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All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

Shielded Heavy-Ion Environment Linear Detector (SHIELD): an experiment for the Radiation and Technology Demonstration (RTD) Mission

Radiological assessment of the many cosmic ion species of widely distributed energies requires the use of theoretical transport models to accurately describe diverse physical processes related to nuclear reactions in spacecraft structures, planetary atmospheres and surfaces, and tissues. Heavy-ion transport models that were designed to characterize shielded radiation fields have been validated through comparison with data from thick-target irradiation experiments at particle accelerators. With the RTD Mission comes a unique opportunity to validate existing radiation transport models and guide the development of tools for shield design. For the first time, transport properties will be measured in free-space to characterize the shielding effectiveness of materials that are likely to be aboard interplanetary space missions. Target materials composed of aluminum, advanced composite spacecraft structure and other shielding materials, helium (a propellant) and tissue equivalent matrices will be evaluated. Large solid state detectors will provide kinetic energy and charge identification for incident heavy-ions and for secondary ions created in the target material. Transport calculations using the HZETRN model suggest that 8 g cm -2 thick targets would be adequate to evaluate the shielding effectiveness during solar minimum activity conditions for a period of 30 days or more.

NASA Center LaRC↗
NASA NTRS2010

Optical Imaging and Radiometric Modeling and Simulation

OPTOOL software is a general-purpose optical systems analysis tool that was developed to offer a solution to problems associated with computational programs written for the James Webb Space Telescope optical system. It integrates existing routines into coherent processes, and provides a structure with reusable capabilities that allow additional processes to be quickly developed and integrated. It has an extensive graphical user interface, which makes the tool more intuitive and friendly. OPTOOL is implemented using MATLAB with a Fourier optics-based approach for point spread function (PSF) calculations. It features parametric and Monte Carlo simulation capabilities, and uses a direct integration calculation to permit high spatial sampling of the PSF. Exit pupil optical path difference (OPD) maps can be generated using combinations of Zernike polynomials or shaped power spectral densities. The graphical user interface allows rapid creation of arbitrary pupil geometries, and entry of all other modeling parameters to support basic imaging and radiometric analyses. OPTOOL provides the capability to generate wavefront-error (WFE) maps for arbitrary grid sizes. These maps are 2D arrays containing digital sampled versions of functions ranging from Zernike polynomials to combination of sinusoidal wave functions in 2D, to functions generated from a spatial frequency power spectral distribution (PSD). It also can generate optical transfer functions (OTFs), which are incorporated into the PSF calculation. The user can specify radiometrics for the target and sky background, and key performance parameters for the instrument s focal plane array (FPA). This radiometric and detector model setup is fairly extensive, and includes parameters such as zodiacal background, thermal emission noise, read noise, and dark current. The setup also includes target spectral energy distribution as a function of wavelength for polychromatic sources, detector pixel size, and the FPA s charge diffusion modulation transfer function (MTF).

Ha, Kong Q.↗
NASA NTRS2017

Methods for Reducing Singly Reflected Rays on the Wolter-I Focusing Figures of the FOXSI Rocket Experiment

In high energy solar astrophysics, imaging hard X-rays by direct focusing offers higher dynamic range and greater sensitivity compared to past techniques that used indirect imaging. The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket payload which uses seven sets of nested Wolter-I figured mirrors that, together with seven high-sensitivity semiconductor detectors, observes the Sun in hard X-rays by direct focusing. The FOXSI rocket has successfully flown twice and is funded to fly a third time in Summer 2018. The Wolter-I geometry consists of two consecutive mirrors, one paraboloid, and one hyperboloid, that reflect photons at grazing angles. Correctly focused X-rays reflect twice, once per mirror segment. For extended sources, like the Sun, off-axis photons at certain incident angles can reflect on only one mirror and still reach the focal plane, generating a pattern of single-bounce photons that can limit the sensitivity of the observation of faint focused X-rays. Understanding and cutting down the singly reflected rays on the FOXSI optics will maximize the instrument's sensitivity of the faintest solar sources for future flights. We present an analysis of the FOXSI singly reflected rays based on ray-tracing simulations, as well as the effectiveness of different physical strategies to reduce them.

FOXSI Rocket Experiment↗
NASA NTRS2020

Extension of the energy range accessible with a TES using bath temperature variations

The energy range of transition-edge sensor (TES) X-ray microcalorimeters with a multiplexed readout depends upon the width and shape of the TES superconducting transition, and also on the dynamic range of the readout. In many detector systems, the multiplexed readout slew rate capability will be the limiting factor for the energy range. In these cases, if we are willing to accept some energy resolution degradation, we can significantly extend the energy range by increasing the bath temperature of operation, essentially creating a second “extended energy range” mode of operation. For example, if we require the very highest energy resolution up to 7 keV, and wish to optimize the design up to this energy, for some measurements it could be very beneficial to have a mode where we can extend the energy range to 15–20 keV even if some energy resolution is sacrificed. In this paper, we explore the trade-off between dynamic range and energy resolution from changing the bath temperature of the TES. We present measurements of TES resolution and slew rate as a function of bath temperature and compare to numerical simulations.

S Beaumont↗
NASA NTRS2024

Thermal Inspection of Low Emissivity Surfaces Using a Pulsed Light Emitting Diodes (PLED) Heat Source

Thermal inspections of a structure typically utilize a flash or quartz lamp heat source located on the same side of an infrared camera. The heat source provides light energy for heating while the infrared camera measures the surface transient temperature response. The inspection can be difficult for low emissivity surfaces for several reasons. First, the high intensity light can reflect off the surface and cause “burn-in” to the camera’s detector. The “burn-in” can take time for the sensors to recover and potentially damage the detector. Secondly, the heat source after pulsing, has a transient cool down component. The cool down component can be reflected and therefore superimposed over the structure’s thermal response which can cause an error (false defect indications) in the inspection. Lastly, the heat source is spectrally broad and therefore while heating, infrared components of the heat source can produce non-uniformity in the measured temperature field. Typically for the inspection of reflective surfaces, paint or other emissivity enhancing coatings are applied before inspection. In this paper, a pulsed light emitting diodes (PLED) heat source is used. The PLED heat source is spectrally narrow, contained within the visible band, and therefore not detectable by the infrared camera. The PLED heat source is configured to reduce any transient cool down components that could produce false defect indications. The PLED thermal inspections are compared to commercially available flash thermography inspections on unpainted aluminum samples with simulated corrosion and additively manufactured Ti-6AL-4V metal specimens.

thermal nondestructive evaluation↗
NASA NTRS2024

Thermal Inspection of Low Emissivity Surfaces Using a Pulsed Light Emitting Diodes (PLED) Heat Source

Thermal inspections of a structure typically utilize a flash or quartz lamp heat source located on the same side of an infrared camera. The heat source provides light energy for heating while the infrared camera measures the surface transient temperature response. The inspection can be difficult for low emissivity surfaces for several reasons. First, the high intensity light can reflect off the surface and cause “burn-in” to the camera’s detector. The “burn-in” can take time for the sensors to recover and potentially damage the detector. Secondly, the heat source after pulsing, has a transient cool down component. The cool down component can be reflected and therefore superimposed over the structure’s thermal response which can cause an error (false defect indications) in the inspection. Lastly, the heat source is spectrally broad and therefore while heating, infrared components of the heat source can produce non-uniformity in the measured temperature field. Typically for the inspection of reflective surfaces, paint or other emissivity enhancing coatings are applied before inspection. In this paper, a pulsed light emitting diodes (PLED) heat source is used. The PLED heat source is spectrally narrow, contained within the visible band, and therefore not detectable by the infrared camera. The PLED heat source is configured to reduce any transient cool down components that could produce false defect indications. The PLED thermal inspections are compared to commercially available flash thermography inspections on unpainted aluminum samples with simulated corrosion and additively manufactured Ti-6AL-4V metal specimens.

thermal nondestructive evaluation↗
NASA NTRS1993

Remote sounding of tropospheric minor constituents

The etalon interferometer, or Fabry-Perot interferometer (FPI), with its high throughput and high spectral resolution was widely used in the remote-sensing measurements of the earth's atmospheric composition, winds, and temperatures. The most recent satellite instruments include the Fabry-Perot interferometer flown on the Dynamics Explorer-2 (DE-2) and the High Resolution Doppler Imager (HRDI) to be flown on the Upper Atmosphere Research Satellite (UARS). These instruments measure the Doppler line profiles of the emission and absorption of certain atmospheric species (such as atomic oxygen) in the visible spectral region. The successful space flight of DE-FPI and the test and delivery of UARS-HRDI demonstrated the extremely high spectral resolution and ruggedness of the etalon system for the remote sensing of earth and planetary atmospheres. Recently, an innovative FPI focal plane detection technique called the Circle-to-Line Interferometer Optical (CLIO) system was invented at the Space Physics Research Laboratory (SPRL). The CLIO simplifies the FPI focal plane detection process by converting the circular rings or fringes into a linear pattern similar to that produced by a conventional spectrometer, while retaining the throughput advantage of the etalon interferometer. CLIO makes the use of linear array detectors more practical and efficient with FPI, the combination of FPI and CLIO represents a very promising new technique for the remote sensing of the lower atmospheres of Earth, Mars, Venus, Neptune, and other planets. The Multiorder Etalon Spectrometer (MOES), as a combination of the rugged etalon and the CLIO, compares very favorably to other spaceborne optical instruments in terms of performance versus complexity. The feasibility of an advanced etalon spectrometer for the remote sensing of tropospheric trace species, particularly carbon monoxide (CO), nitrous oxide (N2O), and methane (CH4) was discussed. The etalon atmospheric spectroscopy techniques are described, instrument design and related technical issues are discussed. The primary objective is to establish the concept of atmospheric spectroscopy with the CLIO and etalon system and its applications for the measurements of tropospheric trace species analyze system requirements and performance, determine the feasibility of components and subsystem implementation with available technology, and develop inversion algorithm for retrieval simulation and data analysis.

Drayson, S. Roland↗
NASA NTRS2020

Improvements of On-orbit Characterization of Terra MODIS Short-wave Infrared Spectral Bands Out-of-Band Responses

The short-wave infrared (SWIR) bands (5-7, 26) of Terra MODIS, which are co-located with the mid-wav e infrared (MWIR) bands (20-25) on the short and mid-wave infrared (SMIR) Focal Plane Assembly (FPA),have a known issue related to 5.3 μm out-of-band (OOB) thermal leak and electronic crosstalk that was identified prelaunch. As a result, a crosstalk correction algorithm was designed and implemented in the MODIS Level 1B (L1B) calibration. Shortly after the Terra launch, extensive efforts were under taken to help characterize and mitigate the impact due to the OOB response and crosstalk on the SWIR on-orbit calibration and, consequently, the associated L1Bdata products. In addition, special night time day mode (NTDM) operations have been regularly scheduled to derive the crosstalk correction coefficients. Since MODIS does not have a spectral band centered at 5.3 μm, its band 28 (7.325 μm) was chosen as the surrogate sending band to simulate the OOB radiances at 5.3 μm. This was largely based on the measurements from the MODIS Airborne Simulator (MAS) spectrometer field campaigns in the early months after the Terra launch. In the case of Aqua MODIS, the magnitude of the SWIR crosstalk was much smaller and band 25 (4.52 μm) was found to be more effective as the sending band for the crosstalk correction. In recent years, the Terra MODIS photovoltaic (PV) long-wave infrared (LWIR) bands (27-30)electronic crosstalk has increased considerably, especially after the spacecraft safe-mode event occurred in February2016. This accentuated degradation in the PV LWIR performance has also impacted the performance of the SWIR crosstalk correction and thus its calibration and data quality. In this paper, we examine the use of band 25 as the sending band for the Terra MODIS SWIR crosstalk correction and compare its performance with that based on band 28 as the sending band. Results indicate an improvement of on-orbit gain stability for the SWIR calibration and reduced detector to detector and sub-frame to sub-frame striping in the calibrated L1B imagery, especially during the period when the PV LWIR electronic crosstalk has become more severe. This approach has been implemented in the forward production of Terra MODIS Collection 6 and Collection 6.1,starting from July 2019, and is planned to be used for the future reprocessing of MODIS L1B and to help improve the mission-long reflectance calibration and trending stability of the SWIR bands.

MODIS, Terra, Aqua, MODIS, SWIR, calibration, out-↗
NASA NTRS2019

Innovative Compact Coronagraph Approach for Balloon-Borne Investigation of Temperature and Speed of Electrons in the Corona (BITSE)

We are developing an innovative compact coronagraph for studying the physical conditions in the solar wind acceleration region. This paper presents the new development of the compact coronagraph for the investigation of temperature and speed of electrons in the solar corona. The proposed compact coronagraph is a one stage externally occulted coronagraph without internal occulter or Lyot stop mask. The key of the new idea is to set the inner field cutoff at the External Occulter (EO) much smaller than the specified inner field cutoff. A second occulter on the surface of the detector array removes the remaining diffraction. The occulter on the detector surface functions similar to an internal occulter with the Inner Field of View Cutoff (IFoVC) exactly the same as specified. For BITSE, the desired inner field cutoff is 3 R, but the cutoff at EO is only 1.5 R☉. The diffraction analysis shows that in the sensor plane, the diffraction intensity at the 3 R☉ is not sensitive to the EO cutoff, either at 1.5 R☉ or close to 3 R☉. The advantage of having a smaller EO cutoff is that the vignetting decreased for the Field of View (FoV) near 3 R☉, therefore, the signal increases. Meanwhile, the diffraction of Point Spread Function is much less in the radial direction, which not only increases the image quality around 3 R☉, but also increases the encircled energy and signal to noise ratio. In other words, the data is useful right at 3 R☉! The BITSE optical design and diffraction analysis will be presented in detail. The simulation shows the signal to noise ratio obtained from the diffraction and vignetting data enables corona temperature and speed measurement.

Gong, Qian↗
NASA NTRS2019

Innovative Compact Coronagraph Approach for Balloon-Borne Investigation of Temperature and Speed of Electrons in the Corona (BITSE)

We are developing an innovative compact coronagraph for studying the physical conditions in the solar wind acceleration region. This paper presents the new development of the compact coronagraph for the investigation of temperature and speed of electrons in the solar corona. The proposed compact coronagraph is a one stage externally occulted coronagraph without internal occulter or Lyot stop mask. The key of the new idea is to set the inner field cutoff at the External Occulter (EO) much smaller than the specified inner field cutoff. A second occulter on the surface of the detector array removes the remaining diffraction. The occulter on the detector surface functions similar to an internal occulter with the Inner Field of View Cutoff (IFoVC) exactly the same as specified. For BITSE, the desired inner field cutoff is 3 R, but the cutoff at EO is only 1.5 R☉. The diffraction analysis shows that in the sensor plane, the diffraction intensity at the 3 R☉ is not sensitive to the EO cutoff, either at 1.5 R☉ or close to 3 R☉. The advantage of having a smaller EO cutoff is that the vignetting decreased for the Field of View (FoV) near 3 R☉, therefore, the signal increases. Meanwhile, the diffraction of Point Spread Function is much less in the radial direction, which not only increases the image quality around 3 R☉, but also increases the encircled energy and signal to noise ratio. In other words, the data is useful right at 3 R☉! The BITSE optical design and diffraction analysis will be presented in detail. The simulation shows the signal to noise ratio obtained from the diffraction and vignetting data enables corona temperature and speed measurement.

Gong, Qian↗
NASA NTRS1999

Hardware and Methods of the Optical End-to-End Test of the Far Ultraviolet Spectroscopic Explorer (FUSE)

The Far Ultraviolet Spectroscopic Explorer (FUSE), currently being tested and scheduled for a 1999 launch, is an astrophysics satellite designed to provide high spectral resolving power (Lambda/(Delta)Lambda = 24,000-30,000) over the interval 90.5-118.7 nm. The FUSE optical path consists of four co-aligned, normal incidence, off-axis parabolic, primary mirrors which illuminate separate Rowland circle spectrograph channels equipped with holographic gratings and delay line microchannel plate detectors. We describe the hardware and methods used for the optical end-to-end test of the FUSE instrument during satellite integration and test. Cost and schedule constraints forced us to devise a simplified version of the planned optical test which occurred in parallel with satellite thermal-vacuum testing. The optical test employed a collimator assembly which consisted of four co-aligned, 15" Cassegrain telescopes which were positioned above the FUSE instrument, providing a collimated beam for each optical channel. A windowed UV light source, remotely adjustable in three axes, was mounted at the focal plane of each collimator. Problems with the UV light sources, including high F-number and window failures, were the only major difficulties encountered during the test. The test succeeded in uncovering a significant problem with the secondary structure used for the instrument closeout cavity and, furthermore, showed that the mechanical solution was successful. The hardware was also used extensively for simulations of science observations, providing both UV light for spectra and visible light for the fine error sensor camera.

Conard, Steven J.↗
NASA NTRS1995

Calibration of the advanced microwave sounding unit-A for NOAA-K

The thermal-vacuum chamber calibration data from the Advanced Microwave Sounding Unit-A (AMSU-A) for NOAA-K, which will be launched in 1996, were analyzed to evaluate the instrument performance, including calibration accuracy, nonlinearity, and temperature sensitivity. The AMSU-A on NOAA-K consists of AMSU-A2 Protoflight Model and AMSU-A1 Flight Model 1. The results show that both models meet the instrument specifications, except the AMSU-A1 antenna beamwidths, which exceed the requirement of 3.3 +/- 10%. We also studied the instrument's radiometric characterizations which will be incorporated into the operational calibration algorithm for processing the in-orbit AMSU-A data from space. Particularly, the nonlinearity parameters which will be used for correcting the nonlinear contributions from an imperfect square-law detector were determined from this data analysis. It was found that the calibration accuracies (differences between the measured scene radiances and those calculated from a linear two-point calibration formula) are polarization-dependent. Channels with vertical polarizations show little cold biases at the lowest scene target temperature 84K, while those with horizontal polarizations all have appreciable cold biases, which can be up to 0.6K. It is unknown where these polarization-dependent cold biases originate, but it is suspected that some chamber contamination of hot radiances leaked into the cold scene target area. Further investigation in this matter is required. The existence and magnitude of nonlinearity in each channel were established and a quadratic formula for modeling these nonlinear contributions was developed. The model was characterized by a single parameter u, values of which were obtained for each channel via least-squares fit to the data. Using the best-fit u values, we performed a series of simulations of the quadratic corrections which would be expected from the space data after the launch of AMSU-A on NOAA-K. In these simulations, the cosmic background radiance corresponding to a cold space temperature 2.73K was adopted as one of the two reference points of calibration. The largest simulated nonlinear correction is about 0.3K, which occurs at channel 15 when the instrument temperature is at 38.09 deg C. Others are less than 0.2K in the remaining channels. Possible improvement for future instrument calibration is also discussed.

Mo, Tsan↗
NASA NTRSDate not supplied

Assessing the feasibility of a spaceborne 3D lightning observing concept

The distribution of electrical charge in thunderclouds results from thermodynamic, microphysical, and kinematic processes, which also modulate thunderstorm evolution. It is no surprise that the connection between lightning and these physical processes is so strong that the increase and vertical growth of lightning activity closely follows the vertical growth of the thundercloud, but unraveling these connections is not trivial and requires observations of the three-dimensional (3D) structure of electrical activity in a cloud. Ground-based 3D lightning mapping networks give excellent 3D flash-level detail but are limited to regional coverage. Satellite-based optical lightning mappers give excellent global coverage but are largely limited to 2D summaries of flash rate and radiant intensity, albeit new flash products and stereographic techniques are chipping away this limitation. New observing strategies are needed to expand and diversify the corpus of 3D lightning datasets and motivate studies that unravel connections lightning has with these key physical processes and the surrounding environment. This study examines the feasibility of using a distributed network of orbing satellites with VHF-based lightning detectors to obtain global maps of 3D lightning activity and assess efficacy of this approach for use in a new, small satellite mission concept called CubeSpark. CubeSpark combines new VHF and high-resolution, bispectral optical instruments on a constellation of low-Earth orbiting (LEO) satellites to globally map the 3D electrical structure of thunderstorms and study how it relates to thunderstorm evolution, extreme weather, nitrogen oxide production and distribution, upper atmospheric electrical phenomena, and how 3D flash observations can complement existing satellite-based lightning mappers and improve decision support tools. To locate lightning discharges, CubeSpark seeks to use the VHF time-of-arrival technique, similar to ground-based total lightning mapping networks. The vertical location accuracy of these satellite retrievals will be of poorer quality compared to a ground-based network, which has non-trivial implications for lightning flash reconstruction and lightning-based interpretations of deep convection. We adapt a Lightning Mapping Array (LMA) simulation framework to an orbiting network and use it to address feasibility of 3D lightning detection from space with particular attention to location accuracy of VHF detections of lightning in the vertical. These simulations inform a constellation design study that defines a realistic orbital configuration and depicts the global coverage for CubeSpark. Results indicate that a 3D location accuracy of <1-2 km for each dimension can be achieved across 300-500 km wide swaths, which suggests that CubeSpark can resolve the charge structure of thunderclouds from the tropics to the mid- and high- latitudes.

Lightning↗
NASA NTRS2016

Optics of Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII): Delay Lines and Alignment

We present the optics of Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) as it gets ready for launch. BETTII is an 8-meter baseline far-infrared (30-90 microns) interferometer mission with capabilities of spatially resolved spectroscopy aimed at studying star formation and galaxy evolution. The instrument collects light from its two arms, makes them interfere, divides them into two science channels (30-50 microns and 60-90 microns), and focuses them onto the detectors. It also separates out the NIR light (1-2.5 microns) and uses it for tip-tilt corrections of the telescope pointing. Currently, all the optical elements have been fabricated, heat treated, coated appropriately and are mounted on their respective assemblies. We are presenting the optical design challenges for such a balloon borne spatio-spectral interferometer, and discuss how they have been mitigated. The warm and cold delay lines are an important part of this optics train. The warm delay line corrects for path length differences between the left and the right arm due to balloon pendulation, while the cold delay line is aimed at introducing a systematic path length difference, thereby generating our interferograms from where we can derive information about the spectra. The details of their design and the results of the testing of these opto-mechanical parts are also discussed. The sensitivities of different optical elements on the interferograms produced have been determined with the help of simulations using FRED software package. Accordingly, an alignment plan is drawn up which makes use of a laser tracker, a CMM, theodolites and a LUPI interferometer.

Dhabal, Arnab↗
NASA NTRS2004

An Acquisition Control for the Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna (LISA) mission is a planned gravitational wave detector consisting of three spacecraft in heliocentric orbit. Laser interferometry is used to measure distance fluctuations between test masses aboard each spacecraft to the picometer level over a 5 million kilometer separation. Each spacecraft has two incoming and two outgoing laser beams for a total of six laser links. These links will have to be established sequentially at the start of the mission, and the spacecraft control systems must aim their lasers at each other with pointing motions less than 8 nanoradians per root Hertz in the frequency band 0.1-100 mHz. This paper presents a strategy for the laser acquisition process. The outgoing beam on one spacecraft is spoiled to provide a wide beam that encompasses the accuracy (noise and mounting bias) of the star tracker. The control system then takes advantage of an array of sensors with increasing sensitivity to quiet the receiving spacecraft and lock the laser. This process is carried out for each of the six links of LISA. A complete analysis and simulation of the acquisition process are presented.

Maghami, Peiman G.↗
NASA NTRS2008

New Insights into the Composition and Texture of Lunar Regolith Using Ultrafast Automated Electron-Beam Analysis

Sieved grain mounts of Apollo 16 drive tube samples have been examined using QEMSCAN - an innovative electron beam technology. By combining multiple energy-dispersive X-ray detectors, fully automated control, and off-line image processing, to produce digital mineral maps of particles exposed on polished surfaces, the result is an unprecedented quantity of mineralogical and petrographic data, on a particle-by-particle basis. Experimental analysis of four size fractions (500-250 microns, 150-90 microns, 75-45 microns and < 20 microns), prepared from two samples (64002,374 and 64002,262), has produced a robust and uniform dataset which allows for the quantification of mineralogy; texture; particle shape, size and density; and the digital classification of distinct particle types in each measured sample. These preliminary data show that there is a decrease in plagioclase modal content and an opposing increase in glass modal content, with decreasing particle size. These findings, together with data on trace phases (metals, sulphides, phosphates, and oxides), provide not only new insights into the make-up of lunar regolith at the Apollo 16 landing site, but also key physical parameters which can be used to design lunar simulants, and compute Figures of Merit for each material produced.

Rickman, Doug↗
NASA NTRS1996

An Evaluation of the Measurement Requirements for an In-Situ Wake Vortex Detection System

Results of a numerical simulation are presented to determine the feasibility of estimating the location and strength of a wake vortex from imperfect in-situ measurements. These estimates could be used to provide information to a pilot on how to avoid a hazardous wake vortex encounter. An iterative algorithm based on the method of secants was used to solve the four simultaneous equations describing the two-dimensional flow field around a pair of parallel counter-rotating vortices of equal and constant strength. The flow field information used by the algorithm could be derived from measurements from flow angle sensors mounted on the wing-tip of the detecting aircraft and an inertial navigation system. The study determined the propagated errors in the estimated location and strength of the vortex which resulted from random errors added to theoretically perfect measurements. The results are summarized in a series of charts and a table which make it possible to estimate these propagated errors for many practical situations. The situations include several generator-detector airplane combinations, different distances between the vortex and the detector airplane, as well as different levels of total measurement error.

Fuhrmann, Henri D.↗
NASA NTRS2005

MOVPE of GaSb/InGaAsSb Multilayers and Fabrication of Dual Band Photodetectors

Metalorganic vapor phase epitaxy (MOVPE) of GaSb/InGaAsSb multilayer thin films and fabrication of bias-selectable dual band photodetectors are reported. For the dual band photodetectors the short wavelength detector, or the upper p- GaSb/n-GaSb junction photodiode, is placed optically ahead of the long wavelength one, or the lower photodiode. The latter is based on latticed-matched In0.13Ga0.87As0.11Sb0.89 with bandgap near 0.6 eV. Specifically, high quality multilayer thin films are grown sequentially from top to bottom as p+-GaSb/p-GaSb/n-GaSb/n-InGaAsSb/p-InGaAsSb/p-GaSb on undoped p-type GaSb substrate, and as n-GaSb/p-GaSb/p-InGaAsSb/n-InGaAsSb/n-GaSb on Te-doped n-type GaSb substrate respectively. The multilayer thin films are characterized by optical microscope, atomic force microscope (AFM), electron microprobe analyses etc. The photodiode mesa steps are patterned by photolithography with wet chemical etching and the front metallization is carried out by e-beam evaporation with Pd/Ge/Au/Ti/Au to give ohmic contact on both n- and p-type Sb based layer surfaces. Dark I-V measurements show typical diode behavior for both the upper and lower photodiodes. The photoresponsivity measurements indicate that both the upper and lower photodiodes can sense the infrared illumination corresponding to their cutoff wavelengths respectively, comparable with the simulation results. More work is underway to bring the long wavelength band to the medium infrared wavelength region near 4 micrometers.

Xiao, Ye-Gao↗