Search NASA⌕ Search

SEARCH · Search NASA

Results for “optical detectors”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

The Characteristics of Total Lightning Activity in Severe Florida Thunderstorms

Severe thunderstorms are defined by specific exceedance criteria regarding either wind speed (greater than or equal to 50 kts), hailstone diameter (greater than or equal to 3/4 inch), the occurrence of a tornado, or any combination thereof. Although traditional radar signatures of severe thunderstorms have been well documented, the characteristics of associated total lightning activity (both intracloud and cloud-to-ground) of severe thunderstorms remain poorly established. The reason for this are (1) less than 1% of all storms are actually severe, (2) intracloud lightning, which is typically the dominant form of electrical discharge within thunderstorms, is not routinely measured or recorded, (3) direct visual observations of intracloud lightning are obscured during the daytime, and (4) the migratory nature of many severe thunderstorms can make the accurate detection and mapping of intracloud lightning difficult when using fixed-location sensors. The recent establishment of LISDAD (Lightning Imaging Sensor Data Acquisition and Display - discussed in Goodman et al, this Meeting) has substantially addressed these limitations in east central Florida (ECFL). Analysis of total lightning flash Count histories using the LDAR (Lightning Detection And Ranging) system for known severe thunderstorms (currently irrespective of seasonal aspects and severe storm-type) has revealed flash rates exceeding 1 per second. This appears to be a necessary, but not sufficient,condition for most ECFL severe storm cases. The differences in radar-observed storm structure for high flash rate storms (to include both severe and non-severe categories) will be described together with the timing of peak flash rate vs. the timing of the severe weather manifestation. Comparisons with the satellite-bases OTD (Optical Transient Detector) overhead passes will also be presented when possible.

Williams, E.↗
NASA NTRS1999

Cross-Sensor Calibration of the GAI Long Range Detection Network

The long range component of the North American Lightning Detection Network has been providing experimental data products since July 1996, offering cloud-to-ground lightning coverage throughout the Atlantic and Western Pacific oceans, as well as south to the Intertropical Convergence Zone. The network experiences a strong decrease in detection efficiency with range, which is also significantly modulated by differential propagation under day, night and terminator-crossing conditions. A climatological comparison of total lightning data observed by the Optical Transient Detector (OTD) and CG lightning observed by the long range network is conducted, with strict quality control and allowance for differential network performance before and after the activation of the Canadian Lightning Detection Network. This yields a first-order geographic estimate of long range network detection efficiency and its spatial variability. Intercomparisons are also performed over the continental US, allowing large scale estimates of the midlatitude climatological IC:CG ratio and its possible dependence on latitude.

Boccippio, Dennis J.↗
NASA NTRS1994

HST Briefing: HST Science Overview

Hubble Space Telescope upgrades are discussed during this overview. Among those discussed are the Space Telescope Imaging Spectograph, the New Infrared Camera, upgrading of instruments with 90's technology, new CCD detectors, corrective optics, reconfiguration of mirrors, reduction in overall exposure time. A question and answer period with Johnson Spaceflight Center, Goddard Spaceflight Center and the press focuses primarily on these upgrades to the Hubble Space Telescope.

Source record↗
NASA NTRS1999

Combined Satellite and Surface-Based Estimation of the Intracloud/ Cloud-to-Ground Lightning Ratio Over the Continental United States

Four years of observations from the NASA Optical Transient Detector (OTD) and Global Atmospherics National Lightning Detection Network (NLDN) are combined to determine the geographic distribution of the intracloud/cloud to ground lightning ratio over the continental United States. The mean ratio over this region is 2.64-2.94, with a standard deviation of 1.1-1.3 and anomalies as low as 1.0 or less over the Rocky and Appalachian mountains and as high as 8-10 in the central-upper midwest. There is some indication that the ratio covaries with ground elevation, although the relationship is nonunique. Little evidence is found to support a latitudinal covariance, despite significant variation in the climatological mean tropopause pressure over the latitudes considered. The dynamic range of local variability is comparable to the range of values cited by previous studies for latitudinal variation from the deep tropics to midlatitudes. Local high anomalies of this ratio in the midwest are coincident with anomalies in the climatological percentage of positive CG occurrence, as well as in the occurrence of large positive CGs characteristic of MCS convective and trailing stratiform regions. This suggests that storm type, morphology and level of organization may dominate over environmental cofactors in the local determination of this ratio.

Boccippio, D. J.↗
NASA NTRS2000

Initial Performance of the Aspect System on the Chandra Observatory: Post-Facto Aspect Reconstruction

The aspect system of the Chandra Observatory plays a key role in realizing the full potential of Chandra's x-ray optics and detectors. To achieve the highest spatial and spectral resolution (for grating observations), an accurate post-facto time history of the spacecraft attitude and internal alignment is needed. The CXC has developed a suite of tools which process sensor data from the aspect camera assembly and gyroscopes, and produce the spacecraft aspect solution. In this poster, the design of the aspect pipeline software is briefly described, followed by details of aspect system performance during the first eight months of flight. The two key metrics of aspect performance are: image reconstruction accuracy, which measures the x-ray image blurring introduced by aspect; and celestial location, which is the accuracy of detected source positions in absolute sky coordinates.

Aldcroft, T.↗
NASA NTRS2000

Global Lightning Variations Caused by Changes in Thunderstorm Flash Rate and by Changes in Number of Thunderstorms

Global lightning activity is highly variable on many time scales. This variability is attributable to changes in the flash rate per thunderstorm, the number of thunderstorms, or a combination. The TRMM Mission offers lightning observations from the Optical Transient Detector (OTD) and the Lightning Imaging Sensor (LIS) in space. Both are used to examine the response of these parameters to thermodynamic forcing of deep convection on the diurnal and annual time scales. On both time scales, the changes in the number of storms dominate the variations in total lightning activity. On the diurnal time scale, the mean flash rate appears to vary with cloud buoyancy, peaking in early afternoon and declining in late afternoon, but the contribution of number of thunderstorms is 2-3 times greater that the mean storm flash rate. On the annual time scale, almost all of the total lightning response is due to changes in the number of storms, with a negligible contribution from flash rate. Evidence is presented that the LIS/OTD 'area' is a meaningful objective identifier for a thunderstorm.

Williams, E. K.↗
NASA NTRS2001

HERO: Program Status and Fist Images from a Balloon-Borne Focusing Hard-X-ray Telescope

HERO is a balloon payload featuring shallow-graze angle replicated optics for hard-x-ray imaging. When completed, the instrument will offer unprecedented sensitivity in the hard-x-ray region, giving thousands of sources to choose from for detailed study on long flights. A recent proof-of-concept flight captured the first hard-x-ray focused images of the Crab Nebula, Cygnus X-1 and GRS 1915+105. Full details of the HERO program are presented, including the design and performance of the optics, the detectors and the gondola. Results from the recent proving flight are discussed together with expected future performance when the full science payload is completed.

Ramsey, B. D.↗
NASA NTRS2003

Global Lightning Activity

Our knowledge of the global distribution of lightning has improved dramatically since the 1995 launch of the Optical Transient Detector (OTD) followed in 1997 by the launch of the Lightning Imaging Sensor (LIS). Together, these instruments have generated a continuous seven-year record of global lightning activity. These lightning observations have provided a new global perspective on total lightning activity. For the first time, total lightning activity (CG and IC) has been observed over large regions with high detection efficiencies and accurate geographic location. This has produced new insights into lightning distributions, times of occurrence and variability. It has produced a revised global flash rate estimate (46 flashes per second) and has lead to a new realization of the significance of total lightning activity in severe weather. Accurate flash rate estimates are now available for large areas of the earth (+/- 72deg latitude) Ocean-land contrasts as a function of season are clearly revealed, as are orographic effects and seasonal and interannual variability. The data set indicates that air mass thunderstorms, not large storm systems dominate global activity. The ability of LIS and OTD to detect total lightning has lead to improved insight into the correlation between lightning and storm development. The relationship between updraft development and lightning activity is now well established and presents an opportunity for providing a new mechanism for remotely monitoring storm development. In this concept, lightning would serve as a surrogate for updraft velocity. It is anticipated hat this capability could lead to significantly improved severe weather warning times and reduced false warning rates.

Christian, Hugh↗
NASA NTRS2003

Understanding the Wind/Ejecta of ETA Carinae with the STIS

The mysterious Luminous Blue Variable, Eta Carinae, and its ejecta, the Homunculus, are yielding much new and exciting information to the Space Telescope Imaging Spectrograph (STIS). With 0.05 in. pixels and R=5000, we have followed changes in the Central Source and the surrounding ejecta over the past 5.5 year period in the spectral region 1650-10300A. Neutral, partially-ionized and highly-excited nebular regions have been resolved. With 0.025 in., R=30000 and R=120000 between 1180 and 3160A, we are following the changes in the Central Source, the wind structure and at least 20 absorption components, thought to be within the wall of the Humunculus. The Central Source proves to be slightly extended at HST angular resolutions. Theoretical models of the nebular line fluxes are providing new insight to the sources of excitation. Photo-excitation, line and continuum pumping, plus shocks, all play a role. Laboratory measurements of Fe-like elements and improved atomic data bases prove necessary for proper interpretation. Increased angular resolution by larger-aperture space telescopes and improved optics and detectors will lead to new understanding of massive stars, their wind structures and history of ejecta.

Gull, Theodore R.↗
NASA NTRS2004

Global Lightning Activity

Our knowledge of the global distribution of lightning has improved dramatically since the advent of spacebased lightning observations. Of major importance was the 1995 launch of the Optical Transient Detector (OTD), followed in 1997 by the launch of the Lightning Imaging Sensor (LIS). Together, these instruments have generated a continuous eight-year record of global lightning activity. These lightning observations have provided a new global perspective on total lightning activity. For the first time, total lightning activity (cloud-to-ground and intra-cloud) has been observed over large regions with high detection efficiency and accurate geographic location. This has produced new insights into lightning distributions, times of occurrence and variability. It has produced a revised global flash rate estimate (44 flashes per second) and has lead to a new realization of the significance of total lightning activity in severe weather. Accurate flash rate estimates are now available over large areas of the earth (+/- 72 deg. latitude). Ocean-land contrasts as a function of season are clearly reveled, as are orographic effects and seasonal and interannual variability. The space-based observations indicate that air mass thunderstorms, not large storm system dominate global activity. The ability of LIS and OTD to detect total lightning has lead to improved insight into the correlation between lightning and storm development. The relationship between updraft development and lightning activity is now well established and presents an opportunity for providing a new mechanism for remotely monitoring storm development. In this concept, lightning would serve as a surrogate for updraft velocity. It is anticipated that this capability could lead to significantly improved severe weather warning times and reduced false warning rates. This talk will summarize our space-based lightning measurements, will discuss how lightning observations can be used to monitor severe weather, and present a concept for continuous geostationary-based lightning observations.

Christian, Hugh J.↗
NASA NTRS2004

Total Lightning Activity as Observed from Space

Our knowledge of the global distribution of lightning has improved dramatically since the 1995 launch of the Optical Transient Detector (OTD), followed in 1997 by the launch of the Lightning Imaging Sensor (LIS). Together, these instruments have generated a continuous seven-year record of global lightning activity. These lightning observations have provided a new global perspective on total lightning activity. For the first time, total lightning activity (CG and IC) has been observed over large regions with high detection efficiencies and accurate geographic location. This has produced new insights into lightning distributions, times of occurrence and variability. It has produced a revised global flash rate estimate (44 flashes per second) and has lead to a new realization of the significance of total ligh&g activity in severe weather. Accurate flash rate estimates are now available for areas of the earth (+/- 72 deg. latitude). Ocean-land contrasts as a function of season are clearly reveled, as are orographic effects and seasonal and interannual variability. The data set indicates that air mass thunderstorms, not large storm system dominate global activity. The ability of LIS and OTD to detect total lightning has lead to improved insight into the correlation between lightning and storm development. The relationship between updraft development and lightning activity is now well established and presents an opportunity for providing a new mechanism for remotely monitoring storm development. In this concept, lightning would serve as a surrogate for updraft velocity. It is anticipated that this capability could lead to significantly improved severe weather warning times and reduced false warning rates.

Christian, Hugh J.↗
NASA NTRS2004

Total Lightning Activity as Observed from Space

ABSTRACT: Our knowledge of the global distribution of lightning has improved dramatically since the 1995 launch of the Optical Transient Detector (OTD), followed in 1997 by the launch of the Lightning Imaging Sensor (LIS). Together, these instruments have generated a continuous seven-year record of global lightning activity. These lightning observations have provided a new global perspective on total lightning activity. For the first time, total lightning activity (CG and IC) has been observed over large regions with high detection efficiencies and accurate geographic location. This has produced new insights into lightning distributions, times of occurrence and variability. It has produced a revised global flash rate estimate (44 flashes per second) and has lead to a new realization of the significance of total ligh&g activity in severe weather. Accurate flash rate estimates are now available for areas of the earth (+/- 72 deg. latitude). Ocean-land contrasts as a function of season are clearly reveled, as are orographic effects and seasonal and interannual variability. The data set indicates that air mass thunderstorms, not large storm system dominate global activity. The ability of LIS and OTD to detect total lightning has lead to improved insight into the correlation between lightning and storm development. The relationship between updraft development and lightning activity is now well established and presents an opportunity for providing a new mechanism for remotely monitoring storm development. In this concept, lightning would serve as a surrogate for updraft velocity. It is anticipated that this capability could lead to significantly improved severe weather warning times and reduced false warning rates.

Christian, Hugh J.↗
NASA NTRS2000

A Passive Method for Detecting Vegetation Stress from Orbit: Chlorophyll Fluorescence Spectra from Fraunhofer Lines

Solar-stimulated chlorophyll fluorescence measured with the Fraunhofer line depth method has correlated well with vegetation stress in previous studies. However, the instruments used in those studies were limited to a single solar absorption line (e.g. 656.3 nm), obviating the red/far-red ratio (R/FR) method. Optics and detector technology have reached the level whereby multiple, very narrow Fraunhofer lines are resolvable. Thirteen such lines span the visible spectrum in the red to far-red region where chlorophyll fluorescence occurs. Fluorescence intensities at the 13 Fraunhofer line wavelengths were used to model emission spectra. The source data were collected for summer and fall bean crops (Phaseolus vulgaris L.) subjected to various levels of nitrogen fertilization. The intensities were adjusted to account for Fraunhofer line depth and atmospheric transmittance. Multiple R/FR fluorescence ratios, calculated from the modeled fluorescence spectra, correlated strongly with leaf chlorophyll concentration and well with applied nitrogen. The ratio yielding the best correlation with chlorophyll utilized red fluorescence at the 694.5 nm Fraunhofer line and farred fluorescence at the 755.6 nm Fraunhofer line. Twenty R/FR ratios, each evaluated for the maximum differential between low and high (optimal) nitrogen treatments, ranked higher in some cases and lower in others, possibly related to the time of year the crops were grown and the stage of growth of the crops. Ratios with 728.9 nm and 738.9 nm in the denominator consistently ranked in the lowest and next lowest quartile, respectively. Ratios of the 656.3 nm Fraunhofer line and the 755.6 nm line consistently ranked highest for the summer crop. Ratios with 755.6 nm in the denominator ranked in the upper quartile for 10 out of 12 measurement dates. Differences in ratio ranking indicate that physiological conditions may be estimated using selected ratios of Fraunhofer lines within the context of R/FR analysis. A passive instrument designed to monitor R/FR chlorophyll fluorescence (i.e. vegetation stress) from orbit could be built today.

Theisen, Arnold F.↗
NASA NTRS2004

Lightning Optical Pulse Statistics from Storm Overflights During the Altus Cumulus Electrification Study

The Altus Cumulus Electrification Study (ACES) was conducted during the month of August, 2002 in an area near Key West, Florida. One of the goals of this uninhabited aerial vehicle (UAV) study was to collect time resolved optical pulse data from thunderstorms. During the month long campaign, we acquired 5294 lightning generated optical pulses. Most of these observations were made while close to the top of the storms. We divided our data into two amplitude groups based on prior NASA U2 aircraft optical data and our pulse characteristics. The group of large pulses with radiance greater than 2.1 mW /sq m sr had mean and median 10 - 10% optical pulse widths of 765 and 735 microns respectively, the 50-50% pulse widths of 396 and 355 microns respectively, and 10-90% rise times of 290 and 260 microns. These values are very similar to the previous U2 based optical results The other group of pulses consisting of slightly more than a quarter of the total pulses observed had radiances less than the minimum values detected in the U2 study. The small pulses were narrower than the large pulses with 5040% mean and median values of 198 and 160 ps respectively. Only 12 % of the flashes contained only small pulses, minimizing the impact of this data on the estimates of detection efficiencies of the orbital instruments, the Lightning Imaging Sensor and Optical Transient Detector.

Mach, D. M.↗
NASA NTRS2005

Global Lightning Activity

Our knowledge of the global distribution of lightning has improved dramatically since the advent of space-based lightning observations. Of major importance was the 1995 launch of the Optical Transient Detector (OTD), followed in 1997 by the launch of the Lightning Imaging Sensor (LIS). Together, these instruments have generated a continuous nine-year record of global lightning activity. These lightning observations have provided a new global perspective on total lightning activity. For the first time, total lightning activity (cloud-to-ground and intra-cloud) has been observed over large regions with high detection efficiency and accurate geographic location. This has produced new insights into lightning distributions, times of occurrence and variability. It has produced a revised global flash rate estimate (45 flashes per second) and has lead to a new realization of the significance of total lightning activity in severe weather. Additional information is included in the original extended abstract.

Christian, Hugh J., Jr.↗
NASA NTRS2005

Global Lightning Activity

Our knowledge of the global distribution of lightning has improved dramatically since the advent of space-based lightning observations. Of major importance was the 1995 launch of the Optical Transient Detector (OTD), followed in 1997 by the launch of the Lightning Imaging Sensor &IS). Together, these instruments have generated a continuous nine-year record of global lightning activity. These lightning observations have provided a new global perspective on total lightning activity. For the first time, total lightning activity (cloud-to-ground and intra-cloud) has been observed over large regions with high detection efficiency and accurate geographic location. This has produced new insights into lightning distributions, times of occurrence and variability. It has produced a revised global flash rate estimate (45 flashes per second) and has lead to a new realization of the significance of total lightning activity in severe weather.

Christian, H. J.↗
NASA NTRS2005

Pre-Launch GOES-R Risk Reduction Activities for the Geostationary Lightning Mapper

The GOES-R Geostationary Lightning Mapper (GLM) is a new instrument planned for GOES-R that will greatly improve storm hazard nowcasting and increase warning lead time day and night. Daytime detection of lightning is a particularly significant technological advance given the fact that the solar illuminated cloud-top signal can exceed the intensity of the lightning signal by a factor of one hundred. Our approach is detailed across three broad themes which include: Data Processing Algorithm Readiness, Forecast Applications, and Radiance Data Mining. These themes address how the data will be processed and distributed, and the algorithms and models for developing, producing, and using the data products. These pre-launch risk reduction activities will accelerate the operational and research use of the GLM data once GOES-R begins on-orbit operations. The GLM will provide unprecedented capabilities for tracking thunderstorms and earlier warning of impending severe and hazardous weather threats. By providing direct information on lightning initiation, propagation, extent, and rate, the GLM will also capture the updraft dynamics and life cycle of convective storms, as well as internal ice precipitation processes. The GLM provides information directly from the heart of the thunderstorm as opposed to cloud-top only. Nowcasting applications enabled by the GLM data will expedite the warning and response time of emergency management systems, improve the dispatch of electric power utility repair crews, and improve airline routing around thunderstorms thereby improving safety and efficiency, saving fuel and reducing delays. The use of GLM data will assist the Bureau of Land Management (BLM) and the Forest Service in quickly detecting lightning ground strikes that have a high probability of causing fires. Finally, GLM data will help assess the role of thunderstorms and deep convection in global climate, and will improve regional air quality and global chemistry/climate modeling. The GLM has a robust design that benefits and improves upon its strong heritage of NASA-developed LEO predecessors, the Optical Transient Detector (OTD) and the Lightning Imaging Sensor (LIS). GLM will have a substantially larger number of pixels within the focal plane, two lens systems, and multiple Real-Time Event Processors REPS for on-board event detection and data compression to provide continuous observations of the Americas and adjacent oceans.

Goodman, S. J.↗
NASA NTRS2005

Lightning NO(x) Production and Its Consequences for Tropospheric Chemistry

Cloud-resolving case-study simulations of convective transport and lightning NO production have yielded results which are directly applicable to the design of lightning parameterizations for global chemical transport models. In this work we have used cloud-resolving models (the Goddard Cumulus Ensemble Model (GCE) and MMS) to drive an off-line cloud-scale chemical transport model (CSCTM). The CSCTM, in conjunction with aircraft measurements of NO(x) in thunderstorms and ground-based lightning observations, has been used to constrain the amount of NO produced per flash. Cloud and chemistry simulations for several case studies of storms in different environments will be presented. Observed lightning flash rates have been incorporated into the CSCTM, and several scenarios of NO production per intracloud (IC) and per cloud-to-ground (CG) flash have been tested for each storm. The resulting NO(x) mixing ratios are compared with aircraft measurements taken within the storm (typically the anvil region) to determine the most likely NO production scenario. The range of values of NO production per flash (or per meter of lightning channel length) that have been deduced from the model will be shown and compared with values of production in the literature that have been deduced from observed NO spikes and from anvil flux calculations. Results show that on a per flash basis, IC flashes are nearly as productive of NO as CG flashes. This result simplifies the lightning parameterization for global models (ie., an algorithm for estimating the ICKG ratio is not necessary). Vertical profiles of lightning NOx mass at the end of the 3-D storm simulations have been summarized to yield suggested profiles for use in global models. Estimates of mean NO production per flash vary by a factor of three from one simulated storm to another. When combined with the global flash rate of 44 flashes per second from NASA's Optical Transient Detector (OTD) measurements, these estimates and the results from other techniques yield global NO production rates of 2-9 TgN/year. Simulations of the photochemistry over the 24 hours following a storm has been performed to determine the additional ozone production which can be attributed to lightning NO. Convective transport of HO(x), precursors leads to the generation of a HO(x), plume which substantially aids the downstream ozone production.

Pickering, Kenneth E.↗