Satellite-borne Microwave Radiometers Research Articles

Demonstration of Ultrawideband Hyperspectral Microwave Interferometer by True Time Delay

JAXA has conducted earth observations using the AMSR series of satellite-borne microwave radiometers (MWRs) for 20 years. Although design of the AMSR series poses problems in terms of lifetime, spatial resolution, and artificial radio frequency interference (RFI), the same design has been carried over to the AMSR2 successor (AMSR3) currently under development. In response to the need for technological development to solve these problems, we began studying a satellite-borne MWR based on a new concept. We recently developed a new-type MWR, DSμRAD. This is the world's first hyperspectral MWR that can receive microwave power from L- to Ka-band with an ultrawideband antenna and measure the spectrum of brightness temperature in high-frequency resolution by ultrahigh-speed A/D converters. Based on the development of DSμRAD, we believe that a hyperspectral microwave interferometer (MWI) with a phased array antenna (PAA) composed of ultrawideband antennas is the most ideal MWR that can solve all the problems at once. This MWI controls pointing by true time delay (TTD) taking advantage of ultrahigh-speed sampling. As the first step to demonstrate this idea, we developed and tested the most basic two-element ultrawideband hyperspectral MWI based on DSμRAD. As a result, the sharp main lobe of the MWI was simultaneously formed over the 4-16 GHz range at the same position, and its pointing and width could be controlled as intended. In particular, the azimuthal shift of the main lobe caused by the time delay given to the signal of either element did not depend on the frequency.

Spatial Resolution Enhancement Algorithm Based on the Backus–Gilbert Method and Its Application to GCOM-W AMSR2 Data

The greatest advantage of a satellite-borne microwave radiometer (MWR) is that it can obtain information on the Earth’s surface with less influence by rain and clouds compared to an infrared radiometer; however, it has the disadvantage that the observing area is large so the spatial resolution is low. One example is the Advanced Microwave Scanning Radiometer-2 (AMSR2) loaded on the GCOM-W satellite. Hardware technologies to overcome this disadvantage are in progress, but software processings to improve spatial resolution are also needed, especially to process previously obtained data. The author was inspired by the experience of developing the L1R product for the AMSR2 and has investigated optimizing the Backus–Gilbert (BG) method to enhance spatial resolution. First, the BG method was used to calculate weighting coefficients to synthesize the brightness temperature equivalently obtained in a small observation area from those obtained in multiple larger areas. Next, a viable algorithm was developed and implemented in order to solve the problem appearing in this simple application of the BG method. The algorithm was verified and shown to reduce the observation area by about 17%, enhancing the spatial resolution.

Comparison Analysis of Total Precipitable Water of Satellite-Borne Microwave Radiometer Retrievals and Island Radiosondes

Total precipitable water (TPW) of satellite-borne microwave radiometer retrievals is compared with the data that were collected from 49 island radiosonde stations for the period 2007–2015. Great consistency was found between TPW measurements made by radiosonde and eight satellite-borne microwave radiometers, including SSMI-F13, SSMI-F14, SSMIS-F16, SSMIS-F17, AMSR-E, AMSR-2, GMI, and WindSat. Mean values of the TPW differences for eight satellites ranged from −0.51 to 0.38mm, both root mean square errors and standard deviations were around 3mm, and all of the correlation coefficients between satellite TPW retrievals and radiosonde TPW for each satellite can reach 0.99. Subsequently, an analysis of the comparison results was conducted, which revealed three problems in the satellite TPW retrieval and two problems in radiosonde data. For TPW retrievals of satellite, when the values are above 60 mm, the precision of TPW retrieval significantly decreases with a distinct dry bias, which can reach 4 mm; additionally, abias related to wind speed and the uncertainty with the TPW retrieval in the presence of rain, which is stronger than 1mm/h, was found. The TPW measurements of radiosonde made by the type of IM-MK3 from India were quite unreliable, and almost all of the radiosonde data during the daytime were plagued by a dry bias.

Contemporary methods for retrieving the integrated atmospheric water-vapor content and the total cloud liquid-water content

A review of contemporary methods for determining integrated parameters of the water content in the atmosphere―atmospheric water-vapor content and cloud liquid-water content―is presented. Fields of these parameters can only be mapped spatially on the basis of using data of satellite measurements. The least errors of the retrieval of atmospheric water-vapor content and cloud liquid-water content is provided by methods based on using measurements of the satellite-borne scanning multichannel microwave radiometers over the ice-free ocean areas in the absence of precipitation. Most methods for retrieving the atmospheric water-vapor content and cloud liquid-water content from the data of microwave radiometers are based on results of numerical simulation of brightness temperatures of the upwelling microwave radiation of the ocean–atmosphere system. The evolution of satellite-borne microwave radiometers and methods for the retrieval of integrated parameters of water content is presented.

Ground-Based L-Band Emission Measurements at Dome-C Antarctica: The DOMEX-2 Experiment

In recent years, there has been growing interest on the part of the remote sensing community in using the Antarctic area for calibrating and validating data of low-frequency satellite-borne microwave radiometers. In particular, the East Antarctic Plateau appears to be suited for this purpose. The reasons for this interest are the size, structure, spatial homogeneity, and thermal stability of this area. This is particularly interesting for low-frequency microwave radiometers since, due to the low extinction of dry snow, the upper ice-sheet layer is almost transparent and the brightness temperature variability is therefore extremely small. In the context of calibration and validation activities of the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) satellite, an experiment called DOMEX-2, which included radiometric L-band measurements, was carried out at the Italian-French base of Concordia located at Dome C in the East Antarctic Plateau from December 2008 to December 2010. Ground measurements (i.e., snow temperature at different depths, snow structure, meteorological data, etc.) were also collected during the experiment. This paper presents information on the experimental campaign, the characteristics of the radiometric measurements, and the main results. A comparison with SMOS data is also presented.

Sensitivity Analysis of Ocean Microwave Radiation Characteristics

Ocean salinity is one of the important parameters in marine ecosystem and climate system. Satellite-borne microwave radiometer observing system is the only feasible method to study the distributions and changes of the global ocean salinity. Numbers of factors make a great impact on the sea salinity inversion accuracy based on the microwave radiation technology. These factors include the sea surface temperature, distribution range of the salinity, the observation angle, polarization, as well as the microwave frequency of the microwave radiometer. The present paper analyzed the effect of microwave frequency, incidence angle and polarization mode on the sensitivity of the ocean salinity based on the microwave radiation simulated data of calm see surface. Furthermore, it studied the effect of the seawater temperature and salinity distributions on the salinity sensitivity. The results indicated that the sensitivity between the ocean surface microwave radiation and the water salinity decreased with increased microwave frequency; the influence of the incident angle on the sensitivity is related to the polarization mode. The sensitivity increased with the increasing of the incident angle under V-polarization mode, however, the sensitivity decreased under H-polarization mode. The water temperature had an effect of amplification on the sensitivity. The higher the temperature of water is, the stronger the sensitivity is. However, the changing trend of the sensitivity was not related with the polarization, frequency, and incident angle.

Thermal Design, Analysis and Experimental Verification of Electronic Equipment of a Satellite Borne Microwave Radiometer

Thermal design, finite element analysis and experiment verification of electronic equipment of a satellite borne microwave radiometer are introduced. Some methods were adopted to help heat conduct and a finite element model was built. The analysis results show that the temperature scopes of the main structures are from 45°C to63.9°C in the digital control equipment and 45°C to 68.7°C in the receiver equipment and all of junction temperatures of the components are lower than the derated maximum junction temperatures themselves and leave enough design margins, which match the requirements of thermal analysis. The experimental results show that the computing values are close to experimental values and the largest error is 10.1°C, which is allowed for engineering application.

A Review:The Calibration of Ground-based,Airborne and Satellite-borne Microwave Radiometers

It has been proven that there exists heat radiation from any object whose temperature is greater than absolute zero by therories and studies.Brightness temperature is usually adopted to describe this radiation characteristic,which varies with the geometry and dielectric characteristics of targets.Microwave radiometer is a category of instrument,which is applied to measure the heat radiation characteristic of one target and very useful in observing the earth from satellite.In fact,it is one receiver with high sensitivity since the radiation signal is extremely weak,even weaker than the receiver background noise.This receiver consists of antenna,broad bandwidth receiver,and data recorder.\;Studies in the past have shown that the collected data are of very confusion if microwave radiometers are not calibrated and validated before application,so microwave radiometer calibration techniques are very important.Microwave radiometers can be utilized on ground-based,airborne and satellite-borne platforms,and there are different demands and difficulties about microwave radiometer calibration accordingly,so it is necessary to discuss the calibration on different platforms.In this paper,the calibration techniques of ground-based,airborne and satellite-borne microwave radiometers are discussed,and some problems are also mentioned.

Analysis of the system linearity caused by gain variation of satellite-borne microwave radiometer

针对星载微波辐射计中检波器的工作特性进行分析. 观测固定目标时, 通过调节AGC (Automatic Gain Control)的大小, 定量分析系统伏安特性曲线, 对星载微波辐射计系统线性度进行定性分析. 结果表明, 通过调节AGC大小, 系统增益发生变化, 导致检波器工作区间发生微弱变化, 通过两点定标不能完全消除由于增益引起的非线性误差. 对一定亮温范围的观测目标进行分析, 结果证明, AGC变化时反算亮温与实际目标亮温差值随观测亮温的变化而成非线性变化. 从而证明, 系统增益变化导致的亮温变化通过两点线性定标并不能完全消除. 在系统正常工作或实时定标过程中, 要尽量避免调节AGC值, 保证微波辐射计系统工作在线性区间.

Lightning-based propagation of convective rain fields

Abstract. This paper describes a new multi-sensor approach for continuously monitoring convective rain cells. It exploits lightning data from surface networks to propagate rain fields estimated from multi-frequency brightness temperature measurements taken by the AMSU/MHS microwave radiometers onboard NOAA/EUMETSAT low Earth orbiting operational satellites. Specifically, the method allows inferring the development (movement, morphology and intensity) of convective rain cells from the spatial and temporal distribution of lightning strokes following any observation by a satellite-borne microwave radiometer. Obviously, this is particularly attractive for real-time operational purposes, due to the sporadic nature of the low Earth orbiting satellite measurements and the continuous availability of ground-based lightning measurements – as is the case in most of the Mediterranean region. A preliminary assessment of the lightning-based rainfall propagation algorithm has been successfully made by using two pairs of consecutive AMSU observations, in conjunction with lightning measurements from the ZEUS network, for two convective events. Specifically, we show that the evolving rain fields, which are estimated by applying the algorithm to the satellite-based rainfall estimates for the first AMSU overpass, show an overall agreement with the satellite-based rainfall estimates for the second AMSU overpass.

Review of Current Situation and Problem towards Global Earthquake Detection using Satellite-borne Microwave Radiometer

We have investigated a new approach for global earthquake detection based on an experimental fact that microwave signals are generated by rock failures. We already developed an analysis method to detect rock failure microwave signals from the data of a microwave radiometer, the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) aboard the Aqua satellite, and confirmed that characteristic microwave signals were emitted from the faults and fissures around the epicenter in association with the 2004 Morocco Earthquake. However, at the same time, these results have revealed some problems to be solved before we can move our methodology closer to practical use. In this paper, we review the results obtained in AMSR-E's data analysis phase and summarize the current problems to be solved.

Microwave single-scattering properties of randomly oriented soft-ice hydrometeors

Abstract. Large ice hydrometeors are usually present in intense convective clouds and may significantly affect the upwelling radiances that are measured by satellite-borne microwave radiometers – especially, at millimeter-wavelength frequencies. Thus, interpretation of these measurements (e.g., for precipitation retrieval) requires knowledge of the single scattering properties of ice particles. On the other hand, shape and internal structure of these particles (especially, the larger ones) is very complex and variable, and therefore it is necessary to resort to simplifying assumptions in order to compute their single-scattering parameters. In this study, we use the discrete dipole approximation (DDA) to compute the absorption and scattering efficiencies and the asymmetry factor of two kinds of quasi-spherical and non-homogeneous soft-ice particles in the frequency range 50–183 GHz. Particles of the first kind are modeled as quasi-spherical ice particles having randomly distributed spherical air inclusions. Particles of the second kind are modeled as random aggregates of ice spheres having random radii. In both cases, particle densities and dimensions are coherent with the snow hydrometeor category that is utilized by the University of Wisconsin – Non-hydrostatic Modeling System (UW-NMS) cloud-mesoscale model. Then, we compare our single-scattering results for randomly-oriented soft-ice hydrometeors with corresponding ones that make use of: a) effective-medium equivalent spheres, b) solid-ice equivalent spheres, and c) randomly-oriented aggregates of ice cylinders. Finally, we extend to our particles the scattering formulas that have been developed by other authors for randomly-oriented aggregates of ice cylinders.

Explaining discrepancies in passive microwave cloud-radiation databases in microphysical context from two different cloud-resolving models

Mesoscale Cloud Resolving Models (CRMs) are often used to generate descriptions of the microphysical properties of precipitating clouds for the purpose of guiding precipitation retrieval algorithms designed for satellite-borne passive microwave radiometers. However, CRMs were not originally designed for that purpose. Notably, individual CRMs have adopted different bulk microphysical schemes to optimize the dynamical evolution of storms and accumulated rainfall, rather than optimizing for simulations of radiative properties – which are greatly affected by the microphysical details and vertical distributions of liquid and frozen hydrometeors. Thus, in principle, the simulated upwelling passive microwave (PMW) brightness temperatures (TBs) and associated precipitation retrievals generated by means of different CRMs with different microphysical parameterizations may be significantly different – even when the different CRMs prognostically adhere to the main dynamical and precipitation characteristics of a given storm.

Deriving Sea Surface Salinity and Density Variations From Satellite and Aircraft Microwave Radiometer Measurements: Application to Coastal Plumes Using STARRS

Using brightness temperature <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tb</i> measurements from L-band airborne microwave radiometers, with independent sea surface temperature (SST) observations, sea surface salinity (SSS) can be remotely determined with errors of about 1 psu in temperate regions. Nonlinearities in the relationship between <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tb</i> , SSS, and SST produce variations in the sensitivity of salinity <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> to variations in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tb</i> and SST. Despite significant efforts devoted to SSS remote sensing retrieval algorithms, little consideration has been given to deriving density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> from remotely sensed SSS and SST. Density is related to <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> through the equation of state. It affects the ocean's static stability and its dynamical response to forcings. By chaining together two empirical relationships (flat-sea emissivity and equation of state) to form an inversion algorithm for sea surface density (SSD) in terms of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tb</i> and SST, we develop a simple L-band SSD retrieval algorithm. We use this to investigate the sensitivity of SSD retrievals to observed <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tb</i> and SST and infer errors in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> for typical sampling configurations of the airborne Salinity, Temperature, And Roughness Remote Scanner (STARRS) and satellite-borne Soil Moisture and Ocean Salinity (SMOS) and Aquarius radiometers. We then derive <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> from observations of river plumes obtained using STARRS and demonstrate several oceanographic applications: the observations are used to study variations in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> effects on <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> in the Mississippi plume, and the across-shelf density gradient is used to infer surface geostrophic shear and subsurface geostrophic current in the Plata plume. Future basin-scale applications of SSD retrievals from satellite-borne microwave radiometers such as SMOS and Aquarius are anticipated.

Global Precipitation Map Using Satellite-Borne Microwave Radiometers by the GSMaP Project: Production and Validation

This paper documents the production and validation of retrieved rainfall data obtained from satellite-borne microwave radiometers by the Global Satellite Mapping of Precipitation (GSMaP) Project. Using various attributes of precipitation derived from Tropical Rainfall Measuring Mission (TRMM) satellite data, the GSMaP has implemented hydrometeor profiles derived from Precipitation Radar (PR), statistical rain/no-rain classification, and scattering algorithms using polarization-corrected temperatures (PCTs) at 85.5 and 37 GHz. Combined scattering-based surface rainfalls are computed depending on rainfall intensities. PCT85 is not used for stronger rainfalls, because strong depressions of PCT85 are related to tall precipitation-top heights. Therefore, for stronger rainfalls, PCT37 is used, with PCT85 used for weaker rainfalls. With the suspiciously strong rainfalls retrieved from PCT85 deleted, the combined rainfalls correspond well to the PR rain rates over land. The GSMaP algorithm for the TRMM Microwave Imager (TMI) is validated using the TRMM PR, ground radar [Kwajalein (KWAJ) radar and COBRA], and Radar Automated Meteorological Data Acquisition System (AMeDAS) precipitation analysis (RA). Monthly surface rainfalls retrieved from six microwave radiometers (GSMaP_MWR) are compared with the gauge-based dataset. Rain rates retrieved from the TMI (GSMaP_TMI) are in better agreement with the PR estimates over land everywhere except over tropical Africa in the boreal summer. Validation results of the KWAJ radar and COBRA show a good linear relationship for instantaneous rainfall rates, while validation around Japan using the RA shows a good relationship in the warm season. Poor results, connected to weak-precipitation cases, are found in the cold season around Japan.

Multifrequency Microwave Emission Fromthe Dome-C Area on the East Antarctic Plateau: Temporal and Spatial Variability

The Antarctic plateau that extends for several hundred kilometers with an average altitude of close to 3000 m a.s.l. is the highest part of the east Antarctic ice cap. This area provides unique opportunities for various scientific disciplines, including glaciology and atmospheric and earth sciences. In addition, there is growing interest in using the Antarctic plateau, for calibrating and validating data of satellite-borne microwave radiometers, thanks to the size, structure, and spatial homogeneity of this area, and the thermal stability of deeper snow layers. In this paper, we analyze the temporal and spatial variabilities of multifrequency microwave emission from the area surrounding the Dome-C scientific station using Advanced Microwave Scanning Radiometer data collected throughout 2005. Moreover, a multilayer coherent electromagnetic model is used for estimating the contribution of snow layers to emission at various frequencies. The results are consistent with the physical structure of the ice sheet and with its seasonal and spatial variations.

Diurnal Variation of the AMSU-A Brightness Temperatures Over the Amazon Rainforest

Brightness temperatures over the Amazon rainforest are obtained from the Advanced Microwave Sounding Units (AMSU-A and AMSU-B) instruments onboard three NOAA satellites (NOAA-15, -16, and -17, respectively) for the months of July, August, and October, 2002. The three AMSU-A instruments provided six daily measurements, separated by 2.5-5.5 h of the diurnal time intervals, over the Amazon rainforest region, and these measurements offer a unique opportunity for investigation of the diurnal variation of the brightness temperatures over the Amazon rainforests. The angular distributions of brightness temperatures over the Amazon rainforest are very stable and can be simulated with a radiative transfer model, which consists of an atmospheric radiative component and a rainforest-canopy model that treats the rainforest as a uniform layer with an effective canopy temperature. The simulated results agree well with the observations. The diurnal variation of brightness temperatures over the Amazon rainforest is simulated with a Fourier-series model. It shows that a second order of Fourier series can reproduce the observed pattern of diurnal variation of the brightness temperatures at zenith angles of 0deg, 28.7deg, and 58.1deg, respectively. In a practical application, the coefficients of Fourier-series expansion can be used to generate the brightness temperatures as a function of diurnal hours. These results can be applied to postlaunch calibration of satellite-borne microwave radiometer with different equator crossing time. In addition, the results presented in this paper indicate that the Amazon rainforest can be used as a hot calibration reference target. The availability of a land calibration target is important for calibration and validation of spaceborne microwave radiometers

DOMEX 2004: An Experimental Campaign at Dome-C Antarctica for the Calibration of Spaceborne Low-Frequency Microwave Radiometers

Satellite data are the most suitable tools for monitoring time and spatial variations of snow covered areas and for studying snow characteristics on a global scale. Current knowledge of the microwave emission from the deep ice sheet in Antarctica is limited by the lack of low-frequency satellite sensors and by their inadequate knowledge of the physical effects governing microwave emission at wavelengths exceeding 5 cm. On the other hand, in addition to the interest related to climatic changes and to glaciological and hydrological applications, there is growing interest, on the part of the remote sensing community, in using the Antarctic and, in particular, the Dome-C plateau where the Concordia station is located, for calibrating and validating data of satellite-borne microwave and optical radiometers. This is because of the size, structure, spatial homogeneity, and thermal stability of this area. With a view to the future launches of two new low-frequency spaceborne sensors Soil Moisture and Ocean Salinity mission and Aquarius, an experiment was carried out at Dome-C, thanks to financial support from European Space Agency, aimed at evaluating the stability and the absolute value of the L- and C-band brightness temperature Tb. This paper presents a report on the experimental campaign, the characteristics of the radiometric measurements, and on the main results. The C-band Tb data indicated a diurnal cycle amplitude of a few kelvin. It was confirmed that this takes place as a consequence of observed variability in the physical temperature of the top 4 m of the snowpack around the mean surface value of -24degC. In contrast, the L-band data indicated extremely stable Tb values of 192.32 K (1sigma=0.18 K) and 190.77 K (1sigma=0.57 K) at thetas=45deg and thetas=56deg, respectively

Comparing microphysical/dynamical outputs by different cloud resolving models: impact on passive microwave precipitation retrieval from satellite

Abstract. Mesoscale cloud resolving models (CRM's) are often utilized to generate consistent descriptions of the microphysical structure of precipitating clouds, which are then used by physically-based algorithms for retrieving precipitation from satellite-borne microwave radiometers. However, in principle, the simulated upwelling brightness temperatures (TB's) and derived precipitation retrievals generated by means of different CRM's with different microphysical assumptions, may be significantly different even when the models simulate well the storm dynamical and rainfall characteristics. In this paper, we investigate this issue for two well-known models having different treatment of the bulk microphysics, i.e. the UW-NMS and the MM5. To this end, the models are used to simulate the same 24-26 November 2002 flood-producing storm over northern Italy. The model outputs that best reproduce the structure of the storm, as it was observed by the Advanced Microwave Scanning Radiometer (AMSR) onboard the EOS-Aqua satellite, have been used in order to compute the upwelling TB's. Then, these TB's have been utilized for retrieving the precipitation fields from the AMSR observations. Finally, these results are compared in order to provide an indication of the CRM-effect on precipitation retrieval.

Oceanic whitecaps: Sea surface features detectable via satellite that are indicators of the magnitude of the air-sea gas transfer coefficient

Stage A whitecaps (spilling wave crests) have a microwave emissivity of close to 1. Thus if even a small fraction of the sea surface is covered by these features there will be a detectable enhancement in the apparent microwave brightness temperature of that surface as determined by satellite-borne microwave radiometers. This increase in the apparent microwave brightness temperature can as a consequence be routinely used to estimate the fraction of the sea surface covered by stage A whitecaps. For all but the very lowest wind speeds it has been shown in a series of controlled experiments that the air-sea gas transfer coefficient for each of a wide range of gases, including carbon dioxide and oxygen, is directly proportional to the fraction of the sea surface covered by these stage A whitecaps.