Search NASA⌕ Search

SEARCH · Search NASA

Results for “Scan Mirror”

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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS2022

Thermal Design of the Thermal Infrared Spectrometer (TIRS) Instrument on PREFIRE

The Polar Radiant Energy in the Far-Infrared Experiment (PREFIRE) consists of two 6U CubeSats that each host a Thermal Infrared Spectrometer (TIRS) instrument. The CubeSats are scheduled for launch in March and October of 2023. The purpose of the TIRS instrument is to make spectrally resolved measurements of the Earth’s thermal radiation from the top of the atmosphere. PREFIRE will document, for the first time, variability in spectral fluxes from 4 to 54 microns on hourly to seasonal timescales. The primary mission is 12 months in a polar orbit with inclination of 82 to 98 at altitudes between 450 km and 650 km. The TIRS instrument is an Offner spectrometer that uses a thermopile detector array and a Schwarzchild telescope. In addition, there is a scan mirror which is periodically actuated for calibration. The instrument concept of operation is to collect radiation emitted towards zenith in a nadir sounding orientation with periodic internal and space calibrations. The TIRS thermal control architecture consists of entirely passive elements. The instrument requires all elements be maintained between 0 C and 50 C during operation for the duration of the mission. An overview of the overall thermal control design approach is presented.

Andrade, Andres↗
NASA NTRSDate not supplied

Using Scattered Light From Nadir Port to Aid the Terra and Aqua MODIS Reflective Solar Band Calibration

The MODIS instruments on the Terra and Aqua spacecraft employ a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) system to calibrate their reflective solar bands (RSBs), covering a spectral range from 0.4 to 2.1 µm. The UV exposure of the SD, from its sun-view port as well as the scattered light (sunlight reflected from top-of-atmosphere), has led to a wavelength dependent degradation of the SD, with larger degradation observed at shorter wavelengths. The scatter off the diffuser onto the scan mirror is in the forward direction, whereas the scatter off the diffuser onto the SDSM fold mirror is in the backward direction. Since the outgoing angles (viewed by MODIS detectors) are the same as the scheduled SD calibration, the gain derived from scattering light facilitates monitoring the dependence of the SD’s degradation on incident angles. In this paper, we present a method that uses multiple orbits over each mission to obtain a SD response to the nadir port illumination. The SD degradation estimated from the nadir port illumination is compared with the degradation derived from the sun-illuminated SD. As both Terra and Aqua spacecraft continue to drift from their nominal orbits, the SD calibration mechanism has been adapted to these drifts, especially in terms of characterizing the transmission screen function. This paper also presents the utility of this scattering light data to support the RSB calibration in the post-nominal orbit drift era of operations.

MODIS↗
NASA NTRS1997

Point Relay Scanner Utilizing Ellipsoidal Mirrors

A scanning system uses a polygonal mirror assembly with each facet of the polygon having an ellipsoidal mirror located thereon. One focal point of each ellipsoidal mirror is located at a common point on the axis of rotation of the polygonal mirror assembly. As the mirror assembly rotates. a second focal point of the ellipsoidal mirrors traces out a scan line. The scanner can be utilized for scanned output display of information or for scanning information to be detected.

Manhart, Paul K.↗
NASA NTRS2011

LIDAR Metrology for Prescription Characterization and Alignment of Large Mirrors

We describe the use of LIDAR, or "laser radar," (LR) as a fast, accurate, and non-contact tool for the measurement of the radius of curvature (RoC) of large mirrors. We report the results of a demonstration of this concept using a commercial laser radar system. We measured the RoC of a 1.4m x 1m spherical mirror with a nominal RoC of 4.6 m with a manufacturing tolerance of 4600mm +/- 6mm. The prescription of the mirror is related to its role as ground support equipment used in the test of part of the James Webb Space Telescope (JWST). The RoC of such a large mirror is not easily measured without contacting the surface. From a position near the center of curvature of the mirror, the LIDAR scanned the mirror surface, sampling it with 1 point per 3.5 sq cm. The measurement consisted of 3983 points and lasted only a few minutes. The laser radar uses the LIDAR signal to provide range, and encoder information from angular azimuth and elevation rotation stages provide the spherical coordinates of a given point. A best-fit to a sphere of the measured points was performed. The resulting RoC was within 20 ppm of the nominal RoC, also showing good agreement with the results of a laser tracker-based, contact metrology. This paper also discusses parameters such as test alignment, scan density, and optical surface type, as well as future possible application for full prescription characterization of aspherical mirrors, including radius, conic, off-axis distance, and aperture.

Eegholm, B.↗
NASA NTRS1988

IR-dust observations of Comet Tempel 2 with CRAF VIMS

Measurement strategies are now being planned for using the Visual and Infrared Mapping Spectrometer (VIMS) to observe the asteroid Hestia, and the nucleus, and the gas and dust in the coma of comet P/Tempel 2 as part of the Comet Rendezvous Asteroid Flyby (CRAF) mission. The spectral range of VIMS will cover wavelengths from 0.35 to 5.2 micrometers, with a spectral resolution of 11 nm from 0.35 to 2.4 micrometers and of 22 nm from 2.4 to 5.2 micrometers. The instantaneous field of view (IFOV) provided by the foreoptics is 0.5 milliradians, and the current design of the instrument provides for a scanning secondary mirror which will scan a swath of length 72 IFOVs. The CRAF high resolution scan platform motion will permit slewing VIMS in a direction perpendicular to the swath. This enables the building of a two dimensional image in any or all wavelength channels. Important measurements of the dust coma will include the onset of early coma activity, the mapping of gas and dust jets and correlations with active nucleus areas, observations of the dust coma from various scattering phase angles, coverage of the low wavelength portion of the thermal radiation, and the 3.4 micrometer hydrocarbon emission. A description of the VIMS instrument is presented.

Combi, Michael R.↗
NASA NTRS1992

Conically scanned lidar telescope using holographic optical elements

Holographic optical elements (HOE) using volume phase holograms make possible a new class of lightweight scanning telescopes having advantages for lidar remote sensing instruments. So far, the only application of HOE's to lidar has been a non-scanning receiver for a laser range finder. We introduce a large aperture, narrow field of view (FOV) telescope used in a conical scanning configuration, having a much smaller rotating mass than in conventional designs. Typically, lidars employ a large aperture collector and require a narrow FOV to limit the amount of skylight background. Focal plane techniques are not good approaches to scanning because they require a large FOV within which to scan a smaller FOV mirror or detector array. Thus, scanning lidar systems have either used a large flat scanning mirror at which the receiver telescope is pointed, or the entire telescope is steered. We present a concept for a conically scanned lidar telescope in which the only moving part is the HOE which serves as the primary collecting optic. We also describe methods by which a multiplexed HOE can be used simultaneously as a dichroic beamsplitter.

Schwemmer, Geary K.↗
NASA NTRS2020

Assessment of MODIS On-orbit Polarization Impact on Detector Relative Calibration

The Moderate-Resolution Imaging Spectroradiometer (MODIS) has been maintained its nominal operations since it was launched in December 1999 on Terra and May 2002 on Aqua spacecraft, respectively. MODIS is a scanning radiometer with a two-sided mirror that scans the Earth over an angular range from -55° to +55° off nadir and covers a wavelength range from 0.4 to 14.4 μm in 36 spectral bands. The sensitivity to polarization was fully characterized during prelaunch characterization for each reflective solar band (RSB), detector and mirror side as a function of scan angle. It was observed that while on-orbit polarization sensitivity for Aqua MODIS has been stable over the course of mission, the Terra MODIS polarization sensitivity at a few short wavelengths showed a significant increase near the end of scan angle after a few years in the mission. Our previous study examined the impact of polarization on the measured top-of-atmosphere reflectances obtained over the pseudo-invariant desert sites. There are up to 25% change in gain and 30% seasonal variation in select Terra RSBs due to the increase in polarization sensitivity. In this study, we examine relative changes in the polarization sensitivity of detectors within a band. Results from prelaunch tests indicate that there is a noticeable detector dependent polarization sensitivity and it is expected that such a dependency could change on orbit, particularly for Terra MODIS. Results of this study provide useful information on polarization impact and implementation of the correction algorithm.

MODIS↗
NASA NTRS1980

Design challenges of the thematic mapper

The paper discusses the Thematic Mapper (TM), a multispectral earth resources sensor that will be launched on the Landsat-D satellite. The TM will operate in a circular, near-polar orbit of 750-km altitude and will scan a swath of earth 185 km wide. One TM mirror is a moving scan mirror, active during the forward and reverse scans; the final design of this component incorporates a mirror which has the required linearity and is not influenced by structural vibration. Another TM component that presented a design difficulty was the optical metering structure which has to be thermally stable across the temperature range in the instrument's orbital environment and duty cycle.

Blanchard, L. E.↗
NASA NTRS1993

Image analysis of the AXAF VETA-1 X-ray mirror

Initial core scan data of the VETA-1 X-ray mirror proved disappointing, showing considerable unpredicted image structure and poor measured FWHM. 2-D core scans were performed, providing important insight into the nature of the distortion. Image deconvolutions using a raytraced model PSF was performed successfully to reinforce our conclusion regarding the origin of the astigmatism. A mechanical correction was made to the optical structure, and the minor was tested successfully (FWHM 0.22 arcsec) as a result.

Freeman, M.↗
NASA NTRS1992

Image analysis of the AXAF VETA-I x ray mirror

Initial core scan data of the VETA-I x-ray mirror proved disappointing, showing considerable unpredicted image structure and poor measured FWHM. 2-D core scans were performed, providing important insight into the nature of the distortion. Image deconvolutions using a ray traced model PSF was performed successfully to reinforce our conclusion regarding the origin of the astigmatism. A mechanical correction was made to the optical structure, and the mirror was tested successfully (FWHM 0.22 arcsec) as a result.

Freeman, Mark D.↗
NASA NTRS1986

Quantitative thermal diffusivity measurements of composites

A remote radiometric technique for making quantitative thermal diffusivity measurements is described. The technique was designed to make assessments of the structural integrity of large composite parts, such as wings, and can be performed at field sites. In the measurement technique, a CO2 laser beam is scanned using two orthogonal servo-controlled deflecting mirrors. An infrared imager, whose scanning mirrors oscillate in the vertical and the horizontal directions, is used as the detector. The analysis technique used to extract the diffusivity from these images is based on a thin infinite plate assumption, which requires waiting a given period of time for the temperature to equilibrate throughout the thickness of the sample. The technique is shown to be accurate to within two percent for values of the order of those for composite diffusivities, and to be insensitive to convection losses.

Heath, D. M.↗
NASA NTRS1986

Photocurrent Imaging Detects Solar-Module Defects

Raster-scanned laser beam excites photocurrents in thin-film amorphous silicon devices. Solar-cell laser scanner uses two galvanometer-driven mirrors to scan laser-beam spot over surface of module or cell under test. Position signals from scan controllers used to index storage of photocurrent signal data in scan-converter image memory. Stored image displayed on television monitor.

Kim, Q.↗
NASA NTRS2009

Assessment of the Short-Term Radiometric Stability between Terra MODIS and Landsat 7 ETM+ Sensors

The Landsat 7 (L7) Enhanced Thematic Mapper (ETM+) sensor was launched on April 15th, 1999 and has been in operation for over nine years. It has six reflective solar spectral bands located in the visible and shortwave infrared part of the electromagnetic spectrum (0.5 - 2.5 micron) at a spatial resolution of 30 m. The on-board calibrators are used to monitor the on-orbit sensor system changes. The ETM+ performs solar calibrations using on-board Full Aperture Solar Calibrator (FASC) and the Partial Aperture Solar Calibrator (PASC). The Internal Calibrator Lamp (IC) lamps, a blackbody and shutter optics constitute the on-orbit calibration mechanism for ETM+. On 31 May 2003, a malfunction of the scan-line corrector (SLC) mirror assembly resulted in the loss of approximately 22% of the normal scene area. The missing data affects most of the image with scan gaps varying in width from one pixel or less near the centre of the image to 14 pixels along the east and west edges of the image, creating a wedge-shaped pattern. However, the SLC failure has no impacts on the radiometric performance of the valid pixels. On December 18, 1999, the Moderate Resolution Imaging Spectroradiometer (MODIS) Proto-Flight Model (PFM) was launched on-board the NASA's EOS Terra spacecraft. Terra MODIS has 36 spectral bands with wavelengths ranging from 0.41 to 14.5 micron and collects data over a wide field of view angle (+/-55 deg) at three nadir spatial resolutions of 250 m, 500 in 1 km for bands 1 to 2, 3 to 7, and 8 to 36, respectively. It has 20 reflective solar bands (RSB) with spectral wavelengths from 0.41 to 2.1 micron. The RSB radiometric calibration is performed by using on-board solar diffuser (SD), solar diffuser stability monitor (SDSM), space-view (SV), and spectro-radiometric calibration assembly (SRCA). Through the SV port, periodic lunar observations are used to track radiometric response changes at different angles of incidence (AOI) of the scan mirror. As a part of the AM Constellation satellites, Terra MODIS flies approximately 30 minutes behind L7 ETM+ in the same orbit. The orbit of L7 is repetitive, circular, sunsynchronous, and near polar at a nominal altitude of 705 km (438 miles) at the Equator. The spacecraft crosses the Equator from north to south on a descending node between 10:00 AM and 10:15 AM. Circling the Earth at 7.5 km/sec, each orbit takes nearly 99 minutes. The spacecraft completes just over 14 orbits per day, covering the entire Earth between 81 degrees north and south latitude every 16 days. The longest continuous imaging swath that L7 sensor can collect is for a 14-minute subinterval contact period which is equivalent to 35 full WRS-2 scenes. On the other hand, Terra can provide the entire corresponding orbit with wider swath at any given ETM+ collection without contact time limitation. There are six spectral matching band pairs between MODIS (bands 3, 4, 1, 2, 6, 7) and ETM+ (bands 1, 2, 3, 4, 5, 7) sensor. MODIS has narrower spectral responses than ETM+ in all the bands. A short-term radiometric stability was evaluated using continuous ETM+ scenes within the contact period and the corresponding half orbit MODIS scenes. The near simultaneous earth observations (SNO) were limited by the smaller swath size of ETM+ (187 km) as compared to MODIS (2330 km). Two sets of continuous granules for MODIS and ETM+ were selected and mosaiced based on pixel geolocation information for non cloudy pixels over the North American continent. The Top-of- Atmosphere (TOA) reflectances were computed for the spectrally matching bands between ETM+ and MODIS over the regions of interest (ROI). The matching pixel pairs were aggregated from a finer to a coarser pixel resolution and the TOA reflectance values covering a wide dynamic range of the sensors were compared and analyzed. Considering the uncertainties of the absolute calibration of the both sensors, radiometric stability was verified for the band pairs. The Railroad Valley Playa, Nada (RVPN) was included in the path of this continuous orbit, which served as a verification point between the shortterm and the long-term trending results from previous studies. This work focuses on monitoring the short-term on-orbit stability of MODIS and the ETM+ RSB. It also provides an assessment of the absolute calibration differences between the two sensors over their wide dynamic ranges.

Choi, Taeyoung↗
NASA NTRS2008

Range Imaging without Moving Parts

Range-imaging instruments of a type now under development are intended to generate the equivalent of three-dimensional images from measurements of the round-trip times of flight of laser pulses along known directions. These instruments could also provide information on characteristics of targets, including roughnesses and reflectivities of surfaces and optical densities of such semi-solid objects as trees and clouds. Unlike in prior range-imaging instruments based on times of flight along known directions, there would be no moving parts; aiming of the laser beams along the known directions would not be accomplished by mechanical scanning of mirrors, prisms, or other optical components. Instead, aiming would be accomplished by using solid-state devices to switch input and output beams along different fiber-optic paths. Because of the lack of moving parts, these instruments could be extraordinarily reliable, rugged, and long-lasting. An instrument of this type would include an optical transmitter that would send out a laser pulse along a chosen direction to a target. An optical receiver coaligned with the transmitter would measure the temporally varying intensity of laser light reflected from the target to determine the distance and surface characteristics of the target. The transmitter would be a combination of devices for generating precise directional laser illumination. It would include a pulsed laser, the output of which would be coupled into a fiber-optic cable with a fan-out and solid-state optical switches that would enable switching of the laser beam onto one or more optical fibers terminated at known locations in an array on a face at the focal plane of a telescope. The array would be imaged by the telescope onto the target space. The receiver optical system could share the aforementioned telescope with the transmitter or could include a separate telescope aimed in the same direction as that of the transmitting telescope. In either case, light reflected from the target would be focused by the receiver optical system onto an array of optical fibers matching the array in the transmitter. These optical fibers would couple the received light to one or more photodetector( s). Optionally, the receiver could include solid-state optical switches for choosing which optical fiber(s) would couple light to the photodetector(s). This instrument architecture is flexible and can be optimized for a wide variety of applications and levels of performance. For example, it is scalable to any number of pixels and pixel resolutions and is compatible with a variety of ranging and photodetection methodologies, including, for example, ranging by use of modulated (including pulsed and encoded) light signals. The use of fixed arrays of optical fibers to generate controlled illumination patterns would eliminate the mechanical complexity and much of the bulk of optomechanical scanning assemblies. Furthermore, digital control of the selection of the fiber-optic pathways for the transmitted beams could afford capabilities not seen in previous three-dimensional range-imaging systems. Instruments of this type could be specialized for use as, for example, proximity detectors, three-dimensional robotic vision systems, airborne terrain-mapping systems, and inspection systems.

Blair, J. Bryan↗
NASA NTRS2003

Ladar System Identifies Obstacles Partly Hidden by Grass

A ladar-based system now undergoing development is intended to enable an autonomous mobile robot in an outdoor environment to avoid moving toward trees, large rocks, and other obstacles that are partly hidden by tall grass. The design of the system incorporates the assumption that the robot is capable of moving through grass and provides for discrimination between grass and obstacles on the basis of geometric properties extracted from ladar readings as described below. The system (see figure) includes a ladar system that projects a range-measuring pulsed laser beam that has a small angular width of radians and is capable of measuring distances of reflective objects from a minimum of dmin to a maximum of dmax. The system is equipped with a rotating mirror that scans the beam through a relatively wide angular range of in a horizontal plane at a suitable small height above the ground. Successive scans are performed at time intervals of seconds. During each scan, the laser beam is fired at relatively small angular intervals of radians to make range measurements, so that the total number of range measurements acquired in a scan is Ne = / .

Castano, Andres↗
NASA NTRS2012

Ultra-Miniature Lidar Scanner for Launch Range Data Collection

The most critical component in lidar is its laser scanner, which delivers pulsed or CW laser to target with desirable field of view (FOV). Most existing lidars use a rotating or oscillating mirror for scanning, resulting in several drawbacks. A lidar scanning technology was developed that could achieve very high scanning speed, with an ultra-miniature size and much lighter weight. This technology promises at least a 10x performance improvement in these areas over existing lidar scanners. Features of the proposed ultra-miniature lidar scanner include the ability to make the entire scanner <2 mm in diameter; very high scanning speed (e.g. 5 - 20 kHz, in contrast to several hundred Hz in existing scanners); structure design to meet stringent requirements on size, weight, power, and compactness for various applications; and the scanning speed and FOV can be altered for obtaining high image resolutions of targeted areas and for diversified uses.

Geng, Jason↗
NASA NTRS2014

Scientific Impact of MODIS C5 Calibration Degradation and C6+ Improvements

The Collection 6 (C6) MODIS (Moderate Resolution Imaging Spectroradiometer) land and atmosphere data sets are scheduled for release in 2014. C6 contains significant revisions of the calibration approach to account for sensor aging. This analysis documents the presence of systematic temporal trends in the visible and near-infrared (500 m) bands of the Collection 5 (C5) MODIS Terra and, to lesser extent, in MODIS Aqua geophysical data sets. Sensor degradation is largest in the blue band (B3) of the MODIS sensor on Terra and decreases with wavelength. Calibration degradation causes negative global trends in multiple MODIS C5 products including the dark target algorithm's aerosol optical depth over land and Ångstrom exponent over the ocean, global liquid water and ice cloud optical thickness, as well as surface reflectance and vegetation indices, including the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI). As the C5 production will be maintained for another year in parallel with C6, one objective of this paper is to raise awareness of the calibration-related trends for the broad MODIS user community. The new C6 calibration approach removes major calibrations trends in the Level 1B (L1B) data. This paper also introduces an enhanced C6C calibration of the MODIS data set which includes an additional polarization correction (PC) to compensate for the increased polarization sensitivity of MODIS Terra since about 2007, as well as detrending and Terra- Aqua cross-calibration over quasi-stable desert calibration sites. The PC algorithm, developed by the MODIS ocean biology processing group (OBPG), removes residual scan angle, mirror side and seasonal biases from aerosol and surface reflectance (SR) records along with spectral distortions of SR. Using the multiangle implementation of atmospheric correction (MAIAC) algorithm over deserts, we have also developed a detrending and cross-calibration method which removes residual decadal trends on the order of several tenths of 1% of the top-of-atmosphere (TOA) reflectance in the visible and near-infrared MODIS bands B1-B4, and provides a good consistency between the two MODIS sensors. MAIAC analysis over the southern USA shows that the C6C approach removed an additional negative decadal trend of Terra (Delta)NDVI approx.0.01 as compared to Aqua data. This change is particularly important for analysis of vegetation dynamics and trends in the tropics, e.g., Amazon rainforest, where the morning orbit of Terra provides considerably more cloud-free observations compared to the afternoon Aqua measurements.

Terra and Aqua↗
NASA NTRS1989

A segmented mirror antenna for radiometers

An antenna is designed for the radiometer application of the planned NASA Earth Science Geostationary Platforms in the 1990's. The antenna consists of two parts: a regular parabolic dish of 5 meters in diameter which converts the radiation from feeds into a collimated beam, and a movable mirror that redirects the beam to a prescribed scan direction. The mirror is composed of 28 segmented planar conducting plates, mostly one square meter in size. The secondary pattern of the antenna was analyzed based on a physical optics analysis. For frequencies between 50 and 230 GHz, and for a scan range of + or -8 deg (270 beamwidths scan at 230 GHz), the worst calculated beam efficiency is 95 percent. To cover such a wide frequency and scan range, each of the 28 plates is individually controlled for a tilting less than 4 deg, and for a sliding less than 0.5 cm. The sliding is done at discrete steps. At 230 GHz, a step size of 2 mil is sufficient. The plate positions must be reset for each frequency and for each scan direction. Once the position is set, the frequency bandwidth of the antenna is very narrow.

Lee, S. W.↗