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The passive and active L-band system (PALS) is an airborne microwave instrument suite that combines a separate radar and radiometer airborne instrument with a common antenna. PALS has been used for salinity, wind speed, and soil moisture (SM) measurements, which is critical for calibration and validation of the SM active passive satellite brightness temperature measurements. The combined 1.413 GHz radiometer and 1.26 GHz radar measurements can provide high-accuracy SM measurements over vegetated areas. The new upgraded version of the PALS instrument employs a novel single-chain receiver topology and combines the radar and radiometer instruments. The reduced-noise single-chain receiver PALS instrument was deployed on a DC-3 aircraft for SM measurement in the summer of 2019.

A new forward model (FM) was developed to characterize the influence of precipitation on L-band passive ocean surface measurements. The FM, which relates rain-induced brightness temperature (TB) variations to the rain rate and wind speed (WS), was established through a machine learning approach (referred to as the ML-FM). The soil moisture active passive (SMAP) data matched with integrated multisatellite retrievals for global precipitation measurement (IMERG) rain rate data and cross-calibrated multiplatform (CCMP) wind data were binned as a function of the rain rate, WS, and wind direction. The ML-FM was validated by comparing the simulated top-of-atmosphere (TOA) TB values with SMAP measurements. The results showed favorable agreement between the ML-FM outputs and SMAP data, with a root mean square error (RMSE) smaller than 0.55 K for both the horizontal and vertical polarizations. The validation results for ensuring more reasonable rainfall intensity distributions showed that the ML-FM returned stable results with a slightly reduced RMSE of ∼0.75 K for both the horizontal and vertical polarizations. Based on the ML-FM, we found that sea surface emission exhibited significant dependence on the rain rate for both polarizations. In addition, the ML-FM demonstrated signal saturation when the rain rate exceeded 45 mm/h, while precipitation slightly affected the directional characteristics of sea surface emission. These effects accounted for ∼0.3 K at a rain rate of 50 mm/h. Overall, our analyses demonstrated that the proposed ML-FM achieved superior performance in retrieving the TOA TB for both the vertical and horizontal polarizations with a higher accuracy than existing models.

Passive microwave remote sensing is a vital tool for acquiring valuable information regarding the Earth's surface, with significant applications in agriculture, water management, forestry, and various environmental disciplines. Precision agricultural (PA) practices necessitate the availability of field-scale, high-resolution remote sensing data products. This study focuses on the design and development of a cost-effective, portable L-band microwave radiometer capable of operating from an unmanned aircraft system platform to measure high-resolution surface brightness temperature ($T_{B}$). This radiometer consists of a dual-polarized (Horizontal polarized, H-pol and Vertical polarized, V-pol) antenna and a software-defined radio-based receiver system with a 30 MHz sampling rate. The post-processing methodology encompasses the conversion of raw in-phase and quadratic (I&Q) surface emissions to radiation $T_{B}$ through internal and external calibrations. Radiometric measurements were conducted over an experimental site covering both bare soil within an agricultural field and a large water body. The results yielded a high-resolution $T_{B}$ map that effectively delineated the boundaries between land and water, and identified land surface features. The radiometric temperature measurements of the sky and blackbody demonstrated a standard deviation of 0.95 K for H-pol and 0.57 K for V-pol in the case of the sky and 0.39 K for both H-pol and V-pol in the case of the blackbody observations. The utilization of I&Q samples acquired via the radiometer digital back-end facilitates the generation of different time–frequency (TF) analyses through short-time Fourier transform and power spectral density (PSD). The transformation of radiometer samples into TF representations aids in the identification and mitigation of radio frequency interference originating from the instrument itself and external sources.

Subsurface electromagnetic sensing techniques that can measure material properties of hidden layers are useful for applications such as security screening. A system combining active (radar) and passive (radiometer) sensing into one unit, or a pactive sensor, can address drawbacks of extant single mode sensors by reducing measurement ambiguity and resolving features better, thereby leading to improved identification of the hidden layer. This work investigates a technique to noninvasively extract the complex permittivity and thickness of hidden dielectrics using a pactive sensor. A proof-of-concept demonstration of an optimization-based inversion technique is used to extract the properties of an unknown, hidden layer in a multi-layered dielectric structure. The technique uses active data to estimate dielectric constant and thickness, and passive data to estimate loss tangent. The experimental setup is selected to represent a simplified single-voxel security scenario consisting of multiple dielectric layers backed by a human phantom layer (heated water). Tests using K-band prototype radar and radiometer systems yielded results with less than 5% error in the extracted dielectric constant, loss tangent and thickness of the unknown layer.

An eight-channel phased array (PA) radiometer front-end is developed to serve video-rate passive millimeter wave (PMMW) imaging at ${V}$ -band. The front-end is implemented by multichip module (MCM), in which all active microwave functions are realized with GaAs monolithic microwave integrated circuits (MMICs) while passive devices and connection lines are designed on printed circuit board (PCB). All receiver channels are linearly arranged and equipped with waveguide-fed dielectric rod antennas (DRAs). By introducing a surface metallized bent structure into the DRA, the phase center of element antenna can be changed, which provides an effective approach to suppress grating lobes without a significant increase in structural complexity of the sparse array. Moreover, chip assembly details are presented together with the methods taken to mitigate the mismatch caused by wire-bond interconnections and MMICs. Single-channel (SC) and overall-module measurements are applied to the front-end, and the results reveal that the front-end achieves 50-dB gain, 5-dB noise figure (NF), ±24° field of view (FoV) and −10-dB side/grating lobes within the frequency band of 52–56 GHz. Finally, a passive imaging system is established using the proposed front-end and the primary imaging results verify the front-end design in system level. [ABSTRACT FROM AUTHOR]

This article extends the application of an existing change-detection-based, time-series soil moisture retrieval algorithm to nonconcurrent active and passive measurements from WindSat/AMSR2 and the Soil Moisture Active Passive radar, which was active from late April until mid-July of 2015. A time series of L-band radar backscatter observations was used to populate an underdetermined matrix equation whose optimal solution was derived via a bounded linear least squares estimator, and whose bounds were derived from a time series of radiometer-derived soil moisture estimates (taken by either WindSat or AMSR2). Surface soil moisture estimates are compared with in-situ measurement probes, which were treated as ground truth. Error statistics and time-series results for the validation sites are presented here and conclusions derived therefrom. The overall RMSE and unbiased RMSE for the retrieval algorithm, taken across all reference pixels considered in the study, were 0.070 $\mathbf{m}^{3}/\mathbf{m}^{3}$ and 0.067 $\mathbf{m}^{3}/\mathbf{m}^{3}$, respectively, when using WindSat to constrain the algorithm. When using AMSR2 to constrain the algorithm, the RMSE and unbiased RMSE were 0.093 $\mathbf{m}^{3}/\mathbf{m}^{3}$ and 0.090 $\mathbf{m}^{3}/\mathbf{m}^{3}$, respectively.

Medical microwave radiometry is a near-field passive thermometry technique that requires highly sensitive and stable receiver to gather and process the ultra-low power thermal radiation from biological tissues (−174 dBm/Hz). The noise measurements are susceptible to radio frequency interference (RFI) and influenced by variations in power reflection between the near-field antenna and radiometer input ($\rho $), instrument electronic noise ($\boldsymbol {T}_{ \boldsymbol {rec}}$), amplifier noise ($\boldsymbol {T}_{ \boldsymbol {A}}$), and system gain ($\boldsymbol {G}$). Design of a self-balanced dual frequency band Dicke radiometer is reported in this work with real time calibration to eliminate the influence of $\rho $ , ${ \boldsymbol {T}}_{ \boldsymbol {rec}}, \boldsymbol {T}_{ \boldsymbol {A}}$ and gain ($\boldsymbol {G}$) on the unknown source temperature ($\boldsymbol {T}_{ \boldsymbol {S}}$). The radiometer is designed for operation over 1.2-1.5 GHz (low frequency/LF) and 2.8-3.1 GHz (high frequency/HF) for deep and superficial tissue temperature measurements, respectively. The radiometer measurements are characterized using matched load and ultra-wideband spiral antenna using tissue mimicking phantoms. The linearity of source temperature measurements for the matched load is close to 1 with root mean square error of 0.22 and 0.37 °C in LF and HF bands, respectively. Radiometer noise measurements using UWB antenna in tissue mimicking phantom confirm stable measurements and immunity to ambient RFI. [ABSTRACT FROM AUTHOR]

Sea surface salinity (SSS) in coastal zones with high temporal and spatial resolutions is also needed for scientific research and marine economy. However, one of the limitations of airborne L-band real-aperture microwave radiometers and synthesis aperture interferometric radiometers is high mass and volume or the high complexity of the system. To overcome the limitations, an L-band phased array radiometer is proposed as an alternative solution to monitor SSS in coastal zones from a small unmanned aerial vehicle (UAV). The L-band phased array radiometer is introduced, which only comprises a phased array and a single receiver. In addition, the calibration methods of the L-band phased array, radiometer that the external calibration method and the internal calibration method, are also introduced. A series of experiments was performed to assess the performance of the L-band phased array radiometer. The results indicate that the L-band phased array radiometer show a good performance in low pointing angles and low scanning angles. As a conclusion, the L-band phased array radiometer can be an alternative solution to monitor SSS in coastal zones with high temporal and spatial resolutions from a small UAV.

National Aeronautics and Space Administration's soil moisture active–passive (SMAP) mission potential to produce high-resolution soil moisture suffered adversely due to its L-band synthetic-aperture radar (SAR) failure. Other satellite-based L-/C-band SAR observations can be used within the SMAP active–passive algorithm. In this article, we evaluated the capability of ingesting ISRO's Radar Imaging Satellite-1 (RISAT-1) C-band SAR observations in the SMAP active–passive algorithm to obtain soil moisture at 1, 3, and 9 km over the agricultural region dominant by paddy that experiences seasonal flooding. We also improved the SMAP mission active–passive algorithm with a dynamic surface water bodies (ponding conditions) masking approach using the native RISAT-1 observations. The article shows that the use of surface water masks helps in mitigating the negative impact of surface water bodies in the active–passive disaggregation process. The SMAP–RISAT soil moisture retrievals at 1 and 3 km resolutions are found to have high unbiased root-mean-square error (ubRMSE) greater than 0.06 m3/m3 during very wet and high vegetative conditions. However, at low and moderate soil moisture states, the ubRMSE is below 0.06 m3/m3. Comparison of soil moisture retrievals at 9 km resolution with upscaled ground-based soil moisture measurements shows ubRMSE less than 0.04 m3/m3. This article is a precursor for estimating soil moisture for the upcoming RISAT-1A dataset over India. The findings will further help in the implementation of a microwave active–passive algorithm to retrieve soil moisture for future satellite missions involving radiometer-SAR instruments, and challenging geophysical conditions (i.e., dynamic surface water bodies).

The Temporal Experiment for Storms and Tropical Systems Technology Demonstration (TEMPEST-D) instrument is a five-frequency millimeter-wave radiometer operating from 87 to 181 GHz. The cross-track scanning radiometer has been operating on a 6U CubeSat in low Earth orbit since September 5, 2018. The direct-detection architecture of the radiometer reduces its mass and power consumption by eliminating the need for a local oscillator and mixer, also reducing system complexity. The instrument includes a scanning reflector and ambient calibration target. The reflector rotates continuously to scan the antenna beams in the cross-track direction, first across the blackbody calibration target, then toward the Earth over the full range of incidence angles, and finally to cosmic microwave background radiation at 2.73 K. This enables precision end-to-end calibration of the millimeter-wave receivers during every 2-s scan period. The TEMPEST-D millimeter-wave radiometers are based on 35-nm indium phosphide (InP) high-electron-mobility transistor (HEMT) low-noise amplifiers. This article describes the instrument and its characterization prior to launch. [ABSTRACT FROM AUTHOR]

An active–passive microwave retrieval algorithm for simultaneous determination of soil surface roughness parameters [vertical root-mean-square (RMS) height (${s}$) and horizontal correlation length (${l}$)] is presented for bare soils. The algorithm is based on active–passive microwave covariation, including the improved Integral Equation Method (I2EM), and is tested with global soil moisture active passive (SMAP) observations. The estimated retrieval results for ${s}$ and ${l}$ are overall consistent with values in the literature, indicating the validity of the proposed algorithm. Sensitivity analyses showed that the developed roughness retrieval algorithm is independent of permittivity for ${\varepsilon }_{s} > 10$ [-]. Furthermore, the physical model basis of this approach (I2EM) allows the application of different autocorrelation functions (ACF), such as Gaussian and exponential ACFs. Global roughness retrieval results confirm bare areas in deserts such as Sahara or Gobi. However, the type of ACF used within roughness parameter estimation is important. Retrieval results for the Gaussian ACF describe a rougher surface than retrieval results for the exponential ACF. No correlations were found between roughness results and the amount of precipitation or the soil texture, which could be due to the coarse spatial resolution of the SMAP data. The extension of this approach to vegetated soils is planned as an add-on study. [ABSTRACT FROM AUTHOR]

A 3-D printed antenna subsystem, which proves its effectiveness in cost- & weight-reduction and reliability improvement over conventional CNC fabrication, is proposed in this paper. The proposed structure which integrates an orthogonal-mode transducer (OMT) and a corrugated horn is fully compatible to fabrication using high temperature resistant resin Acrylonitrile Butadiene Styrene (ABS) with 3-D printing technology, as well as coating using copper with electroless metallization method. Detailed design procedures are provided and a prototype operating at X-band is designed, fabricated and measured. Over the whole operating frequency range of 11.7 GHz~12.7 GHz, measured reflection coefficients are better than -22 dB and -12 dB for different polarization directions. A very good isolation better than 60 dB is achieved between two input ports. The cross polarization level is at least 30 dB lower than the co-polarization. Moreover, the effectiveness of the fabricated antenna is also verified in a polarimetric radiometer system.

Image reconstruction from discrete samples is fundamental in remote sensing. The reconstruction problem is more complicated when the sample locations are irregularly spaced and employ different aperture functions. Further, not all 2-D sampling configurations permit full image reconstruction of band-limited signals. In this article, 2-D signal reconstruction from irregular samples with variable apertures is considered using theory and examples. Exact reconstruction requires that the signal be band-limited and the sampling matrix be invertable. The results are sensitive to the sample locations and noise in the measurements. Illustrative examples are provided using simulation and actual data from the L-band Soil Moisture Active Passive (SMAP) radiometer. The general approach can be employed with other sensors. [ABSTRACT FROM AUTHOR]

We present a co-designed Dicke switch and low-noise amplifier (LNA) in which the switch incorporates a transformer-based topology and serves as the input matching network of the LNA. This topology is configured to minimize the amplifier gain mismatch between the two switching states caused by process variations while providing a low noise figure (NF). The circuit is implemented in a 0.13-μm SiGe BiCMOS technology, and it achieves more than 20-dB gain and minimum NF values of 4.5 and 0.58 dB at 300 and 20 K, respectively. It consumes a dc power of 15 mW. The front-end switch presents a peak isolation of 17 dB, and the input return loss is better than 15 dB across 45–70 GHz. A noise source is also coupled to the front-end circuitry to enable on-chip calibration. The noise performance and reliability of several on-chip p-n junctions were characterized, and the one with the highest excess noise ratio was coupled to the front-end circuitry. To the best of our knowledge, the lowest NF of an integrated switch and LNA at V-band frequencies is achieved in the present work, along with the first reliability study of on-chip noise sources in a SiGe BiCMOS technology. [ABSTRACT FROM AUTHOR]

When using traditional methods to measure the microwave/millimeter-wave emissivity of small targets, errors arise due to that the antenna's main lobe and side lobe cannot be completely covered by the target. To eliminate the errors exist in traditional measurement methods, this paper first analyzes the main sources of errors and gives the analytical expression of the errors. On this basis, an improved method named voltage method is proposed. To test the effectiveness of the improved method, the horizontal polarization emissivity and the vertical polarization emissivity of a metal plate coat with stealthy nano-materials were measured at different observation angles by voltage method and traditional method, respectively. A dick radiometer working in 35 GHz is used in the experiments. Simultaneously, the accurate emissivities of the target are obtained by standard arch method. The results show that the voltage method improves the measurement accuracy largely compared to the traditional method.

Crookes radiometers have been the subject of numerous theoretical, numerical, and experimental studies because of the complicated forces they exhibit as well as their potential applications to light sensing and actuation. The majority of these studies have focused on classical radiometers, which function under low vacuum pressures. In contrast, here we report a radiometer with microengineered vanes that rotates at atmospheric pressure. Its functionality at pressures thousands of times higher than previous light mills is due to unique attributes of the nanocardboard that forms its vanes: 1) the extremely low areal density (0.1 mg/cm2) of nanocardboard reduces the vane masses by two orders of magnitude; 2) its lower thermal conductivity allows a greater cross-vane temperature difference; and 3) its microchannels dramatically increase the thermal transpiration flow that drives the rotation. Intriguingly, the experimentally observed rotation speeds are substantially higher than those theoretically expected. Our device demonstrates new possibilities for micromanipulation, propulsion of aerial vehicles, and light-powered generators. [2020-0165] [ABSTRACT FROM AUTHOR]

To monitor sea surface salinity (SSS) in coastal zones with high temporal and spatial resolution, an L-band phased array radiometer is developed. However, it is well known that the L-band radiometers suffer from serious radio frequency interference (RFI) effects. To mitigate the RFI effects, an RFI detection and mitigation algorithm is proposed for the L-band phased array radiometer. The experiments in coastal zones are performed to assess the performance of the RFI algorithm. The results indicate that the detection and mitigation algorithm exhibits good performance and can effectively mitigate the RFI effects. [ABSTRACT FROM AUTHOR]

Solar diffusers (SDs) have often been used as the onboard calibrators for the radiometric calibration of reflective solar band imaging sensors. After being spaceborne, the reflectance of SDs is observed to degrade with spectral dependence due to exposure to solar UV and energetic particle radiation. Long-term spectral reflectance data of SDs onboard multiple LEO imaging sensors, such as the Moderate Resolution Imaging Spectroradiometer (MODIS) on Terra and Aqua and the Visible Infrared Imaging Radiometer Suite (VIIRS) on SNPP, are analyzed. The reflectance of SDs on these three instruments degrades faster for the shorter wavelength (0.4– $0.6~\mu \text{m}$) bands than the longer wavelength bands. The Surface Roughness-induced Rayleigh Scattering (SRRS) model is applied to simulate the SD degradation on these instruments, and the growth of the surface roughness parameter of the SDs is derived. It is determined that the change of surface roughness scale length is ~tens of nanometers. To show the consistency of roughness growth rates among the SDs on Terra/Aqua MODIS and SNPP VIIRS instruments, the functional dependences of the growth rates are characterized according to the SD exposure time and the stage of surface roughness. It is also found that the flattening or reverse in the growth trend of the surface roughness for these three SDs occurred around the same interval between October 2013 and October 2015. The confirmation of the applicability of SRRS model with the long-term spectral reflectance data from three independent spaceborne SDs facilitates a better understanding of the origin and physical processes of the SD degradation. [ABSTRACT FROM AUTHOR]

The performance of radio frequency interference (RFI) detection and mitigation algorithms depends on the properties of the RFI signals against which they are used. This letter presents the bandwidth, duration, and center frequency of the RFI signals that the Soil Moisture Active Passive (SMAP) radiometer has observed in the course of one week (June 3–9, 2018) over the entire world. It has been shown that L-band RFI is a significant problem on a global scale, and SMAP multidomain RFI detection approach supported by its digital backend is well justified as the bandwidth and duration characteristics of the RFI environment may vary significantly. [ABSTRACT FROM AUTHOR]

Temporal and spatial variation of sea ice type in the Arctic is an indicator of regional and global change. Arctic sea ice can be classified into two major categories: multiyear ice (MYI) and first-year ice. In this paper, classification method based on machine learning is established and applied to produce daily sea ice classification data set during the winter (November–April) from 2002 to 2017 using active microwave data from QuikSCAT and Advanced Scatterometer as well as passive microwave data from Advanced Microwave Scanning Radiometer for EOS, Special Sensor Microwave Imager/Sounder, and Advanced Microwave Scanning Radiometer 2 radiometer. First, the open water area is flagged out using brightness temperature (Tb) from the passive microwave sensor. Then, K-means algorithm is applied to identify the clusters of the two ice types in the Tb/backscatter parameter space and finally assign pixels to each class. Two optimization methods based on the movement of MYI and marginal ice zone are used to correct the misclassification of MYI. The results have shown a decrease of MYI in winter from 2002 to 2017, especially in 2008 and 2013 with a remarkable recovery in 2014. The classifications are consistent with results by visual interpretation from synthetic aperture radar images in the Canadian Arctic Archipelago with overall classification accuracy over 93%. Comparison with classifications from previous studies and products shows that our method could reflect more differences in MYI declining trend interannually and less anomalous fluctuations in certain years. [ABSTRACT FROM AUTHOR]

A next-generation real-aperture spaceborne radiometer system for high-quality ocean measurements is discussed. Instead of illuminating the antenna reflector by a classical feed array of horn antennas in a one-feed-per-beam configuration, a multi-feed-per-beam configuration is chosen. Each antenna beam is thus created by adding the outputs from many small antenna elements in the feed array, thus providing an antenna beam of unsurpassed quality. This solves the classical polarization purity and land/sea contamination issues. The concept requires many microwave receivers and fast analog-to-digital converters as well as fast digital signal processing onboard the satellite. This is discussed, and resource budgets, especially concerning power, are provided. [ABSTRACT FROM AUTHOR]

Active and passive low-frequency microwave measurements from a number of space- and airborne instruments are used to estimate soil moisture. Each of the sensing approaches has distinct advantages and disadvantages. There is increasing interest in combining active and passive measurements in order to realize the advantages and alleviate the disadvantages. In order to combine active and passive measurements, their covariations with respect to soil moisture need to be known. The covariation is dependent on how the active and passive microwaves interact with vegetation canopy and soil surface. In this paper, we introduce a physics-based model for the covariation of active and passive microwaves over soil surfaces with vegetation cover. The analytical form for a covariation function is derived which depends on the scattering and absorption of microwaves by soil and vegetation with different orientations, structures, and water contents. The main finding is that the covariation function $\beta $ is related to the roughness and vegetation losses in the two measurements. An increase in soil roughness or in vegetation cover leads to less negative values of $\beta $ , which is pronounced for dense and moist vegetation. Both the soil and vegetation components introduce a polarization dependence of $\beta $ that is caused by polarization-induced differences in soil scattering and oriented plant structures. The forward modeled covariations are plotted together with statistically derived covariation estimates from two months of global active and passive L-band observations of the Soil Moisture Active Passive mission. The physically modeled and statistically derived estimates of covariation are comparable in magnitude and scale. [ABSTRACT FROM AUTHOR]

Conventional scatterometer wind retrieval has been proved effective for low and moderate wind conditions. Under high wind conditions, especially for typhoons, the air–sea interactions are fierce, and usually the sea surface backscattering is complicated by rain, leading to degraded scatterometer wind quality. Under such circumstances, radiometers in different frequencies can provide information of rains and the complex air–sea interface. In this paper, an artificial neural network (ANN) was employed to combine the observations of the two sensors onboard HY-2A satellite: radiometer and scatterometer (HSCAT) to achieve wind field retrieving under typhoon conditions. The ANN was trained by using global/regional assimilation and prediction system (GRAPES) wind field data as true values. The established network is then verified by an independent dataset excluded from the training data. It is shown that the wind speeds and directions retrieved from the ANN are better than those of the operational HSCAT products as compared to GRAPES winds. Further comparison with the H*Wind product proves that the proposed ANN is effective in terms of high wind field retrieval. The research of this paper provides a nice reference for the typhoon wind retrieval from the HY-2A satellite and for the data processing of the coming HY-2 satellite series. [ABSTRACT FROM AUTHOR]

The Soil Moisture Active Passive (SMAP) mission was launched on January 31, 2015. It is a combined L-band active/passive system envisioned for the measurement of soil moisture over land. In addition to the soil moisture measurement, the SMAP data readily permit retrievals of ocean surface winds and sea surface salinity. In the previous work, we have found that the SMAP radiometer displays sensitivity to ocean surface wind all the way up to the most extreme wind speeds, possibly as high as 70 m/s, far beyond what is capable with typical $C$ - and $Ku$ -bands ocean wind scatterometers. In this letter, we use the Rapid Scatterometer and stepped frequency microwave radiometer to further validate these SMAP radiometer-only high-wind speed retrievals. In addition, we consider the size of the retrieved high wind speeds, validating them with the Automated Tropical Cyclone Forecasting system B-deck files. [ABSTRACT FROM AUTHOR]

A wideband (30% relative bandwidth) correlation and detection module based on substrate-integrated waveguide (SIW) technology intended for a radio astronomy polarimeter is presented. The SIW circuit is a six-port network with two input ports that are correlated in two hybrid couplers and their corresponding output signals are routed to Schottky diode detectors, which are designed using microstrip technology and assembled within the same system. The designed SIW structure includes hybrid couplers, power dividers, a 90° phase shifter, and 90° bends, providing a real implementation of a functional system with improved bandwidth performance from 35 to 47 GHz. Experimental results are in concordance with simulations, and they validate the module operation for the proposed application. [ABSTRACT FROM AUTHOR]

Moderate Resolution Imaging Spectroradiometer (MODIS) reflective solar bands (RSBs) are calibrated on-orbit using a solar diffuser (SD) with its degradation tracked by an onboard SD stability monitor (SDSM). The SDSM has nine detectors with wavelengths from 0.41 to 0.94 \mu \textm . It is operated during each scheduled SD calibration event, making alternate observations of the sun and the SD. Due to erroneous design parameters, which led to the misalignment of the key elements in the SDSM, there are significant ripples in the sun view responses as the solar viewing angle changes. At the mission beginning, the effect of the ripples was eliminated by normalizing each SDSM detector response to the response of detector 9 (D9) at 0.94 \mu \textm , assuming that D9 had no degradation. However, D9 degradation increases over MODIS operation times. Degradation of up to 2% has recently been observed in D9 for the Terra MODIS. A newly implemented approach reduces the sun view ripples using a lookup table constructed using SDSM data carefully selected from a short period early in the mission lifetime. In this paper, we provide an overview of different approaches that have been applied over the years by the MODIS Characterization Support Team to track the on-orbit SD degradation. We evaluate the overall SD and SDSM on-orbit performance for both Terra and Aqua MODIS, as well as the impact on the MODIS RSB calibration uncertainty. [ABSTRACT FROM PUBLISHER]

This paper describes the analysis, design, and characterization of a high-sensitivity millimeter-wave total-power radiometer front-end integrated into a 0.25- \mu \textm SiGe:C BiCMOS technology. This prototype is composed of a two cascode stage low-noise amplifier (LNA) and a voltage-driven common-emitter square-law detector. The LNA is interfaced to the detector through a low transformation ratio (i.e., high-impedance node) to achieve an efficient wideband signal transfer. The front end achieves both a low $1/f$ -noise corner and a low noise-equivalent power (NEP) by combining a large area, high resistive value load resistor together with a minimum size heterojunction bipolar transistor. At 56 GHz and optimum bias, the prototype provides a 61-MV/W responsivity which combined with a 194-nV/ $\surd $ Hz white noise level result in a 3.2-fW/ $\surd $ Hz NEP when the input power is modulated with a frequency above the 30-Hz flicker noise corner. The achieved 3-dB NEP bandwidth is 6 GHz. [ABSTRACT FROM PUBLISHER]

In this paper, an operational retrieval algorithm is described to infer sea surface temperature (SST) using measurements from the Advanced Microwave Scanning Radiometer-2 (AMSR-2) aboard the Global Change Observation Mission-Water. The algorithm exploits AMSR-2 observations from 12 channels (6–36 GHz, horizontally and vertically polarized), after being corrected for residual calibration biases, to retrieve SST using a statistical based scheme. The Algorithm performance is assessed and results are compared to models and other independent data products. [ABSTRACT FROM PUBLISHER]

A robust physics-based combined active–passive (C-AP), or active–passive, surface soil moisture and roughness estimation methodology is presented. Soil moisture and roughness retrieval is performed via optimization, i.e., minimization, of a joint objective function, which constrains similar resolution radar and radiometer observations simultaneously. A data-driven and noise-dependent regularization term has also been developed to automatically regularize and balance corresponding radar and radiometer contributions to achieve optimal soil moisture retrievals. It is shown that in order to compensate for measurement and observation noise, as well as forward model inaccuracies, in C-AP estimation, surface roughness can be considered a free parameter. Extensive Monte Carlo numerical simulations and assessment using field data have been performed both to evaluate the algorithm’s performance and to demonstrate soil moisture estimation. Unbiased root mean squared errors range from 0.18 to 0.03 cm3/cm3 for two different land-cover types of corn and soybean. In summary, in the context of soil moisture retrieval, the importance of consistent forward emission and scattering development is discussed and presented. [ABSTRACT FROM PUBLISHER]

Manufacture and in-service assessment of composite materials can be challenging since there is, yet, no standard method for testing them. Nevertheless, regular inspection is necessary to maintain the structural integrity. This paper describes a nondestructive, broadband noise mapping method that uses a millimeter-wave radiometer to detect defects in composite materials. The $W$ -band system is configured for transmission mode measurement in which an amplified photonic source illuminates a glass fiber reinforced polymer containing machined defects. A high sensitivity radiometer is used for imaging the sample. The radiometer consists of a Schottky diode-based frequency tripler and a fundamental mixer. [ABSTRACT FROM AUTHOR]

A multichannel radiometer frontend, which can improve the radiometer sensitivity and utilization of the frequency band dramatically, is proposed based on the principle of bandwidth synthetic technology. The multichannel radiometer, called multichannel bandwidth synthetic radiometer (MCBWSR), consists of multiple conventional radiometer channels, and the frequency bands of all the channels are synthesized by the bandwidth synthesizer before the detector. The synthesized bandwidth of the synthesizer can reach a maximum of the sum of all the bandwidths. The operating frequency can also be selected flexibly using the frequency conversion technology. An analysis of the sensitivity \Delta Tsyn of the MCBWSR is deduced in terms of the conventional single-channel radiometer sensitivity \Delta Ti . The relationship of sensitivity between the conventional single-channel radiometer and MCBWSR is validated using an RF simulator. To verify the analysis, three experiments are implemented on a direct detection radiometer, a heterodyne radiometer, and a dual-channel bandwidth synthetic radiometer (DCBWSR) receiver, respectively. The experimental results are presented, and the sensitivity among them is compared. These results indicate that the DCBWSR can improve the sensitivity by about 1/\sqrt 2 , which is consistent with the theoretically calculated results. Compared with the conventional radiometers, the MCBWSR has the advantages of higher sensitivity, lower power consumption, and integration. [ABSTRACT FROM AUTHOR]

The single-event transient (SET) response of a W-band (75–110 GHz) radar receiver front-end is investigated in this paper. A new technique to facilitate the SET testing of the high frequency transceivers is proposed and demonstrated experimentally. The entire radar receiver front-end, including the high frequency signal sources and modulators, were designed and fully integrated in 90 nm 300 GHz SiGe process technology (Global Foundries SiGe 9HP). Two-photon absorption (TPA) laser pulses were utilized to induce transient currents in different devices in various circuit blocks. The study shows how short transient pulses from the high frequency tuned circuits are propagated throughout the receiver and are broadened while passing through low-pass filters present at supply nodes and the low-pass filter following the down-conversion mixer, thus affecting the digital data at the output of the receiver. The proposed methodology allows the study of the effect of SETs on the recovered digital data at the output of the high frequency receivers, thus allowing bit error rate calculations. Comprehensive device and circuit level simulations were also performed, and a close agreement between the measurement results and simulation data was demonstrated. To the authors’ best knowledge, this is the first study of SET on full receiver at millimeter-wave (mmW) frequencies. [ABSTRACT FROM PUBLISHER]

The single-event transient (SET) response of two different silicon-germanium (SiGe) X-band (8–12 GHz) low noise amplifier (LNA) topologies is fully investigated in this paper. The two LNAs were designed and implemented in 130nm SiGe HBT BiCMOS process technology. Two-photon absorption (TPA) laser pulses were utilized to induce transients within various devices in these LNAs. Impulse response theory is identified as a useful tool for predicting the settling behavior of the LNAs subjected to heavy ion strikes. Comprehensive device and circuit level modeling and simulations were performed to accurately simulate the behavior of the circuits under ion strikes. The simulations agree well with TPA measurements. The simulation, modeling and analysis presented in this paper can be applied for any other circuit topologies for SET modeling and prediction. [ABSTRACT FROM PUBLISHER]

In this paper, we introduce the combined active/passive (CAP) data product for the Soil Moisture Active Passive mission. We develop the algorithms for a radiometer-only salinity product, a radar-only vector wind product, and a CAP vector wind and salinity product. We show that the performance of the radiometer-only salinity product nears but is still inferior to the Aquarius salinity accuracy performance when aggregated on a monthly timescale. Then, we show that the radar-only vector wind product has reasonable accuracy away from the nadir track while suffering from inadequate measurement geometry in the middle of the swath. Finally, we demonstrate that the CAP salinity and vector wind performance is superior to individual algorithms and provides wind vectors nearly as good as RapidScat for low-to-moderate winds and possibly superior to traditional scatterometers for wind speeds larger than 12.5 m/s. [ABSTRACT FROM PUBLISHER]

The L-band passive microwave data from the Soil Moisture Active Passive (SMAP) observatory are investigated for remote sensing of ocean surface winds during severe storms. The surface winds of Joaquin derived from the real-time analysis of the Center for Advanced Data Assimilation and Predictability Techniques at Penn State support the linear extrapolation of the Aquarius and SMAP geophysical model functions (GMFs) to hurricane force winds. We apply the SMAP and Aquarius GMFs to the retrieval of ocean surface wind vectors from the SMAP radiometer data to take advantage of SMAP's two-look geometry. The SMAP radiometer winds are compared with the winds from other satellites and numerical weather models for validation. The root-mean-square difference (RMSD) with WindSat or Special Sensor Microwave Imager/Sounder is 1.7 m/s below 20-m/s wind speeds. The RMSD with the European Center for Medium-Range Weather Forecasts direction is 18° for wind speeds between 12 and 30 m/s. We find that the correlation is sufficiently high between the maximum wind speeds retrieved by SMAP with a 60-km resolution and the best track peak winds estimated by the National Hurricane Center and the Joint Typhoon Warning Center to allow them to be estimated by SMAP with a correlation coefficient of 0.8 and an underestimation by 8%–18% on average, which is likely due to the effects of spatial averaging. There is also a good agreement with the airborne Stepped-Frequency Radiometer wind speeds with an RMSD of 4.6 m/s for wind speeds in the range of 20–40 m/s. [ABSTRACT FROM PUBLISHER]

The global measurement of sea-surface salinity and more precise measurements of soil moisture from space has been enabled by L-band observations from the Soil Moisture and Ocean Salinity (SMOS) mission, the Aquarius mission, and the Soil Moisture Active Passive (SMAP) mission. These measurements are key components of the global water cycle and the ability to resolve climate scale variations in these variables is extremely valuable for improved understanding of how the hydrologic cycle varies in a changing climate system. It is therefore imperative that the radiometer systems making these measurements be calibrated to remove any spurious instrument temporal drifts. In this paper, models are developed over Antarctica and rainforest regions to track the gain and offset drifts in spaceborne L-band radiometer brightness-temperature (TB) measurements. The Antarctica region is found to be best for tracking small variations (0.1 K over 10 days) for vertically polarized observations near the Brewster angle. The rainforest regions are found to be best for tracking longer term variations ($ > $60 days) in all channels, due to larger uncertainty in the surface temperature knowledge, and excellent for tracking shorter term variations between channels ($ < $10 days). These reference regions were used to separate a long-term gain drift and a quasi-monthly offset variation in the Aquarius radiometer. These observations eventually led to the characterization of the root cause for the drift and are contributing to improved correction methods based on models for the hardware behavior. [ABSTRACT FROM PUBLISHER]