Showing papers on "Radiometer published in 1990"

SSM/I instrument evaluation

Abstract: The Special Sensor Microwave/Imager (SSM/I) instrument and scan geometry are briefly described. The results of investigations of the stability of the gain, calibration targets and spin rate, the radiometer noise and sensitivity, the coregistration, the beam width and main-beam efficiency of the antenna beams, and the absolute calibration and geolocation of the instrument are presented. The results of this effort demonstrate that the SSM/I is a stable, sensitive, and well-calibrated microwave radiometric system capable of providing accurate brightness temperatures for microwave images of the Earth and for use by environmental product retrieval algorithms. It is predicted that this SSM/I and the 11 future ones currently built or to be built will provide high-performance microwave measurements for determination of global weather and critical atmospheric, oceanographic, and land parameters to operational forecasters and users and the research community for the next two decades. >

Optimum interpolation of imaging microwave radiometer data

Abstract: A technique of interpolating imaging microwave radiometer data is presented as an application of the Backus-Gilbert theory. The interpolation process is optimal in the sense that it attempts to preserve the spatial resolution of the antenna gain function associated with the sampled radiometer data. The technique is applied to the Special Sensor Microwave/Imager (SSM/I) satellite data and is found to enhance the high-resolution features of the imagery. >

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

Abstract: 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.

Atmospheric water vapour over oceans from SSM/I measurements

Abstract: A statistical retrieval technique is developed to derive the atmospheric water vapor column content from the Special Sensor Microwave/Imager (SSM/I) measurements. The radiometer signals are simulated by means of radiative-transfer calculations for a large set of atmospheric/oceanic situations. These simulated responses are subsequently summarized by multivariate analyses, giving water-vapor coefficients and error estimates. Radiative-transfer calculations show that the SSM/I microwave imager can detect atmospheric water vapor structures with an accuracy from 0.145 to 0.17 g/sq cm. The accuracy of the method is confirmed by globally distributed match-ups with radiosonde measurements.

Determination from Space of Atmospheric Total Water Vapor Amounts by Differential Absorption near 940 nm: Theory and Airborne Verification

Abstract: A new technique is proposed to estimate atmospheric total water vapor amounts from space. The technique consists of viewing the Earth's surface in two spectral channels, one narrow, the other wide, centered on the same wavelength at the water vapor absorption maximum near 940 nm. With these characteristics, the ratio of the solar radiance measured in the two channels is independent of the surface reflectance and yields a direct estimate of the water vapor amount integrated along the optical path. To test the technique, we designed and built a two-channel radiometer based on the above concept. Airborne experiments carried out with the new device demonstrate the technique's feasibility under clear sky conditions over both sea and land. Over the ocean and in the presence of thick aerosol layers, however, total water vapor amounts may be underestimated by as much as 20%. Compared to satellite microwave techniques, which are applicable under most weather conditions, the proposed technique has the adva...

Sea Surface Temperature

Abstract: Satellite Sea Surface Temperature (SST) determinations from ocean surface emission in the thermal IR portion of the spectrum have been made for a number of years (McClain, 1979; Alishouse and McClain, 1985). A number of research and operational environmental satellites have carried scanning radiometer systems that measure emitted terrestrial and atmospheric IR radiation. The various sensors and their characteristics have been described in Section IV and by McClain (1980a, b). In most cases the atmospheric window from 10.5 to 12.5 µm has been used, but its use for determining absolute sea surface temperature has often been secondary to its use for imaging of clouds and thermal fronts associated with ocean currents and upwelling (Legeckis, 1978). Radiometers of improved quality, such as the AVHRR, with two and three atmospheric window channels, have permitted development of methods to retrieve absolute SST of far better accuracy than was possible with single window systems (Strong and McClain, 1984).

Initial Results In The Development Of A Synthetic Aperture Microwave Radiometer

Abstract: A radiometer that measures the complex correlation of the voltage from pairs of antennas at many different baselines is being developed. Each baseline produces a sample point in the Fourier transform of the scene, and a map of the scene is obtained after all measurements have been made by inverting the transform. A substantial reduction in the antenna collecting area required compared to a conventional imaging radiometer can be obtained in this manner. An aircraft prototype being developed is a hybrid which uses real aperture antennas to obtain resolution along-track (stick antennas) and uses aperture synthesis to obtain resolution across-track. The prototype was flight-tested aboard the NASA P-3 in June 1988. During this flight a map was made of the Delmarva Peninsula south of NASA's Wallops Flight Facility. This initial map shows the major land/water features and compares very favorably with a Landsat image of the area, suggesting a bright outlook for the development of this technique in the future.

Determination of the Spectral Absorption of Solar Radiation by Marine Stratocumulus Clouds from Airborne Measurements within Clouds

Abstract: A multiwavelength scanning radiometer has been used to measure the angular distribution of scattered radiation deep within a cloud layer at discrete wavelengths between 0.5 and 2.3 pm. The relative angular distribution of the intensity field at each wavelength is used to determine the similarity parameter, and hence single scattering albedo, of the cloud at that wavelength using the diffusion domain method. In addition to the spectral similarity parameter, the analysis provides a good estimate of the optical thickness of the cloud beneath the :raft. Results of such analysis are presented for a 50 km section of clean marine stratocumulus clouds off the coast of California on 10 July 1987. These observations were obtained from the University of Washington Convair C- 13 1 A aircraft as part of the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE). In addition to the radiation measurements, we obtained microphysical and thermodynamic measurements from which the expected similarity parameter spectrum was calculated using accepted values of the refractive index of liquid water and the transmission function of water vapor. The measured absorption for this case of clean maritime clouds is close to, but persistently larger than, theoretical predictions. Finally, measurements are presented of the spectral albedo of the ocean surface under overcast conditions.

Remotely Sensed Measurements of Stratocumulus Properties during FIRE Using the C130 Aircraft Multi-channel Radiometer

Abstract: This paper gives a description of a narrow-hand near-infrared radiometer that was operated during the marine stratocumulus intensive field observation program of FIRE in 1987 from the United Kingdom Meteorological Office C130 aircraft. All data with small solar zenith angles and continuous stratocumulus sheets have been analysed. A method to determine cloud optical depth and effective radius of the drop size distribution is presented from both reflectance and transmittance data. The data demonstrate that different cloud droplet sizes were successfully distinguished and realistic optical depths were obtained. Absolute comparison with in situ droplet effective radii measurements showed an inconsistency, remotely retrieved values being up to 50% larger than the in situ values.

The regional optimization of infrared measurements of sea surface temperature from space

Abstract: The accuracy with which well-calibrated satellite infrared radiometers can measure sea surface temperature is limited by the validity of the correction applied for the modification of the electromagnetic radiation before it reaches the radiometer. An accurate numerical line-by-line model of the radiative transfer through the atmosphere is used to simulate measurements of the advanced very high resolution radiometer (AVHRR/2) on the NOAA series of near-polar-orbiting satellites for conditions of the region of the Greenland, Iceland, and Norwegian Sea. A set of regionally optimized zenith-angle dependent coefficients for the “split-window” algorithm is derived and its error characteristics are discussed. While the benefit of using such coefficients is demonstrated, the errors resulting from failing to account properly for seasonal changes in this particular region are shown to be relatively small.

The along track scanning radiometer for ERS-1-scan geometry and data simulation

Abstract: The first European remote-sensing satellite (ERS-1) will carry the along track scanning radiometer (ATSR). which has been specifically designed to give accurate satellite measurements of sea surface temperature (SST). Details of the novel scanning technique used by the ATSR are given, and data from the NOAA-9 AVHRR instrument are used to simulate raw ATSR imagery. Because of the high precision of the onboard blackbodies, the active cooling of the detectors, 12-b digitization, and dual-angle capability, the ATSR promises to achieve higher-accuracy satellite-derived SSTs than are currently available. >

Cirrus Structure and Radiative Parameters from Airborne Lidar and Spectral Radiometer Observations: The 28 October 1986 FIRE Study

Abstract: Remote sensing lidar and imaging spectral radiometer observations were obtained from the ER-2 high-altitude research aircraft during the 1986 FIRE cirrus missions. The dual polarization lidar measurements were nadir directed with 7.5 m vertical and 40 m horizontal resolution, and clearly depicted structure at the top and within the cirrus. Simultaneous radiometric cloud top images were acquired with 5 mrd resolution at ten visible channels, three infrared window channels, and four near-infrared channels. The combined lidar and radiometer data were analyzed for the cirrus structure, radiative parameters, and inferred microphysical properties. On 28 October 1986 a cirrus formation crossed Wisconsin. The results indicate that for the eastern edge of the formation there was a cirrus layer at 9 to 11 km altitude, and a separate lower cloud at 7 to 8 km. The lidar depolarization indicated the upper layer was ice crystals, the lower layer was ice in some areas, and water or possibly mixed phase in other...

The Magellan Venus Radar Mapping Mission.

Abstract: The NASA Magellan Venus Radar Mapper spacecraft, which will be placed into orbit around Venus on August 10, 1990, is described and its mission is discussed. The orbiter's 12-cm wavelength, multimode radar system is examined and the applications of its modes are addressed. In the SAR mode, it can image most of the Venus surface at a resolution of better than 300 m, approaching 120 m over more than half the planet. In the altimeter mode, the radar will determine topographic relief to a vertical accuracy of better than 50 m averaged over a surface resolution cell approximately 10 km in diameter. In the radiometer mode, the radar receiver can determine the surface radio emission brightness temperature with an absolute accuracy of 20 K, at a resolution of 2 K. The nature of the data products and the archiving plans are also considered.

The beam filling error in the Nimbus 5 electronically scanning microwave radiometer observations of Global Atlantic Tropical Experiment rainfall

Abstract: A comparison of rain rates retrieved from the Nimbus 5 electronically scanning microwave radiometer brightness temperatures and observed from shipboard radars during the Global Atlantic Tropical Experiment (GATE) phase I shows that the beam filling error is the major source of discrepancy between the two. When averaged over a large scene (the GATE radar array, 400 km in diameter), the beam filling error is quite stable, being 50 percent of the observed rain rate. This suggests the simple procedure of multiplying retrieved rain rates by 2 (correction factor). A statistical model of the beam filling error is developed by envisioning an idealized instrument field-of-view that encompasses an entire gamma distribution of rain rates. A modeled correction factor near 2 is found for rain rate and temperature characteristics consistent with GATE conditions. The statistical model also suggests that the correction factor varies from 1.5 to 2.5 for suppressed to enhanced tropical convective regimes, and decreases to 1.5 as the freezing level and average depth of the rain column decreases to 2.5 km.

A high-resolution chirp transform spectrometer for microwave measurements

Abstract: A microwave heterodyne spectrometer consists of a radiometer front-end and a real-time spectrum analyser back-end. Common spectrometer types are filterbanks, autocorrelators and acousto-optic spectrometers (AOS). A high-resolution chirp transform spectrometer using reflective array compressor (RAC) filters with a time bandwidth product of 6400 is presented. The spectrometer has 1600 channels within a 40 MHz input bandwidth and a linear dynamic range of 63 dB for a CW input. It seems to be very promising for future space applications.

The sensitivity of synthetic aperture radiometers for remote sensing applications from space

Abstract: Aperture synthesis offers a means of realizing the full potential of microwave remote sensing from space by helping to overcome the limitations set by antenna size. The result is a potentially lighter, more adaptable structure for applications in space. However, because the physical collecting area is reduced, the signal-to-noise ratio is reduced and may adversely affect the radiometric sensitivity. Sensitivity is an especially critical issue for measurements to be made from low earth orbit because the motion of the platform limits the integration time available for forming an image. The purpose is to develop expression for the sensitivity of remote sensing systems which use aperture synthesis. The objective is to develop basic equations general enough to be used to obtain the sensitivity of the several variations of aperture synthesis which were proposed for sensors in space. The conventional microwave imager (a scanning total power radiometer) is treated as a special case and a comparison of three synthetic aperture configurations with the conventional imager is presented.

Boundary layer diagnostics by means of an infrared scanning radiometer

Abstract: A computerized infrared (IR) scanning radiometer is employed to characterize the boundary layer development over a model wing, having a Gottingen 797 cross-section, by measuring the temperature distribution over its heated surface. The Reynolds analogy is used to relate heat transfer measurements to skin friction. The results show that IR thermography is capable of rapidly detecting location and extent of transition and separation regions of the boundary layer over the whole surface of the tested model wing. Thus, the IR technique appears to be a suitable and effective diagnostic tool for aerodynamic research in wind tunnels.

Passive microwave remote sensing of cloud liquid water over land regions

Abstract: Techniques for cloud liquid water retrieval over land are developed using data from the Special Sensor Microwave/Imager 85.5 GHz (3.5 mm) channels. To minimize the effect of surface emittance variability on the calculations, the surface emittance was estimated with the aid of surface skin temperature retrievals from the Visible Infrared Spin Scan Radiometer in geosynchronous orbit. The high sensitivity of the 85.5 GHz channels to cloud liquid water allows for the estimation of integrated cloud liquid water based on the microwave brightness temperature depression caused by attenuation and emission of microwave radiation at the colder ambient temperature of the cloud. The method assumes nonscattering radiative processes are dominant, therefore only nonprecipitating cloud liquid water is considered. Integrated cloud liquid water retrievals show good qualitative agreement with other available data sources. Numerical error sensitivity analysis show integrated cloud liquid water error estimates of 0.05-0.50 kg m{sup {minus}2} depending on the contrast of cloud over background.

An automated cloud screening algorithm for daytime advanced very high resolution radiometer imagery

Abstract: Valid estimates of sea surface temperature (SST) from the advanced very high resolution radiometer (AVHRR) are critically dependent upon the identification and removal of clouds from parent images. Previous cloud-screening algorithms have demonstrated varying degrees of success, the effectiveness of the particular method used being critically dependent upon cloud type. Contamination from cirrus clouds, for example, has been a major deficiency in previous methods. A robust new algorithm for cloud masking has now been developed which is very effective at removing cloud contamination from images regardless of cloud type. The method, which evaluates every pixel in the image, is statistically reproducible, computationally efficient, and requires no subjective inputs or knowledge of cloud type. The results also show that the algorithm is not regionally specific. The basis for this new method is presented and applications to global AVHRR data, including comparisons with previously used methods, are shown.

The absolute temperature of the sky and the temperature of the cosmic background radiation at 600 MHz

Abstract: The absolute temperature of the sky at delta = 465 deg has been measured at 600 MHz (50 cm wavelength) using a differential radiometer and a cold calibrator at liquid helium temperature To separate the cosmic background radiation (CBR) from the remaining components of the sky temperature, observations at other frequencies, now under way, are necessary A preliminary estimate of the CBR temperature at 600 MHz made using data of T(sky) at 600 MHz and maps of the sky at 408 MHz already in the literature gives T(cbr) = (30 + or - 12) K, in good agreement with previous measurements at nearby frequencies 45 refs

Multiparameter Radar And Microwave Radiative Transfer Modeling Of Nonspherical Atmospheric Ice Particles

Abstract: Multiparameter radar and polarized microwave radiative transfer models for arbitrarily shaped particles are developed. The ice crystals modeled are horizontally oriented hexagonal plates, columns, and needles. The ice water content is fixed at 0.1 g/cu m, and a realistic size distribution is used. The radar modeling is done for S through K-band, and the passive microwave calculations are at 37, 85, and 157 GHz. The modeling results show that particle shape is important for both multiparameter radar and passive microwave radiometry. Radar reflectivity and upwelling microwave brightness temperatures depend strongly on the individual particle volume, which in turn depends on the ice crystal shape. Radar differential reflectivity is high for the plates and lower for needles and columns. Linear depolarization calculations indicate that oblate ice crystals such as plates can be distinguished from prolate crystals such as columns. At 85 and 157 GHz, significant polarization brightness temperature differences are calculated for plates and columns. The particle bulk density strongly affects the radar and radiometer observables. Recent measurements of ice crystals are discussed to show that the model results have practical applicability.

In-situ atmospheric water-vapor retrieval in support of AVIRIS validation

Abstract: A comparison is made between two ground-based atmospheric water-vapor measurement techniques, each of which uses data from a solar-pointing radiometer. One technique uses visible wavelength channels to retrieve aerosol loading, surface pressure readings for Rayleigh scattering analyses, and the 0.94 micron channel to extract water vapor from the residual of total versus scattering opacity. The other technique requires only the ratio of channels centered at 0.94 and 0.87 micron. Results are given for the April 13, 1989 AVIRIS in-flight calibration.

A study of the geolocation errors of the Special Sensor Microwave/Imager (SSM/I)

Abstract: Geolocation errors in excess of 20-30 km have been routinely observed in SSM/I data. Potential error sources due to inaccurate spacecraft ephemeris, spacecraft predict ephemeris, and the geolocation algorithm are examined. In addition, the effects of possible misalignment of the SSM/I instrument about its spin axis or to the spacecraft and potential spacecraft attitude errors are considered. It is found that the use of accurate spacecraft ephemerides removes about half of the geolocation error. In the cases studied, the remaining error can be reduced to below 7 km by employing a fixed set of effective spacecraft attitude corrections in the geolocation algorithm; 85-GHz imagery is presented to provide a visual evaluation of the geolocation errors along with the reduction of these errors. >

Determination of surface reflectance and estimates of atmospheric optical depth and single scattering albedo from Landsat Thematic Mapper data

Abstract: Ground reflectance data of selected targets for calibration of a Landsat Thematic Mapper (TM) image of Wind River Basin, Wyoming, acquired November 21, 1982, were analysed. The calibration lines, one for each band, are derived from linear regression analysis of scatter plots of ground bidirectional spectral reflectance versus the Thematic Mapper (TM) scanner response. The field-measured reflectances were obtained with a portable spectrometer at 10 nm spectral resolution in November 1983. The empirical equations derived were used to predict the ground reflectance of five unknown targets scattered throughout the Wind River and adjacent Bighorn Basins. Subsequently, in July 1984, field measurements were made over these targets with field spectrometers and with a hand-held radiometer with filters matching those of the TM. The agreement between predicted and measured quantities, mostly within the uncertainties of the field observations, was obtained, suggesting that the ground reflectance properties o...

Retrieval of Precipitation Profiles from Airborne Radar and Passive Radiometer Measurements: Comparison with Dual-Frequency Radar Measurements

Abstract: Precipitation-rate profiles derived from a single-frequency radar and a radiometer are compared with profiles derived from a dual-frequency radar, using data obtained during the 1985-1986 Communication Research Laboratory/NASA rain measuring experiment over the Atlantic Ocean from a dual-frequency X- and Ka-band radars and an X-band passive radiometer. Profiles were retrieved using a modification of the Hitschfeld-Bordan (1954) algorithm constrained by a measurement of the path integrated extinction derived from passive radiometry. It was found that the two techniques under comparison yielded average rainfall rates that agreed within about 25 percent. It is suggested that, while a single-frequency radar operating in conjunction with multifrequency passive radiometers might provide cost-effective spaceborne measurements of tropical rainfall distributions, a dual-frequency radar might be able to remove some ambiguities that were encountered in this study.