Global Navigation Satellite System reflectometry (GNSS-R) has emerged as a valuable tool for Earth observations, providing low-cost measurements with high spatiotemporal resolution. This study presents the first application of Spire Global Inc. near-nadir (NN) GNSS-R observations for sea ice detection and sea ice extent (SIE) mapping, covering the Arctic and Antarctic. The delay and Doppler maps (DDMs) of Spire NN GNSS-R show a significant difference over various surfaces. For a test period from February to July 2024, results show that the Arctic’s SIE decreases from 584600 to 333400 km2, and the Antarctic’s SIE increases from 142825 to 584750 km2. Compared to Advanced Microwave Scanning Radiometer 2 (AMSR2) estimates, Spire NN GNSS-R retrievals achieve a probability of detection (Pd) of 94%–96% and the probability of false alarm (Pfa) and probability of error (Pe) below 0.23%. These results demonstrate the high sensitivity, accuracy, and operational potential of Spire NN GNSS-R for sea ice remote sensing.

Measurements have been made at P-band (0.707 GHz) to construct a model for the dielectric constant of sea water and extend the model for the dielectric constant to high salinity [$S \gt 50$practical salinity scale (pss)]. The measurements are part of research to develop a model for the dielectric constant suitable for future wide-bandwidth (BW) remote sensing of salinity and for application to water bodies, such as the Great Salt Lake with salinity significantly above that found in the open ocean. Measurements have been made at temperatures from 2 °C to 30 °C and salinity from 0 to 138 pss. The data have been fit to a Debye model for the dielectric constant with a single relaxation mode as has been employed at L-band (1.413 GHz), where remote sensing of salinity is currently done. Comparison with contemporary models developed from data at L-band indicates that the L-band model and new P-band model do well at both frequencies for salinity less than 50 pss, but at higher values of salinity, the L-band models diverge from the data. The data have also been used to test at high salinity the mathematical relationship between salinity and conductivity, which is the basis for the pss.

above article [1], the corrections are needed for signs in (3b) and (5a) and for the location of the decimal point in the first term in (8b). Equations (3b) and (5a) with the correct signs are \begin{align*} & \varepsilon _{s}\left ({{S, T}}\right) =\varepsilon _{s}\left ({{0, T}}\right)\left [{{1-\text {SR}\left ({{S, T}}\right)}}\right]\tag {3b}\\[8pt] & \tau \left ({{0, ~T }}\right) = \left [{{17.9539-0.6955 ~T+ 0.0177 ~T ^{2}-0.0002 ~T ^{3}}}\right]10^{-12.} \tag {5a}\end{align*}

The paper reports the measurements of the dielectric constant of seawater at the P-band frequency on both typical ocean low salinity waters and the high salinity waters such as occur in the Great Salt Lake, Utah. The measurements are compared with the predictions from the GW 2022 model that is originally built based on the low salinity measurements at L-band and modified to apply to high salinity. For low salinity (below 40 psu), the GW 2022 model demonstrates excellent agreement with the P-band measurements within the temperature range 5°C to 35°C.. At higher salinity levels (50, 75, 100, and 150 psu), slight divergence is observed between the model predictions and the measurements, particularly as temperature increases. If the measurements are correct, this divergence indicates that the GW 2022 model will require modification for P-band especially for high salinity and high temperatures.

This article explores the potential of P-band and L-band radiometry for estimating soil moisture and temperature profiles. A total of 3977 hourly in situ soil data were collected at depths (d) of 5–60 cm in Beltsville, MD, USA. Using the data, a coherent model was used to generate synthetic brightness temperatures at an incidence angle of 40° for frequencies of 0.8, 0.9, 1.1, and 1.4 GHz. These synthetic brightness temperatures facilitated the estimation of soil moisture ($m_{v}$) and temperature (T) profiles, which were modeled as quadratic functions with three coefficients. The inversion problem was formulated as a least-squares problem and optimized by the adaptive simulated annealing (ASA) algorithm. Regression analysis highlighted the sensitivity of the soil moisture and temperature function coefficients on the brightness temperature at 1.4 GHz and the differential between 1.4 and 0.8 GHz ($R^{2}=0.62-0.97$), yielding the best-fit models to describe the relationships. The application of the ASA algorithm incorporating the best-fit models to constrain the search spaces showed the RMSE of soil moisture${m_{v}}\leq 0.04~\rm cm^{3}/cm^{3}$and soil temperature${T}\leq 1.35~^{\circ }\text {C}$for depths$d\leq 20~ \rm cm$, and${m_{v}}\leq 0.10~\rm cm^{3}/cm^{3}$and${T}\leq 1.60~^{\circ }\text {C}$for$d\leq 60 ~\rm cm$. A streamlined inversion method was also investigated which used the best-fit models as the retrieval formula and solely relied on the V-pol data. The streamlined inversion method achieved comparable retrieval accuracy but reducing the runtime from 770 to 0.54 s for one inversion. This approach simplifies the data collection process by eliminating the need for H-pol data, potentially broadening its flexibility and applicability.

This paper is concerned with the accurate measurement of the dielectric constant of seawater at P band. The measurement of seawater salinity at L band (1.414 GHz) by satellite becomes less accurate as the seawater temperature decreases. At lower frequencies such as 707 MHz, the sensitivity to salinity of the radiometer measurements increases. Designers of P band radiometers for ocean sensing will need to know how the dielectric constant of seawater varies with salinity (S) and temperature (T). To determine this relationship, a P band cavity, based on an L band design, has been constructed. Initial measurements are made at S=35 psu and T= 10°, 20°, 30°C. The results are compared with the values obtained from L band models evaluated at P band (f=707MHz.). The model predictions are close to the measurement results.

This paper describes a new system for measuring the dielectric constant of seawater near 700 MHz. The purpose of this research is to obtain data at P-band for the dielectric constant of seawater that can be combined with previous L-band data for developing a model function valid from 500 MHz to 2000 MHz. A transmission-type cylindrical cavity has been constructed with the$\text{TM}_{010}$mode resonance occurring near 700 MHz. The seawater is introduced via a capillary quartz tube. The dimensions of the cavity have been determined by electromagnetic modelling. The model takes the cavity wall loss into account and can provide accurate estimation of resonance condition for both the empty cavity and cavity with seawater. The errors between the theory and measurement are about 0.07% for the resonant frequency and 0.4% for the quality factor,$Q$. The cavity design, experimental setup and measurement schedule will be presented in the paper.

This paper demonstrates the potential use of P and L band passive measurements to determine root zone soil moisture (SM) and soil temperature (ST). SM and ST data have been taken as a function of depth during the NASA GSFC PLEX 19 experiment in the summer of 2019 at Beltsville, MD, USA. Using these data, a coherent model has been used to compute H and V brightness temperatures at frequencies of 0.8 and 1.4 GHz with an observation angle of 35 degrees. These synthetic brightness data are then used to estimate the SM and ST profiles which are represented by linear polynomials. The inversion problem is formulated as a least square problem that is solved by a global optimization method known as the Adaptive Simulated Annealing (ASA) method. Four inversion examples having different SM and ST profiles are presented. Selected results show that the standard deviation between the retrieved and measured data is less than 0.077$\text{cm}^{3}/\text{cm}^{3}$for SM, and 2.245 °C for ST.

A decomposition and reconstruction theorem (DRT) is introduced to advance computation and provide physical understanding for the scattering from fractal surfaces (FPs). The profile of FP is decomposed into test profiles (TPs), and the scattering of FP is reconstructed using the scattering of TPs to enhance the computational efficiency. A new method that applies DRT on the extended boundary condition method combined with the truncated singular value decomposition technique (TEBCM) is presented and referred to as TEBCM-DRT. The method of TEBCM-DRT is employed to solve the scattering from realistic soil surfaces, and the results are validated against other scattering models. Moreover, the efficiency of TEBCM-DRT and its validity range are investigated. The result shows that TEBCM-DRT improves computational efficiency to$1.95\times 10^{5}$and$7.35\times 10^{2}$times, respectively, compared to TEBCM and the conventional method of moments for a wide range of roughness. In addition, TEBCM-DRT indicates that the amplitude and direction of propagation of scattering modes are dependent on deterministic TPs. This relationship benefits for obtaining the accurate height of an arbitrary point on the profile from bistatic scattering coefficients.

This paper proposes a closed-form asymptotic solution for the bistatic coherent scattering of a multilayered rough surface structure based upon its effective dielectric constant (EDC) in specular direction. The EDC is modeled by establishing an equivalence of the coherent scattering between the multilayered rough surface structure and a half-space homogeneous medium. The scattering solution is then solved using the scalar Kirchhoff approximation (SKA) method. This new method is referred to as the SKA-EDC method, and it is applied to analyze the sensitivity of the EDC and coherent reflectivity to bare soils with realistic parameters at P- and L- bands. The result indicates that EDC can give different responses to the soil moisture variations with respect to the coherent reflectivity, enabling the potentials of root-zone soil moisture retrieval. At incidence angle smaller than 35°, EDC gives the same value for both polarizations, and the coherent reflectivity can show a significant response to soil at depths < 15-50 cm at 0.80 GHz and <5-15 cm at 1.57 GHz, depending on soil moisture. The simulation also demonstrates that subsurface roughness has a trivial effect on the EDC and coherent reflectivity for three-rough-surfaces separated by continuously dielectric profiles. Subsurface can thus be assumed to be flat for reducing uncertainty in soil moisture inversion algorithms when the subsurface roughness is unknown.

Fractal geometry function provides a practical way to simulate natural rough surface for electromagnetic scattering calculation of microwave remote sensing. In our study, we generate three-dimensional random rough surface based on band-limited Weierstrass-Mandelbrot model and carry out a series of quantitative analyses of the relation between fractal parameters and rough surface parameters. In order to eliminate the artifical reflection from the truncated boundary of rough surface model, a tapered incident wave model is built to illuminate the surface profile model with the use of sampling strategy. The tapered incident wave is introduced into the classical Two-Scale Model to calculate backscattering coefficient. The proposed model is applied to simulate the scattering of sea clutter based on the fractal sea surface model. Its simulation results are compared with real sea surface scatterometer measurement experiments in Beihai, Guangxi province and the overall deviation is less than 10%. The results validate the efficiency of the proposed model in obtaining the scattering simulation data on sea surface for parameter inversion of active microwave remote sensing.

The transmission line method for measuring the complex permittivity of material via the S-parameter has been widely researched. In this paper, the rationale of the transmission line method is derived clearly, especially for the optimized solutions for the multivalued issue and the thickness resonance are presented. The revised algorithm is employed to evaluated the complex permittivities of soil in terms of various of water content at frequencies between 500MHz and 10GHz. The experiment shows that there not exist the multivalued issue and the thickness resonance in the retrieving results, it is indicated that the complex permittivities of soil can be obtained by the advanced transmission line method.

A novel formulation for the calculation of equivalent dielectric constant of multilayer medium with slightly rough interfaces is proposed. The formulation is based on the volumetric perturbation of inhomogeneous medium and the closed-form solution to the problem of electromagnetic scattering from the simplified geometric structure of layered media. And the derivation relies on the reciprocity principle which can use the perturbation of dielectric constant to replace the height fluctuation of rough interface contours. According to the calculation results, the validity of macroscopic dielectric constant model is tested to a certain extent. Furthermore, the simulation of four-layer soils and ice-covered regions will be directly applicable for active microwave remote sensing inversion.

In this paper, a novel method is proposed to evaluate the scattered field from rough surfaces by decomposition-reconstitution. The extended boundary condition method (EBCM) is employed in conjunction with a Weierstrass Mandelbrot (WM) function for fractal surface profile and its components. WM function can be decomposed into a series of sinusoidal profile. The properties of the sinusoidal profile and WM function to be periodic and almost periodic allow the deriving respective amplitudes of the scattered Floquet modes. The superposition of scattered fields from each sinusoidal component is consistent with the scattered field from WM surface. The result has compared with AIEM in backscattering coefficients. Thus, by calculating the scattered information for each sinusoidal component of the rough surface, we can obtain the overall scattered field.

The research on electromagnetic scattering from random rough surface plays an important role on the scientific investigation and engineering applications of the microwave remote sensing. In this paper, finite difference time domain (FDTD) is employed to calculate the backscattering coefficients from soil surface in terms of angular, frequency, RMS height, correlation length and moisture, the errors are less than 3dB between the numerical simulation and advanced integral equation model (AIEM) in most cases. This technique is also used to solve the backscattering coefficient of the bare soil surface of Qionglai, Sichuan, the angular trends are generally in good consistency with the ground-based scatterometer data.

In this paper, the decomposition-reconstitution theorem is introduced to solve the electromagnetic scattering field from the random rough surface. The random rough surface is decomposed into a series of fractal described by sinusoidal basis functions and the scattering fields are computed for each fractal. The scattering field of surface is reconstituted by vector-superposing the scattering fields of fractal. The theorem is demonstrated by the numerical simulation using FDTD. It shows that the difference of both results is a little in the range of the calculation error. This theorem was applied to solving the backscattering coefficient of the soil surface of Qionglai Sichuan, the result generally coincides with the ground-based scatter-meter measurement data and AIEM.

In this paper, a method of Numerical Maxwell Model in 3D Simulations (NMM3D) is used to evaluate the backscattering information of fractal rough surfaces. Weierstrass Mandelbrot function is employed to generate natural rough surfaces. Quantitative analyses are demonstrated on the relation of fractal parameters and rough surface parameters. In order to eliminate the unwanted edge effects, a tapered incident wave model is built to illuminate the surface models with the use of sampling strategy. Method of Moments is employed to evaluate the backscattered fields. The results are demonstrated and show a good agreement with AIEM model.