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At least 199 records · Page 11

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NASA NTRS1983

Low level water vapor fields from the VISSR Atmospheric Sounder (VAS) 'split window' channels

The use of the 11 and 12-micron IR-radiometer channels of the VISSR Atmosphere Sounder (VAS) on the GOES to detect water vapor in the lowest 300-400 mb of the troposphere is reported. An algorithm is developed to eliminate the background temperature, allowing the calculation of precipitable water (PW) over both land and water via a single-layer radiative model. This 'split-window' method is demonstrated in a case study, covering the US on July 13, 1981. PW values were calculated from five VAS images and compared with those from radiosonde data and surface measurements. It is shown that the VAS PW images have good resolution (15 km), reveal vivid, continuously evolving details, and differentiate deep, convection-supporting layers from shallow ones. PW ranged from 1.7 to 5.5 g/sq cm (+ or - 1.0 g/sq cm), in good agreement with other measurements. Since this method can detect mesoscale water-vapor fields in relatively clear air, it is considered of great potential value for numerical forecasting.

Robinson, W. D.↗

NASA NTRS1983

Electron collisional rate coefficients for low-level transitions in hydrogen

There exists presently no general agreement concerning the collisional excitation rates of atomic hydrogen. The present investigation is concerned with the results reported by various investigators, taking into account the semiempirical rates of Johnson (1972), a study conducted by Aggarwal (1983), empirically based rates of Crandall et al. (1974), and the rates tabulated in Osterbrock (1974), based on the close-coupling calculations of Burke et al. (1967). It is found that there is a major conflict between the 'best' theoretical and Johnson values on the one hand, and the empirical data on the other. A study of the data shows that the Johnson value for the 1 to 2 rate agrees well with other theoretical calculations, and within 25 percent of empirical measurements, but that the Johnson 1 to 3 rate is 40-75 percent of the 'best' theoretical calculations.

Drake, S. A.↗

NASA NTRS1985

Anomalous low level of cosmic ray intensity decreases observed during 1980

Past studies have revealed solar cycle changes in the sunspot activity, as well as in many other solar parameters, such as, solar flares and solar coronal holes. These solar features in turn produce the observed cyclic variations in the interplanetary plasma and fields. Both the cosmic ray intensity as well as the intensity of geomagnetic disturbances are affected by the interplanetary changes and produce 11/22 year periodicity. An anomalous situation has been noticed during the year 1980 (period of high sunspot activity), when both the geomagnetic disturbance index Ap, as well as the magnitude and number of Forbush decreases as small. Such an anomaly occurs, in spite of the fact that both the sunspot numbers and the energetic solar flares are almost maximum during the present solar cycle.

Jain, A. K.↗

NASA NTRS1983

Low-level microburst wind structure using Doppler radar and PAM data

The Joint Airport Weather Studies (JAWS) observation network used at an airport in Denver, CO consisted of one S-band and two C-band radars, together with 27 Portable Automated Mesonet (PAM) stations. Each of the three microburst case studies observed during the JAWS experiment occurred within the surface PAM network, and these surface data were compared with the radar analyses. It emerges that if microburst vertical motions are to be accurately resolved, the spacing of the surface stations must be considerably less than 4 km.

Kessinger, C.↗

NASA NTRS1985

Amplifier for measuring low-level signals in the presence of high common mode voltage

A high common mode rejection differential amplifier wherein two serially arranged Darlington amplifier stages are employed and any common mode voltage is divided between them by a resistance network. The input to the first Darlington amplifier stage is coupled to a signal input resistor via an amplifier which isolates the input and presents a high impedance across this resistor. The output of the second Darlington stage is transposed in scale via an amplifier stage which has its input a biasing circuit which effects a finite biasing of the two Darlington amplifier stages.

Lukens, F. E.↗

NASA NTRS1987

A nondestructive technique for predicting the strength remaining in filament wound composites subjected to low-level impact

A model for predicting the fracture strength of homogeneous materials is proposed. Impacted FWC samples were evaluated using ultrasonic testing and an X-ray dye penetration method. The ability of the model to measure fracture strength was also examined. The relation between attenuation and velocity measurements is studied. It is observed that the X-ray method is not useful for predicting fracture strength because the dye could not penetrate the matrix. It is noted that fracture strength predictions derived from the fracture mechanical model and the ultrasonic measurements correlate well with actual measured fracture strengths.

Madaras, E. I.↗

NASA NTRS1987

Rapid, low-level X-ray variability in active late-type dwarfs

A sensitive new statistical analysis method was used to establish certain properties of quiescent X-ray variability in a sample of 19 late-type dwarfs. Sixteen stars proved to be significantly variable. The typical amplitude of the fluctuations was about 30 percent. Variability time scales ranged from about 150 s for 40 Eri C to lower limits as large as 2000 s. Neither the amplitudes nor the time scales of the X-ray variability appeared to depend on position along the main sequence.

Ambruster, Carol W.↗

NASA NTRS1988

A piloted simulation investigation of yaw dynamics requirements for turreted gun use in low-level helicopter air combat

A piloted, fixed-base simulation study was conducted to investigate the handling qualities requirements for helicopter air-to-air combat using turreted guns in the near-terrain environment. The study used a version of the helicopter air combat system developed at NASA Ames Research Center for one-on-one air combat. The study focused on the potential trade-off between gun angular movement capability and required yaw axis response. Experimental variables included yaw axis response frequency and damping and the size of the gun-movement envelope. A helmet position and sighting system was used for pilot control of gun aim. Approximately 340 simulated air combat engagements were evaluated by pilots from the Army and industry. Results from the experiment indicate that a highly-damped, high frequency yaw response was desired for Level I handling qualities. Pilot preference for those characteristics became more pronounced as gun turret movement was restricted; however, a stable, slow-reacting platform could be used with a large turret envelope. Most pilots preferred to engage with the opponent near the own-ship centerline. Turret elevation restriction affected the engagement more than azimuth restrictions.

Decker, William A.↗

NASA NTRS1989

Low level remote sensing: Orographic winds

The orographic flow data set was obtained from a flight program to measure the influence of orographic features on turbulence momentum, heat, and moisture fluxes. The NASA B-57 aircraft instrumented with probes for measuring the three fluctuating wind speed components, temperature, and humidity was the primary measuring vehicle. Ancillary measurements were made with several ground-based sensors. These include the NOAA radar wind profilers, the Boulder wind network, the PROFS mesoscale surface network, the Boulder Atmospheric Observatory 300 m tower, special rawinsonde observations, and the NOAA/WPL Doppler lidar. The major objective of the flight program was to provide planetary boundary layer parameter information for new and current general circulation computer models. A numerical code, WINDER, based on a discrete element technique was run to numerically model the water tunnel simulated flow. Comparison of the analytical model with the experimental results is very good. Physical fluid dynamic principles embedded in the computational model and visual and hot wire anemometer measurements from the simulation are being rationalized to develop a physical understanding of the vortex flow. The results will be interpreted as they pertain to full scale atmospheric flows.

Frost, Walter↗

NASA NTRS1989

Low level remote sensing: The Doppler Radar wind profiler

Mesoscale phenomena such as thunderstorm and sea breeze frontal circulations are being investigated using a 50 MHz Doppler wind profiler at the Kennedy Space Center. The profiler installation will begin October 1, 1988 and will be completed by February 17, 1989. The focus of current research and plans for next year include: examination of vertical velocities associated with local thunderstorm activity and sea breeze frontal circulations and compare the vertical velocities to conceptual mesoscale models; implementation of space-time conversion analysis techniques to blend profiler data with National Meteorological Center's model output and other wind data such as jimsphere, windsonde and rawinsonde for mesoscale analysis; development of suggestions for use of wind profiler data in mesoscale analysis and forecasting at Kennedy Space Center; and problems detection in the quality of the profiler data during this research project. Researchers will work closely with MSFC to identify and solve the data quality problems.

Forbes, Gregory S.↗

NASA NTRS1990

Thermal advection and stratification effects on surface winds and the low level meridional mass transport

Statistical tests are performed on the Seasat scatterometer observations to examine if and to what degree thermal advection and stratification effects manifest themselves in these remotely sensed measurements of mean wind and wind stress over the ocean. On the basis of a two layer baroclinic boundary layer model which is presented, it is shown that the thermal advection and stratification of the entire boundary layer as well as the geostrophic forcing influence the modeled near surface wind and wind stress profiles. Evidence of diurnal variation in the stratification under barotropic conditions is found in the data, with the daytime marine boundary layer being more convective than its nighttime counterpart. The temporal and spacial sampling pattern of the satellite makes it impossible to recover the full diurnal cycle, however. The observed effects of the thermal advection are shown to be statistically significant during the day (and presumed more convective) hours, causing a systematic increase in the poleward transport of mass and heat. The statistical results are in a qualitative agreement with the model simulations and cannot be reproduced in randomized control tests.

Levy, Gad↗

NASA NTRS1991

The perception of surface layout during low level flight

Although it is fairly well established that information about surface layout can be gained from motion cues, it is not so clear as to what information humans can use and what specific information they should be provided. Theoretical analyses tell us that the information is in the stimulus. It will take more experiments to verify that this information can be used by humans to extract surface layout from the 2D velocity flow field. The visual motion factors that can affect the pilot's ability to control an aircraft and to infer the layout of the terrain ahead are discussed.

Perrone, John A.↗

NASA NTRS1992

Survey Parameters and Availability of Low-level Aeromagnetic Data for Geomagnetic Field Modelling

Since aeromagnetic surveying started immediately after World War II, a considerable area of the earth's surface both onshore (in excess of 25 million sq. km) has been magnetically surveyed. For about the first 15 years or so, fluxgate magnetometers were employed in aeromagnetic surveys, but the introduction of proton free-precession magnetometers resulted in absolute readings being recorded. Proton precession magnetometers have now been replaced to a large extent by the more sensitive optical absorption magnetometers. Some care has to be taken to calibrate aeromagnetic survey systems and this is best done using a calibration range tied to a magnetic observatory so that accuracies of 10nT or better are achieved for the total field values recorded. Survey navigation has always posed a problem for aeromagnetic surveys, especially offshore. Over land, vertically pointed 35 mm cameras were initially used to recover the aircraft track using a combination of aerial photos and topographic maps. Over featureless areas, it was necessary to utilize existing electronic positioning systems such as Loran C or set up special navigation systems. The advent of the satellite-based Global Positioning Systems (GPS) has to a larger extent solved the navigational problem because there is now almost continuous worldwide coverage to 10 m accuracy in the differential mode. The resultant aeromagnetic data is normally compiled into contour maps in which the diurnal variation and aircraft heading effects are removed. The resultant digital data are normally made publicly available both in gridded and profile form along with the published contour maps. Most aeromagnetic coverage has been obtained in the developed western countries, but elsewhere a considerable amount of surveying has been carried out; as an example, some 80 percent of Africa has been surveyed mostly as a result of aid programs. The data is usually held by the national geological survey or equivalent organizations, but in a number of cases the data (e.g., for the Magnetic Anomaly Map of North America) is also held by the World Data Centres.

Hood, Peter↗

NASA NTRS1992

Low-level atmospheric flows studied by pulsed Doppler lidar

A pulsed Doppler radar gains a tremendous advantage in studying atmospheric flows when it has the ability to scan. The Wave Propagation Laboratory (WPL) has been operating a scanning, 10.59 micron CO2 system for over 10 years. Recently, the WPL lidar has been a featured instrument in several investigations of mesoscale wind fields in the lowest 3-4 km of the atmosphere. These include four experiments: a study of the initiation and growth of the sea breeze off the coast of California, a study of the snake column of a prescribed forest fire, a study of the nighttime flow over the complex terrain near Rocky Flats, Colorado as it affects the dispersion of atmospheric contaminants, and a study of the wind flow in the Grand Canyon. We have analyzed much data from these experiments, and we have found that the lidar provides new insight into the structure of these flows. Many of these studies took place in rugged or mountainous terrain, thus using one of the major benefits of the lidar: the narrow, 90 microrad beam of the lidar makes it an ideal instrument for studying flow close to topography.

Banta, Robert M.↗

NASA NTRS1992

Natural versus anthropogenic factors affecting low-level cloud albedo over the North Atlantic

Cloud albedo plays a key role in regulating earth's climate. Cloud albedo depends on column-integrated liquid water content and the density of cloud condensation nuclei, which consists primarily of submicrometer-sized aerosol sulfate particles. A comparison of two independent satellite data sets suggests that, although anthropogenic sulfate emissions may enhance cloud albedo immediately adjacent to the east coast of the United States, over the central North Atlantic Ocean the variability in albedo can be largely accounted for by natural marine and atmospheric processes that probably have remained relatively constant since the beginning of the industrial revolution.

Falkowski, Paul G.↗

NASA NTRS1991

Visual cues to geographical orientation during low-level flight

A field study of an operational Emergency Medical Service (EMS) unit was conducted to investigate the relationships among geographical orientation, pilot decision making, and workload in EMS flights. The map data collected during this study were compared to protocols gathered in the laboratory, where pilots viewed a simulated flight over different types of unfamiliar terrain and verbally identified the features utilized to maintain geographical orientation. The EMS pilot's questionnaire data were compared with data from non-EMS helicopter pilots with comparable flight experience.

Battiste, Vernol↗

NASA NTRS1991

Low level measurements of atmospheric DMS, H2S, and SO2 for GTE/CITE-3

This project involved the measurement of atmospheric dimethylsulfide (DMS) and hydrogen sulfide (H2S) as part of the GTE/CITE-3 instrument intercomparison program. The two instruments were adapted for use on the NASA Electra aircraft and participated in all phases of the mission. This included ground-based measurements of NIST-provided standard gases and a series of airborne missions over the Western Atlantic Ocean. Analytical techniques used are described and the results are summarized.

Saltzman, Eric↗

NASA NTRS1992

Visual cues in low-level flight - Implications for pilotage, training, simulation, and enhanced/synthetic vision systems

This paper reviews some of the sources of visual information that are available in the out-the-window scene and describes how these visual cues are important for routine pilotage and training, as well as the development of simulator visual systems and enhanced or synthetic vision systems for aircraft cockpits. It is shown how these visual cues may change or disappear under environmental or sensor conditions, and how the visual scene can be augmented by advanced displays to capitalize on the pilot's excellent ability to extract visual information from the visual scene.

Foyle, David C.↗