The SMAP product provides the soil moisture (SM) computed using three different retrieval algorithms: the single channel H and V algorithms (SCA-H and SCA-V), and the dual-channel algorithm (DCA) which in addition provides the vegetation optical depth (VOD). The roughness model and the roughness parameters play an important role in the determination of the soil moisture and the VOD. In this regard, the SMAP SCA and DCA utilize different approaches to incorporate the effect of roughness. In this work we will summarize those approaches and we will evaluate the effect of the DCA approach on the retrieval of VOD. We will compare the SMAP DCA roughness parameter$h$with topographic parameters such as DEM height, DEM slope, DEM height standard deviation and DEM slope standard deviation.

For understanding englacial hydrology and its impact on ice sheet mass balance, observations of the liquid water content (LWC) within the ice sheets are needed. Earlier studies have shown the complementary nature of multi-frequency microwave radiometer measurements to detect subsurface LWC distribution in addition to surface LWC, which is critical for understanding the seasonal melt dynamics of ice sheets. In this study, we used 1.4 GHz brightness temperature (TB) measurements from the NASA Soil Moisture Active Passive (SMAP) satellite, and 6.9, 10.7, 18.9, and 36.5 GHz TB measurements from the JAXA Global Change Observation Mission-Water Shizuku (GCOM-W) satellite to investigate the multi-frequency response at pan-Greenland scale. The melt indications derived at different frequencies show trends consistent with persistent seasonal subsurface melt water and delayed subsurface refreezing of the seasonal melt water. The result suggests that the seasonal subsurface persistent melt water occurrences that are not captured by the high-frequency retrievals are both temporally and spatially very significant.

The Soil Moisture Active Passive (SMAP) satellite has been developed by NASA to make global soil moisture measurements on the Earth's land surface. It can also distinguish frozen from thawed land surfaces. The SMAP satellite was launched on January 31, 2015, and the science data production began on March 31, 2015. It has both L-band radar and radiometer instruments sharing a rotating 6-m mesh reflector antenna. The SMAP radar failed in July 2015, while its radiometer continues nominal operations. In this paper, the approximately two months of SMAP synthetic aperture radar (SAR) data has been revisited and scrubbed. The SAR bad data (aka outlier) are detected and removed by statistically investigating the time series difference between scatterometer and linearly averaged SAR measurements within the SMAP antenna footprint (∼38 km). It is performed orbit by orbit. The outlier or bad orbits were identified when a data point is more than three scaled median absolute deviations (MAD) away from the median. On average only about 10% or less of all SAR orbits (more than 700), in each polarization, are classified as outliers.

Due to their larger penetration and sensing depth, low frequency microwave measurements have been recently employed to detect ice sheet melt events. In this paper, the response of NASA's SMAP (Soil Moisture Active Passive) L-band measurements to surface melting of the Greenland ice sheet from 2015 through 2021 is investigated. SMAP covers virtually the entire Greenland ice sheet twice a day with its L-band (1.4 GHz) radiometer. The results show that the ice sheet experienced unusually strong surface melting on August 14,2021, which extended the melt area across much of dry snow zone over a period of two days. Moreover, the observational results agree well with model simulations conducted using Glacier Energy and Mass Balance (GEMB) module within the Ice-sheet and Sea-level System Model (ISSM).

This letter presents a new hardware setup to suppress radio frequency interference (RFI) for a recently developed microwave radiometer technique, known as wideband autocorrelation radiometry (WiBAR). WiBAR is a method that can directly measure the thickness of a low-loss layer like lake ice or dry snow on the ground by finding the lag time that corresponds to the transit time of layer in the autocorrelation function (ACF) of the received signal. However, RFI increases the noise floor of the ACF and results in a decreased signal-to-noise ratio (SNR) of the WiBAR delay peak in the ACF. We enhanced a WiBAR instrument with a tunable comb filter having a frequency response with many evenly spaced alternating pass and stop bands. We show the RFI mitigation performance of a WiBAR set-up with a comb filter in the laboratory with a simulation circuit that creates a spectrum polluted with RFI.

The power system, dominated by synchronous generation, is moving toward a higher share of renewable generation. A voltage-source converter (VSC) known as an inverter is usually utilized as an interface between renewable generation units and the power system. Different methods are developed to control the voltage and power of VSCs. This paper evaluates the performance of a voltage controller and an angle droop-based power sharing method for a power system, which is a combination of inverter-based generation units and a synchronous generator. The studied controllers enable 1) accurate power sharing between all inverter-based generation units, 2) terminal voltage regulation by considering the maximum current of renewable generation units, and 3) maintaining frequency of the system at 60 Hz in the steady state independent from communication links. Different simulation case studies are performed in PSCAD/EMTDC software to study the performance of the angle droop-based power sharing algorithm.

This paper studies the transient behavior and resonance mitigation performance of a grid-connected voltage-sourced converter (VSC) under several control strategies using hardware-in-the-loop (HIL) simulation. Many VSCs use LCL filters to increase their power quality. However, an LCL filter may increase the possibility of resonance. Therefore, controllers need to include an active damping feature to improve the transient behavior of the system. There are several active damping current controllers such as 1) internal model control, 2) virtual RC damping control, and 3) notch filter control methods. This paper discusses the overshoot, rise time, and dq-axis current decoupling feature for each controller using HIL simulation case studies performed in the OPAL-RT real-time platform.

This project presents a power sharing and voltage control scheme for multi-terminal HVDC (MTDC) grids. A generalized hierarchical droop-based controller is designed to control the DC-side voltages and dispatched powers. To improve the steady-state performance of the MTDC grid, this project proposes a feedforward mechanism to make the current controller of the voltage-sourced converters (VSC) independent of the AC-side load conditions. The performance of the proposed scheme is evaluated under several case studies. A comparison between the proposed controller and traditional controllers is also made with time-domain simulation studies in PSCAD/EMTDC software. Results show that the proposed controller can successfully control the real powers and DC voltages while having a stable performance during AC-side load changes.

Analytical evaluation and voltage control method of a cascaded step-up switched-capacitor converter (CSU-SCC) under zero current switching (ZCS) operation and continuous conduction mode is studied in this paper. The ZCS condition, which means operation below resonant frequency, makes the soft-switching operation of switching devices. Therefore, it increases the converter efficiency. In addition, due to operating in higher range of switching frequency, the size of proposed converter can be reduced (i.e., higher power density). Compared with the competitive topology of SCCs, it enables the output voltage regulation of the converter under ZCS. The proposed CSU-SCC provides a higher range of output voltage and power by proposing an exponential structure of an SCC. An analytical study is developed to calculate the output current and voltage characteristics of the converter. Based on the derived output voltage characteristic, a closed-loop voltage controller is proposed to regulate the output voltage of the CSU-SCC under different conditions. The simulation results for an input voltage 110 V-160 V and 500 W CSU-SCC converter verify the proposed analysis and the proposed voltage controller.

Seasonal snow cover plays a key role in freshwater resources, water security, natural hazards, and weather and climate. However, accurate estimation of snow-water equivalent (SWE) with remote sensing observations remains a significant challenge. NASA Terrestrial Hydrology Program launched its multi-year SnowEx mission whose primary goal is to develop and test techniques for estimating how much water is stored in some complex Earth's terrestrial snow-covered regions (e.g. forested areas). The first year of the 5-year campaign took place in Colorado during the winter 2016–2017, during which in situ measurements and ground-based remote sensing observations were collected by the scientific community. Throughout February 2017, about 100 people were deployed and over 30 remote sensing instruments were used. This required an exceptional coordination effort, which resulted in collocated in situ measurements from snowpits (e.g. profiles of stratigraphy, density, grain size and type, specific surface area, temperature) and along transects (mainly for snow depth measurements) with ground-based remote sensing observations (microwave radiometers, radar, scatterometers, lidars, etc.). The public release of all these datasets has started (nsidc.org/data/snowex).

Performance analysis and output voltage regulation of a cascaded step-up switched-capacitor converter (SCC) under zero voltage switching (ZVS) condition is analyzed in this paper. The ZVS condition (converter operation above its resonant frequency) provides 1) zero-voltage switching, which leads to a relatively higher efficiency compared to zero current switching (ZCS) under the resonant frequency in MOSFET-based converters and 2) operating in a higher frequency, which reduces the size and increases the power density of the converter. It also provides output voltage regulation compared with previously proposed SCCs in ZCS condition. This paper proposed a cascaded switched-capacitor converter (CSCC) that provides a high voltage gain. For this structure, it proposes a mathematical analysis to calculate the output voltage characteristics of the converter. This study is then employed to design a control loop for the output voltage of the converter. The simulation results for a 110–160 V and 500W SCC converter confirm the proposed analysis and the designed control loop.

NASA SnowEx's goal is estimating how much water is stored in Earth's terrestrial snow-covered regions. To that end, two fundamental questions drive the mission objectives: (a) What is the distribution of snow-water equivalent (SWE), and the snow energy balance, among different canopy and topographic situations?; and (b) What is the sensitivity and accuracy of different SWE sensing techniques among these different areas? In situ, ground-based and airborne remote sensing observations were collected during winter 2016–2017 in Colorado to provide the scientific community with data needed to work on these key questions. An intensive period of observations occurred in February 2017 during which over 30 remote sensing instruments were used. Their observations were coordinated with in situ measurements from snowpits (e.g. profiles of stratigraphy, density, grain size and type, specific surface area, temperature) and along transects (mainly for snow depth measurements). Both remote sensing and in situ data will be archived and publicly distributed by the National Snow and Ice Data Center at nsidc.org/data/snowex.