Showing papers on "Radiometer published in 1992"

Structure in the COBE differential microwave radiometer first year maps

Abstract: Results of the first year of data from the differential microwave radiometers on the Cosmic Background Explorer are presented. Statistically significant structure that is well described as scale-invariant fluctuations with a Gaussian distribution is shown. The rms sky variation, smoothed to a total 10-deg FWHM Gaussian, is 30 +/-5 micro-K for Galactic latitude greater than 20-deg data with the dipole anisotropy removed. The rms cosmic quadrupole amplitude is 13 +/-4 micro-K. The angular autocorrelation of the signal in each radiometer channel and cross-correlation between channels are consistent and give a primordial fluctuation power-law spectrum with index of 1.1 +/-0.5, and an rms-quadrupole-normalized amplitude of 16 +/-4 micro-K. These features are in accord with the Harrison-Zel'dovich spectrum predicted by models of inflationary cosmology.

Signal interception: performance advantages of cyclic-feature detectors

Abstract: The problem of detecting the presence of spread-spectrum phase-shift-keyed signals in variable noise and interference backgrounds is considered, and the performances of four detectors are evaluated and compared. The detectors include the optimum radiometer, the optimum modified radiometer that jointly estimates the noise level and detects the signal, and the maximum-SNR spectral-line regenerator for spectral-line frequencies equal to the chip rate and the doubled carrier frequency. It is concluded that the spectral-line regenerators can outperform both types of radiometers by a wide margin. The performance advantages are quantified in terms of receiver operating characteristics for several noise and interference environments and receiver collection times. >

Measurement of oceanic wind vector using satellite microwave radiometers

Abstract: The possibility of retrieving both wind speed and direction from microwave radiometer measurements of the ocean is studied using Special Sensor Microwave/Imager (SSM/I) measurements collocated with buoy reports from the National Data Buoy Center (NDBC). A physically based algorithm is used to retrieve the wind speed. The RMS difference between the SSM/I and buoy wind speed is 1.6 m/s for 3321 comparisons. It is found that the SSM/I minus buoy wind speed difference is correlated with wind direction. When this wind direction signal is removed, the RMS difference between the SSM/I and buoy winds reduces to 1.3 m/s. The wind direction signal is used to make global, low-resolution maps of the monthly mean oceanic vector. The wind direction sensing capability of a prospective two-look satellite radiometer is also processed. >

Geostationary satellite detection of bio mass burning in South America

Abstract: This paper presents the results of using Geostationary Operational Environmental Satellite (GOES) Visible Infrared Spin Scan Radiometer Atmospheric Sounder (VAS) data to monitor biomass burning associated with deforestation and grassland management in South America. The technique of Matson and Dozier has been adapted to GOES VAS short-wave and long-wave infrared window data to determine ihe size and temperature of fires associated with these activities. Although VAS data do not offer the spatial resolution available with Advanced Very High Resolution Radiometer (AVHRR) data (7 km versus I km) this decreased resolution does not seem to hinder the ability of the VAS instrument to delect fires; in some cases it proves to be advantageous, in that saturation does not occur as often. Sequences of VAS visible data are helpful in verifying that the hot spots sensed in the infrared are actually related to fires. Furthermore, the smoke of the fires can be tracked in time to determine their motion and traje...

Interpretation of the Cosmic Microwave Background radiation anisotropy detected by the COBE differential microwave radiometer

Abstract: The large-scale cosmic background anisotropy detected by the COBE Differential Microwave Radiometer (DMR) instrument is compared to the sensitive previous measurements on various angular scales, and to the predictions of a wide variety of models of structure formation driven by gravitational instability. The observed anisotropy is consistent with all previously measured upper limits and with a number of dynamical models of structure formation. For example, the data agree with an unbiased cold dark matter (CDM) model with H0 = 50 km/s Mpc and Delta-M/M = 1 in a 16 Mpc radius sphere. Other models, such as CDM plus massive neutrinos (hot dark matter (HDM)), or CDM with a nonzero cosmological constant are also consistent with the COBE detection and can provide the extra power seen on 5-10,000 km/s scales.

Preliminary separation of galactic and cosmic microwave emission for the COBE Differential Microwave Radiometer

Abstract: Preliminary models of microwave emission from the Milky Way Galaxy based on COBE and other data are constructed for the purpose of distinguishing cosmic and Galactic signals. Differential Microwave Radiometer (DMR) maps, with the modeled Galactic emission removed, are fitted for a quadrupole distribution. Autocorrelation functions for individual Galactic components are presented. When Galactic emission is removed from the DMR data, the residual fluctuations are virtually unaffected, and therefore they are not dominated by any known Galactic emission component.

The Nimbus 7 solar total irradiance: A new algorithm for its derivation

Abstract: A new analysis is presented of the Nimbus-7 cavity radiometer measurements of the solar total irradiance from November 1978 to July 1991. Several problems concerning Nimbus 7 measurements are identified, and a new algorithm is developed for deriving the solar irradiance from Nimbus-7 raw data, which removes more of the instrumental and geometrical influences on the measurements than did previous algorithms. Compared to previous analyses of Nimbus-7 radiometer, the new values are higher and somewhat less variable than the older values. Compared to SMM measurements, the new values agree with SMM data quite well as long as any solar activity is present, but when the sun is quiet and its irradiance variability is less than the Nimbus radiometer resolution, the comparison breaks down.

Errors in Net Radiometry: Comparison and Evaluation of Six Radiometer Designs

Abstract: Net radiation is a fundamental variable in meteorology, but net radiometers have not been used extensively since the 1960s because of what is commonly considered as unreliable function. This study was conducted to determine whether this problem was caused by design, by calibration, or if some more fundamental problem was associated with net radiometry. Comparative measurements between six different types of net radiometers, calibrated in two wares were carried out under different climatic conditions. Calibration factors obtained from shadowing varied between 75% and 129% of the manufacturer's. The range was 85%–161% for calibrations relative to reference measurements of individual radiation components. Differences between the reference and individual meters were between 6% and 20%. After correction for different responsivities for longwave and shortwave radiation, the differences decreased markedly, especially in nighttime, except for two meters that had bad cosine responses. Three meters showed ...

Spatial resolution enhancement of terrestrial features using deconvolved SSM/I microwave brightness temperatures

Abstract: A method for enhancing the 19-, 22-, and 37-GHz measurements of the SSM/I (Special Sensor Microwave/Imager) to the spatial resolution and sampling density of the high-resolution 85-GHz channel is presented. An objective technique for specifying the tuning parameter, which balances the tradeoff between resolution and noise, is developed in terms of maximizing cross-channel correlations. Various validation procedures are performed to demonstrate the effectiveness of the method, which, it is hoped, will provide researchers with a valuable tool in multispectral applications of satellite radiometer data. >

Determination of Precipitable Water from Solar Transmission

Abstract: A method of determining precipitable water to within 10% from solar radiometer data has been developed. The method uses a modified Langley technique to obtain the water vapor optical depth, and a model developed at the University of Arizona is used to convert this to a precipitable water amount. The method is applied to two-and three-channel radiometric data and is compared to results obtained from empirical methods and to radiosonde data.

Actinometer and Eppley radiometer measurements of the NO2 photolysis rate coefficient during the Mauna Loa Observatory photochemistry experiment

Abstract: Measurements of the apparent first-order rate coefficient for NO2 photodissociation (jNO2) were made using a chemical actinometer during the month of May 1988 at the Mauna Loa Observatory, Hawaii. Simultaneous measurements of the ultraviolet irradiance (E), obtained with an Eppley radiometer, allowed extensive testing of the semi-empirical relationships between E and jNO2 proposed by Madronich (1987a). More than 3700 simultaneous measurements of jNO2 and E were obtained for solar zenith angles ranging from 4–90 degrees, and for different sky conditions (including clear skies, partial cloud cover, arid valley clouds below the horizon). For overhead clear skies, the NO2 photodissociation rate coefficient derived from Eppley radiometer data, here denoted j′, was in good agreement with actinometric measurements, j′/jNO2=1.01±0.05(1σ). The actinometer-radiometer relationship holds reasonably well even when low-lying valley clouds are present. For the periods of overhead intermittent clouds, the j′values track jNO2 values well, but the observed ratio shows significantly more scatter and the average is somewhat less than unity: 0.93 ± 0.09 (1σ). Measurements taken with and without upward scattered and reflected radiation show that valley clouds can contribute to the jNO2 values for the conditions encountered during the Mauna Loa Observatory Photochemistry Experiment.

The advanced very high resolution radiometer (AVHRR) - A brief reference guide

Abstract: The Advanced Very High Resolution Radiometer, carried by NOAA's Polar Orbiting Environmental Satellites, has unique characteristics of spectral response, image geometry, frequency of coverage, and accessibility that make it useful for applications in oceanography, terrestrial sciences, and meteorology. We review the history, engineering characteristics, data availability, applications, and future of this sensor. With cited references, this paper constitutes a starting point for more extensive investigations of the data and their applications.

COBE Differential Microwave Radiometers - Preliminary systematic error analysis

Abstract: The techniques available for the identification and subtraction of sources of dynamic uncertainty from data of the Differential Microwave Radiometer (DMR) instrument aboard COBE are discussed. Preliminary limits on the magnitude in the DMR 1 yr maps are presented. Residual uncertainties in the best DMR sky maps, after correcting the raw data for systematic effects, are less than 6 micro-K for the pixel rms variation, less than 3 micro-K for the rms quadruple amplitude of a spherical harmonic expansion, and less than 30 micro-(K-squared) for the correlation function.

Numerical sensitivity analysis of passive EHF and SMMW channels to tropospheric water vapor, clouds, and precipitation

Abstract: Potential uses of specific extremely high frequency (EHF) and submillimeter-wave (SMMW) channels at 90, 166, 283, 220, 325, 340, and 410 GHz for passive spaceborne remote sensing of the troposphere and lower stratosphere are investigated using an iterative numerical radiative transfer model. Collectively, these channels offer potential for high spatial resolution imaging using diffraction-limited apertures of practical size, along with the ability to profile water vapor, map precipitation beneath optically opaque cloud cover, and to measure nonprecipitating cloud (e.g. cirrus) parameters. A passive airborne imaging instrument for tropospheric meteorological sensing at 90, 150, 183+or-1, 3, 7, 220, and 325+or-1, 3, 9 GHz, called the Millimeter-wave Imaging Radiometer (MIR), is described. >

Radiometric determination of water vapor and liquid water and its validation with other techniques

Abstract: For remote sensing applications in the field of atmospheric physics it is necessary to know the highly variable part of the tropospheric water vapor absorption. With a dual-channel microwave radiometer, operating at 21.3 and 31.5 GHz, the estimation of the water vapor and liquid water content is possible. The retrieval is performed iteratively with an algorithm using simplified model profiles of the troposphere and a millimeter wave absorption model. This site-independent profile algorithm changes a model atmosphere until the calculated and measured sky brightness temperatures do agree. Integrated precipitable water vapor (IWV) and integrated liquid water (ILW) are obtained by integrating over the corresponding profiles. By the use of the absorption model sky brightness temperatures, opacities or propagation delays at other frequencies up to 1000 GHz can be calculated. Simulations with a few thousand test profiles showed that the rms error, excluding that of the absorption model, is about 3% for the IWV and between 10% and 20% for the ILW (ILW > 0.1 mm), while for the predicted opacities at 90, 142, and 204 GHz the error ranges between 15% and 25%. The performance of the algorithm and the water vapor radiometer has been checked with independent brightness temperature measurements at 142 and 204 GHz, respectively, IR spectroscopic measurements and radiosonde data. The independent measurements showed all a good agreement with the retrieved quantities of the profile algorithm.

Narrowband to Broadband Conversion with Spatially Autocorrelated Reflectance Measurements

Abstract: A new technique for estimating broadband reflectance from Advanced Very High-Resolution Radiometer (AVHRR) narrowband reflectances in channel 1 and 2 is developed. The data used are simultaneous and coincident narrowband and broadband measurements made by the AVHRR and Earth Radiation Budget Experiment (ERBE) radiometers aboard NOAA-9 during four days in July 1985 in the region north of 60°N. The limitations and inefficiency of classical regressional methods when applied to datasets with high spatial auto-correlation, which is often the case for remotely sensed data, are discussed. A statistical variable, Moran's I, is introduced, which is specifically designed for testing against a null hypothesis of spatial independence. On the basis of Moran's I and a correlogram analysis of the spatial autocorrelation of measured reflectances, the data are sampled to provide a spatially independent dataset. In addition to sampling, the data are also screened with respect to spatial homogeneity. Both scene-dep...

Characterization of an absolute cryogenic radiometer as a standard detector for radiant-power measurements.

Abstract: An active cavity radiometer of the electrical substitution type with a cone receiver that operates at 2-4 K has been developed for measuring radiant fluxes in the dynamic range of 20 nW to 100 microW within an uncertainty of +/-1% (2sigmalevel). It is a broadband absolute detector with a flat overall absorption efficiency that is >99% for radiation from the visible to long-wavelength IR. The system is designed based on thermal modeling and experimental measurements of concepts. It has been installed in the cryogenic chamber for low-background infrared radiation calibrations at the National Institute of Standards and Technology (NIST) for testing cryogenic blackbody sources, detectors, and optical components. Its time constant, responsivity, and nonequivalence error have been measured. They are in agreement with design predictions. Radiant power measurements of an amplitude-stabilized He-Ne laser beam with the radiometer and an industry standard photodiode detector, QED-200, have been intercompared and found to be in agreement. The intercomparison ratio of the measurements with the absolute cryogenic radiometer and QED-200 was 1.004 in the 75-100-microW range with an uncertainty of 0.5% (the 3sigma level).

COBE differential microwave radiometers - Calibration techniques

Abstract: The COBE spacecraft was launched November 18, 1989 UT carrying three scientific instruments into earth orbit for studies of cosmology. One of these instruments, the Differential Microwave Radiometer (DMR), is designed to measure the large-angular-scale temperature anisotropy of the cosmic microwave background radiation at three frequencies (31.5, 53, and 90 GHz). This paper presents three methods used to calibrate the DMR. First, the signal difference between beam-filling hot and cold targets observed on the ground provides a primary calibration that is transferred to space by noise sources internal to the instrument. Second, the moon is used in flight as an external calibration source. Third, the signal arising from the Doppler effect due to the earth's motion around the barycenter of the solar system is used as an external calibration source. Preliminary analysis of the external source calibration techniques confirms the accuracy of the currently more precise ground-based calibration. Assuming the noise source behavior did not change from the ground-based calibration to flight, a 0.1-0.4 percent relative and 0.7-2.5 percent absolute calibration uncertainty is derived, depending on radiometer channel.

Microwave radiometry in living tissue: what does it measure?

Abstract: The sensitivity of microwave radiometry for detecting subcutaneous targets was studied both experimentally and theoretically. The radiometer used a dielectric loaded rectangular waveguide antenna in contact with a lossy dielectric medium. A cylindrical target with dielectric properties and/or temperature different from that of the surrounding medium was located beneath this surface. The results indicate that microwave radiometry responds to the temperature averaged over the field pattern of the antenna with very strong weighting of regions near the surface. A simple quasi-static analysis provides a good indication of the sensitivity of the technique for detecting cylindrical targets whose dielectric properties are different from those of the surrounding medium. >

A simple instrument and technique for measuring columnar water vapor via near-IR differential solar transmission measurements

Abstract: A simple two-channel solar radiometer and data retrieval technique is described for sensing the columnar content of atmospheric water vapor via differential solar transmission measurements in and adjacent to the 940-nm water vapor absorption band. The instrument features two parallel channels for simultaneous measurements in and out of the absorption band to eliminate temporal variability effects in the differential comparison of the data from the two channels. The water vapor transmittance is determined by a modified Langley plot analysis of the ratio of the two channel signals. A statistical band model which closely follows the square-root law is then used to extract the columnar water vapor amount from the water vapor band transmittance. Error analyses and experimental results indicate that the instrument/technique can be reasonably employed to retrieve water vapor amounts with an error of 10% or less. >

Water‐Vapour Continuum Absorption In the Tropics: Aircraft Measurements and Model Comparisons

Abstract: Measurements of water-vapour continuum absorption in the 10-12 μm window have been made in the tropical atmosphere using a multi-channel narrow field-of-view radiometer mounted on an aircraft. Comparisons with two radiative transfer models (GENLN 2 and LOWTRAN7) are made which show that the model underestimates the absorption by some thirty percent. Two methods of modifying the existing empirical continuum parametrization to obtain better agreement with the aircraft measurements are assessed: one involves increasing the self-broadened absorption coefficient, the other increasing the temperature-dependence parameter.

Atmospheric water vapor measurements: Comparison of microwave radiometry and lidar

Abstract: The NASA/GSFC Crustal Dynamics Project microwave water vapor radiometer (J03) is evaluated in terms of measurements of the integrated precipitable water vapor content of a particular column of the troposphere. The measurements were taken during the Atmospheric Moisture Intercomparison Study (ATMIS) held at Wallops Island (Virginia) during April 1989. Water vapor sensing instruments used during ATMIS, included radiometers, radiosondes, and the NASA/GSFC Raman lidar. Comparisons between water vapor measurements by the radiometer and the lidar yielded a correlation coefficient of 0.998 and rms differences for three nights of -0.2 +/- 0.2 mm (April 11-12), -0.8 +/- 0.5 mm (April 16-17), and -0.4 +/- 0.3 mm (April 17-18). The integrated precipitable water vapor measurements for these three nights were approximately 5, 10 and 21 mm, respectively. This study shows that the radiometer provides accurate continuous measurements of the water vapor integrated through the depth of the atmosphere.

Galileo Photopolarimeter/Radiometer Experiment

Abstract: The Photopolarimeter/Radiometer (PPR) is a remote sensing instrument on the Galileo Orbiter designed to measure the degree of linear polarization and the intensity of reflected sunlight in ten spectral channels between 410 and 945 nm to determine the physical properties of Jovian clouds and aerosols, and to characterize the texture and microstructure of satellite surfaces. The PPR also measures thermal radiation in five spectral bands between 15 and 100 μm to sense the upper tropospheric temperature structure. Two additional channels which measure spectrally integrated solar and solar plus thermal radiation are used to determine the planetary radiation budget components. The PPR photopolarimetric measurements utilize previously flown technology for high-precision polarimetry using a calcite Wollaston prism and two silicon photodiodes to enable simultaneous detection of the two orthogonal polarization components. The PPR radiometry measurements are made with a lithium tantalate pyroelectric detector utilizing a unique arrangement of radiometric stops and a scene/space chopper blade to enable a warm instrument to sense accurately the much colder scene temperatures.

All-solid-state radiometers for environmental studies to 700 GHz

Abstract: We report results with an all-solid-state radiometer for measurements of the ClO molecule at 649 GHz. The project is part of a program to provide low-noise, low-weight, low-power radiometers for space operation, and special effort has been expended on the development of high-efficiency solid-state frequency multipliers and Schottky-barrier mixers with low local oscillator power requirements. The best measured system noise temperature was 1750 K with the mixer and preamplifier cooled to 77 K. The mixer diode was easily pumped into saturation, indicating that the design has excellent prospects of operating at higher frequencies - our present design goal being 1 THz. We comment on the principal design features of such systems and will report on stratospheric measurements performed with this system.

Precise ground-based solar photometry and variations of total irradiance

Abstract: Several empirical models of sunspot and facular irradiance effects were tested by assessing the degree of correlation between variations in the total solar irradiance, as measured by the active cavity radiometer irradiance monitor on the SMM and the measures of magnetic activity on the solar disk. This was done by analyzing images made during 21 days between June 20 and July 14, 1988. The paper also describes the instruments and the methods used to gather the ground-based photometric images, as well as the analysis procedure.

Active and passive microwave measurements of soil moisture in FIFE

Abstract: During the intensive field campaigns of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) in May-October of 1987, several nearly simultaneous measurements were made with low-altitude flights of the L-band radiometer and C- and X-band scatterometers over two transects in the Konza Prairie Natural Research Area, some 8 km south of Manhattan, Kansas. These measurements showed that although the scatterometers were sensitive to soil moisture variations in most regions under the flight path, the L-band radiometer lost most of its sensitivity in regions unburned for many years. The correlation coefficient derived from the regression between the radar backscattering coefficient and the soil moisture was found to improve with the increase in antenna incidence angle. This is attributed to a steeper falloff of the backscattering coefficient as a function of local incidence at angles near nadir than at angles greater than 30 deg.

Evaluating hand‐held radiometer derived vegetation indices for estimating above ground biomass

Abstract: The potential for using vegetation indices developed from remote sensing devices, such as the GSFC Mark‐II hand‐held radiometer, to evaluate biomass has been examined in a number of studies. Relationships range from very good (r2=0.96) to very poor (r2=0.029). One problem with most of the studies is that the data were only collected during a single year, therefore, the usefulness of the relationships within and between years is difficult to evaluate. This study examined the usefulness of vegetation indices, calculated from data gathered by the GSFC Mark‐II radiometer, in assessing the amount of biomass on native grasslands. Vegetation indices derived from data gathered over two years (1989 and 1990) and during four periods throughout each growing season indicate that the ratio between near‐infrared and red spectral responses (ratio vegetation index) performed better then either the difference between the two bands or the normalized difference vegetation indices. Poor relationships were observed (...

Radiative heat feedback in a toluene pool fire

Abstract: A purged radiometer was used to measure directional heat flux incident on the fuel surface in a 30 cm diameter toluene fire A new approximate method for the treatment of the effects of turbulence on radiation was evaluated using these data The average emissive power and the average transmittance are the two local properties needed for the approximate method These quantities were obtained from transient local measurements of temperatures and soot volume fractions based on emission at two wavelengths Soot volume fractions based on absorption were also measured for comparison The results showed that a large fraction of the soot particles observed by the absorption probe were at relatively low temperatures The predictions of directional heat fluxes showed systematic errors with angle when compared to measurements These errors are related to the absorption of energy by fuel vapor in the central core and the higher spatial resolution needed in the necking-in region of the fire The directional total flux data and predictions were integrated to obtain total radiative heat feedback to the surface Comparisons between measurements and predictions of total heat flux were reasonably good