Showing papers on "Radiometer published in 1998"

Radiometric profiling of temperature, water vapor and cloud liquid water using various inversion methods

Abstract: We have developed a tunable temperature profiler based on a highly stable synthe- sizer that can observe at multiple frequencies on the shoulder of the 60 GHz atmospheric oxygen feature. We are developing a similar radiometer to obtain the vertical distribution of water vapor by making observations on the pressure broadened water vapor line from 22 to 29 GHz. Informa- tion on cloud liquid water profiles is also contained in these two wavebands. Various mathemati- cal retrieval methods for temperature, water vapor, and cloud liquid water profiles were tested based on these radiometer designs. These include neural networking, Newtonian iteration of sta- tistically retrieved profiles, and Bayesian "most probable" retrievals. Based on realistic radiome- ter errors and performance, very good retrieval capability is demonstrated. The performance of the various retrieval methods are presented and compared. Examples of profile retrievals are also presented. Data were not binned into seasons to reduce computer time; better retrieval results for all methods would be expected with binning. 1. Motivation for Radiometric Atmospheric Profiling Radiosonde observations (RAOBs) are the fundamental method for atmospheric temperature, wind, and water vapor profiling, in spite of their inaccuracies, cost, sparse temporal sampling and logistical difficulties. A better technology has been sought for decades, but until now, no accurate continuous all weather technology has been demonstrated. Laser radars (LIDARS) and Fourier transform infrared spectrometers can profile temperature and water vapor, but not in the presence of cloud. Our highly stable frequency agile radiometric temperature and water vapor profilers give continuous unattended profile measurements. They also have the capability to profile cloud liquid water, a capability absent in RAOBs and all other systems except for in situ aircraft devices. Applications for this passive radiometric profiling include: weather forecasting and now- casting; detection of aircraft icing and other aviation related meteorological hazards; determina- tion of density profiles for artillery trajectory and sound propagation; refractivity profiles for radio ducting prediction; corrections to radio astronomy; satellite positioning and Global Posi- tioning System (GPS) measurements; atmospheric radiation flux studies; estimation and predic-

Three months of continuous monitoring of atmospheric water vapor with a network of Global Positioning System receivers

Abstract: Three months of continuous data from the Global Positioning System (GPS) using 20 sites in Sweden and 5 sites in Finland have been used to estimate the integrated amount of atmospheric water vapor. The quality of the data has been assessed by comparisons with a microwave radiometer (water vapor radiometer (WVR)) at the Onsala Space Observatory and with data from four different radiosonde stations. We found the agreement in integrated water vapor (IWV) between the GPS estimates and the radiometer data to be 1–2 kg/m2 in terms of daily root-mean-square (rms) differences. A major part of these rms differences were caused by a bias between the data sets. This bias (WVR-GPS) varied from day to day between −1.0 and +2.5kg/m2 with a mean value of +1.3kg/m2. Comparisons with radiosonde data showed rms differences around or slightly above 2kg/m2 for each station using the entire 3 month data set. Also here the GPS estimates were, on the average, below the radiosonde results. We show that the radomes used to protect the GPS antennas are likely to cause a large part of the observed bias. Spatial structure functions were calculated by using the GPS and the radiosonde data. An overall consistency between the GPS-based and the radiosonde-based structure functions indicates that the spatial correlations between the GPS estimates are not affected by the estimation process used in the GPS data analysis.

Retrieval of aerosol optical depth over land using two-angle view satellite radiometry during TARFOX

Abstract: A new aerosol optical depth retrieval algorithm is presented that uses the two-angle view capability of the Along Track Scanning Radiometer 2 (ATSR-2). By combining the two-angle view and the spectral information this so-called dual view algorithm separates between aerosol and surface contributions to the top of the atmosphere radiance. First validation of the dual view algorithm was performed during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX), which was conducted at the mid-Atlantic coast of the United States in July 1996. The satellite retrieved spectral aerosol optical depth is in good agreement with the aerosol optical depth from ground-based Sun/sky radiometers in three out of four cases. This shows the potential of aerosol retrieval over land using two-angle view satellite radiometry. Copyright 1998 by the American Geophysical Union.

The USDA Ultraviolet Radiation Monitoring Program

Abstract: The U.S. Department of Agriculture's Ultraviolet (UV) Radiation Monitoring Program has been measuring UV radiation since 1994. The initial network of 12 stations employed broadband meters to measure UVB irradiance and included ancillary measurements of temperature, humidity, and irradiance at seven wavelengths in the visible produced by a Multi-Filter Rotating Shadowband Radiometer (MFRSR). Since that beginning the network has expanded to more than 20 stations and the broadband meters have been supplemented with a seven-wavelength Ultraviolet Multi-Filter Rotating Shadowband Radiometer (UV-MFRSR). The network has been designed to include 30 stations, each with a full complement of instrumentation. Annual characterizations of the network's filter radiometers indicate that gradual shifts in instrument response are manageable but must be accounted for to achieve accurate and precise measurements of UV irradiance. The characterization and calibration of the filter instruments is discussed along with ...

The ScaRaB Earth Radiation Budget Dataset

Abstract: Following an overview of the scientific objectives and organization of the French–Russian–German Scanner for Radiation Budget (ScaRaB) project, brief descriptions of the instrument, its ground calibration, and in-flight operating and calibration procedures are given. During the year (24 February 1994–6 March 1995) of ScaRaB Flight Model 1 operation on board Meteor-3/7, radiometer performance was generally good and well understood. Accuracy of the radiances is estimated to be better than 1% in the longwave and 2% in the shortwave domains. Data processing procedures are described and shown to be compatible with those used for the National Aeronautics and Space Administration's (NASA) Earth Radiation Budget Experiment (ERBE) scanner data, even though time sampling properties of the Meteor-3 orbit differ considerably from the ERBE system orbits. The resulting monthly mean earth radiation budget distributions exhibit no global bias when compared to ERBE results, but they do reveal interesting strong r...

Radiometric sensitivity computation in aperture synthesis interferometric radiometry

Abstract: This paper is concerned with the radiometric sensitivity computation of an aperture synthesis interferometric radiometer devoted to Earth observation. The impact of system parameters and the use of simultaneous redundant measurements are analyzed. The interferometric radiometer uncertainty principle is presented; it quantifies the relationship between radiometric sensitivity and angular resolution.

Modeling of Submillimeter Passive Remote Sensing of Cirrus Clouds

Abstract: The scattering properties of cirrus clouds at submillimeter-wave frequencies are analyzed and characterized in this paper. This study lays a theoretical foundation for using radiometric measurements to investigate and monitor cirrus properties from high-flying aircraft or satellite. The significance of this capability is that it would provide data on the global distribution of cloud ice mass that is currently required to validate climate models. At present, these needs remain unmet by existing and planned observational systems. In this study the brightness temperature depression (DTb) of upwelling radiation due to cirrus clouds is simulated at 150, 220, 340, 500, 630, and 880 GHz. The effects of a range of size distributions, eight ice particle shapes, and different atmospheric profiles are modeled. The atmospheric transmission is high enough in the submillimeter windows to allow upper-tropospheric sensing from space, but absorption by water vapor reduces the sensitivity to lower cirrus clouds in a simply predictable manner. It is shown that frequencies above 500 GHz have adequate sensitivity to measure cirrus cloud properties. For these higher frequencies, the DTb is closely proportional to ice water path (IWP) for median mass equivalent sphere diameters (Dme) above 125 mm. The differing sensitivity with frequency allows two channels to determine particle size. A two-channel Bayesian algorithm is developed to assess retrieval accuracy with a Monte Carlo error analysis procedure. Particle shape, size distribution width, and receiver noise are considered as error sources. The rms errors for a nadir view with 630/880 GHz are less than 40% for IWP . 5gm 22 and Dme . 100 mm, while using an oblique viewing angle of 738 results in the same accuracy down to an IWP o f1gm 22 (visible optical depth less than 0.1). The two-channel algorithm and error analysis methods are used to show how submillimeter radiometer and millimeter radar measurements may be combined.

Fast generic millimeter-wave emissivity model

Abstract: In recent year san increasingly diverse range of passive microwave satellite data has become available for applications in numerical weather forecasting, climate studies and environmental monitoring. Top of atmosphere radiance is measured, which as originated from both the surface and the atmosphere. When retrieving atmospheric quantities such as temperature, humidity or cloud liquid water content near the surface it is necessary to account correctly for the contribution to the measured radiance from the surface. This depends on the surface emissivity, which varies widely with surface type, roughness and temperature. For real time applications such as numerical weather forecasting, it is necessary to be able to model the surface emissivity very quickly. There is therefore a need for a fast surface emissivity model has therefore been developed. This is a semi-empirical mode. Some aspects are physically based, for example many surface scan be assumed to be di- electric media. Other aspects such as geometric roughness have been parameterized for speed. For some complex or poorly understood aspects of the electromagnetic interaction empirical adjustments are made to fit observed values. The model has been compared with emissivities derived from aircraft radiometer measurements at 24, 50, 89 and 157 GHz. It is also intended to compare with data from the special sensor microwave imager and advanced microwave sounding unit instruments. For the ocean surface the fast model fits estimated emissivities from airborne radiometers two within 1-2 percent, and it fits the unparameterized model to within 0.1-3 percent.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Use of cloud model microphysics for passive microwave-based precipitation retrieval : Significance of consistency between model and measurement manifolds

Abstract: Precipitation estimation from passive microwave radiometry based on physically based profile retrieval algorithms must be aided by a microphysical generator providing structure information on the lower portions of the cloud, consistent with the upper-cloud structures that are sensed. One of the sources for this information is mesoscale model simulations involving explicit or parameterized microphysics. Such microphysical information can be then associated to brightness temperature signatures by using radiative transfer models, forming what are referred to as cloud‐radiation databases. In this study cloud‐radiation databases from three different storm simulations involving two different mesoscale models run at cloud scales are developed and analyzed. Each database relates a set of microphysical profile realizations describing the space‐time properties of a given precipitating storm to multifrequency brightness temperatures associated to a measuring radiometer. In calculating the multifrequency signatures associated with the individual microphysical profiles over model space‐time, the authors form what are called brightness temperature model manifolds. Their dimensionality is determined by the number of frequencies carried by the measuring radiometer. By then forming an analogous measurement manifold based on the actual radiometer observations, the radiative consistency between the model representation of a rain cloud and the measured representation are compared. In the analysis, the authors explore how various microphysical, macrophysical, and environmental factors affect the nature of the model manifolds, and how these factors produce or mitigate mismatch between the measurement and model manifolds. Various methods are examined that can be used to eliminate such mismatch. The various cloud‐radiation databases are also used with a simplified profile retrieval algorithm to examine the sensitivity of the retrieved hydrometeor profiles and surface rainrates to the different microphysical, macrophysical, and environmental factors of the simulated storms. The results emphasize the need for physical retrieval algorithms to account for a number of these factors, thus preventing biased interpretation of the rain properties of precipitating storms, and minimizing rms uncertainties in the retrieved quantities.

Design and preflight performance of ASTER instrument protoflight model

Abstract: The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is an advanced multispectral imager with high spatial, spectral, and radiometric resolution, built to fly on the EOS-AM1 spacecraft along with four other instruments, which will be launched in 1998. The ASTER instrument covers a wide spectral region, from visible to thermal infrared with 14 spectral bands. To meet the wide spectral coverage, optical sensing units of ASTER are separated into three subsystems: visible and near-infrared (VNIR) subsystem, shortwave infrared (SWIR) subsystem, and thermal infrared (TIR) subsystem. ASTER also has an along-track stereoscopic viewing capability using one of the near-infrared bands. To acquire the stereo data, the VNIR subsystem has two telescopes, one for nadir and another for backward viewing. Several new technologies are adopted as design challenges to realize high performance. Excellent observational performances are obtained by a pushbroom VNIR radiometer with a high spatial resolution of 15 m, a pushbroom SWIR radiometer with high spectral resolution, and a whiskbroom-type TIR radiometer with high spatial, spectral, and radiometric resolutions. The preflight performance is evaluated through a protoflight model (PFM).

A New Look at Calibration and Use of Eppley Precision Infrared Radiometers. Part I: Theory and Application

Abstract: The calibration and accuracy of the Eppley precision infrared radiometer (PIR) is examined both theoretically and experimentally. A rederivation of the fundamental energy balance of the PIR indicates that the calibration equation in common use in the geophysical community today contains an erroneous factor of the emissivity of the thermopile. If a realistic value (0.98) for the emissivity is used, then this leads to errors in the total flux of 5–10 W m−2. The basic precision of the instrument is found to be about 1.5% of the total IR irradiance when the thermopile voltage and both dome and case temperatures are measured. If the manufacturer’s optional battery-compensated output is used exclusively, then the uncertainties increase to about 5% of the total (20 W m−2). It is suggested that a modern radiative transfer model combined with radiosonde profiles can be used as a secondary standard to improve the absolute accuracy of PIR data from field programs. Downwelling IR fluxes calculated using the ...

The Optical Properties of Equatorial Cirrus from Observations in the ARM Pilot Radiation Observation Experiment

Abstract: The optical properties of equatorial cirrus were studied during a three-week period of the ARM Pilot Radiation and Observation Experiment at Kavieng, Papua New Guinea, in January and February 1993. The experiment consisted of vertical lidar (532 nm) and passive infrared filter radiometer (10.84 μm) observations of cirrus clouds. The observations gave values of cloud height, depth, structure, infrared emittance, infrared absorption, and visible optical depth and linear depolarization ratio. A standard lidar–radiometer analysis, with some improvements, was used to calculate these quantities. The cirrus was found to vary in altitude from a maximum cloud top of 17.6 km to a minimum cloud base of 6 km with equivalent temperatures of −82°C to −7°C respectively. The cirrus also varied widely in depth (0.7 to 7.5 km). The mean emittance (for each temperature interval) of the cooler clouds was found to be higher than that observed previously at tropical and midlatitude sites and at equivalent temperatures...

The potential for temperature retrieval from an angular‐scanning single‐channel microwave radiometer and some comparisons with in situ observations

Abstract: During December 1993 a microwave radiometer belonging to the Central Aerological Observatory (CAO) Moscow was operated at the Meteorological Research Unit, Cardington, UK. The radiometer has a single channel with a central frequency of 61.0 GHz, bandwidth of 2 GHz and sensitivity 0.04 K for a 1 s integration time. The antenna (beam-width 6°) was scanned from nadir to horizontal in 9° steps. Calibration was done every scan cycle using an electrically switched load. The information potentially available from such a radiometer is discussed. The measurements made at Cardington are compared with in situ observations made by radiosondes and by the Meteorological Office's tethered balloon system. Microwave brightness temperatures calculated from the radiosonde ascents are compared with observations. These were found to agree with a mean difference of 0.1 K and with a standard deviation of <0.3 K. Similarly retrieved temperature profiles up to 500 m were found to agree with the radiosonde profiles within 1 K rms. During the comparison period there were no strong low-level inversions to test fully the radiometer's retrieval capability. Copyright © 1998 Royal Meteorological Society

A study of regional aerosol radiative properties and effects on ultraviolet‐B radiation

Abstract: A field experiment was conducted in western North Carolina to investigate the relationship between aerosol optical properties and atmospheric transmission. Two research measurement sites in close horizontal proximity but at different altitudes were established to measure the transmission of UV radiation through a slab of atmosphere. An identical set of radiation sensing instruments, including a broadband UV-B radiometer, a direct Sun pyrheliometer, a shadowband radiometer, and a spectral photometer, was placed at both sites, a mountaintop site (Mount Gibbes 35.78°N, 82.29°W, 2004 m elevation) and a valley site (Black Mountain, North Carolina 35.66°N, 82.38°N, 951 m elevation). Aerosol size distribution sampling equipment was located at the valley site. Broadband solar pseudo-optical depth and aerosol optical depths at 415 nm, 500 nm, and 673 nm were measured for the lowest 1-km layer of the troposphere. The measurements exhibited variations based on an air mass source region as determined by back trajectory analysis. Broadband UV-B transmission through the layer also displayed variations relating to air mass source region. Spectral UV transmission revealed a dependence upon wavelength, with decreased transmission in the UV-B region (300–320 nm) versus UV-A region (320–363.5 nm). UV-B transmission was found to be negatively correlated with aerosol optical depth. Empirical relations were developed to allow prediction of solar noon UV-B transmission if aerosol optical depth at two visible wavelengths (415 and 500 nm) is known. A new method was developed for determining aerosol optical properties from the radiation and aerosol size distribution measurements. The aerosol albedo of single scatter was found to range from 0.75 to 0.93 and the asymmetry factor ranged from 0.63 to 0.76 at 312 nm, which is close to the peak response of human skin to UV radiation.

Extension of the clean technique to the microwave imaging of continuous thermal sources by means of aperture synthesis radiometers

Abstract: The derivation of soil moisture and ocean salinity contents can be performed by means of passive observations at L-band. The global coverage, the low revisit time (1-3 days), the radiometric resolution (1 K) and the spatial resolution required (10-30 Km) can be achieved by means of low orbit interferometric radiometers. Interferometric radiometers synthesize a thin beam by correlating the outputs of a sparse array of small antennas. The imaging algorithms used in radioastronomy can not be directly applied to Earth observation because of the large number of baselines, the proximity between the antennas, which increases coupling effects, and the antennas' wide beams required to cope with the large field-of-view (FOV), about 120°. Since most receiver errors and drifts can be hardware or software calibrated, inversion algorithms are focused mainly in the correction of the aberrations introduced by antenna voltage pattern mismatches and antenna positioning errors. This paper presents an extension of the CLEAN technique used in radioastronomy to map quasi-point thermal sources, i.e., stars, to the generation of brightness temperature maps of extended thermal sources, i.e., the Earth. The iterative approach used allows to account for antenna voltage pattern mismatches within the large FOV. The proposed algorithm has been optimized to process hexagonally sampled visibilities given by Y-shaped-arrays, as it is the case of the MIRAS instrument (Microwave Imaging Radiometer by Aperture Synthesis), an interferometric radiometer currently under study at the European Space Agency.

Characterization of photodiodes as transfer detector standards in the 120 nm to 600 nm spectral range

Abstract: Using spectrally dispersed synchrotron radiation of continuously tuneable wavelength as delivered by the ultraviolet (UV) and vacuum ultraviolet (VUV) calibration facility of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage ring BESSY I in Berlin, various types of silicon photodiode have been examined for their radiometric performance in the 120 nm to 600 nm spectral range. Their absolute spectral responsivity was determined with a typical relative uncertainty of 0.7 % using the synchrotron-radiation cryogenic electricalsubstitution radiometer, SYRES, as primary detector standard. Particular emphasis has been given to the study of radiation-damage effects at wavelengths below 250 nm. In addition, the reflectance of the photodiodes was measured to determine their internal quantum efficiency. Using a physical model for the internal losses, the mean energy to create an electron-hole pair in silicon was derived.

Approach for satellite slant path remote sensing

Abstract: It is theoretically and experimentally shown how angular smoothing of the sky brightness temperature caused by the radiometer antenna pattern can be used in conjunction with the spectral characteristics of the amplitude scintillations of a beacon receiver to allow unambiguous determination of the wind speed of a turbulent layer and its height, helping in the estimation of the refractive index structure constant (C/sub n//sup 2/).

Design and Performance Analysis of an Automated 10-Channel Solar Radiometer Instrument

Abstract: An automated multichannel solar radiometer has been designed and fabricated by the Atmospheric Remote Sensing Laboratory at The University of Arizona. The automated radiometer has 10 separate silicon-photodiodebased channels that allow near-simultaneous solar spectral measurements through narrow bandpass filters (approximately 10 nm) from the visible to near-IR regions. The photodiode detectors are temperature stabilized using a heating temperature controller circuit. The instrument is pointed toward the sun via an autotracking system that actively tracks the sun with a 60.058 tracking accuracy. The instrument can continuously collect data for about 22 h at once per minute sample rate. This paper presents instrument design features as well as some performance and experimental results for the automated solar radiometer.

Angular resolution of two‐dimensional, hexagonally sampled interferometric radiometers

Abstract: A theoretical analysis of the angular resolution of two-dimensional interferometric radiometers for Earth observation from low-orbit satellites and its degradation due to spatial decorrelation effects is presented. The analysis extends basic effects known in the context of radio astronomy (application with narrow field of view, very few baselines) and in one dimension (ESTAR L band, few baselines interferometric radiometer) to the wide-field-of-view, many-baseline, high-resolution two-dimensional system required by Earth observation applications and computes beam width, encircled energy (or main beam efficiency), and side lobe level as a function of windowing (apodization) to allow for an optimum angular versus radiometric resolution trade-off. It is found that the extension of the Barlett window (which has a poor performance in one dimensional signal processing) to two dimensions produces high-quality results, comparable or better than those of Gaussian and Blackmann windows. Theory is extended to hexagonally sampled systems based on a ? or Y-shaped instrument, with hexagonal- and star-shaped support regions in the visibility space, respectively. The superior performance of the latter over the former for the same number of antennas and correlators is quantified and details of the angular resolution of one instrument of this kind, MIRAS, under development by the European Space Agency, are presented. For this radiometer Gaussian or Barlett windows should be used for good radiometric sensitivity or spatial resolution, respectively. In both cases the effects of decorrelation within the small alias-free field of view are negligible. It is also found that the impact of hardware imperfections which exist within the strict requirements of the specifications have a negligible effect on the angular resolution. Finally, experimental angular resolution results with a laboratory breadboard in a focused near-field setup are presented and compared to the theoretical predictions.

Corrections for detector nonlinearities and calibration inconsistencies of the infrared channels of the advanced very high resolution radiometer

Abstract: The data received from the thermal infrared channel 4 (∼10.3–11.3 μm) and channel 5 (∼11.5–12.5 μm) of the advanced very high resolution radiometer (AVHRR) flown on the National Oceanic and Atmospheric Administration (NOAA) Polar-Orbiting Operational Environmental Satellites provide only a linear estimate of the actual radiance. We describe here a simple procedure, which incorporates data from prelaunch calibration tests, to correct the linear estimates for the nonlinear response of the channels 4 and 5 Mercury-Cadmium-Telluride (Hg-Cd-Te) sensors. The procedure applies a “nonzero radiance of space” concept to specify the form of the linear radiance estimate. This linear radiance is nearly independent of the operating temperature of the AVHRR and is the sole input to the correction algorithm. Additionally, it is demonstrated with NOAA 14 data that this calibration procedure resolves discrepancies found in the prelaunch data which can affect the calibration accuracy of channel 3 (∼3.55–3.95 μm), which possesses a linear response, as well as channels 4 and 5. When applied to independent sets of prelaunch calibration data, this procedure reproduces the laboratory-measured temperature data to within an accuracy of 0.1°–0.2°K. Comparison with nonlinearity corrections based on different procedures points to the superior applicability of the present results over the entire range of Earth-scene temperatures measured by the AVHRR in orbit. This accuracy is particularly important when these three infrared channels are used in multichannel algorithms to generate environmental parameters such as sea surface temperature. The algorithm coefficients and values of the nonzero radiance of space required to calculate the nonlinearity radiance corrections are given for the AVHRRs on NOAA 7, 9, 10, 11, 12, and 14 spacecraft.

The NIST EOS thermal-infrared transfer radiometer

Abstract: A portable thermal-infrared transfer radiometer (TXR) has been developed for use in comparisons and scale verifications of sources used to calibrate thermal-infrared (TIR) channels of the National Aeronautics and Space Administration's (NASA's) Earth Observing System (EOS) flight instruments. The TXR is designed to measure the radiance temperature of large-area black-body sources in cryogenic vacuum environments, either at the National Institute of Standards and Technology (NIST) or at the EOS instrument-calibration facilities. It can be operated in ambient conditions of room temperature and pressure, or in EOS instrument thermal/vacuum chambers at temperatures as low as 77 K. The TXR is a liquid-nitrogen-cooled filter radiometer with two channels: one centred at 5 µm based on a photovoltaic InSb detector, and the other centred at 10 µm based on a photovoltaic HgCdTe (MCT) detector. The spectral, spatial and temporal characterization of the TXR using state-of-the-art NIST ambient-infrared instrumentation is reported.

Weighted mean tropospheric temperature and transmittance determination at millimeter-wave frequencies for ground-based applications

Abstract: A simple model for the radiometric determination of tropospheric transmittance is based on an isothermal troposphere In this model the key parameter is the weighted mean tropospheric temperature Tm, which characterizes the radiation and temperature properties of the troposphere Statistical approaches in modeling this parameter are presented here by using ground temperature, ground relative humidity, and radiometer data In order to determine the statistical coefficients for Tm modeling and the parameters used in the transmittance retrieval algorithm, radiosonde data were used in a millimeter-wave propagation model for a site in the Swiss central plane and an Alpine site Various observing geometries at different millimeter-wave frequencies were considered A determination of Tm from ground temperature was achieved with a rms error between 4–5 K for the low-altitude site and 3–4 K for the high-altitude site By incorporating relative humidity or radiometer data, an improvement of up to 25% relative to these values results, depending on frequency and site The zenith transmittance estimations for the low-altitude site with our best model have a rms error of 05% at 38 GHz, 1% at 94, 110, and 142 GHz, 15% at 115 GHz, 2% at 204 GHz, and 35% at 279 GHz, whereas for the high-altitude site all rms errors are below 1% The inclusion of radiometric information at 20 and 31 GHz did not provide any additional improvement, which was confirmed by actual measurements at 142 GHz

Ground-based microwave radiometry and long-term observations of atmospheric water vapor

Abstract: Microwave radiometer data and radiosonde data from the time period 1981-1995 have been used to study long-term trends in the integrated precipitable water vapor (IPWV) The two instruments have operated 37 km apart on the Swedish west coast Model parameters are estimated for the entire data sets as well as for subsets of the data The IPWV model parameters are a mean value, a linear drift with time, and the amplitude and phase of an annual component The radiosonde data, which are uniformly sampled in time, show an increase in the IPWV of 003 mm/yr with a statistical standard deviation of 001 mm The microwave radiometer data, which are not at all uniformly sampled in time, show -002+/-001 mm/yr We show that the disagreement is caused by the different sampling of the data for the two instruments When the two data sets are reduced to include only data that are sampled simultaneously, we find an agreement between all estimated model parameters, given their statistical uncertainties This suggests that if the microwave radiometer had also been operating continuously over the 15-year period, its data would have implied a linear trend similar to the result obtained from the radiosonde data The general quality of the data, in terms of the short time scatter, has been improved over the time period The root mean square (RMS) difference between the IPWV measured by the radiometer and by the radiosondes was 21 mm during the first 5 years and was reduced to 16 mm during the last 4 years These values include the real difference in the IPWV between the two sites The bias, radiometer-radiosonde, was 01 mm for the whole data set and varied between -02 and 09 mm for smaller data sets of a few years

Passive ground-based remote sensing of atmospheric temperature, water vapor, and cloud liquid water profiles by a frequency synthesized microwave radiometer;Passive bodengebundene atmosphärische Fernerkundung von Temperatur-, Wasserdampf- und Wolkenwasserprofilen mit eine frequenzsynthetisierten Mikrowellen-Radiometer

Abstract: Summar . An advanced microwave radiometer for profiling atmospheric parameters is described in this paper. The method accomplishing profiling is described. The innovations of the radiometer system, including utilization of a stable frequency synthesizer and the calibration system are described. The performance of several mathematical retrieval methods and representative profiles are presented, as well as several comparisons with radiosondes.

The measurement of solar ultraviolet radiation

Abstract: High skin cancer rates, stratospheric ozone depletion and increased public interest and concern have resulted in a strong demand for solar ultraviolet radiation measurements and information. The Australian Radiation Laboratory (ARL) has been involved since the mid-1980s in the measurement of solar ultraviolet radiation (UVR) using spectroradiometers (SRM) and a network of broadband detectors at 18 sites in Australia and Antarctica and in Singapore through a collaborative agreement with the Singapore Institute of Science and Forensic Medicine. Measurement locations range from equatorial (Singapore, 1.3 degrees N) through tropical (Darwin, 12.4 degrees S) to polar (Mawson, 67.6 degrees S) and as a result there are many difficulties associated with maintenance and calibration of the network detectors, and transfer of data to ensure an accurate and reliable data collection. Calibration procedures for the various detectors involve the comparison with simultaneous spectral measurements using a portable SRM incorporating a double monochromator, calibrated against traceable standard lamps. Laboratory measurements of cosine response and responsivity are also made. Detectors are intercompared at the Yallambie site for a number of months before installation at another location. As an additional check on the calibrations, computer models of solar UVR at the earth's surface for days with clear sky and known ozone are compared with the UV radiometer measurements.

Calibration of a visible and near-infrared portable transfer radiometer

Abstract: A portable transfer radiometer covering the spectral range 400 nm to 900 nm has been built and calibrated. This radiometer was designed to measure the output of spherical integrating sources with large apertures such as those used to calibrate spacecraft sensors for the Earth Observing System (EOS) of the National Aeronautics and Space Administration (NASA). The radiometer is a simple, robust, narrowband multifilter instrument using a silicon trap detector. The temperature of the filters, apertures, detectors and electronics is controlled slightly above ambient. There are no imaging optics and the radiometer throughput is controlled by Invar-spaced apertures. The radiometer has been calibrated by two methods. First, the radiometer was calibrated in an irradiance mode by use of a National Institute of Standards and Technology (NIST) FEL lamp. Second, a Solar-Radiation-Based Calibration (SRBC) was carried out, in which the radiometer viewed a panel, calibrated with a bidirectional reflectance distribution function (BRDF), illuminated by the Sun. Differences between the NIST calibration and the SRBC method for a recent solar spectrum are quite small: less than 2.1% for the seven bands between 412 nm and 868 nm, and well within the estimated uncertainties for the two calibration methods.

Spectral Reflectance of Whitecaps: Instrumentation, Calibration, and Performance in Coastal Waters

Abstract: A measurement system for determining the spectral reflectance of whitecaps in the open ocean is described. The upwelling radiance is obtained from a ship by observing a small region of the water surface over time using a six-channel radiometer (410, 440, 510, 550, 670, and 860 nm) extended from the bow of the ship. Downwelling irradiance is simultaneously measured and used to provide surface reflectance. The system includes a TV camera mounted beside the radiometer that provides a visual reference of surface events. Air/water temperature and wind speed/direction are also measured along with global positioning system data. Calibration procedures and radiometric characterization of the system for operation under different sky conditions and solar zenith angles are emphasized so that full advantage is taken of ship time whenever whitecap events occur. The radiometer was operated at sea and examples of the spectral reflectance of different foam types (thick foam layers to thin residual patches) generated by the ship’s bow in coastal waters are presented and found to vary spectrally. The presence of submerged bubbles in the foam measurement results in a lower reflectance at the longer wavelengths. For wavebands in the visible region, the spectral reflectance values tend to equalize with higher reflecting foam from thicker foam layers.