Passive Microwave Systems Research Articles

A Bayesian machine learning method to explain the error characteristics of global-scale soil moisture products

Estimating accurate surface soil moisture (SM) dynamics from space, and knowing the error characteristics of these estimates, is of great importance for the application of satellite-based SM data throughout many Earth Science/Environmental Engineering disciplines. Here, we introduce the Bayesian inference approach to analyze the error characteristics of widely used passive and active microwave satellite-derived SM data sets, at different overpass times, acquired from the Soil Moisture Active Passive (SMAP), Soil Moisture and Ocean Salinity (SMOS), and Advanced Scatterometer (ASCAT) missions. In particular, we apply Bayesian hierarchical modeling (BHM) and triple collocation analysis (TCA) to investigate the relative importance of different environmental factors and human activities on the accuracy of satellite-based data.To start, we compare the BHM-based sensitivity analysis method to the classic multiple regression models using a frequentist approach, which includes complete pooling and no-pooling models that have been widely used for sensitivity analysis in the field of remote sensing and demonstrate the BHM's adaptability and great potential for providing insight into sensitivity analysis that can be used by various remote sensing research communities.Next, we conduct an uncertainty analysis on BHM's model parameters using a full range of uncertainties to assess the association of various environmental factors with the accuracy of satellite-derived SM data. We focus on investigating human-induced error sources such as disturbed surface soil layers caused by irrigation activities on microwave satellite systems, naturally introduced error sources such as vegetation and soil organic matter, and errors related to the disregard of SM retrieval algorithmic assumptions - such as the thermal equilibrium passive microwave systems. Based on the BHM-based sensitivity analysis, we find that assessments of SM data quality with a single variable should be avoided, since numerous other factors simultaneously influence their quality. As such, this provides a useful framework for applying Bayesian theory to the investigation of the error characteristics of satellite-based SM data and other time-varying geophysical variables.

A web-based system for satellite-based high-resolution global soil moisture maps

Nowadays, a wide range of applications require near-real-time Surface Soil Moisture (SSM) data at high spatial resolution. However, operational passive microwave systems like SMOS and SMAP can only acquire such information at a relatively coarser resolution. Therefore, several downscaling algorithms have been developed to address this issue and provide SSM maps at a finer spatial scale. Users may, however, find it difficult to implement the downscaling algorithm due to the complexity of integrating various data sources. Disaggregation based on Physical and Theoretical scale Change (DisPATCh) is one of the algorithms that is widely accepted to downscale passive microwave SSM observations. But the complexity of modeling, the variety of data sources and formats of input data make it very difficult for users to implement the algorithm. Thus, we developed a Satellite-based Hydrological Monitoring System (SHMS), which facilitates this gap through the implementation of the DisPATCh algorithm to generate large-scale SSM maps with high resolution, which is achieved by combining SMAP and MODIS products. The System Usability Scale (SUS) method was used to evaluate the system's strengths and weaknesses. The SUS evaluation results show that 74.75% of SHMS users are satisfied with the system's performance.

Multi-Parameter Regularization Method for Synthetic Aperture Imaging Radiometers

Synthetic aperture imaging radiometers (SAIRs) are powerful passive microwave systems for high-resolution imaging by use of synthetic aperture technique. However, the ill-posed inverse problem for SAIRs makes it difficult to reconstruct the high-precision brightness temperature map. The traditional regularization methods add a unique penalty to all the frequency bands of the solution, which may cause the reconstructed result to be too smooth to retain certain features of the original brightness temperature map such as the edge information. In this paper, a multi-parameter regularization method is proposed to reconstruct SAIR brightness temperature distribution. Different from classical single-parameter regularization, the multi-parameter regularization adds multiple different penalties which can exhibit multi-scale characteristics of the original distribution. Multiple regularization parameters are selected by use of the simplified multi-dimensional generalized cross-validation method. The experimental results show that, compared with the conventional total variation, Tikhonov, and band-limited regularization methods, the multi-parameter regularization method can retain more detailed information and better improve the accuracy of the reconstructed brightness temperature distribution, and exhibit superior noise suppression, demonstrating the effectiveness and the robustness of the proposed method.

Microwave Imaging Using Noise Signals

A new method of active incoherent microwave imaging is presented that uses noise signals and spatial frequency sampling. Building on interferometric spatial frequency sampling arrays developed in radio astronomy, active incoherent microwave imaging utilizes the transmission of noise signals to implement the first active imaging method that samples in the spatial frequency domain. In comparison to passive microwave imaging systems, active incoherent imaging requires receivers with far less sensitivity, and thus less overall cost. We present the theory behind the imaging technique and show experimental results from a 5.85-GHz system imaging in one and two dimensions. For 1-D images, the transmitter consisted of two noise generators, while the receive array was a synthesized linear array with elements placed in $1\lambda $ increments with the widest spacing of $15\lambda $ . For 2-D images, the transmitter consisted of three noise generators, while the receive array was a synthesized T-array with elements placed in $0.5\lambda $ increments. We demonstrate the reconstruction of 1-D and 2-D scenes consisting of spherical reflecting targets, using only 25 MHz of signal bandwidth and 10 $\mu \text{s}$ of integration time, both of which are an order of magnitude less than passive microwave and millimeter-wave imaging systems.

Single-conductor co-planar quasi-symmetry unequal power divider based on spoof surface plasmon polaritons of bow-tie cells

In this paper, the spoof surface plasmon polaritons (SSPPs) transmission line (TL) of periodical grooved bow-tie cells is proposed. The complex propagation constant and characteristic impedance of the SSPPs TLs and microstrip lines (MLs) are extracted using the analytical method of generalized lossy TL theory. The properties of the SSPPs TLs with different substrates and the same geometrical configuration are experimented. Then, for comparison, two ML counterparts are also experimented, which shows that the SSPPs TL is less sensitive to the thickness, dielectric constant and loss tangent of the chosen substrate below the cutoff frequency, compared with the ML ones. The single-conductor co-planar quasi-symmetry unequal power divider based on this SSPPs TL is presented in microwave frequencies. For experimental validation, the 0-dB, 2-dB, and 5-dB power dividers are designed, fabricated, and measured. Both simulated and measured results verify that the unequal power divider is a flexible option, which offers massive advantages including single-conductor co-planar quasi-symmetry structures, wide-band operation, and convenient implementations of different power-dividing ratios. Hence, it can be expected that the proposed unequal power dividers will inspire further researches on SSPPs for future design of novel planar passive and active microwave components, circuits and systems.

Evaluation of AMSR2 soil moisture products over the contiguous United States using in situ data from the International Soil Moisture Network

High quality soil moisture datasets are required for various environmental applications. The launch of the Advanced Microwave Scanning Radiometer 2 (AMSR2) on board the Global Change Observation Mission 1—Water (GCOM-W1) in May 2012 has provided global near-surface soil moisture data, with an average revisit frequency of two days. Since AMSR2 is a new passive microwave system in operation, it is very important to evaluate the quality of AMSR2 products before widespread utilization of the data for scientific research. In this paper, we provide a comprehensive evaluation of the AMSR2 soil moisture products retrieved by the Japan Aerospace Exploration Agency (JAXA) algorithm. The evaluation was performed for a three-year period (July 2012–June 2015) over the contiguous United States. The AMSR2 soil moisture products were evaluated by comparing ascending and descending overpass products to each other as well as comparing them to in situ soil moisture observations of 598 monitoring stations obtained from the International Soil Moisture Network (ISMN). The accuracy of AMSR2 soil moisture product was evaluated against several types of monitoring networks, and for different land cover types and ecoregions. Three performance metrics, including mean difference (MD), root mean squared difference (RMSD), and correlation coefficient (R), were used in our accuracy assessment. Our evaluation results revealed that AMSR2 soil moisture retrievals are generally lower than in situ measurements. The AMSR2 soil moisture retrievals showed the best agreement with in situ measurements over the Great Plains and the worst agreement over forested areas. This study offers insights into the suitability and reliability of AMSR2 soil moisture products for different ecoregions. Although AMSR2 soil moisture retrievals represent useful and effective measurements for some regions, further studies are required to improve the data accuracy.

The Terra Nova Bay (Antarctica) polynya observed by MODIS ice surface temperature imagery from May to June 2009

We have examined the development of the Terra Nova Bay polynya (TNBP) using a two-month sequence of Moderate Resolution Imaging Spectroradiometer (MODIS) ice surface temperature (IST) images, complemented by a set of synthetic aperture radar (SAR) images. Results of a comparison between the clear-sky ISTs and the contemporaneous SAR images revealed that the polynya is characterized in the ISTs by a warm core and in the SARs by a bright backscattering area, due to the presence of seawater and grease ice. The activity of the polynya during the period was highly dynamic, with opening and closing phases that had similar temporal length scales, sometimes of less than 24 h. The recent activity of the polynya is evidenced in the SARs by alternate bright and dark backscatter bands formed during openings and calm periods and transported offshore by the ice drift. The thermal fronts that separate the warm core of the polynya from the surrounding ice sheet appeared sharp during the opening phase and smooth during the closing phase. The spatial resolution of 1 km of MODIS ISTs, compared with space-borne passive microwave systems with maximum resolution of approximately 10 km, provides a smaller area for the TNBP than do passive microwave data. The polynya area was detectable up to 80% of time with a revisit time of a few hours, giving an average area of ∼1300 km2 and peaks up to 3000 km2. The polynya is forced by katabatic winds that have a warm signature, which is also visible in the IST sequence. Katabatic airflows descending from the Reeves Glacier valley contribute to the opening of the polynya in the central part of the bay, while airflows descending from the surrounding glacier valleys contribute to the opening in the north and south of the bay.

Overview of Soil Moisture Retrieval from Passive Microwave Remote Sensing

The surface soil moisture is a quantity resulting from the water and energy exchanges at the land-surface/atmosphere interface. Measuring this variable has potential application in hydrology and meteorology. Passive microwave remote sensing techniques have great potential for providing estimates of soil moisture with good temporal repetition on a daily basis and on a regional scale (~10 km).Compared with optical and thermal-infrared sensors, although passive microwave measurements are of lower spatial resolution than optical and infrared measurements, they are less influenced by solar illumination, aerosols, and clouds, and they are responsive to different dynamic ranges of vegetation structure and biomass. Compared with active microwave sensors, the passive microwave systems include frequent coverage, low data rates, simpler data processing. However, the effects of vegetation cover, soil temperature, snow cover, topography, and soil surface roughness also play a significant role in the microwave emission from the surface. Different soil moisture retrieval approaches have been developed to account for the various parameters contributing to the surface microwave emission. In this paper, approaches for monitoring soil moisture using passive microwave remote sensing were discussed and compared, especially the retrieval approaches which will have to be used in the future operational application.

Monitoring of rainfall by ground-based passive microwave systems: models, measurements and applications

Abstract. A large set of ground-based multi-frequency microwave radiometric simulations and measurements during different precipitation regimes are analysed. Simulations are performed for a set of frequencies from 22 to 60 GHz, representing the channels currently available on an operational ground-based radiometric system. Results are illustrated in terms of comparisons between measurements and model data in order to show that the observed radiometric signatures can be attributed to rainfall scattering and absorption. An inversion algorithm has been developed, basing on the simulated data, to retrieve rain rate from passive radiometric observations. As a validation of the approach, we have analyzed radiometric measurements during rain events occurred in Boulder, Colorado, and at the Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains (SGP) site in Lamont, Oklahoma, USA, comparing rain rate estimates with available simultaneous rain gauge data.

Spatial resolution and processing tradeoffs for HYDROS: application of reconstruction and resolution enhancement techniques

Recent developments in reconstruction and resolution enhancement for microwave instruments suggest a possible tradeoff between computation, resolution, and downlink data rate based on postcollection reconstruction/resolution enhancement processing. The Hydrospheric State (HYDROS) mission is designed to measure global soil moisture and freeze/thaw state in support of weather and climate prediction, water, energy, and carbon cycle studies, and natural hazards monitoring. It will use an active and passive L-band microwave system that optimizes measurement accuracy, spatial resolution, and coverage. The active channels use synthetic aperture radar-type processing to achieve fine spatial resolution, requiring a relatively high downlink data rate and ground processor complexity. To support real-time applications and processing, an optional postcollection reconstruction and resolution enhancement method is investigated. With this option, much lower rate real-aperture radar data are used along with ground-based postprocessing algorithms to enhance the resolution of the observations to achieve the desired 10-km resolution. Several approaches are investigated in this paper. It is determined that a reconstruction/resolution enhancement technique combining both forward- and aft-looking measurements enables estimation of 10-km resolution or better backscatter values at acceptable accuracy. Key tradeoffs to achieve this goal are considered.

Satellite earth observation in operational oceanography

The role and contribution of satellite data in operational oceanography is reviewed, with emphasis on northern European seas. The possibility to observe various ocean parameters and processes by existing satellite sensors, such as optical instruments, infrared radiometers, passive microwave radiometers, and active microwave systems (altimeter, scatterometer, SAR) is discussed. The basic parameters are: sea-surface temperature observed by infrared radiometers, ocean colour by spectrometers, sea-surface elevation by altimeters, and surface roughness by active and passive microwave systems, which can be used to derive surface wind and waves. A number of ocean processes can be derived from synoptic mapping of the basic parameters of larger sea areas, such as current patterns, fronts, eddies, water mass distribution, and various water quality parameters (chlorophyll, surface slicks, suspended sediments). The suitability of existing satellite data to fulfil the operational requirements for temporal and spatial coverage, data delivery in near-real-time, and long-term access to data is discussed in light of the fact that optical/infrared data in northern Europe are severely hampered by frequent cloud cover, while microwave techniques can provide useful data independent of weather and light conditions. Finally, the use of data assimilation in oceanographic models is briefly summarised, indicating that this technique is under development and will soon be adopted in operational oceanography.

Potential applications of 1–5 GHz radar backscatter measurements of seasonal land snow cover

Lower frequency microwave radar observations can help to measure the properties of snow cover on land by providing information about the soil‐snow boundary condition. In this theoretical study, we examine the sensitivities of microwave radar measurements to soil and snow characteristics, and we compare a simple model with previously published data. Depending on the surface roughness, copolarized ratios or single polarization time ratios of radar backscattering may be affected only by the incidence angle and the dielectric contrast at the soil‐snow boundary. These measurements can reduce the number of unknowns in any corresponding higher‐frequency observations that are also affected by the lower boundary condition. The copolarized ratio is sensitive, first of all, to the snow density, which offers the possibility of measuring this parameter directly if a separate measurement of the soil temperature from a passive microwave system is also available. The thermal insulation provided by snow cover can have a powerful effect on the soil‐snow boundary by altering the soil temperature and therefore changing the dielectric contrast. Because the microwave response of snow cover is so sensitive to the conditions of the underlying frozen soil, it is important for future ground truth campaigns that measure snow conditions to also collect data on the temperature, water content, and texture of the soil.

Some results of new microwave remote sensing experiments

Within the last two decades, active and passive microwave systems for remote sensing have been tremendously improved in their performance. On one hand this is due to the availability of better analogue components and circuits and on the other hand due to the impressive progress in the field of digital signal processors. Furthermore, new concepts based on understanding physical phenomena have been developed. Airborne and ground based experiments are usually forerunners of spacebome systems. In this paper, image data of recent airborne polarimetric SAR and polarimetric SAR interferometer experiments will be explained with respect to its new information content. In addition, some remarks will be made about ERS1/ERS-2 Tandem data for SAR interferometry with respect to the ground location of the image pixels and the errors caused by environmental changes in the time interval between the multi-pass data takes. The improvements expected by the upcoming SRTM (single-pass) will be mentioned, too. In the field of imaging radiometers, studies and experiments are focussed on imaging aperture synthesis radiometers which take advantage of the multi-beam capabilities of thinned arrays, in order to cover pixel by pixel the sites of interest. Promising images have been achieved. Reaults Will be shoyyn.

Stop, look and listen: Auditory perception analogies for radar remote sensing

As active microwave instruments become more advanced and the application of radar images more prolific there is an ever present need for a wider audience to appreciate the workings and subtleties of radar remote sensing systems. Many problems associated with the use and interpretation of such data are a consequence of users failing to properly conceptualise the radar system and the manner in which it gathers information about the target. This paper attempts to provide a conceptual framework for radar systems based on an auditory perception analogy that should allow for a clearer appreciation of the subtle differences between passive optical and active microwave systems. Through a comparison between how eyes and ears function, and by looking at their very different means of gathering information, a few of the difficult concepts that are important in gaining a deeper understanding of radar image production are discussed.

RF characterization of an inflatable parabolic torus reflector antenna for space-borne applications

Space-borne satellite applications provide a vast array of services extending from global connectivity to Earth observation systems. The soil moisture radiation mission is a proposed space-borne passive microwave system complementary to the existing Earth observing system operating at low microwave frequencies and requiring an antenna with multibeam, high-beam efficiency, and dual polarization capabilities. To achieve both the large reflector size and the multibeam pattern at the operational frequencies an innovative multibeam reflector antenna design was needed. The advances in inflatable antenna technology has been proposed to overcome the launch vehicle size and weight restrictions. This paper describes a novel offset parabolic torus reflector antenna design that produces the desired multibeam pattern and is compatible with the inflatable antenna technology. Using the system requirements of this mission as an example, the design process for an inflatable parabolic torus reflector antenna is outlined, the development of suitable distortion models is given, and representative RF characteristics are presented. These RF characteristics include far-field patterns, beam contour patterns, beam efficiency, and other key performance parameters. The development of an advanced analytical modeling/numerical tool in support of the design effort is also detailed.

Assessment of sea‐ice conditions using two‐channel active and passive microwave systems1

Abstract An algorithm is developed and tested for assessing the types and concentrations of sea ice on the basis of data obtained from two‐band active and passive microwave sensors, using a priori values of the scattering parameters and natural emission of basic types of ice for the corresponding characteristics of the observation system—frequency, polarization, and observation angle—and a linear mixture model for the measured radiobrightness temperature and specific effective scattering area. The results of satellite monitoring with a side‐looking radar with a real SLR aperture and an RM‐08 microwave radiometer, of the region of the Barents‐Kara‐Laptev seas and the adjacent Arctic Ocean, were used in organizing a satellite database for the period October 1995–March 1996, in compiling digital mosaic charts of the ice cover, and in making a comparative analysis with the corresponding charts prepared by the U.S. Navy and National Oceanic and Atmospheric Administration.

High temperature superconductors and their application in passive antenna systems

The basic physics of superconductivity is reviewed and related to the high-temperature superconductors. The high-frequency characteristics of the new materials are examined, and the two-fluid model is introduced and used to derive an empirical expression for the conductivity of superconductors. The advantages and disadvantages of using these superconductors in the construction of high-frequency transmission lines and passive microwave and millimeter-wave antenna systems are discussed. >

Möglichkeiten und grenzen von mikrowellensystemen zur fernerkundung von satelliten

(Possibilities and Limitations of Microwave Systems for Satellite Remote Sensing)—Microwaves are applied in satellite remote sensing in both active, i.e. radar systems and in passive, i.e. microwave radiometer systems. The advantages and disadvantages of active and passive microwave systems in comparison to each other and to optical systems are discussed shortly. It becomes evident that radar has advantages over microwave radiometry due to its ranging and velocity-measurement capability, due to its SAR-capability with its extremely fine resolution and due to the fact that no major theoretical limitations exist for the obtainable measurement accuracy. The most important radar systems for Earth and ocean observations from satellites are altimeter, scatterometer and imaging radars with real or synthetic aperture. The physical and technical basics of these types of radar are presented together with some applications. Examples for remote sensing data and results obtained from different microwave remote sensing systems show the present state of the art and the present possibilities and limitations of microwave systems for satellite remote sensing.

Passive microwave remote sensing of the ocean?A review

This paper reviews the current status of passive microwave remote sensing of the ocean. The physics of emission and instrumentation are highlighted in order to establish a relationship between the thermal emission and retrieved geophysical parameters. A discussion then follows on measurements of temperature, salinity, windspeed, etc. using passive microwave systems. These measurements are related to the accuracy and spatial resolution required by the users. The status of passive microwave remote sensing is summarized and recommendations for future research are presented.

Microwave remote sensing

Microwave remote sensing is rapidly reaching the stage af maturity enjoyed by optical and infra-red systems. The value of microwaves is that the atmosphere is almost transparent within this band. Microwaves therefore offer a view of the earth surface that is completely unobstructed by clouds. Both passive and active microwave systems have been utilized for measuring several geophysical phenomena. Microwave radiometers have, for example, measured ocean temperature and salinity, winds over the ocean, soil moisture through vegetation, and the age of arctic ice. Active microwave systems (scatterometers, short pulse altimeters, and imaging radars) have proven their capability to measure wind speed and direction over the ocean, ocean wave height, the topography of the earth, and the conditions of crops. Numerous other applications can be found in the recent technical literature. As detailed examples of the value of microwave remote sensing, this article describes two systems which are approaching an operational stage.