent test of this theory is to measure, independently and from the same surface area, the normalized microwave cross section (σ0) and the Bragg wave spectral density, and compare them using the theory. In this paper, we use a calibrated optical slope imaging system in a wind-wave tank to measure the two-dimensional wavenumber spectrum of short waves. From these spectra, we calculate both the pure Bragg scattering σ0 which neglects longwave effects and the more complex composite surface σ0. The results are compared with σ0 obtained from backscatter measurements at X band (10 GHz) and Ka band (35 GHz) made between 28° and 68° incidence angle. We find that composite surface theory generally shows better agreement with experiment at both frequencies than pure Bragg scattering theory. The agreement seems best for friction velocities above 40 cm s−1. For all friction velocities up to 70 cm s−1, however, composite surface theory somewhat underpredicts the actual σ0 in a majority of the cases. This is especially true for horizontal polarization at large incidence angles. We conclude that while composite surface theory accounts for much of the backscatter at both frequencies in the incidence angle range we examined, the discrepancy between the predicted and measured cross sections is sufficiently large that contributions from other scattering processes cannot be ruled out.

Satellite sensor requirements for monitoring essential biodiversity variables of coastal ecosystems

Abstract The biodiversity and high productivity of coastal terrestrial and aquatic habitats are the foundation for important benefits to human societies around the world. These globally distributed habitats need frequent and broad systematic assessments, but field surveys only cover a small fraction of these areas. Satellite‐based sensors can repeatedly record the visible and near‐infrared reflectance spectra that contain the absorption, scattering, and fluorescence signatures of functional phytoplankton groups, colored dissolved matter, and particulate matter near the surface ocean, and of biologically structured habitats (floating and emergent vegetation, benthic habitats like coral, seagrass, and algae). These measures can be incorporated into Essential Biodiversity Variables (EBVs), including the distribution, abundance, and traits of groups of species populations, and used to evaluate habitat fragmentation. However, current and planned satellites are not designed to observe the EBVs that change rapidly with extreme tides, salinity, temperatures, storms, pollution, or physical habitat destruction over scales relevant to human activity. Making these observations requires a new generation of satellite sensors able to sample with these combined characteristics: (1) spatial resolution on the order of 30 to 100‐m pixels or smaller; (2) spectral resolution on the order of 5 nm in the visible and 10 nm in the short‐wave infrared spectrum (or at least two or more bands at 1,030, 1,240, 1,630, 2,125, and/or 2,260 nm) for atmospheric correction and aquatic and vegetation assessments; (3) radiometric quality with signal to noise ratios (SNR) above 800 (relative to signal levels typical of the open ocean), 14‐bit digitization, absolute radiometric calibration <2%, relative calibration of 0.2%, polarization sensitivity <1%, high radiometric stability and linearity, and operations designed to minimize sunglint; and (4) temporal resolution of hours to days. We refer to these combined specifications as H4 imaging. Enabling H4 imaging is vital for the conservation and management of global biodiversity and ecosystem services, including food provisioning and water security. An agile satellite in a 3‐d repeat low‐Earth orbit could sample 30‐km swath images of several hundred coastal habitats daily. Nine H4 satellites would provide weekly coverage of global coastal zones. Such satellite constellations are now feasible and are used in various applications.

Testing the expanding‐contracting polar cap paradigm

The expanding-contracting polar cap (ECPC) paradigm is tested. Under the ECPC paradigm ionospheric convection in the polar cap is driven by the combined effects of magnetic field dayside merging and nightside reconnection, as opposed to being mapped down from higher altitudes. The ECPC paradigm is tested by separately examining the cross-polar cap potential when the polar cap is expanding versus contracting. The open magnetic flux is estimated from SuperDARN observations of the convection reversal boundary (CRB) made simultaneously at different local times. Sotirelis et al. [2005] established the CRB as a proxy for the open-closed boundary. The correlation of the ionospheric convection potential, determined from SuperDARN, with solar wind/IMF driving is indeed found to depend on whether the polar cap is expanding or contracting. Specifically, when the polar cap is expanding, ionospheric convection potential correlates best (0.86) with the most recent 10 minutes of solar wind/IMF driving (versus 0.57 for contracting). When contracting, convection potential correlates best (0.87) with 90-minute averages of solar wind/IMF driving (versus 0.51 for expanding). This result is consistent with the expectations of the ECPC paradigm.

A spaceborne visible-NIR hyperspectral imager for coastal phenology

The temporal variability, or phenology, of animals and plants in coastal zone and marine habitats is a function of geography and climatic conditions, of the chemical and physical characteristics of each particular habitat, and of interactions between these organisms. These conditions play an important role in defining the diversity of life. The quantitative study of phenology is required to protect and make wise use of wetland and other coastal resources. We describe a low cost space-borne sensor and mission concept that will enable such studies using high quality, broad band hyperspectral observations of a wide range of habitats at Landsat-class spatial resolution and with a 3 day or better revisit rate, providing high signal to noise observations for aquatic scenes and consistent view geometry for wetland and terrestrial vegetation scenes.

Wide-bandwidth time decay signatures from UXO targets

This paper presents wide bandwidth, time decay signatures from recent unexploded ordnance (UXO) field experiment at a US Government UXO test site. While current technologies have shown the ability to detect buried metal objects, they tend to fail in discriminating the UXOs from metal objects that pose no risk. Metal target time decay measurements have been shown to be an excellent method for target classification and identification. The present paper addresses the research communitys need for accurate, wide-bandwidth UXO target signatures. Metal target signatures for a number of important UXO targets are presented in the paper for both vertical and horizontal magnetic field excitation. Target time decay signatures from about 30 microseconds to 8 milliseconds are presented. Target signatures are also characterized using a non-linear parameterization scheme in an effort to develop a compact target signature library.