David A. Imel

Measurements of ocean surface waves and currents using L- and C-band along-track interferometric SAR

Along-track interferometric synthetic aperture radar (ATI-SAR) is an active coherent imaging system, utilizing two antennas separated along the platform flight direction. The phase information of ATI-SAR from the Doppler shift of the backscattered signal represents the line-of-sight velocity of the water scatterers. While the advent of ATI-SAR provided us with a potentially powerful technique for ocean surface current and wave mapping, the surface current has not been measured exactly from the ATI-SAR velocity because the Doppler shift is not simply proportional to the component of the mean surface current. It also includes other types of contributions associated with the phase velocity of the Bragg waves and orbital motions of all ocean waves that are longer than Bragg waves. In this paper, we review how the phase difference measured by ATI-SAR is related to the mean Doppler frequency, and we develop a new and practically useful method to extract the surface current component utilizing simultaneously measured L- and C-band ATI-SAR data. Since the measured ATI-SAR velocity shows a different value at a different radar-frequency, we investigate the influence of Bragg-resonant waves and long ocean wave motions on the ATI-SAR velocity according to the radar frequency. The Bragg-wave phase velocity component, which is a significant source of error for extracting the surface current, can be effectively eliminated by using L- and C-band ATI-SAR. The method is applied to L- and C-band ATI-SAR measurements acquired at the Ulsan coast in the southeastern part of the Korean peninsula. The resulting ocean surface current vectors are compared with in situ measurements collected by recording current meter. We furthermore extract ocean surface wave information from the ATI-SAR phase image using a quasi-linear transform.

Design, construction and performance of a liquid xenon and liquid argon ionization chamber

Abstract We have constructed an ionization chamber for liquid xenon and liquid argon and have conducted studies of initial ion-electron recombination. Using a 392 keV 113Sn source, we find in xenon a charge collection of (90±5)% at an electric drift field of 1.6 kV/cm and (10±3)% at 10 V/cm. In argon we collect (78±5)% of the charge in a field of 1.6 kV/cm. We have developed a method for measuring the concentration of electronegative impurities in the detector liquid. The purity of the liquid was always better than 2 ppb O2 equivalent. Under these conditions, the energy resolution observed with either liquid in the detector is (12±1)% FWHM at 380 keV in an electric drift field of 1.8 kV/cm for argon and 1.6 kV/cm for xenon. This appears to be a fundamental limit imposed by the statistics of delta electron production.

Accuracy of the residual-delay absolute-phase algorithm

An absolute-phase algorithm is used in an interferometric radar mapper to determine the unknown constant number of cycles of phase difference after phase unwrapping and, thus, the absolute topography. Using a model and a simulation, the author computes the accuracy of one such algorithm and compares it to the results for an actual system. The algorithms accuracy for other systems is predicted.

Event identification with a time projection chamber in a double beta decay experiment on 136Xe

A xenon time projection chamber has been constructed to study double beta decay in 136Xe. The excellent track reconstruction capability of the TPC provides a powerful means of background identification. We have demonstrated the novel technique of measuring the directionality of low energy (MeV range) electrons by the difference of dEdx between their beginnings and ends. Rejection efficiencies are 100% for alpha particles, cosmic muons and multiple Compton scattering, and more than 98% for single electrons, while the identification efficiencies for events due to neutrinoless double beta decay are 76% and 65% for the mass mechanism and right-handed currents modes, respectively. Very sensitive limit has been achieved for the half-life of the 0νββ channel. The tracking performance of the TPC as well as the event analysis and background rejection procedures of the experiment are discussed.

Progress report on the NASA/JPL airborne synthetic aperture radar system

AIRSAR has served as a test-bed for both imaging radar techniques and radar technologies for over a decade. In fact, the polarimetric, cross-track interferometric, and along-track interferometric radar techniques were all developed using AIRSAR. In this paper, we present the up-to-date system configuration and expected performance in the standard radar modes. In addition, we describe the various experimental modes available to researchers. Finally, we discuss on-going improvements with AIRSAR and future direction of the program.

Providing comprehensive and consistent access to astronomical observatory archive data: the NASA archive model

Since the turn of the millennium a constant concern of astronomical archives have begun providing data to the public through standardized protocols unifying data from disparate physical sources and wavebands across the electromagnetic spectrum into an astronomical virtual observatory (VO). In October 2014, NASA began support for the NASA Astronomical Virtual Observatories (NAVO) program to coordinate the efforts of NASA astronomy archives in providing data to users through implementation of protocols agreed within the International Virtual Observatory Alliance (IVOA). A major goal of the NAVO collaboration has been to step back from a piecemeal implementation of IVOA standards and define what the appropriate presence for the US and NASA astronomy archives in the VO should be. This includes evaluating what optional capabilities in the standards need to be supported, the specific versions of standards that should be used, and returning feedback to the IVOA, to support modifications as needed. We discuss a standard archive model developed by the NAVO for data archive presence in the virtual observatory built upon a consistent framework of standards defined by the IVOA. Our standard model provides for discovery of resources through the VO registries, access to observation and object data, downloads of image and spectral data and general access to archival datasets. It defines specific protocol versions, minimum capabilities, and all dependencies. The model will evolve as the capabilities of the virtual observatory and needs of the community change.

AIRSAR automated web-based data processing and distribution system

In this paper, we present an integrated, end-to-end synthetic aperture radar (SAR) processing system that accepts data processing requests, submits processing jobs, performs quality analysis, delivers and archives processed data. This fully automated SAR processing system utilizes database and internet/intranet web technologies to allow external users to browse and submit data processing requests and receive processed data. It is a cost-effective way to manage a robust SAR processing and archival system. The integration of these functions has reduced operator errors and increased processor throughput dramatically.

Ionization spectra in solid xenon

Abstract A simple radial ionization chamber for solid and liquid xenon and argon has been built and tested, and solid xenon ionization spectra are presented for the first time. Energy resolution for external, uncollimated γ-rays is 14% FWHM at 341 keV in solid and liquid argon and 16% FWHM at 0.5 MeV in solid and liquid xenon. This design may have applications in nuclear ββ decay.