High-resolution soil moisture (SM) observations are critical for agricultural monitoring, forestry management, and hazard prediction, yet current satellite passive microwave missions are unable to directly provide retrievals at tens-of-meter spatial scales. Unmanned aerial vehicle (UAV)–mounted microwave radiometry presents a promising alternative, but most evaluations to date have focused on agricultural settings, with limited exploration across other land covers and few efforts to quantify retrieval uncertainty. This study addresses both gaps by evaluating SM retrievals from a drone-based Portable L-band Radiometer (PoLRa) across shrubland, bare soil, and forest strips in Central Illinois, U.S., using a 10-day field campaign in 2024. Controlled UAV flights at altitudes of 10 m, 20 m, and 30 m were performed to generate brightness temperatures (TB) at spatial resolutions of 7 m, 14 m, and 21 m. SM retrievals were carried out using multiple tau-omega-based algorithms, including the single channel algorithm (SCA), dual channel algorithm (DCA), and multi-temporal dual-channel algorithm (MT-DCA). A Bayesian inference framework was then applied to provide probabilistic uncertainty characterization for both SM and vegetation optical depth (VOD). Results show that the gridded TBdistributions consistently capture dry-wet gradients associated with vegetation density variations, and spatial correlations between polarized observations are largely maintained across scales. Validation againstin situmeasurements indicates that PoLRa-derived SM retrievals from the SCA-V and MT-DCA algorithms achieve unbiased root-mean-square errors (ubRMSE) generally below 0.04 m3/m3across different land covers. Bayesian posterior analyses confirm that reference SM values largely fall within the derived uncertainty intervals, with mean uncertainty ranges around ± 0.02 m3/m3and ± 0.11 m3/m3for SCA and DCA-related retrievals. These findings underscore the potential of UAV-mounted PoLRa for high-resolution SM retrieval across varied landscapes and emphasize the need for standardized calibration and uncertainty quantification frameworks to support broader scientific and operational adoption.

The use of coarse scale and temporally static effective roughness and single scattering albedo parameters in the Soil Moisture Active Passive (SMAP) operational algorithms falls short in achieving high spatial resolution in soil moisture (SM) retrieval. To address this issue, this study leverages in-situ SM measurements from 2016 to 2018 at the SMAP core validation sites to derive monthly parameters. These parameters were then applied to estimate SM for subsequent periods. Our findings suggest that SM retrievals from these monthly-adjusted parameters exhibit a marginal improvement in mean absolute error relative to the standard SMAP retrievals. This scheme will be applied to develop high spatiotemporal resolution parameters for passive microwave SM retrieval from instruments aboard uncrewed aerial systems (UAS). The proposed method also allows for these parameters to be tailored to targeted agricultural and forested areas for applications ranging from precision agriculture to intelligent wildfire management. This adaptation promises to refine our ability to retrieve SM with greater accuracy and resolution.

A physics-based microwave emission model for layered vegetation (MEMLV) is developed to simulate the vegetation optical depth (VOD) and scattering albedo of coniferous forests from P- to Ka-band (0.4 GHz–37 GHz). This study aims to use physics-based forward modeling to guide and support multifrequency VOD retrieval. The MEMLV consists of three major components: 1) the single-layer discrete scatter model (SL-DSM) that calculates the VOD and single scattering albedo of a single layer; 2) a new Tree Structure Model (TSM) that represents forests by stratified multilayer media; and 3) the Two-Stream microwave emission model (2S-MEM) that combines SL-DSM and TSM to calculate the brightness temperature of forests and their corresponding effective VOD,$\tau _{\text {eff}}$, and effective scattering albedo,$\omega _{\text {eff}}$. Simulation of an exemplary coniferous forest shows that$\tau _{\text {eff}}$increases as frequency increases until reaching saturation at K-band. Meanwhile,$\omega _{\text {eff}}$increases rapidly at low frequencies until reaching a peak at S-band, then decreases until reaching X-band, and finally increases monotonically as frequency increases. Notably,$\omega _{\text {eff}}$is negligible at P-band for most cases. Sensitivity analyses demonstrate the saturation of$\tau _{\text {eff}}$when canopy height is substantial, and the decreasing trend of$\omega _{\text {eff}}$as canopy height or the areal fraction of canopy gap increases. The MEMLV has the potential to improve the parameterization of retrieval algorithms and to enhance the understanding of the retrieved VOD over a wide frequency range.

The future of agriculture is moving towards a more technological and efficient way to produce food for the world’s population. One key resource for that is water. A radiometer design is introduced that will be used to determine the feasibility of an 800g C-Band (6.8 GHz) radiometer for fixed-wing UAVs, giving agronomists a new cutting-edge tool to measure soil and kernel moisture in cereal grain pods. Simulation tools and computer-aided optimization were used to develop multiple design iterations, which were manufactured and measured to assess the performance of the proposed radiometer. A custom HEMT LNA was designed using ADS EM co-simulation, resulting in a 10dB gain and 1dB noise figure. The proposed system design has a theoretical NEΔT of 0.28 K and a preliminary measured NEΔT of 0.535K at 36.9ms integration time, respectively.

A second-generation portable L-band radiometer is presented for use in turfgrass management applications and golf course soil moisture mapping. The "turfRad" has been optimized for ease-of-use and for the conditions in operational turfgrass management. This is the first known use of a vehicle-mounted L-band radiometer for turfgrass soil moisture mapping for irrigation decision-making. Preliminary results, including dual-polarization brightness temperatures and retrieved soil moisture, are presented. The single-parameter soil moisture retrieval using the Tau-Omega emission model with constant vegetation optical depth is chosen due to the high level of vegetation uniformity in the fairway areas.

This article experimentally investigates relationships between copol backscattering at a wide range of frequencies (L- to Ka-bands) and snow–ground state parameters (SPs) in different evolution phases during the full winter cycle of 2019/2020. Backscattering coefficients from 1 to 40 GHz,in situsnow–ground SPs, and meteorological data are measured at the Davos-Laret Remote Sensing Field Laboratory (Switzerland). Relative strengths of the snow–ground system’s three primary scattering elements (air–snow interface, snow volume, and snow–ground interface) on backscattering are assessed. An anticorrelation between reasonably high snow wetness and backscattering coefficient is found, especially at higher microwave frequencies. For small amounts of snow wetness, backscatter coefficients at L- and S-bands are intensified via increasing snow volume and snow surface scattering. Snow–ground SPs influence backscattering according to their characteristic time scales of temporal evolution. Under dry snow conditions and at low and intermediate frequencies, ground permittivity is the major influencer of backscatter at a time scale of roughly two weeks. Snowfall is the major influencer of backscatter at a time scale of a few hours to a few days. The findings of this article are valuable to the development of retrieval algorithms using machine learning while maintaining a grasp on the ongoing physical processes. Another key message is that multifrequency active microwave measurements are critical to maximize the number of retrievable SPs and their estimation accuracy. For example, while Ka-band performs well in the detection of snow cover, L-band measurements are more responsive to changes of snow water equivalent (SWE) under moist or wet snow conditions.

We examine melt over Greenland using two passive microwave-based melt detection algorithms. A recently developed L-band (1.4 GHz) liquid water quantification method, and a well known method based on 19 and 37 GHz brightness temperatures known as the cross polarization gradient ratio (XPGR). We discuss spatial patterns of total number of melt days, and temporal trends in total melt area. The L-band method provides quantified liquid water as opposed to melt flagging, but for this study we simplify the retrievals to a binary melt flag for the sake of comparison with the XPGR. We find similar spatial patterns between the two methods, but some notable differences such as melt on the Jacobsavn Isbrae is more apparent in the L-band retrievals as opposed to the XPGR.

We introduce a recently developed cost effective, low mass, portable L-band radiometer for aerial applications on drones or for ground-based applications. The radiometer has been tested on a multicopter drone and for a winter seasons measuring snow at the Davos-Laret Remote Sensing Field Site in Switzerland. The radiometer measures dual-linear polarizations, has a brightness temperature resolution of about 0.15 K for 1 sec integration time, and the antenna has a half-power full beamwidth of 37 degrees. The mass of the combined system is 3.8 kg and the power consumption is less than 3 W. We present the antenna and radiometer instrument, and initial results from these test phases.

In January 2019 a Project Authorization Request was submitted to the IEEE standards association with the title "Standard for Calibration of Microwave Radiometers in the 300 MHz to 1 THz Frequency Range for Geoscience Applications". An open committee is being assembled to draft this standard with the purpose of unifying and documenting calibration procedures for a wide range of microwave radiometers. The committee includes members, collaborators, and contributors from academia, international government and private industry. We include ground-based, air-borne, and space-borne systems. The standard will also define standardized terminology, and address procedures required to obtain traceability to fundamental units or constants. The scope of the standard encompasses various radiometer geometries, Dicke switching, total power, and differential, as well as different polarization configurations including fully polarized (full Stoke's) radiometers. The standard will also separately address free-space and single-mode (e.g. transmission-line) radiometer calibration techniques.

We demonstrate an application of the L-band specific Microwave Emission Model of Layered Snowpacks (LS-MEMLS) to snow over the Greenland Ice Sheet to retrieve density and snow wetness from SMOS satellite data. Retrievals of these parameters have previously been demonstrated using ground-based L-band radiometry over snow covered ground in alpine conditions. Existing passive microwave data products over ice sheets include empirical and threshold-based techniques but until recently have not used radiative-transfer physics-based modelling. We discuss the retrieval method based on inversion of LS-MEMLS and present a comparison between our retrievals and in-situ weather station air temperature data. We also mention ongoing work investigating the applicability of the novel retrieval method for mapping the ablation zone of the Greenland Ice Sheet.

Microwave coaxial resonator sensors are used to retrieve snow complex dielectric permittivity εS= εSi·εSfrom sensors frequency response. In addition to the conventional analytical approach, electromagnetic simulation software HFSS is used to devise a unified εSretrieval approach applicable to all resonator sensors based on the simulation of their frequency response. εSretrievals are performed using sensor measurements, conducted with a portable Vector Network Analyzer (VNA), in Davos-Laret Remote Sensing Field Laboratory and six locations over the Greenland ice sheet. Results indicate improved εSretrieval accuracy achieved using the simulation-based approach. The use of VNA technology together with the simulation-based εSretrieval approach allows for more reliable electromagnetic characterization of snow cover over large areas, which is of critical importance for remote sensing applications.

Infrared (IR) emissivity is a critical parameter for modeling and predicting heat transfer by radiation. Microwave absorbing materials, having a high emissivity in the microwave spectrum, are crucial in a wide array of applications, such as electromagnetic interference mitigation, stealth technology, and microwave remote sensing and radiometer calibration. Accurate knowledge of the thermal properties of these materials is necessary for efficient design and optimization of these types of systems. Typical microwave absorbing materials consist of a dielectric epoxy material impregnated with a lossy material, such as iron or carbon. We study a novel cryogenically compatible epoxy-based absorber material that has been loaded with varying concentrations of carbonyl iron powder (CIP). We study six materials with CIP concentrations of 0%, 5%, 10%, 20%, 30%, and 50% by tap volume. We use a commercial IR camera with sensitivity in the range 7.5-13 μm to measure the radiance of the samples and a waterbath IR blackbody at ten temperatures between about 19 °C and 45 °C. A linear Deming fitting is performed, considering uncertainties in both the measured parameters, and the slope of the linear fit is shown to be the IR emissivity, averaged over the spectral response of the camera. The emissivity ranges between 0.868 and 0.757, decreasing monotonically as a function of iron carbonyl concentration between 0% and 50%. The uncertainty of the emissivity determination method is derived and presented. The uncertainty of the presented method is shown to be no larger than 3.3% for all measured samples.

We discuss the analytical derivation of the absolute brightness temperature uncertainty of a hollow conical blackbody source for radiometer calibration. We introduce a Monte Carlo uncertainty propagation analysis method to quantify uncertainty contributions from nonideal emissivity, physical temperature, and antenna pattern, which are the three major factors contributing to the uncertainty of the brightness temperature radiation from the source. The low reflectance of the hollow conical geometry depends on multiple bounces on the absorber surface. To quantify total brightness temperature over a nonuniform temperature surface, each individual bounce must be considered. We derive a recursive analytical relationship to quantify this multiple-bounce effect as a function of a view angle. The resulting effective blackbody brightness temperature uncertainty is a function of frequency, temperature, antenna pattern, measurement distance, and the measurement environment. We also propagate the uncertainty to the antenna flange of a radiometer viewing the conical blackbody. This includes additional effects, such as spillover, illumination efficiency, and antenna efficiency. We demonstrate the propagation method with an example case. We use fictional input values to investigate the response of the uncertainty to input variables and distance. We find that as the distance between antenna and blackbody increases, the uncertainty due to spillover dominates, but at close distances, the dominant uncertainty contributor is linked to the physical temperature of the absorber.

A conical cavity has been designed and fabricated for use as a broadband passive microwave calibration source, or blackbody, at the National Institute of Standards and Technology. The blackbody will be used as a national primary standard for brightness temperature and will allow for the prelaunch calibration of spaceborne radiometers and calibration of ground-based systems to provide traceability among radiometric data. The conical geometry provides performance independent of polarization, minimizing reflections, and standing waves, thus having a high microwave emissivity. The conical blackbody has advantages over typical pyramidal array geometries, including reduced temperature gradients and excellent broadband electromagnetic performance over more than a frequency decade. The blackbody is designed for use between 18 and 230 GHz, at temperatures between 80 and 350 K, and is vacuum compatible. To approximate theoretical blackbody behavior, the design maximizes emissivity and thus minimizes reflectivity. A newly developed microwave absorber is demonstrated that uses cryogenically compatible, thermally conductive two-part epoxy with magnetic carbonyl iron (CBI) powder loading. We measured the complex permittivity and permeability properties for different CBI-loading percentages; the conical absorber is then designed and optimized with geometric optics and finite-element modeling, and finally, the reflectivity of the resulting fabricated structure is measured. We demonstrated normal incidence reflectivity considerably below -40 dB at all relevant remote sensing frequencies.

This paper discusses the design of a microwave blackbody to be used as a primary laboratory standard for passive remote sensing applications. This temperature adjustable design is required to operate and be fully characterized from 10 to 220 GHz. We discuss the challenges involved in designing this type of calibration source and address how improvements can be made to increase performance over blackbodies typically flown on airborne and space-borne instruments. A simplified electromagnetic model for absorber layer optimization is introduced as the precursor to a finite-element, full-wave solution for the calculation of emissivity. A temperature simulation predicts the physical temperature of the blackbody and surrounding chamber. The simulated data are used as inputs to a rigorous calculation of the microwave brightness temperature radiated by the blackbody source. This calculation provides an estimate of the offset between measured physical temperature and radiometrically measured brightness temperature.

We introduce a method to determine the gradient between measured physical temperature and true radiating surface temperature of a passive microwave calibration target (load or blackbody). An empirical cooling-curve fit is employed to determine heat-transfer coefficients that then allow commercial finite-element software to solve for the physical temperature at the surface of the target. Only gradients in the direction parallel to the target's pyramidal structures are determined. Two target insulation thicknesses are investigated and a mean surface radiating temperature is determined. This surface temperature differs from the internally measured physical temperature by a maximum of 0.3 K in an ambient environment. Use of a thicker insulation assembly decreases this temperature bias by 0.1 K.

We describe and demonstrate a standard radiometer for making microwave brightness-temperature measurements that are traceable to fundamental noise standards. The standard radiometer is based on a National Institute of Standards and Technology (NIST) waveguide radiometer for 18-26.5 GHz, fitted with an antenna to measure radiated power. The fraction of the antenna pattern subtended by the radiating target is determined by anechoic-chamber measurements in which we vary the temperature of the target and measure the received power. Sample measurement results with uncertainties are presented. The typical standard uncertainty for a brightness temperature of around 340 K is about 1 K. The approach should be extendable to other waveguide bands where NIST has radiometers and standards.

We compare three methods of quantifying illumination efficiency (IE). The ratio IE describes the contribution of energy emitted from a blackbody target to the total energy measured at an antenna aperture in a free-space microwave calibration target radiometric measurement. Measurements are compared at three frequencies: 18 GHz, 22.5 GHz, and 26 GHz. An antenna pattern integration method is compared with a recently developed target-temperature fitting method. These two experimental approaches are also compared to a computational antenna pattern simulation. Results show that the simulation agrees with the experimental fitting method more closely at far-field distances, whereas the antenna pattern integration and experimental fitting method agree at closer distances.

We report our recent progress toward the development of microwave brightness-temperature (BT) standards. As one of the crucial parameters, the target illumination efficiency (IE) was traditionally determined from the relative antenna pattern. We propose a measurement technique to extract the target IE solely by the use of passive radiometric measurements for characterizing the BT of the blackbody radiator. Such a technique allows us to skip the complexities that are often encountered during the measurement and calculation of the antenna pattern. Taking advantage of the variable heating capability available on most blackbody targets, we varied the temperature of a heated blackbody target and ran a series of radiometric measurements when the target was separated at different distances away from the the antenna with the radiometer operating at a few frequencies. Our experimental results show excellent measurement accuracy on the IE, with uncertainty of about 1% at close separation distance between the antenna and the target. We further measured and computed the BT of the blackbody target at the locations where we had measured the extracted IE. The BT was slightly lower than the physical temperature of the target and exhibited 0.7 K to about 1 K uncertainty when the target was located no more than 1 m away from the antenna. A measurement uncertainty of 1 K already meets the accuracy requirements of some climate variables, and such results reflect a significant step toward the establishment of BT standards at microwave frequencies.

We report on the characterization of blackbody target reflections as part of the recent progress on the development of brightness temperature standards for microwave remote sensing at the National Institute of Standards and Technology. The very low reflections from the blackbody targets used in airborne or satellite remote-sensing systems present challenges on how to extract reflection coefficients from the measurements. A full calibration technique was developed for this study by use of a flat aluminum plate used as a known standard in combination with measurements of the empty anechoic chamber. The theoretical basis and measurement procedures are presented. Calibration results validate the method by showing its independence from measurement hardware and conditions. A comparison between the theoretical prediction of reflection coefficients of a free-standing dielectric slab with well documented physical parameters and the de-embedded reflection coefficients from experiments confirms good calibration accuracy. The specific blackbody target used in this study shows well matched properties with a power reflectivity below -40 dB over the entire measurement band (18 GHz to 26 GHz).

We report on the characterization of blackbody target reflections as part of the recent progress on the development of brightness temperature standards for microwave remote sensing at the National Institute of Standards and Technology. The very low reflections from the blackbody targets used in airborne or satellite remote sensing systems present challenges on how to extract reflection coefficients from the measurements. A full calibration technique is developed for this study by the use of a flat aluminum plate used as a known standard in combination with measurements of the empty anechoic chamber. The theoretical basis and measurement procedures, along with the uncertainty analysis, are presented. Calibration results validate the method by showing its independence from measurement hardware and conditions. A comparison between the theoretical prediction of reflection coefficients of a free-standing dielectric slab with well-documented physical parameters and the de-embedded reflection coefficients from experiments confirms good calibration accuracy. The specific blackbody target used in this paper shows well-matched properties with a power reflectivity below -40 dB over the entire measurement band (18 to 26 GHz).

We report on the characterization of blackbody reflections as a part of the recent progress on the development of brightness standards for microwave remote sensing at National Institute of Standards and Technology (NIST). Three blackbody targets at variable temperatures used for airborne and/or satellite systems along with an aluminum plate were measured in terms of their reflection coefficients by horn antennas in connection with a vector network analyzer (VNA) in the WR-42 waveguide band. Precision measurements of reflection are needed for blackbody emissivity computation to check against the brightness temperature measurement of blackbody targets. All experiments were conducted in two distance ranges by free-space methods in an anechoic chamber. Preliminary results show negligible reflections from the calibration targets, indicating near ideal blackbody characteristics in the measured frequency range.

Accurate characterization of the brightness temperature (TB) of black-body targets used for calibrating microwave remote-sensing radiometers includes many inputs: antenna pattern and loss, target temperature, target emissivity, mechanical alignment, and radiometric TBmeasurements, all of which must be calibrated against physical standards. Here, we describe measurements made using several black-body targets and two different antennas within the WR-42 (18 to 26.5 GHz) waveguide band. Uncertainty estimates are also shown for the retrieved target TBmeasurements.