Showing papers on "Radiometer published in 1997"

Total Solar Irradiance Trend During Solar Cycles 21 and 22

Abstract: Results from Active Cavity Radiometer Irradiance Monitor (ACRIM) experiments show an upward trend in total solar irradiance of 0.036 percent per decade between the minima of solar cycles 21 and 22. The trend follows the increasing solar activity of recent decades and, if sustained, could raise global temperatures. Trends of total solar irradiance near this rate have been implicated as causal factors in climate change on century to millennial time scales.

The TRMM 'Day-1' Radar/Radiometer Combined Rain-Profiling Algorithm

Abstract: The Tropical Rainfall Measuring Mission (TRMM)'s 'day-1' combined rada/radiometer algorithm uses a rain-profiling approach which gives as much importance to the measurement of the TRMM satellite's precipitation radar (PR) and the TRMM microwave imager (TMI) as their respective intrinsic ambiguities warrant, which avoids any ad hoc shortcuts that might introduce large biases in the rain estimates, yet which is simple enough to be operatrional with TRMM is launched in late 1997.

Heterodyne methods in millimetre wave plasma diagnostics with applications to ECE, interferometry and reflectometry

Abstract: Basic principles of heterodyne techniques are introduced and the various components of a heterodyne system are summarized. Special applications in ECE, interferometry and reflectometry are discussed after introducing the diagnostic principles. Realized systems as described in the literature are briefly outlined. Ordering principles are radiometer types in the case of ECE, mixing scheme and generation and stabilization of local oscillator and intermediate frequency signals in the case of interferometry and reflectometry. Special techniques and their impact on the performance of the diagnostic instruments are discussed.

Observations of Ice Nucleation and Propagation in Plants Using Infrared Video Thermography

Abstract: We evaluated the use of infrared (IR) video thermography to observe directly ice nucleation and propagation in plants. An imaging radiometer with an HgCdTe long-wave (8-12 [mu]m) detector was utilized to image the thermal response of plants during freezing. IR images were analyzed in real time and recorded on videotape. Information on the videotape was subsequently accessed and analyzed utilizing IR image analysis software. Freezing of water droplets as small as 0.5 [mu]L was clearly detectable with the radiometer. Additionally, a comparison of temperature tracking data collected by the radiometer with data collected with thermocouples showed close correspondence. Monitoring of an array of plant species under different freezing conditions revealed that ice nucleation and propagation are readily observable by thermal imaging. In many instances, the ice nucleation-active bacterium Pseudomonas syringae placed on test plants could be seen to initiate freezing of the whole plant. Apparent ice nucleation by intrinsic nucleators, despite the presence of ice nucleation-active bacteria, was also evident in some species. Floral bud tissues of peach (Prunus persica) could be seen to supercool below the temperature of stem tissues, and ice nucleation at the site of insertion of the thermocouple was frequently observed. Rates of propagation of ice in different tissues were also easily measured by thermal imaging. This study demonstrates that IR thermography is an excellent method for studying ice nucleation and propagation in plants.

Cloud detection and derivation of cloud properties from POLDER

Abstract: POLDER (POLarization and Directionality of the Earth's Reflectances) is a new instrument devoted to the globalobservation of the polarization and directionality of solar radiation reflected by the Earth surface-atmosphere system. This radiometer has been on board the Japanese ADEOS platform since August 1996. This paper describes the main algorithms of the POLDER 'Earth radiation budget (ERB) & clouds' processing line used to derive products on a routine basis in the early phase of the mission. In addition to the bidirectional reflectance and polarization distribution functions, the main products will be the cloud optical thickness, pressure (from two different methods) and thermodynamic phase. Airborne POLDER observations support the present algorithms for the cloud detection and the derivation of cloud properties.

Sensing integrated water vapor along GPS ray paths

Abstract: We demonstrate sensing of integrated slant-path water vapor (SWV) along ray paths between Global Positioning System (GPS) satellites and receivers. We use double differencing to remove GPS receiver and satellite clock errors and 85-cm diameter choke ring antennas to reduce ground-reflected multipath. We compare more than 17,000 GPS and pointed radiometer double difference observations above 20° elevation and find 1.3 mm rms agreement. Potential applications for SWV data include local and regional weather modeling and prediction, correction for slant wet delay effects in GPS surveying and orbit determination, and synthetic aperture radar (SAR) imaging. The method is viable during all weather conditions.

Effects of soiling on PV module and radiometer performance

Abstract: The question of "how much does the output of PV modules and radiometers decrease due to soiling?" is often asked, but data are rarely available to answer the question. Related questions are: a) how effectively does rain clean soiled devices; and b) how often should radiometers and modules be cleaned? Answers to these questions are essential to predict PV module performance or the accuracy (uncertainty) of PV module performance data. Results of module and radiometer soiling data from three independent applications are presented in this paper. Data cover soiling periods from 16 months to five years, and results are consistent for all three applications.

Polarized radiance distribution measurements of skylight. I. System description and characterization.

Abstract: A new system to measure the natural skylight polarized radiance distribution has been developed. The system is based on a fish-eye lens, CCD camera system, and filter changer. With this system sequences of images can be combined to determine the linear polarization components of the incident light field. Calibration steps to determine the system 's polarization characteristics are described. Comparisons of the radiance measurements of this system and a simple pointing radiometer were made in the field and agreed within 10 % for measurements at 560 and 670 nm and 25 % at 860 nm. Polarization tests were done in the laboratory. The accuracy of the intensity measurements is estimated to be 10 %, while the accuracy of measurements of elements of the Mueller matrix are estimated to be 2 %.

Synchrotron-radiation-operated cryogenic electrical-substitution radiometer as the high-accuracy primary detector standard in the ultraviolet, vacuum-ultraviolet, and soft-x-ray spectral ranges.

Abstract: The accuracy of detector calibration in the UV, vacuum-ultraviolet, and soft-x-ray spectral ranges could be significantly improved by the use of the synchrotron radiation electrical substitution radiometer (SYRES) as the primary detector standard. The SYRES radiometer is optimized for use with spectrally dispersed synchrotron radiation as supplied by two monochromator beam lines in the radiometry laboratory of the Physikalisch-Technische Bundesanstalt at the Berlin electron-storage ring (BESSY). Wavelength ranges from 0.8 to 25 nm and from 35 to 400 nm are covered. The typically available radiant power of approximately 1-10 microW can be measured with the SYRES radiometer with a standard relative uncertainty of less than 0.2%. The spectral responsivity of qualified photodiodes for use as secondary detector standards is determined by direct comparison with the primary detector standard at an arbitrary wavelength. At present, the scale of spectral responsivity is realized with a standard relative uncertainty of well below 1% in the spectral ranges 0.8-3.5 nm, 5-25 nm, and 120-400 nm. We provide a comprehensive description of the SYRES radiometer and of the two facilities for detector calibration in the UV and vacuum-ultraviolet spectral ranges and in the soft-x-ray spectral range, respectively, and we discuss the achievable uncertainties in the calibration of detectors.

Microphysical retrievals over stratiform rain using measurements from an airborne dual-wavelength radar-radiometer

Abstract: The need to understand the complementarity of the radar and radiometer is important not only to the Tropical Rain Measuring Mission (TRMM) program but to a growing number of multi-instrumented airborne experiment that combine single or dual-frequency radars with multichannel radiometers. The method of analysis used in this study begins with the derivation of dual-wavelength radar equations for the estimation of a two-parameter drop size distribution (DSD). Defining a "storm model" as the set of parameters that characterize snow density, cloud water, water vapor, and features of the melting layer, then to each storm model there will usually correspond a set of range-profiled drop size distributions that are approximate solutions of the radar equations. To test these solutions, a radiative transfer model is used to compute the brightness temperatures for the radiometric frequencies of interest. A storm model or class of storm models is considered optimum if it provides the best reproduction of the radar and radiometer measurements. Tests of the method are made for stratiform rain using simulated storm models as well as measured airborne data. Preliminary results show that the best correspondence between the measured and estimated radar profiles usually can be obtained by using a moderate snow density (0.1-0.2 g/cm/sup -3/), the Maxwell-Garnett mixing formula for partially melted hydrometeors (water matrix with snow inclusions), and low to moderate values of the integrated cloud liquid water (less than 1 kg/m/sup -/2). The storm-model parameters that yield the best reproductions of the measured radar reflectivity factors also provide brightness temperatures at 10 GHz that agree well with the measurements. On the other hand, the correspondence between the measured and modeled values usually worsens in going to the higher frequency channels at 19 and 34 GHz. In searching for possible reasons for the discrepancies, it is found that changes in the DSD parameter /spl mu/, the radar constants, or the path-integrated attenuation can affect the high frequency channels significantly. In particular, parameters that cause only modest increases in the median mass diameter of the snow, and which have a minor effect on the radar returns or the low frequency brightness temperature, can produce a strong cooling of the 31 GHz brightness temperature.

Airborne video thermal radiometry as a tool for monitoring microscale structures of the urban heat island

Abstract: An advance thermal video radiometer (TVR) was mounted onboard a helicopter and used to collect data over Tel-Aviv, Israel, from an altitude of 7000 feet The sensor, sensitive to the 3-12 mu m spectral region, consisted of an onboard calibration device and a 18 mRad instantaneous field of view (IFOV), which enabled direct assessment of surface radiometric temperature patterns in very high spatial and thermal resolutions The TVR capability was examined by studying the microscale structures of the urban heat island (UHI) of Tel-Aviv, Israel, during a stable calm night on 28 February 1995 The TVR data were found to be of high quality in terms of signal to noise ratio, reproducibility, stability, thermal sensitivity and spatial resolution capabilities The radiometric data were analysed against air temperature acquired during the flight time by four mobile traverses that crossed the city from north to south A significant heat island was identified over the inner city areas, which was found to be

Influence of land-cover category on brightness temperature of snow

Abstract: A helicopter-borne multifrequency radiometer (24, 34, 48 and 94 GHz vertical polarization) was used to investigate the behavior of the brightness temperature of snow in Sodankyla (latitude: 67.41 N, longitude: 26.58 E), Northern Finland. The measurements were carried out during dry snow, wet snow, and snow-free conditions. The angle of incidence was 45/spl deg/ in all measurements. The measurements and the main results are presented. The analysis is focused on the effect of vegetation and land type on the brightness temperature of snow. The main topics of this paper are: (a) the general behavior of the brightness temperature of snow for different land types, (b) the effect of forest vegetation on the brightness temperature of snow, and (c) the capability of the radiometer system to monitor snow extent in forests during the melting period.

GPS surveying with 1 mm precision using corrections for atmospheric slant path delay

Abstract: Multipath and atmospheric effects can limit GPS surveying precision. We surveyed a 43 km baseline using large diameter choke ring antennas to reduce multipath and pointed radiometer and barometric data to correct for atmospheric slant delay. Based on 11 daily solutions, atmospheric slant delay corrections improved vertical precision to 1.2 mm rms and horizontal precision to sub-mm. Applications for high precision GPS surveying include deformation monitoring associated with earthquake and volcanic processes, subsidence, isostasy, and sea level measurements; monitoring of atmospheric water vapor for climate and global change research, and to improve the resolution of synthetic aperture radar; calibration of satellite altimeters; and precise satellite orbit determination.

Large-Scale Analysis of Cirrus Clouds from AVHRR Data: Assessment of Both a Microphysical Index and the Cloud-Top Temperature

Abstract: An algorithm that allows an automatic analysis of cirrus properties from Advanced Very High Resolution Radiometer (AVHRR) observations is presented. Further investigations of the information content and physical meaning of the brightness temperature differences (BTD) between channels 4 (11 μm) and 5 (12 μm) of the radiometer have led to the development of an automatic procedure to provide global estimates both of the cirrus cloud temperature and of the ratio of the equivalent absorption coefficients in the two channels, accounting for scattering effects. The ratio is useful since its variations are related to differences in microphysical properties. Assuming that cirrus clouds are composed of ice spheres, the effective diameter of the particle size distribution can be deduced from this microphysical index. The automatic procedure includes first, a cloud classification and a selection of the pixels corresponding to the envelope of the BTD diagram observed at a scale of typically 100 × 100 pixels. ...

Impact of receiver errors on the radiometric resolution of large two‐dimensional aperture synthesis radiometers

Abstract: The specifications of the subsystems that compound a radiometer interferometer devoted to Earth observation are of main concern because they set the viability and final performance of such an instrument. The importance of these errors is related to the exact way they are generated, since this determines if a particular calibration procedure is capable of removing them or if they remain as residual errors. This paper presents a general method to analyze system errors. This method is used to analyze in detail the amplitude and phase errors of the receivers and their impact on the radiometric resolution. Special attention has been paid to nonseparable errors, since foreseen calibration procedures can only deal with separable phase and amplitude terms. Finally, the results have been used to set the receiver requirements of the instrument called MIRAS (microwave imaging radiometer by aperture synthesis), which is currently being developed by the European Space Agency (ESA).

Total Solar Irradiance Variations: The Construction of a Composite and its Comparison with Models

Abstract: Measurements of the total solar irradiance (TSI) during the last 18 years from spacecraft are reviewed. Corrections are determined for the early measurements made by the HF radiometer within the ERB experiment on NIMBUS 7 and the factor to refer active cavity radiometer irradiation monitoring (ACRIM) 2 to the ACRIM 1 irradiance scale. With these corrections, a composite TSI is constructed with a model that combines a magnetic brightness proxy with observed sunspot darkening and explains nearly 90 percent of the observed short and long term variance. Possible, but still unverified degradation of the radiometers hampers conclusions about irradiance changes on decadal time scales and longer.

Development of a detector-based absolute spectral irradiance scale in the 380–900-nm spectral range

Abstract: A detector-based absolute scale for spectral irradiance in the 380-900-nm wavelength region has been developed and tested at the Helsinki University of Technology (HUT). Derivation of the scale and its use for photometric and colorimetric measurements are described. A thorough characterization of a filter radiometer, constructed from a reflection trap detector, a precision aperture, and a set of seven temperature-controlled bandpass filters, is presented. A detailed uncertainty analysis of the scale indicates a relative standard uncertainty of approximately 0.2% throughout most of the wavelength region. The standard uncertainties obtained in measurements of correlated color temperature and luminous intensity of three Osram Wi41/G tungsten-halogen lamps are 2 K and 0.3%, respectively. The spectral irradiance scale is compared with the HUT luminous intensity scale. The agreement of the results at the 0.1% level is well within the combined standard uncertainty of the two scales.

Microwave radiometric technique to retrieve vapor, liquid and ice. I. Development of a neural network-based inversion method

Abstract: With the advent of the microwave radiometer, passive remote sensing of clouds and precipitation has become an indispensable tool in a variety of meteorological and oceanographical applications. There is wide interest in the quantitative retrieval of water vapor, cloud liquid, and ice using brightness temperature observations in scientific studies such as Earth's radiation budget and microphysical processes of winter and summer clouds. Emission and scattering characteristics of hydrometeors depend on the frequency of observation. Thus, a multifrequency radiometer has the capability of profiling cloud microphysics. Sensitivities of vapor, liquid, and ice with respect to 20.6, 31.65 and 90 GHz brightness temperatures are studied. For the model studies, the atmosphere is characterized by vapor density and temperature profiles and layers of liquid and ice components. A parameterized radiative transfer model is used to quantify radiation emanating from the atmosphere. It is shown that downwelling scattering of radiation by an ice layer results in enhancement at 90 GHz brightness temperature. Once absorptive components such as vapor and liquid are estimated accurately, then it is shown that the ice water path can be retrieved using ground-based three-channel radiometer observations. In this paper the authors developed two- and three-channel neural network-based inversion models. Success of a neural network-based approach is demonstrated using a simulated time series of vapor, liquid, and ice. Performance of the standard explicit inversion model is compared with an iterative inversion model. In part II of this paper, actual radiometer, and radar field measurements are utilized to show practical applicability of the inverse models.

Further Developments in Estimating Cloud Liquid Water over Land Using Microwave and Infrared Satellite Measurements

Abstract: Refinements and improvements of an earlier technique to retrieve the cloud liquid water path (LWP) of nonprecipitating clouds over land surfaces using Special Sensor Microwave/Imager (SSM/I) 85.5-GHz measurements are presented. These techniques require estimates of the microwave surface emissivity, which are derived in clear-sky regions from SSM/I measurements and window infrared measurements from the Visible and Infrared Spin Scan Radiometer on GOES-7. A comparison of forward model calculations with SSM/I measurements in clear regions demonstrates that over a 7-day period the surface emissivities are stable. To overcome limitations in the single-channel retrieval method under certain situations, a new method is developed that uses a normalized polarization difference (NPD) of the brightness temperatures. This method has the advantages of providing estimates of the LWP for low clouds and being extremely insensitive to the surface skin temperature. Radiative transfer simulations also show that the...

Storm-Associated Microwave Radiometric Signatures in the Frequency Range of 90–220 GHz

Abstract: Radiometric measurements were made by a millimeter-wave imaging radiometer (MIR) at the frequencies of 89, 150, 183.3 +/- 1, 183.3 +/- 3, 183.3 +/- 7, and 220 GHz aboard the NASA ER-2 aircraft at an altitude of about 20 km over two rainstorms: one in the western Pacific Ocean on 19 January 1993 and another in southern Florida on 5 October 1993. These measurements were complemented by nearly simultaneous observations by other sensors aboard the same aircraft and another aircraft flying along the same path. Analysis of data from these measurements, aided by radiative transfer and radar reflectivity calculations of hydrometeor profiles, which arc generated by a general cloud ensemble model, demonstrates the utility of these frequencies for studying the structure of frozen hydrometeors associated with storms. Particular emphasis is placed on the three water vapor channels near 183.3 GHz. Results show that the radiometric signatures measured by these channels over the storm-associated scattering media bear a certain resemblance to those previously observed over a clear and fairly dry atmosphere with a cold ocean background. Both of these atmospheric conditions are characterized by a small amount of water vapor above a cold background. Radiative transfer calculations were made at these water vapor channels for a number of relative humidity profiles characterizing dry atmospheres over an ocean surface. The results are compared with the measurements to infer some characteristics of the environment near the scattering media. Furthermore, radiometric signatures from these channels display unique features for towering deep convective cells that could be used to identify the presence of such cells in storms.

Characterization of the length-modulated radiometer

Abstract: The development of the length-modulated radiometer (LMR), which is a new type of correlation radiometer for remote sounding, is described. Factors that affect the performance of the LMR are identified and, whenever possible, are minimized. Laboratory measurements of the radiometric response of a LMR filled with carbon monoxide are presented and compared with theoretical calculations. An analysis of the sources of error in these measurements and calculations is presented.

Ice detection using radiometers

Abstract: A radiometer system (20) detects naturally occurring atmospheric microwave radiation (50, 52, 54) in three bands at approximately 20, 30, and 90 GHz. Signals representing radiation intensity at each of these bands are converted into a brightness temperature and used as a system input to a processing system that yields system outputs (56) including the atmospheric ice water path, liquid water path, and water vapor content. A parameterized radiative transfer model (P102) is used to quantify radiation emanating from the atmosphere. The applied signal processing technique provides measurement of the total ice column in the atmosphere, and simultaneously provides accurate measurement of water vapor and liquid water. A calibration technique (P110) enhances the accuracy of ice detection measurements by correcting for the effects of absorptive atmospheric components, such as water vapor and liquid water.

Global Oceanic Precipitation: A Joint View by TOPEX and the TOPEX Microwave Radiometer

Abstract: The TOPEX/POSEIDON mission offers the first opportunity to observe rain cells over the ocean by a dual-frequency radar altimeter (TOPEX) and simultaneously observe their natural radiative properties by a three-frequency radiometer (TOPEX microwave radiometer (TMR)). This work is a feasibility study aimed at understanding the capability and potential of the active/passive TOPEX/TMR system for oceanic rainfall detection. On the basis of past experiences in rain flagging, a joint TOPEX/TMR rain probability index is proposed. This index integrates several advantages of the two sensors and provides a more reliable rain estimate than the radiometer alone. One year's TOPEX/TMR data are used to test the performance of the index. The resulting rain frequency statistics show quantitative agreement with those obtained from the Comprehensive Ocean-Atmosphere Data Set (COADS) in the Intertropical Convergence Zone (ITCZ), while qualitative agreement is found for other regions of the world ocean. A recent finding that the latitudinal frequency of precipitation over the Southern Ocean increases steadily toward the Antarctic continent is confirmed by our result. Annual and seasonal precipitation maps are derived from the index. Notable features revealed include an overall similarity in rainfall pattern from the Pacific, the Atlantic, and the Indian Oceans and a general phase reversal between the two hemispheres, as well as a number of regional anomalies in terms of rain intensity. Comparisons with simultaneous Global Precipitation Climatology Project (GPCP) multisatellite precipitation rate and COADS rain climatology suggest that systematic differences also exist. One example is that the maximum rainfall in the ITCZ of the Indian Ocean appears to be more intensive and concentrated in our result compared to that of the GPCP. Another example is that the annual precipitation produced by TOPEX/TMR is constantly higher than those from GPCP and COADS in the extratropical regions of the northern hemisphere, especially in the northwest Pacific Ocean. Analyses of the seasonal variations of prominent rainy and dry zones in the tropics and subtropics show various behaviors such as systematic migration, expansion and contraction, merging and breakup, and pure intensity variations. The seasonality of regional features is largely influenced by local atmospheric events such as monsoon, storm, or snow activities. The results of this study suggest that TOPEX and its follow-on may serve as a complementary sensor to the special sensor microwave/imager in observing global oceanic precipitation.

Verification plan of ocean color and temperature scanner atmospheric correction and phytoplankton pigment by moored optical buoy system

Abstract: A moored optical buoy system has been developed by the National Space Development Agency of Japan (NASDA) for verification of ocean-observing remote sensing sensors of the Advanced Earth Observing Satellite (ADEOS). An underwater spectral radiometer was specifically designed for this optical system. The two upward radiance and downward irradiance correctors were connected with fiber optical system and were placed at the depth of 1.5 m and 6.5 m. The collector parts of the radiometer were extended from the center of the buoy system in order to avoid self shading with 3 m and 1.2 m arms at the depth of 1.5 m and 6.5 m, respectively. Spectral incident irradiance was measured at top of the buoy system. Phytoplankton pigment concentrations were monitored by Aquatracka-III fluorometer placed at the bottom of the surface float. Atmospheric temperature, pressure, humidity, wind speed and direction, water temperature at a depth of 1.2 m and 6.5 m, and wave height and direction were also monitored. In this paper, we present results from test deployment during January 16 to March 15, 1995, in the Suruga Bay, off Shizuoka, Japan, and compare the water-leaving radiance obtained by buoy with one observed by ship. The chlorophyll a concentrations calculated from water-leaving radiance were also compared with chlorophyll fluorescence measured by fluorometer. It was proved that the self shading and wind affected the radiance and irradiance in the sea. It becomes clear that the quality check of the buoy data is very important for verification of the satellite data. We present also the verification plan of atmospheric correction and phytoplankton pigment for ADEOS/ocean color and temperature scanner (OCTS) using the sea truth data obtained from this buoy system and ships.

Thermomechanical properties of polymers at high rates of strain

Abstract: The objective of this project was to develop a suitable testing technique which will allow the measurement of the bulk surface temperature of a specimen during high strain-rate compression testing. A fast response infra-red radiometer was developed which has the capability of measuring bulk surface temperature changes with an estimated accuracy of ±2°C and with a response time of 1.4μs. The radiometer is capable of measuring temperature changes above 20°C, when the signal-to-noise ratio becomes greater than 1. The properties of polymeric materials are extremely temperature sensitive, particularly when the glass transition temperature is approached. Models such as Eyring predict the yield of a polymer at constant temperature; the yield stress being unaffected by temperature rises. Temperature as well as mechanical data will allow models to be revised to include strain and dynamic temperature terms to predict the flow under adiabatic conditions. Strain-rates of ca. 2500s -1 were achieved when testing specimens and this rate was obtained using a split Hopkinson pressure bar. A substantial number of preliminary tests were conducted in order to obtain a suitable specimen size which was then used in the temperature measurement process. Quasistatic, intermediate and high strain-rate tests were performed; the last utilised the radiometer for temperature measurement. An Eyring plot was constructed from which fundamental values for activation volumes and enthalpies were obtained. Full descriptions of the testing techniques used have been included and a brief photoelastic analysis has been carried out on a partially deformed specimen which shows molecular alignment.