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NASA NTRS1992

Satellite retrievals of marine stratiform cloud systems

The initial validation of a satellite-based cloud retrieval scheme of the spatial-coherence type that is being used to create a marine stratiform cloudiness (MSC) climatology is discussed. Initial results are presented which describe correlations between MSC properties and SST, which could be useful for providing guidelines for assessing potential climate feedbacks regarding these cloud types. The correlation of triangle-technique- and spatial-coherence-method-derived fractional cloud covers together with the regression line for all data points is shown.

Molnar, Gyula I.↗

NASA NTRS1991

Infrared lidars for atmospheric remote sensing

Lidars using pulsed TEA-CO2 transmitters and coherent receivers have been developed at JPL and used to measure atmospheric backscatter and extinction at wavelengths in the 9-11 micron region. The global winds measurement application of coherent Doppler lidar requires intensive study of the global climatology of aerosol and cloud backscatter and extinction. An airborne lidar was recently flown on the NASA DC-8 research aircraft for operation during two Pacific circumnavigation missions. The instrument characteristics, as well as representative measurement results, are discussed.

Menzies, Robert T.↗

NASA NTRS1993

Spatial correlations of interdecadal variation in global surface temperatures

We have analyzed spatial correlation patterns of interdecadal global surface temperature variability from an empirical perspective. Using multitaper coherence estimates from 140-yr records, we find that correlations between hemispheres are significant at about 95 percent confidence for nonrandomness for most of the frequency band in the 0.06-0.24 cyc/yr range. Coherence estimates of pairs of 100-yr grid-point temperature data series near 5-yr period reveal teleconnection patterns consistent with known patterns of ENSO variability. Significant correlated variability is observed near 15 year period, with the dominant teleconnection pattern largely confined to the Northern Hemisphere. Peak-to-peak Delta-T is at about 0.5 deg, with simultaneous warming and cooling of discrete patches on the earth's surface. A global average of this pattern would largely cancel.

Mann, Michael E.↗

NASA NTRS1992

Phase difference statistics related to sensor and forest parameters

The information content of ordinary synthetic aperture radar (SAR) data is principally contained in the radiometric polarization channels, i.e., the four Ihh, Ivv, Ihv and Ivh backscattered intensities. In the case of clutter, polarimetric information is given by the four complex degrees of coherence, from which the mean polarization phase differences (PPD), correlation coefficients or degrees of polarization can be deduced. For radiometric features, the polarimetric parameters are corrupted by multiplicative speckle noise and by some sensor effects. The PPD distribution is related to the sensor, speckle and terrain properties. Experimental results are given for the variation of the terrain hh/vv mean phase difference and magnitude of the degree of coherence observed on bare soil and on different pine forest stands.

Lopes, A.↗

NASA NTRS1993

Scattering by aggregates with and without an absorbing mantle - Microwave analog experiments

We present angular scattering functions for loosely packed aggregates of 250 and 500 identical spheres near the Rayleigh size limit before and after the application of successive layers of an absorbing mantle. All measurements were obtained by using the microwave analog technique. Gross features of the scattering by aggregates without a mantle can be interpreted in terms of coherent scattering from the unit spheres acting independently of each other. The largest deviations from this approximation occur after the first minimum in forward scattering and extend to a scattering angle of 60 or 80 deg for our models. This intermediate range is also where the largest differences occur in the scattering from one aggregate to another. The angular extent of the range is largest for aggregates with the smallest dimensions. The scattering function is usually flat in the backscattering hemisphere and has little or no backscattering increase. The coherent scattering approximation breaks down when the aggregates are coated, and an equivalent spheres approximation becomes a better representation. The maximum degree of polarization near a scattering angle of 90 deg first decreases and then increases again as the mantle grows thicker.

Zerull, R. H.↗

NASA NTRS1985

A comparison of Doppler lidar wind sensors for Earth-orbit global measurement applications

Now, there are four Doppler lidar configurations which are being promoted for the measurement of tropospheric winds: (1) the coherent CO2 Lidar, operating in the 9 micrometer region using a pulsed, atmospheric pressure CO2 gas discharge laser transmitter, and heterodyne detection; (2) the coherent Neodymium doped YAG or Glass Lidar, operating at 1.06 micrometers, using flashlamp or diode laser optical pumping of the solid state laser medium, and heterodyne detection; (3) the Neodymium doped YAG/Glass Lidar, operating at the doubled frequency (at 530 nm wavelength), again using flashlamp or diode laser pumping of the laser transmitter, and using a high resolution tandem Fabry-Perot filter and direct detection; and (4) the Raman shifted Xenon Chloride Lidar, operating at 350 nm wavelength, using a pulsed, atmospheric pressure XeCl gas discharge laser transmitter at 308 nm, Raman shifted in a high pressure hydrogen cell to 350 nm in order to avoid strong stratospheric ozone absorption, also using a high resolution tandem Fabry-Perot filter and direct detection. Comparisons of these four systems can include many factors and tradeoffs. The major portion of this comparison is devoted to efficiency. Efficiency comparisons are made by estimating the number of transmitted photons required for a single pulse wind velocity estimate of + or - 1 m/s accuracy in the middle troposphere, from an altitude of 800 km, which is assured to be reasonable for a polar orbiting platform.

Menzies, Robert T.↗

NASA NTRS1985

The WINDSAT concept for measuring the global wind field

The Wave Propagation Laboratory of the National Oceanic and Atmospheric Administration's (NOAA) Environmental Research Laboratories has investigated the feasibility of measuring the global wind field by using an infrared coherent laser radar under a joint program with the U.S. Air Force Space Division Defense Meteorological Satellite Program (DMSP). These studies considered both the analytical and hardware feasibility of a spaceborne global wind measuring coherent laser radar (WINDSAT). Objectives and requirements of the Air Force Defense Meteorological Satellite Program were used in the study. The vertical distributions of the horizontal wind field were required throughout the troposphere with 300 km square horizontal and 1 km vertical resolution with a measurements accuracy of 1 m/s. Complete global coverage was required. The lidar system performance should also be scalable to operational satellite conditions. The analytical studies were performed for both a 300 km altitude Space Shuttle orbit and an operational polar orbit of 800 km altitude (Huffaker, 1978; Huffaker et al., 1980). A hardware definition study was performed for a Space Shuttle demonstration test flight (Lockheed Missiles and Space Co., 1981). Studies have also been conducted to determine the feasibility of mounting a WINDSAT payload on an Advanced TIROS-N spacecraft (RCA Corporation, 1983).

Huffaker, R. Milton↗

NASA NTRS1985

Doppler lidar sampling strategies and accuracies: Regional scale

It has been proposed that a Doppler lidar be placed in a polar orbit and scanned to provide estimates of lower tropospheric winds twice per day and with a spatial resolution of 300 km. Initial feasibility studies conducted primarily by NOAA and NASA presented an optimistic outlook for a space based lidar. The technology appeared within reach and initial computer simulations suggested that acceptable accuracies could be obtained. Those early studies exposed, however, several potential problem areas which included: (1) the algorithms for computing the wind vectors did not perform well when there were coherent gradients in the wind fields; and (2) the lifetime and power requirements of the lidar put severe restrictions on the pulse repetition frequency (PRF). These two basic problems are currently being addressed by a Doppler lidar simulation study focussed upon three primary objectives: (1) to develop optimum scan parameters and shot patterns for a satellite-based Doppler lidar; (2) to develop robust algorithms for computing wind vectors from lidar returns; and (3) to evaluate the impact of coherent mesoscale structures (wind gradients, clouds, aerosols) on up-scale wind estimates. An overview is provided of the simulation efforts with particular emphasis upon rationale and methodology. Since this research is currently underway, any results shown are meant only as evidence of progress.

Emmitt, G. D.↗

NASA NTRS1993

Enhancement effects in polarimetric radar returns: Phase difference statistics

The probability density functions (pdfs) of the co- and cross-polarized phase differences are derived for backscatter from vegetation using the coherent and incoherent scattering theories. Unlike previous derivations, no assumptions or observations other than the applicability of the Central Limit Theorem (CLT), the low fractional volume of the medium, the reciprocity of the scatterers, and the azimuthal symmetry of the scatterer's orientation statistics are employed. Everything else follows logically via the mathematics. The difference between the coherent theory and the incoherent theory is referred to as the backscatter enhancement effect. The influence of this enhancement effect on the phase difference pdfs is examined and found to be important under combined conditions of scatterer anisotropy and appropriate reflection coefficient values.

Lang, R. H.↗

NASA NTRS1993

Broken symmetry in ideal magnetohydrodynamic turbulence

A numerical study of the long-time evolution of a number of cases of inviscid, isotropic, incompressible, three-dimensional fluid, and magneto-fluid turbulence has been completed. The results confirm that ideal magnetohydrodynamic turbulence is non-ergodic if there is no external magnetic field present. This is due essentially to a canonical symmetry being broken in an arbitrary dynamical representation. The broken symmetry manifests itself as a coherent structure, i.e., a non-zero time-averaged part of the turbulent magnetic field. The coherent structure is observed, in one case, to contain about eighteen percent of the total energy.

Shebalin, John V.↗

NASA NTRS1993

Direction angle sensitivity of agricultural field backscatter with AIRSAR data

This paper presents a study of the direction angle sensitivity of radar backscatter from agricultural fields. The direction angle is defined as the angle between the incident plane and the perpendicular to the field row direction. Previous studies have concentrated on comparing the backscatter of agricultural fields imaged with 0, 45, and 90 degree direction angles. In contrast, this study concentrates on the backscatter changes occurring when the direction angle is modified by a few degrees or even by fractions of a degree. This is possible by using the output of the NASA/JPL AIRSAR processor, in which sixteen independent frames are formed, each one corresponding to the same radar scene imaged with a slightly different squint angle. The studied data set is an agricultural area in La Mancha, Spain acquired in June and July 1991 during the EFEDA experiment. This paper describes the observed backscatter variations of the agricultural fields with direction angle measured at P, L, and C bands. As expected, the backscatter is maximum for a 0 degree direction angle. For several fields, the backscatter at P and L bands drops by more than 10 dB for a 5 degree change in direction angle. Furthermore, the sensitivity to the direction angle decreases with increasing vegetation. The variations in backscatter are compared with model predictions. One model, which agrees with scatterometer data, underestimates the observed backscatter variations with direction angle by more than 10 dB. It does not take into account the possible coherent component of the radar signal. We believe the strong direction sensitivity of agricultural field backscatter obtained with SAR data is due to a Bragg resonant effect, resulting in a strong coherent return when the direction angle is zero. The observations are then projected to the case of spaceborne SAR data.

Dubois, Pascale C.↗

NASA NTRS1993

Development, refinement, and testing of a short term solar flare prediction algorithm

Progress toward performance of the tasks and accomplishing the goals set forth in the two year Research Grant included primarily analysis of digital data sets and determination of methodology associated with the analysis of the very large, unique, and complex collection of digital solar magnetic field data. The treatment of each magnetogram as a unique set of data requiring special treatment was found to be necessary. It is determined that a person familiar with the data, the analysis system, and logical, coherent outcome of the analysis must conduct each analysis, and interact with the analysis program(s) significantly sometimes many iterations for successful calibration and analysis of the data set. With this interaction, the data sets yield valuable, coherent analyses. During this period, it was also decided that only data sets taken inside heliographic longitudes (Central Meridian Distance) East and West 30 degrees (within 30 degrees of the Central Meridian of the Sun). If the total data set is then found to be numerically inadequate for the final analysis, 30 - 45 degrees Central Meridian Distance data will then be analyzed. The Optical Data storage system (MSFC observatory) was found appropriate for use both in intermediate storage of the data (preliminary to analysis), and for storage of the analyzed data sets for later parametric extraction.

Smith, Jesse B., Jr.↗

NASA NTRS1993

Instability modes excited by natural screech tones in a supersonic rectangular jet

The evolution of hydrodynamic instability modes self-excited by harmonically related natural screech tones was experimentally investigated. A convergent rectangular nozzle with an aspect ratio of 9.63 was used to produce a supersonic shock containing jet. Measurements in the flow-field were made using standard hot-film probes positioned only in the subsonic (outer) portions of the flow. The hydrodynamic instability mode observed in the shear layer at the screech frequency was observed to be antisymmetric (sinuous) about the smaller dimension of the jet, whereas its harmonic was observed to be symmetric (varicose). In addition, the near-field noise measurements indicated that the radiated screech tone noise was out of phase on either side of the small jet dimension whereas its harmonic was in phase over the same region. To our knowledge such an observation on the nature of the harmonic has thus far gone unreported and therefore is the focus of the present work. The hydrodynamic instability modes occurring at the screech frequency and its harmonic satisfied the conditions for resonance. Detailed measurements of the coherent wave evolution in the streamwise and spanwise directions indicated that strong spanwise variations were present beyond x/h = 8. Details of the screech noise radiated by the coherent instability modes are also presented in this paper.

Raman, Ganesh↗

NASA NTRS1993

Dynamical characteristics of cirrus clouds from aircraft and radar measurements

Cirrus clouds play an important role in climate and in the development of other types of clouds. Although there are many studies of clouds within the boundary layer, cirrus clouds have been neglected up until the last decade. New tools and in-situ measurements of various physical and dynamical parameters permit us to now study cirrus clouds in much greater detail. Physical and dynamical structures of cirrus clouds were studied in detail by Heymsfield using aircraft measurements. He emphasized the importance of interactions among physical and dynamical processes. Cirrus clouds often exhibit complex physical and dynamical structure. Upper tropospheric flows contain not only coherent structures, but also chaotic movements. The coherent structures (organized movements) transfer significant amounts of heat and momentum while their form, size, and intensity depend strongly on environmental instability. Various dynamical structures including cells, waves, and turbulence are studied in order to understand cirrus cloud formation and development.

Gultepe, I.↗

NASA NTRS1994

Modulation and synchronization technique for MF-TDMA system

This report addresses modulation and synchronization techniques for a multi-frequency time division multiple access (MF-TDMA) system with onboard baseband processing. The types of synchronization techniques analyzed are asynchronous (conventional) TDMA, preambleless asynchronous TDMA, bit synchronous timing with a preamble, and preambleless bit synchronous timing. Among these alternatives, preambleless bit synchronous timing simplifies onboard multicarrier demultiplexer/demodulator designs (about 2:1 reduction in mass and power), requires smaller onboard buffers (10:1 to approximately 3:1 reduction in size), and provides better frame efficiency as well as lower onboard processing delay. Analysis and computer simulation illustrate that this technique can support a bit rate of up to 10 Mbit/s (or higher) with proper selection of design parameters. High bit rate transmission may require Doppler compensation and multiple phase error measurements. The recommended modulation technique for bit synchronous timing is coherent QPSK with differential encoding for the uplink and coherent QPSK for the downlink.

Faris, Faris↗

NASA NTRS1994

Numerical simulation of two-dimensional spatially-developing mixing layers

Two-dimensional, incompressible, spatially developing mixing layer simulations are performed at Re = 10(exp 2) and 10(exp 4) with two classes of perturbations applied at the inlet boundary; combinations of discrete modes from linear stability theory, and a broad spectrum of modes derived from experimentally measured velocity spectra. The effect of the type and strength of inlet perturbations on vortex dynamics and time-averaged properties are explored. Two-point spatial velocity and autocorrelations are used to estimate the size and lifetime of the resulting coherent structures and to explore possible feedback effects. The computed time-averaged properties such as mean velocity profiles, turbulent statistics, and spread rates show good agreement with experimentally measured values. It is shown that by forcing with a broad spectrum of modes derived from an experimental energy spectrum many experimentally observed phenomena can be reproduced by a 2-D simulation. The strength of the forcing merely affected the length required for the dominant coherent structures to become fully-developed. Thus intensities comparable to those of the background turbulence in many wind tunnel experiments produced the same results, given sufficient simulation length.

Wilson, R. V.↗

NASA NTRS1994

Lidar performance analysis

Section 1 details the theory used to build the lidar model, provides results of using the model to evaluate AEOLUS design instrument designs, and provides snapshots of the visual appearance of the coded model. Appendix A contains a Fortran program to calculate various forms of the refractive index structure function. This program was used to determine the refractive index structure function used in the main lidar simulation code. Appendix B contains a memo on the optimization of the lidar telescope geometry for a line-scan geometry. Appendix C contains the code for the main lidar simulation and brief instruction on running the code. Appendix D contains a Fortran code to calculate the maximum permissible exposure for the eye from the ANSI Z136.1-1992 eye safety standards. Appendix E contains a paper on the eye safety analysis of a space-based coherent lidar presented at the 7th Coherent Laser Radar Applications and Technology Conference, Paris, France, 19-23 July 1993.

Spiers, Gary D.↗

NASA NTRS1994

A method of measuring the amplitude-modulated vacuum field near a conducting mirror

Electromagnetic fields of the vacuum mode near a conducting mirror are modified with respect to those in free space, with their amplitudes having a sinusoidal spatial dependence from the mirror. Therefore if we combine this spatially amplitude-modulated vacuum field mode and intense coherent light with a beam splitter, we may detect this fluctuation of the vacuum mode in a homodyne detection scheme. It will give a new method to produce squeezed states of light with a single mirror placed close to an unused port of a beam splitter. We show that the amplitude fluctuation of the combined light can be reduced by a factor of 2 below that of the coherent light. We also discuss the limitations due to the finite line width of the laser and the effective absorption length of the photodiodes.

Youn, Sun-Hyun↗