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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.

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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Irradiance calibration with solar diffuser

The sun's energy is used in combination of movable and fixed diffuser plates, windows and apertures which are positioned in a series of test sequences (modes) for reflectance monitoring and calibration without the use of man-made sources. There are three embodiments, or implementations, of the invention--one embodiment uses two diffusers--a working diffuser and a secondary diffuser--the second embodiment uses three diffusers, a working diffuser, a secondary diffuser and a reference diffuser--and the third embodiment uses two diffusers--a working diffuser and a secondary diffuser, the latter also functioning as a cover for the working diffuser. The movable diffusers are mounted on rotatable cones and, in all embodiments, the sun is blocked from reaching the diffusers when not in use. Thus, the sun is used as a stable source for calibration and monitoring and the sun/diffuser combination is used in such a way that the response of all elements of the optical subsystem of the TOMS can be unambiguously and efficiently characterized with high accuracy and precision.

Haring, Robert E.↗
NASA NTRS2021

Sensor Calibration Impacts on Dust Detection Based On MODIS And VIIRS Thermal Emissive Bands

Dust detection using remotely sensed measurements has been one of the challenging problems encountered by atmospheric scientists. MODerate Resolution Imaging Spectroradiometer (MODIS) on the Terra (T)and Aqua (A) platforms have been a versatile sensor for well over 21 and 18 years respectively and have been extremely useful in the retrieval of aerosol information over the entire globe. The MODIS radiances from the Level1B in general are expected to be within 5% accuracy in the reflective wavelengths and within 1% in the thermal emissive wavelengths. In this paper, we evaluate the sensitivity of previously developed dust detection technique based on thermal emissive wavelengths, which correspond to MODIS bands 20, 29, 31, and 32 respectively. The Thermal Emissive Dust Index (TEDI) performed very comparably to the traditional Aerosol Optical Thickness (AOT) retrievals by MODIS reflective channels. Since the MODIS Thermal Emissive Bands (TEB) are well calibrated on-orbit using a BlackBody (BB) source, the calibration of these long wave infrared bands is quite robust. As A-MODIS continues to perform well beyond its designed lifetime of 6 years, the instrument has undergone various levels of degradation during its mission time. As a consequence, it is imperative to check the impacts of calibration on the higher-level retrievals. In this paper, we rigorously analyze the sensitivity of TEDI due to the impact of calibration by the afore-mentioned TEB. The perturbation of the dominant (linear) calibration term demonstrated the following: first, there was a correlation in the sensitivity of the TEDI due to the uncertainty in the linear calibration term. Based on a perturbation in the linear calibration term for all aforementioned bands over a range of ± 5 % yielded the TEDI sensitivity to vary from approximately -3.2 % to about -3.6 %. When considering the uncertainty in each individual band significant changes were observed. The least change was observed for the perturbation in the calibration of band 20 with the TEDI sensitivity and the largest sensitivity in TEDI was observed in the perturbation of band 31 calibration. Thus, in the case of TEDI, noticeable sensitivity due to calibration uncertainty was observed in bands 29, 31, and 32, reiterating the importance of the TEB calibration in these bands. Also, the dust detection scheme based on A-MODIS was successfully transferred to the follow-on sensors such as Suomi (SNPP) and NOAA 20 (N20) VIIRS. The results presented in this paper would be extremely helpful in understanding impacts of calibration on the higher-level products for both current and future missions based on the MODIS heritage. Finally, the work also identifies the importance of radiometric fidelity in maintaining the accuracy of the dust detection. Results presented will show drastic improvement of the Saharan dust detection after the reduction of the electronic crosstalk in the 8.5 μm channel of T-MODIS.

Sriharsha Madhavan↗
NASA NTRS1988

A digital calibration method for synthetic aperture radar systems

A basic method to calibrate imagery from synthetic aperture radar (SAR) systems is presented. SAR images are calibrated by monitoring all the terms of the radar equation. This procedure includes the use of both external (calibrated reference reflectors) and internal (system-generated calibration signals) sources to monitor the total SAR system transfer function. To illustrate the implementation of the procedure, two calibrated SAR images (X-band, 3.2-cm wavelength) are presented, along with the radar cross-section measurements of specific scenes within each image. The sources of error within the SAR image calibration procedure are identified.

Larson, Richard W.↗
NASA NTRS1976

Effects of edge-rounding on geodesic lenses

A method that eliminates the need for a mathematical model is described for measuring the absolute light intensity radiated from volume chemiluminescent and fluorescent sources. This technique applies to a uniform cylindrical luminous volume, which is the geometry most often practical for the study of flowing chemiluminescent and fluorescent fluids. In this method, the necessary integration over a luminous volume is carried out experimentally, using a calibrated piston source that has as its head a disk of uniform luminosity. The relatively small total error in absolute light intensity measurements with the piston source technique represents a significant improvement over competing comparison methods. The construction of a piston source to fit the cross section of a particular cylindrical volume is straightforward, and its calibration against a standard source can be readily made while exercising normal precautions.

Wood, V. E.↗
NASA NTRS2008

Stable Calibration of Raman Lidar Water-Vapor Measurements

A method has been devised to ensure stable, long-term calibration of Raman lidar measurements that are used to determine the altitude-dependent mixing ratio of water vapor in the upper troposphere and lower stratosphere. Because the lidar measurements yield a quantity proportional to the mixing ratio, rather than the mixing ratio itself, calibration is necessary to obtain the factor of proportionality. The present method involves the use of calibration data from two sources: (1) absolute calibration data from in situ radiosonde measurements made during occasional campaigns and (2) partial calibration data obtained by use, on a regular schedule, of a lamp that emits in a known spectrum determined in laboratory calibration measurements. In this method, data from the first radiosonde campaign are used to calculate a campaign-averaged absolute lidar calibration factor (t(sub 1)) and the corresponding campaign-averaged ration (L(sub 1)) between lamp irradiances at the water-vapor and nitrogen wavelengths. Depending on the scenario considered, this ratio can be assumed to be either constant over a long time (L=L(sub 1)) or drifting slowly with time. The absolutely calibrated water-vapor mixing ratio (q) obtained from the ith routine off-campaign lidar measurement is given by q(sub 1)=P(sub 1)/t(sub 1)=LP(sub 1)/P(sup prime)(sub 1) where P(sub 1) is water-vapor/nitrogen measurement signal ration, t(sub 1) is the unknown and unneeded overall efficiency ratio of the lidar receiver during the ith routine off-campaign measurement run, and P(sup prime)(sub 1) is the water-vapor/nitrogen signal ratio obtained during the lamp run associated with the ith routine off-campaign measurement run. If L is assumed constant, then the lidar calibration is routinely obtained without the need for new radiosonde data. In this case, one uses L=L(sub 1) = P(sup prime)(sub 1)/t(sub 1), where P(sub 1)(sup prime) is the water-vapor/nitrogen signal ratio obtained during the lamp run associated with the first radiosonde campaign. If L is assumed to drift slowly, then it is necessary to postpone calculation of a(sub 1) until after a second radiosonde campaign. In this case, one obtains a new value, L(sub 2), from the second radiosonde campaign, and for the ith routine off-campaign measurement run, one uses an intermediate value of L obtained by simple linear time interpolation between L(sub 1) and L(sub 2).

Leblanc, Thierry↗
NASA NTRS2014

Design and Development of the SMAP Microwave Radiometer Electronics

The SMAP microwave radiometer will measure land surface brightness temperature at L-band (1413 MHz) in the presence of radio frequency interference (RFI) for soil moisture remote sensing. The radiometer design was driven by the requirements to incorporate internal calibration, to operate synchronously with the SMAP radar, and to mitigate the deleterious effects of RFI. The system design includes a highly linear super-heterodyne microwave receiver with internal reference loads and noise sources for calibration and an innovative digital signal processor and detection system. The front-end comprises a coaxial cable-based feed network, with a pair of diplexers and a coupled noise source, and radiometer front-end (RFE) box. Internal calibration is provided by reference switches and a common noise source inside the RFE. The RF back-end (RBE) downconverts the 1413 MHz channel to an intermediate frequency (IF) of 120 MHz. The IF signals are then sampled and quantized by high-speed analog-to-digital converters in the radiometer digital electronics (RDE) box. The RBE local oscillator and RDE sampling clocks are phase-locked to a common reference to ensure coherency between the signals. The RDE performs additional filtering, sub-band channelization, cross-correlation for measuring third and fourth Stokes parameters, and detection and integration of the first four raw moments of the signals. These data are packetized and sent to the ground for calibration and further processing. Here we discuss the novel features of the radiometer hardware particularly those influenced by the need to mitigate RFI.

radio frequency interference↗
NASA NTRS2011

Development and Performance of a Filter Radiometer Monitor System for Integrating Sphere Sources

The NASA Goddard Space Flight Center (GSFC) Radiometric Calibration Laboratory (RCL) maintains several large integrating sphere sources covering the visible to the shortwave infrared wavelength range. Two critical, functional requirements of an integrating sphere source are short and long-term operational stability and repeatability. Monitoring the source is essential in determining the origin of systemic errors, thus increasing confidence in source performance and quantifying repeatability. If monitor data falls outside the established parameters, this could be an indication that the source requires maintenance or re-calibration against the National Institute of Science and Technology (NIST) irradiance standard. The GSFC RCL has developed a Filter Radiometer Monitoring System (FRMS) to continuously monitor the performance of its integrating sphere calibration sources in the 400 to 2400nm region. Sphere output change mechanisms include lamp aging, coating (e.g. BaSO4) deterioration, and ambient water vapor level. The Filter Radiometer Monitor System (FRMS) wavelength bands are selected to quantify changes caused by these mechanisms. The FRMS design and operation are presented, as well as data from monitoring four of the RCL s integrating sphere sources.

Ding, Leibo↗
NASA NTRS2016

Product of the SNPP VIIRS SD Screen Transmittance and the SD BRDF (RSB) From Both Yaw Maneuver and Regular On-Orbit Data

To assure data quality, the Earth-observing Visible Infrared Imaging Radiometer Suite (VIIRS) regularly performs on-orbit radiometric calibrations of its 22 spectral bands. The primary calibration radiance source for the reflective solar bands (RSBs) is a sunlit solar diffuser (SD). During the calibration process, sunlight goes through a perforated plate (the SD screen) and then strikes the SD. The SD scattered sunlight is used for the calibration, with the spectral radiance proportional to the product of the SD screen transmittance and the SD bidirectional reflectance distribution function (BRDF). The BRDF is decomposed to the product of its value at launch and a numerical factor quantifying its change since launch. Therefore, the RSB calibration requires accurate knowledge of the product of the SD screen transmittance and the BRDF (RSB; launch time). Previously, we calculated the product with yaw maneuver data and found that the product had improved accuracy over the prelaunch one. With both yaw maneuver and regular on orbit data, we were able to improve the accuracy of the SDSM screen transmittance and the product for the solar diffuser stability monitor SD view. In this study, we use both yaw maneuver and a small portion of regular on-orbit data to determine the product for the RSB SD view.

Lei, Ning↗
NASA NTRS1971

A measurement of the fast luminescent decay time of P-15 phosphor

During the development of a delayed coincidence apparatus for the study of atomic decays, the need arose for a bright, fast, and inexpensive light source with known decay characteristics. A sample of P-15 phosphor was obtained for use as a calibration light source. A short pulse duration was employed to preferentially excite the short lifetime components relative to the longer-lived ones. The computer analysis revealed that the raw MCA decay curve data could be described well by a sum of three exponentials with decay times of about 88 nsec, about 6.5 nsec, and 1.04 + or - 2%.

Sutton, J. F.↗
NASA NTRS1996

Optical detector calibrator system

An optical detector calibrator system simulates a source of optical radiation to which a detector to be calibrated is responsive. A light source selected to emit radiation in a range of wavelengths corresponding to the spectral signature of the source is disposed within a housing containing a microprocessor for controlling the light source and other system elements. An adjustable iris and a multiple aperture filter wheel are provided for controlling the intensity of radiation emitted from the housing by the light source to adjust the simulated distance between the light source and the detector to be calibrated. The geared iris has an aperture whose size is adjustable by means of a first stepper motor controlled by the microprocessor. The multiple aperture filter wheel contains neutral density filters of different attenuation levels which are selectively positioned in the path of the emitted radiation by a second stepper motor that is also controlled by the microprocessor. An operator can select a number of detector tests including range, maximum and minimum sensitivity, and basic functionality. During the range test, the geared iris and filter wheel are repeatedly adjusted by the microprocessor as necessary to simulate an incrementally increasing simulated source distance. A light source calibration subsystem is incorporated in the system which insures that the intensity of the light source is maintained at a constant level over time.

Strobel, James P.↗
NASA NTRS2010

Phased Array Radiometer Calibration Using a Radiated Noise Source

Electronic beam steering capability of phased array antenna systems offer significant advantages when used in real aperture imaging radiometers. The sensitivity of such systems is limited by the ability to accurately calibrate variations in the antenna circuit characteristics. Passive antenna systems, which require mechanical rotation to scan the beam, have stable characteristics and the noise figure of the antenna can be characterized with knowledge of its physical temperature [1],[2]. Phased array antenna systems provide the ability to electronically steer the beam in any desired direction. Such antennas make use of active components (amplifiers, phase shifters) to provide electronic scanning capability while maintaining a low antenna noise figure. The gain fluctuations in the active components can be significant, resulting in substantial calibration difficulties [3]. In this paper, we introduce two novel calibration techniques that provide an end-to-end calibration of a real-aperture, phased array radiometer system. Empirical data will be shown to illustrate the performance of both methods.

Srinivasan, Karthik↗
NASA NTRS1989

Radiometric Correction and Calibration of SAR Images

A technique for producing radiometrically calibrated SAR image products is described. The output imagery is corrected to represent a measurement of the ground reflectivity or radar cross section. The sources of calibration errors are discussed and the appropriate forms of the radar equation as applied to SAR-image data are reviewed. A key result is the radar equation dependence on the azimuth reference function used in processing. A radiometric correction algorithm for use in an operational SAR correlator is presented. This algorithm has the characteristic that it is fully reversible. Additionally, it can be applied equally to detected or complex SAR images, and it allows for the subtraction of the estimated noise floor in the image but does not require this procedure.

Freeman, Anthony↗
NASA NTRS1989

TOPEX microwave radiometer system calibration - Refining the SMMR heritage

A modified version of the Scanning multichannel Microwave Radiometer (SMMR) will be used for wet tropospheric path-delay corrections to the TOPEX/POSEIDON radar altimeter measurements. A number of the sources of calibration problems encountered by SMMR onboard the Seasat and Nimbus-7 platforms have been identified, and appropriate corrections have been attempted. Calibration hardware corrections include a more representative modeling of the microwave losses and reflections, and a reduction in the thermal gradients expected across this hardware through the use of radomes and sun shades and the choice of pertinent orbit parameters. Antenna calibration corrections include a postlaunch fine tuning of the antenna pattern correction algorithm to accommodate small errors in the prelaunch antenna pattern measurements. This is accomplished by overpasses of ground-based, upward-looking water vapor radiometers. An absolute calibration accuracy of 1.0 K or less is anticipated.

Ruf, Christopher S.↗
NASA NTRS2012

Dispersed Fringe Sensing Analysis - DFSA

Dispersed Fringe Sensing (DFS) is a technique for measuring and phasing segmented telescope mirrors using a dispersed broadband light image. DFS is capable of breaking the monochromatic light ambiguity, measuring absolute piston errors between segments of large segmented primary mirrors to tens of nanometers accuracy over a range of 100 micrometers or more. The DFSA software tool analyzes DFS images to extract DFS encoded segment piston errors, which can be used to measure piston distances between primary mirror segments of ground and space telescopes. This information is necessary to control mirror segments to establish a smooth, continuous primary figure needed to achieve high optical quality. The DFSA tool is versatile, allowing precise piston measurements from a variety of different optical configurations. DFSA technology may be used for measuring wavefront pistons from sub-apertures defined by adjacent segments (such as Keck Telescope), or from separated sub-apertures used for testing large optical systems (such as sub-aperture wavefront testing for large primary mirrors using auto-collimating flats). An experimental demonstration of the coarse-phasing technology with verification of DFSA was performed at the Keck Telescope. DFSA includes image processing, wavelength and source spectral calibration, fringe extraction line determination, dispersed fringe analysis, and wavefront piston sign determination. The code is robust against internal optical system aberrations and against spectral variations of the source. In addition to the DFSA tool, the software package contains a simple but sophisticated MATLAB model to generate dispersed fringe images of optical system configurations in order to quickly estimate the coarse phasing performance given the optical and operational design requirements. Combining MATLAB (a high-level language and interactive environment developed by MathWorks), MACOS (JPL s software package for Modeling and Analysis for Controlled Optical Systems), and DFSA provides a unique optical development, modeling and analysis package to study current and future approaches to coarse phasing controlled segmented optical systems.

Sigrist, Norbert↗
NASA NTRS2014

Radiometric Calibration Techniques for Signal-of-Opportunity Reflectometers

Bi-static reflection measurements utilizing global navigation satellite service (GNSS) or other signals of opportunity (SoOp) can be used to sense ocean and terrestrial surface properties. End-to-end calibration of GNSS-R has been performed using well-characterized reflection surface (e.g., water), direct path antenna, and receiver gain characterization. We propose an augmented approach using on-board receiver electronics for radiometric calibration of SoOp reflectometers utilizing direct and reflected signal receiving antennas. The method calibrates receiver and correlator gains and offsets utilizing a reference switch and common noise source. On-board electronic calibration sources, such as reference switches, noise diodes and loop-back circuits, have shown great utility in stabilizing total power and correlation microwave radiometer and scatterometer receiver electronics in L-band spaceborne instruments. Application to SoOp instruments is likely to bring several benefits. For example, application to provide short and long time scale calibration stability of the direct path channel, especially in low signal-to-noise ratio configurations, is directly analogous to the microwave radiometer problem. The direct path channel is analogous to the loopback path in a scatterometer to provide a reference of the transmitted power, although the receiver is independent from the reflected path channel. Thus, a common noise source can be used to measure the gain ratio of the two paths. Using these techniques long-term (days to weeks) calibration stability of spaceborne L-band scatterometer and radiometer has been achieved better than 0.1. Similar long-term stability would likely be needed for a spaceborne reflectometer mission to measure terrestrial properties such as soil moisture.

calibrating↗
NASA NTRS2011

Assessment of MODIS Thermal Emissive Band On-Orbit Calibration

Sixteen Moderate Resolution Imaging Spectroradiometer thermal emissive bands (TEBs) cover the wavelength from 3.75 to 14.24 m. TEB calibration uses data collected from the detector responses to the onboard blackbody (BB) and space view. The BB was designed to operate either at a constant temperature for detector linear gain calibration or at temperatures varying from ambient ( 270 K) to 315 K for on-orbit characterization of nonlinear coefficients. In this paper, we assess TEB on-orbit calibration performance in two aspects: One is to review the calibration trending on the orbital, daily, and multiyear timescales, and the other is to analyze the on-orbit calibration radiance uncertainty and its impact on the calibration. The calibration trending confirms the detector response dependence on the instrument temperature. The temperature trending and prelaunch characterization provide the basis for determining the calibration radiance source temperature range and uncertainties. An analytical approach was used to assess the impacts of onboard radiance uncertainties. The BB emission uncertainty, resulting from the temperature measurement error and emissivity uncertainty, causes a calibration uncertainty up to 0.3%, a value decreasing with the band wavelength. The BB nonblackness effect is analyzed and found to be insignificant. For the band with the lowest BB emissivity, the nonblackness affects the calibration radiance by less than 0.08%. The cavity emission uncertainty and the scan-mirror emission uncertainty both cause a less than 0.1% calibration uncertainty. The analysis of the nonlinear calibration coefficient uncertainty shows that its effect on the low Earth-view brightness-temperature range varies by band and is generally insignificant.

Chang, Tiejun↗
NASA NTRS2012

Technique for Radiometer and Antenna Array Calibration - TRAAC

Highly sensitive receivers are used to detect minute amounts of emitted electromagnetic energy. Calibration of these receivers is vital to the accuracy of the measurements. Traditional calibration techniques depend on calibration reference internal to the receivers as reference for the calibration of the observed electromagnetic energy. Such methods can only calibrate errors in measurement introduced by the receiver only. The disadvantage of these existing methods is that they cannot account for errors introduced by devices, such as antennas, used for capturing electromagnetic radiation. This severely limits the types of antennas that can be used to make measurements with a high degree of accuracy. Complex antenna systems, such as electronically steerable antennas (also known as phased arrays), while offering potentially significant advantages, suffer from a lack of a reliable and accurate calibration technique. The proximity of antenna elements in an array results in interaction between the electromagnetic fields radiated (or received) by the individual elements. This phenomenon is called mutual coupling. The new calibration method uses a known noise source as a calibration load to determine the instantaneous characteristics of the antenna. The noise source is emitted from one element of the antenna array and received by all the other elements due to mutual coupling. This received noise is used as a calibration standard to monitor the stability of the antenna electronics.

Meyer, Paul↗