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At least 1,441 records · Page 80
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NASA NTRS1990

Nonlinearity corrections in calibration of advanced very high resolution radiometer infrared channels

The IR channels of the AVHRR are calibrated in-flight with data acquired when the AVHRR views space and a warm target on board. This determines the two coefficients of a linear calibration equation. However, in its 11- and 12-micron channels the response of the AVHRR is nonlinear. If not accounted for, the nonlinearity could cause errors as large as 2 C in inferred scene temperatures. Therefore NESDIS computes corrections to the brightness temperatures inferred from the linear calibration. This paper describes how the corrections have been calculated at NESDIS since March 1986 and presents the corrections for the AVHRRs on the NOAA 9, 10, and 11 satellites. The corrections are calculated from results of the prelaunch calibration.

Weinreb, Michael P.↗
NASA NTRS1990

Cloud cover analysis with Arctic Advanced Very High Resolution Radiometer data. II - Classification with spectral and textural measures

The spectral and textural characteristics of polar clouds and surfaces for a 7-day summer series of AVHRR data in two Arctic locations are examined, and the results used in the development of a cloud classification procedure for polar satellite data. Since spatial coherence and texture sensitivity tests indicate that a joint spectral-textural analysis based on the same cell size is inappropriate, cloud detection with AVHRR data and surface identification with passive microwave data are first done on the pixel level as described by Key and Barry (1989). Next, cloud patterns within 250-sq-km regions are described, then the spectral and local textural characteristics of cloud patterns in the image are determined and each cloud pixel is classified by statistical methods. Results indicate that both spectral and textural features can be utilized in the classification of cloudy pixels, although spectral features are most useful for the discrimination between cloud classes.

Key, J.↗
NASA NTRS1990

Identification of Cirrus over Wausau during the 1986 FIRE IFO from ground-based radiometer data

The potential of using irradiation data to indicate episodes of cirrus cloudiness during the daylight hours is explored. Thresholds separating cirrus from other clouds and clear skies are determined using four days of irradiation data, hourly weather observations, sky photographs, sky video, and occasional lidar observations. Data were gathered during the First International Satellite Cloud Climatology Program (ISCCP) Regional Experiment (FIRE) Intensive Field Observations (IFO) cirrus project. Thresholds are tested using data from the remaining 17 days of the IFO. Cirrus episodes are defined as intervals when the sky cover is primarily cirrus. Measurements of incoming shortwave and near infrared full hemispheric and diffuse irradiation, and atmospheric infrared irradiation were made at Wausau, Wisconsin Municipal Airport. Data were collected between October 13 and November 2, 1986 and are one minute averages of ten second samples. Data from October 23 and from October 27 to 29 were used for threshold determination. Using weather observations, lidar output, photographs and video, sky conditions were grouped into three categories: noncirrus cloudy, cirrus, and clear. Irradiation data from all periods falling under each of the sky categories were then examined and thresholds denoting a change from one sky category to another were determined. Variables with only a small amount of overlap between sky categories were selected as key indicators.

Robinson, David A.↗
NASA NTRS1990

Near infrared radiances observed by the UK C130 multi-channel radiometer during the marine stratocumulus IFO and preliminary comparison with model calculations

A preliminary analysis of some of the narrow band radiance data measured on the U.K. Meteorological Office's C130 aircraft during the marine stratocumulus intensive field observation of First ISCCP Regional Experiment (FIRE), San Diego 29 June to 18 July 1987, is presented. The data are compared with Monte Carlo calculations of the reflectance and transmittance of the cloud based upon the observed droplet size distribution. The main scientific question being addressed is whether there is any evidence of anomalous absorption within the cloud which had been observed in similar measurements (Rozenberg et al., 1974; Twomey and Cocks, 1982; Foot, 1988). The measurements also indicate the potential for remotely sensing cloud properties. The data and method of presentation discussed here clearly separates out clouds in terms of the size of the cloud droplets. All of the daytime C130 FIRE flights have been studied and are consistent with the data presented here. There appears to be no peculiarities that might arise, for example if pollution were to be a significant factor in determining cloud absorption. Variation in the inferred size parameters, r sub e, along runs are also very small.

Foot, J. S.↗
NASA NTRS1991

Design, performance evaluation, and investigation of the theoretical capabilities of the NASA Millimeter-wave Imaging Radiometer (MIR)

The development of techniques for passive microwave retrieval of water vapor and precipitation parameters using millimeter- and sub-millimeter wavelength channels is reviewed. Channels of particular interest are in the tropospheric transmission windows at 90, 150, 220, and 340 GHz and centered around the water vapor lines at 183 and 325 GHz. Collectively, these channels have potential application in high-resolution mapping (e.g., from geosynchronous orbit), remote sensing of cloud and precipitation parameters, and retrieval of water vapor profiles. Both theoretical and experimental results to date are discussed.

Gasiewski, Albin J.↗
NASA NTRS1990

Development of components for submillimeter wave heterodyne radiometers at JPL

Components for the key elements of submillimeter wave heterodyne receivers are described. The devices and components are inherently space qualifiable having low power and mass needs and exhibiting robustness. Nb and NbN superconductor insulator superconductor (SIS) tunnel junctions are selected for the nonlinear mixing element because of their robustness. NbN exhibits a large superconducting bandgap, allowing operation in excess of 1500 GHz. Both waveguide mounts, for the lower frequencies and planar antenna mixer mounts for the higher frequency range, are used. Quasi-optical components are used for frequency multiplexing and combining.

Frerking, Margaret A.↗
NASA NTRS1989

Optical alignment equipment and methods for visible and IR imaging meteorological radiometer

This paper describes the test equipment required for precise IR and visible channel microradian alignments for geosynchronous orbiting meteorological instruments. The test equipment design criteria, design, function and use are explained in detail. Wave front error measurements, vibration isolation, thermal compensation and stress free mounting configurations are described.

Herendeen, R.↗
NASA NTRS1992

Remote sensing of the atmosphere by multi-channel radiometers

To characterize Earth/space communication links, the Jet Propulsion Laboratory's (JPL's) Radiowave Propagation Program supports a number of propagation studies. Low-availability satellite links and their propagation vagaries at Ka and millimeter-wave bands constitute a major effort within this program. Since 1987, this program has funded the Wave Propagation Laboratory (WPL) of the National Oceanic and Atmospheric Administration (NOAA) to conduct multiple-frequency radiometric observations of the atmosphere. Measurements of atmospheric emission and attenuation at 20, 31, and 90 GHz at different locations are presented. Single-station and joint-station statistics are derived. A linear regression scheme for the prediction of fade statistics in one channel using fade data from the other two channels is presented.

Davarian, Faramaz↗
NASA NTRS1990

Cirrus cloud characteristics derived from volume imaging lidar, high spectral resolution lidar, HIS radiometer, and satellite

Preliminary measurement results are presented from the Cirrus Remote Sensing Pilot Experiment which used a unique suite of instruments to simultaneously retrieve cirrus cloud visible and IR optical properties, while addressing the disparities between satellite volume averages and local point measurements. The experiment employed a ground-based high resolution interferometer sounder (HIS) and a second Fourier transform spectrometer to measure the spectral radiance in the 4-20 micron band, a correlated high spectral resolution lidar, a volume imaging lidar, a CLASS radiosonde system, the Scripps Whole Sky Imager, and multispectral VAS, HIRS, and AVHRR satellite data from polar orbiting and geostationary satellites. Data acquired during the month long experiment included continuous daytime monitoring with the Whole Sky Imager.

Grund, Christian J.↗
NASA NTRS1990

Processing and analysis of radiometer measurements for airborne reconnaissance

This paper describes selected results of airborne, radiometric imaging measurements at 90 GHz and 140 GHz relevant for the application in reconnaissance. Using a temperature resolution below 0.5 K and an angular resolution of about 1-degree high-quality images show the capability of discriminating between many brightness temperature classes within our natural environment and man-made objects. Measurement examples are given for cloud and fog penetration at 90 GHz, for the detection of vehicles on roads, and for the detection and classification of airports and airplanes. The application of different contour enhancement methods (Marr-Hildreth and Canny) shows the possibility of extracting lines and shapes precisely in order to improve automatic target recognition. The registration of the passive images with corresponding X-band synthetic aperture images from the same area is carried out and the high degree of correlation is discussed.

Suess, Helmut↗
NASA NTRS1992

Comparison of velocity estimates from advanced very high resolution radiometer in the Coastal Transition Zone

Two methods of estimating surface velocity vectors from AVHRR data are applied to the same set of images and the results are compared with in situ and altimeter measurements. The first technique utilizes an automated feature-tracking algorithm and the second uses an inversion of the heat equation. The two methods were comparable in their degree of agreement with the in situ data, giving velocity magnitudes that were 30 to 50 percent less than drifter and acoustic Doppler current profiler velocities measured at 15 to 20 m depth, with rms directional differences of about 60 degrees.

Kelly, Kathryn A.↗
NASA NTRS1987

Space-qualified submillimeter radiometer

The purpose of this research was to develop a reliable submillimeter wave spectrometer for space-borne high frequency spectral line work. The emphasis was on improving the efficiency of frequency multipliers to limit the system components to rugged, low power consumption solid-state devices. This research has allowed Millitech to develop increased efficiency and performance in Millitech's existing line of submillimeter components and systems. Millitech has fabricated and tested a complete solid-state spectrometer front end for use at 560 GHz (the 1(sub 10) to 1(sub 01) transition of water vapor). The spectrometer was designed with the rigors of flight conditions in mind. The spectrometer uses a phase-locked, solid-state Gunn diode oscillator as the local oscillator, employing a tripler to produce about 3 mW of power at 285 GHz, and a low noise second harmonic waveguide mixer which requires less than 2 mW of LO power. The LO (and the signal) is injected into the mixer by means of a quasioptical diplexer. The measured system noise temperature is 2800 K (DSB) over 400 MHz. The whole spectrometer front end is compact (21 cm by 21 cm by 24 cm), light (7.4 kg), and has a power consumption of less than 8 W. Other topics explored in this work include compact frequency agile phase lock loops, optical filters, and InP Gunn oscillators for low noise applications. As a result of this research, the improvement in the design of multipliers and harmonic mixers will allow their use as the LO power for a variety of satellite-borne receivers operating in the 200 to 600 GHz frequency range.

Huguenin, G. R.↗