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Two layers of Saharan dust observed over Leipzig, Germany, in February and March 2021 were used to provide the first-ever lidar measurements of the dust lidar ratio (extinction-to-backscatter ratio) and linear depolarization ratio at all three classical lidar wavelengths (355, 532 and 1064 nm). The pure-dust conditions during the first event exhibit lidar ratios of 47 ± 8, 50 ± 5 and 69 ± 14 sr and particle linear depolarization ratios of 0.242 ± 0.024, 0.299 ± 0.018 and 0.206 ± 0.010 at wavelengths of 355, 532 and 1064 nm, respectively. The second, slightly polluted-dust case shows a similar spectral behavior of the lidar and depolarization ratio with values of the lidar ratio of 49 ± 4, 46 ± 5 and 57 ± 9 sr and the depolarization ratio of 0.174 ± 0.041, 0.298 ± 0.016 and 0.242 ± 0.007 at 355, 532 and 1064 nm, respectively. The results were compared with Aerosol Robotic Network (AERONET) version 3 (v3) inversion solutions and the Generalized Retrieval of Aerosol and Surface Properties (GRASP) at six and seven wavelengths. Both retrieval schemes make use of a spheroid shape model for mineral dust. The spectral slope of the lidar ratio from 532 to 1064 nm could be well reproduced by the AERONET and GRASP retrieval schemes. Higher lidar ratios in the UV were retrieved by AERONET and GRASP. The enhancement was probably caused by the influence of fine-mode pollution particles in the boundary layer which are included in the columnar photometer measurements. Significant differences between the measured and retrieved wavelength dependence of the particle linear depolarization ratio were found. The potential sources for these uncertainties are discussed.

An updated algorithm for removal of thermal noise in Sentinel-1 synthetic aperture radar (SAR) Level-1 Ground Range Detected (GRD) data in cross-polarization is presented. The algorithm is comprised of two steps: correction of the annotated thermal noise magnitude (previously proposed in park2018) and a novel correction of the annotated thermal noise range dependence. The magnitude of the annotated thermal noise is corrected by applying scale and offset coefficients tuned on a few hundred of Sentinel-1 data acquired over surfaces with low backscatter in Interferometric Wide and Extra Wide swath modes in HV and VH polarizations. The values of coefficients for all modes and polarizations for data processed with Instrument Processing Facility (IPF) version 3.1 -3.3 are provided. The range dependence is corrected by minimizing a cost function between the annotated range profiles of thermal noise and antenna pattern gain (APG). An objective validation metric based on comparison of averaged backscatter at inter-swath boundaries is proposed. Validation is performed on hundreds of Sentinel-1 scenes acquired over open ocean, doldrums, deserts and sea ice. It shows that the new algorithm outperforms the standard thermal noise removal algorithm proposed by European Space Agency in almost all cases. Analysis shows that the new algorithm worsens noise correction in cases when the range dependence of the annotated APG does not match with the observed signal, indicating either problems with signal processing on IPF or imprecise annotation of APG.

One of the challenges in ocean surface current retrieval from synthetic aperture radar (SAR) data is the estimation and removal of the wave-induced Doppler centroid (DC). This article demonstrates empirically the relationship between the dc derived from spaceborne X-band InSAR data and the ocean surface wind and waves. In this study, we analyzed over 300 TanDEM-X image pairs. It is found that the general characteristics of the estimated dc follow the theoretically expected variation with incidence angle, wind speed, and wind direction. An empirical geophysical model function (GMF) is fit to the estimated dc and compared to existing models and previous experiments. Our GMF is in good agreement (within 0.2 m/s) with other models and data sets. It is found that the wind-induced Doppler velocity contributes to the total Doppler velocity with about 15% of the radial wind speed. This is much larger than the sum of the contributions from the Bragg waves (~0.2 m/s) and the wind-induced drift current (~3% of wind speed). This indicates a significant (dominant) contribution of the long wind waves to the SAR dc. Moreover, analysis of dual-polarized data shows that the backscatter polarization ratio (PR=σ⁰VV/σ⁰HH) and the dc polarization difference (PD=|dcVV|-|dcHH|) are systematically larger than 1 and smaller than 0 Hz, respectively, and both increase in magnitude with incidence angle. The estimated PR and PD are compared to other theoretical and empirical models. The Bragg scattering theory-based (pure Bragg and composite surface) models overestimate both PR and PD, suggesting that other scattering mechanisms, e.g., wave breaking, are involved. In general, it is found that empirical models are more consistent with both backscatter and Doppler data than theory-based models. This motivates a further improvement of SAR dc GMFs.

As a result of global warming, multiyear ice (MYI) is being replaced by first-year ice (FYI) in the Arctic. Microwave scatterometers in the Ku-band and C-band can provide daily observations of sea ice type. However, their comparative capabilities in mapping ice type have not been thoroughly evaluated. We present a systematic intercomparison of the backscatter signature in VV polarization ( ${σ}_{{vv}}^{{o}})$ and the sea ice classification from three scatterometer systems using the same ice classification approach. The systems are the Ku-band quick scatterometer (QSCAT) and the newly launched Chinese rotating fan-beam scatterometer (RFSCAT) and the C-band advanced scatterometer (ASCAT). Three freezing seasons are used, i.e., 2007/08 and 2008/09 for the QSCAT/ASCAT comparison and 2019/20 for the RFSCAT/ASCAT comparison. With reference to ASCAT, ${σ}_{{vv}}^{{o}}$ bias between QSCAT and RFSCAT results from their different incidence angles. A continuous declining trend of ${σ }_{{vv}}^{{o}}$ from MYI and FYI is observed during winter, with a greater difference between MYI and FYI in the Ku-band. The MYI and FYI extent derived from QSCAT/RFSCAT is highly consistent with that derived from ASCAT, with a difference less than 7% and 3% for MYI and FYI, respectively. The overall accuracy (OA) is around 77% and 80% for the RFSCAT results and ASCAT results, respectively, compared with Sentinel-1 SAR images. The classification results show high consistency (81%-89%) with ice charts from the Canadian Ice Service. The incorporation of ${{Tb}}_{36{h}}$ from AMSR-E/AMSR2 improves the OA of the classification when using ASCAT or RFSCAT by 7%-11%.

With phase-sensitive optical time-domain reflectometry (Φ-OTDR), an optic fiber can be used as a distributed vibration sensor to perceive the external disturbance along the fiber through the phase variation of the Rayleigh backscatter light. However, due to the inevitable interference fading phenomenon, the signal-to-noise ratio (SNR) of Rayleigh backscatter light at certain points is insufficient, thus affecting the fidelity of vibration sensing. In this paper, we propose the spectrum extraction and the rotated-vector-sum (SERVS) method to suppress the interference fading in the pulse-intensity-coding Φ-OTDR, which applies Golay complementary sequence through acousto-optic modulator (AOM). Benefited from the easier implementation, the experimental results demonstrate that the percentage of fading points can be reduced from 2.98% to 0.33% significantly and the standard deviation of differential phase is also decreased from 0.6556 rad to 0.2901 rad. Also the minimum level of averaged power spectral density (PSD) of the differential phase at whole locations along the fiber is reduced about 6 dB/Hz effectively. For the demodulated disturbance signal, no influence on the SNR is observed thus it maintains a SNR improvement of 8.6 dB compared to the traditional single pulse Φ-OTDR system, achieving a noise floor of 2.38 pϵ/√Hz.

A unique data set derived from remote sensing, airborne, and ground-based in situ measurements is presented. This measurement report highlights the known complexity of comparing multiple aerosol optical parameters examined with different approaches considering different states of humidification and atmospheric aerosol concentrations. Mie-theory-based modeled aerosol optical properties are compared with the respective results of airborne and ground-based in situ measurements and remote sensing (lidar and photometer) performed at the rural central European observatory at Melpitz, Germany. Calculated extinction-to-backscatter ratios (lidar ratios) were in the range of previously reported values. However, the lidar ratio is a function of the aerosol type and the relative humidity. The particle lidar ratio (LR) dependence on relative humidity was quantified and followed the trend found in previous studies. We present a fit function for the lidar wavelengths of 355, 532, and 1064 nm with an underlying equation of fLR(RH, γ(λ))=fLR(RH=0,λ)×(1-RH)-γ(λ), with the derived estimates of γ(355 nm) = 0.29 (±0.01), γ(532 nm) = 0.48 (±0.01), and γ(1064 nm) = 0.31 (±0.01) for central European aerosol. This parameterization might be used in the data analysis of elastic-backscatter lidar observations or lidar-ratio-based aerosol typing efforts. Our study shows that the used aerosol model could reproduce the in situ measurements of the aerosol particle light extinction coefficients (measured at dry conditions) within 13 %. Although the model reproduced the in situ measured aerosol particle light absorption coefficients within a reasonable range, we identified many sources for significant uncertainties in the simulations, such as the unknown aerosol mixing state, brown carbon (organic material) fraction, and the unknown aerosol mixing state wavelength-dependent refractive index. The modeled ambient-state aerosol particle light extinction and backscatter coefficients were smaller than the measured ones. However, depending on the prevailing aerosol conditions, an overlap of the uncertainty ranges of both approaches was achieved.

In order to improve the understanding of the dynamical deformation processes of sea ice in the seasonal ice zone (SIZ), measures to detect deformed ice were developed and validated using satellite L-band synthetic aperture radar (ScanSAR) images for the southern Sea of Okhotsk. To approach, sea ice was categorized into three ice types, typical of the sea ice in this region: nilas (thin level), pancake ice (thin rough), and deformed ice (thick rough), and then the measures to classify into these categories were developed using ALOS/Phased Array type L-band Synthetic Aperture Radar (PALSAR) as a function of backscatter coefficients at HH polarization ( $\sigma _{\text {HH}}^{0}$ ) and incidence angle ( $\theta _{i}$ ), based on the field observations. Comparative analysis confirmed that PALSAR can detect deformed ice more efficiently than RADARSAT-2 (C-band SAR). The temporal evolution of the area, judged as deformed ice from these measures, shows significant variability with both time and space, and deformed ice regions appear in relatively linear alignments with a width of a few tens of kilometers in the inner ice pack region, consistent with ice drift convergence. To confirm the results, PALSAR-2 images at HH and HV polarizations were examined as a function of $\theta _{i}$ , based on the four-year field observations in the same area. The results revealed that $\sigma _{\text {HH}}^{0}$ and $\sigma _{\text{HV}}^{0}$ are both subject to floe sizes as well as deformed ice, and $\sigma _{\text{HV}}^{0}$ is more sensitive. This indicates that care should be taken when applying these measures to the ice areas where significantly small floes are dominant like the marginal ice zone.

The resonant fiber optic gyro (RFOG) based on reciprocal modulation-demodulation technique is introduced and the influence of high modulation frequency on resonance and demodulation output is analyzed. The expression of backscattering output in resonant cavity is deduced and the experimental device is built to measure the backscattering coefficient. The demodulation model considering backscattering noise is established under reciprocal sinusoidal modulation, and the optimal modulation parameters are obtained by simulation. Finally, the RFOG system is set up. The results show that the detection sensitivity is improved by 19% with the optimal modulation parameters and the system bias stability improves from 0.067°/s to 0.023°/s.

Introduction. The scattering patterns of non-absorbing coded checkerboard-like meta-coatings (MCs) applied for reducing the radar cross section (RCS) of metal surfaces inevitably contain side diffraction lobes. Therefore, the development of MCs with a low level of diffraction lobes is relevant. For this purpose, it is proposed to use checkerboard-like MCs in the form of a set of several basic flat blocks with the same dimensions. The paper discusses two such basic MC blocks with different coding matrices. The cells of the metasurface contain two coupled elliptical ring resonators and are distinguished by a 2-bit coding of the tilt angle of the anisotropy axis. Coding matrices of the MC blocks are built according to the block principle.Aim. To investigate experimentally and numerically backscatter patterns (BSP) for consonant (co-) and orthogonal (cross-) polarizations of the two developed flat blocks of the 2-bit digital nonabsorbing anisotropic MCs for different planes and polarizations of irradiation.Materials and methods. Full-wave simulation of the MC blocks was carried out using the HFSS software by the finite element method. BSP measurements of the fabricated MC layouts were performed in an anechoic chamber of the Center for Collective Usage “Applied Electrodynamics and Antenna Measurements” of the Southern Federal University using an automated information and computing complex.Results. The RCS reduction for the two blocks under normal irradiation is approximately the same and not less than 12 dB over the 9.8…21 GHz band. A good matching between the simulation and measurement results of backscattering patterns of the blocks in the region of the central lobes for various planes and polarizations of the irradiation is noted. In the principal planes, the blocks cancel the central lobes of the BSP by 10…25 dB; in the sector of angles of around ±40°, the backward RCS of the blocks is lower than that of the reference. In the diagonal planes, there is a cancellation of the RCS by 13…15 dB and an expansion of the central lobe of the BSP for copolarizations, as well as a bifurcation of this lobe for crosspolarizations in the sector of angles ±9°; outside of this sector the RCSs of the blocks are commensurate with the RCS of the reference.Conclusion. The developed blocks of the 2-bit digital nonabsorbing anisotropic MCs can be used for broadband cancellation of the RCS of metal surfaces.

We report on the realization of a novel fiber-optic radio frequency (RF) transfer scheme with the bidirectional frequency division multiplexing (FDM) dissemination technique. Here, the proper bidirectional frequency map used in the forward and backward directions for suppressing the backscattering noise and ensuring the symmetry of the bidirectional transfer RF signals within one telecommunication channel. We experimentally demonstrated a 0.9 GHz signal transfer over a 120 km optical link with the relative frequency stabilities of 2.2E-14 at 1 s and 4.6E-17 at 20,000 s. The implementation of phase noise compensation at the remote site has the capability to perform RF transfer over a branching fiber network with the proposed technique as needed by large-scale scientific experiments., 4 pages, 3 figures, accepted for published in IEEE Photonics Technology Letters

Results of the numerical study of a possibility of compensating for aero-optical aberration distortions of the beam wavefront using the aperture sensing technique based on atmospheric backscatter signals are described. It is shown that the technique makes it possible to partially compensate for aero-optical aberrations. The cross section beam size becomes several times smaller than that of a beam without the wavefront adaptive control, and the intensity distribution becomes more symmetric about the optical axis.

This work demonstrates the ability to extend the positioning range of low-powered RFID tags to distances usually not achievable with other wireless or conventional RFID technologies. The technique is performed through a Received Signal Phase (RSP)-based method on a 5.8 GHz backscatter tunneling tag, in multipath-rich indoor and outdoor environments, at distances up to 35 meters from the reader. Distance errors as low as 0.1% of the total reader-to-tag distance were observed with average errors of 0.8% and 0.6% for indoor and outdoor environments, respectively. Compared to Received Signal Strength (RSS)-based techniques, the average distance estimation accuracy is improved by a factor of 51 and 38 for indoor and outdoor environments, respectively. Moreover, an Effective Isotropic Radiated Power (EIRP) of only 10.5 dBm and a biasing power for the tunneling tag of only $21.3~\mu \text{W}$ at 80 mV promise a low-power, long-range sub-meter scale positioning technique with a projected maximum range over 1 km.

A methodology for the assessment of cell concentration, in the range 5–100 cells/ $\mu \text{L}$ , suitable for in vivo analysis of serous body fluids is presented in this work. This methodology is based on the quantitative analysis of ultrasound images obtained from cell suspensions and considers applicability criteria, such as short analysis times, moderate frequency, and absolute concentration estimation, all necessary to deal with the variability of tissues among different patients. Numerical simulations provided the framework to analyze the impact of echo overlapping and the polydispersion of scatterer sizes on the cell concentration estimation. The cell concentration range that can be analyzed as a function of the transducer and emitted waveform used was also discussed. Experiments were conducted to evaluate the performance of the method using 7- $\mu \text{m}$ and 12- $\mu \text{m}$ polystyrene particles in water suspensions in the 5–100 particles/ $\mu \text{L}$ range. A single scanning focused transducer working at a central frequency of 20 MHz was used to obtain ultrasound images. The method proposed to estimate the concentration proved to be robust for different particle sizes and variations of gain acquisition settings. The effect of tissues placed in the ultrasound path between the probe and the sample was also investigated using 3-mm-thick tissue mimics. Under this situation, the algorithm was robust for the concentration analysis of 12 $\mu \text{m}$ particle suspensions, yet significant deviations were obtained for the smallest particles.

The coupling of Rayleigh backscatter induced phase noise in bi-directional, inline fiber interferometers compromises detection sensitivity for Fourier frequencies less than 1 Hz. Here we report experimental and theoretical work highlighting the coupling of backscatter phase noise, and its subsequent range-gating and suppression using digitally enhanced homodyne interferometry. Between two in-line fiber interferometers, each with a differential arm-length of 8 km, we demonstrate a relative stability of 10 Hz/ $\sqrt{\text{Hz}}$ at a Fourier frequency of 100 mHz.

The analysis of electromagnetic (EM) scattering from a target located in the near zone of an electrically large near-field-focused (NFF) array of antennas is presented. The analysis first examined the EM scattering from a metal sphere, attempting to create a reference for comparison with other targets in a fashion analogous to conventional far-field analysis of radar cross section (RCS). Realistic targets were afterward examined and analyzed to exhibit their RCS characteristics. A numerical simulation-based maximization of EM scattering fields for effective target detection in the near zone of antenna was developed, which allows one to obtain a set of optimum element excitation weightings by applying the steepest ascent method (SAM). RCS phenomena were validated by full-wave simulations and experimental measurements to justify the feasibility of the proposed concept.

The intensity distribution of scattered radiation over the transmitting–receiving aperture of a turbulent lidar is experimentally studied. Sounding was performed along a horizontal path under moderate turbulence with a narrow laser beam located in the center of the receiving aperture of variable size. The relative contribution to the echo signal due to turbulence decreased as the receiving aperture increased. The BSE effect was found to be localized at the center of the receiving aperture: the peak of scattered radiation is located at the axis of the sounding beam, and its size is approximately equal to the beam size; at the periphery, the average intensity of scattered radiation slowly decreased to background values as the distance from the beam axis increased. The result can be used in practice for optimal choice of transceiver parameters when designing a turbulent lidar.

The compact polarimetric (CP) imaging mode is a special, dual-polarization mode in which only one polarization is transmitted and two orthogonal polarizations are simultaneously used to measure the returns. The transmitting polarization can be an arbitrary elliptical wave, and therefore, theoretically, there are numerous possibilities of hybrid dual-pol modes. Since the scattering process is a function of the incident field and scattered field, when we use polarization to measure the backscattering process, radar measurement is polarization-dependent. This leads to difficulties in developing a unified interpretation for target scattering characterization in compact polarimetry, since the scattering models should be modified to take the fact of polarization dependency into account. In this letter, in the framework of a previously proposed CP formalism method, we develop a general model-based target decomposition technique that is applicable to the general CP mode. First, a general link between the fully polarimetric (FP) and the general CP scattering matrix elements is established. Second, the FP models corresponding to a single symmetric scattering mechanism and azimuthally symmetric volume scattering are mapped to the general CP case. Assuming that the 2-D backscatter is composed of a single scattering component and a volume scattering component, the decomposition is finally implemented by solving a quadratic equation. The C-band Shuttle Imaging Radar with Payload C/ X-synthetic aperture radar (SIR C/X-SAR) data are used in the experiment for demonstration.

Introduction. The multipath resistance of GNSS antennas is largely determined by the gain slope of the amplitude radiation pattern at sliding angles (angles close to the horizon). The gain slope of the antenna radiation pattern is determined by the size of its ground plane. This article investigates the dependence between the gain slope and ground plane radius R of a quadrupole antenna.Aim. To analyse the impact of the radius of conventional and high-impedance ground planes on the backscatter radiation pattern of a quadrupole antenna at sliding angles.Materials and methods. Computer simulations were carried out in CAD CST Studio Suite using the methods of finite element analysis (FEM), finite difference time domain (FDTD) and template based post-processing.Results. Quadrupole antennas with a capacitive high-impedance ground plane and a conventional flat ground plane were simulated. The dependence of the average gain slope at sliding angles on the radius of the ground plane was determined at low fн and upper fв GNSS frequencies. The analysis of the down/up ratio, the rolloff gain and the multipath ratio for R= 1…20 of the wavelength of capacitive high-impedance and ground planes conventional flat was performed. It was established that higher gain slopes can be obtained using different types of ground planes; however, lower backscatter radiation values are achievable only using high-impedance structures. It was observed that the same slope of the radiation pattern (about 1 dB/°) for GNSS lower frequencies can be obtained at different R=12λ0, and, presumably, at 20λ0.Conclusion. A high-impedance ground plane with a diameter of 2R=12λ0 is preferable for a quadrupole antenna at low GNSS frequencies. A further increase in the ground plane size will insignificantly improve its characteristics.

This study is concerned with the analysis of the displacement measurements simultaneously retrieved from multiple Doppler radars placed at different positions of a traffic light’s mast. Currently, the structural health monitoring (SHM) works based on portable Doppler radars consider primarily methods for the extraction of displacement measurements. However, other measurements in the form of instantaneous natural frequencies and mode shapes can also be calculated with the use of Doppler radars arranged as an array. In this article, the wind-induced vibration displacement analysis based on a low-cost Doppler radar array is proposed. The sensitivity of the proposed array to changes in the instantaneous natural frequencies is evaluated. In addition, the structural modal analysis based on a Doppler radar array operating in the passive backscattering mode is demonstrated. The accuracy of the measurements obtained from the radars’ demodulated data is compared with the accelerometer-based measurements. The ability of the proposed array to provide instantaneous natural frequency measurements and structural modal analysis is validated by the experimental results.

The microwave scatterometer onboard the China-France Oceanography Satellite (CFOSAT Scatterometer, CSCAT) is the first rotating fan-beam scatterometer (RFSCAT) in operation. It is used to obtain the ocean surface wind field by measuring the backscattering coefficient over a large observation swath. To estimate the backscattering coefficient, the scatterometer needs to measure the scattered signal accurately. The process of removing noise from the echo signal (scattered signal plus noise) is called noise subtraction. Noise subtraction has a great influence on the measurement of the backscattering coefficient and subsequent wind field retrieval. Therefore, accurate noise subtraction is necessary and important. In this article, an improved noise subtraction method for CSCAT is proposed. According to the distribution characteristics of backscattering coefficients, noise correction factors are introduced to improve the noise subtraction of CSCAT. The ocean surface wind fields are obtained using the improved backscattering coefficient data, and the deviation of wind speed and direction are calculated by comparison with the European Centre for Medium Range Weather Forecasts wind fields. The results show that the improved noise subtraction method can lead to improvement of the quality of wind field effectively. In addition, based on the improved method of noise subtraction, a method of estimating the noise correction factor is proposed at a large incidence angle, furtherly improving the noise subtraction method. The results show that the proposed method is effective.

Satellite laser ranging (SLR) with higher repetition rate is the recent trend for its various advantages. Laser backscatter (coincidence between recently transmitted pulses and received pulses near the detector) is found to be a constraint for the repetition rates higher than 20 kHz, due to, overlapping with photons returning from a satellite with the present constellation of most of the SLR systems. Such an overlap occurs at every 75 km satellite distance change at 2 kHz repetition rate, and remains for about 7.5 km; for a 20 kHz system however, it will occur after every 7.5 km and remains for 7.5 km, resulting in constant backscatter overlap – leaving no chance to avoid it. The resulting noise is 5 times more than before causing a serious problem in detection and lowers the signal to noise ratio of the overall SLR system. However, decreasing energy per shot at higher repetition rates – assuming a constant power laser – the resulting backscatter may decrease fractionally.

In this study, a series of time-varying breaking waves are generated by FLUENT. The backscattering normalised radar cross sections of the time-varying braking wave at different incidence angles are analysed by employing the method of moments. The scattering results can present the high polarisation ratios and sea spikes. In addition, the Doppler spectrums of the breaking waves are calculated for different incidence angles, and the computational results can indicate the large Doppler offsets and relatively big bandwidths for large incidence angles. Generally speaking, the scattering results in this study could play a part in explaining the super-scattering events from the breaking wave on the sea surface.

Purpose This study aims to investigate the relationship between backscatter factors and Al-half-value-layers (Al-HVL) by making the quality index (QI) a parameter for diagnostic kilovoltage x-ray beams. Methods Backscatter factors, Bw, for x-ray fluence spectra were calculated from the weighted average of Bw for monoenergetic photons of between 8 and 140 keV with field sizes of 10 cm × 10 cm to 40 cm × 40 cm. The value of Bw for monoenergetic photons was calculated from the ratio of the water kerma at the surface of a water phantom and that at the same point free-in-air using the EGSnrc/cavity code. The weighted averaged backscatter factors were validated by comparing them with those of direct Monte Carlo calculations for the x-ray fluence spectra. The Bw for the x-ray fluence spectra were classified by a QI of 0.35, 0.4, 0.5, 0.6, and 0.7 specified by the ratio of the effective energy and maximum energy. The relationship between Bw and Al-HVL was evaluated for the given QI values. The x-ray fluence spectra were generated for tube voltages of 40–140 kVp with Al-HVLs of 0.5–13.2 mm using the SpekCalc program. Results The weighted averaged backscatter factors for x-ray fluence spectra agreed within 0.7% with those of the direct Monte Carlo calculations. The backscatter factors were represented by the fitting curves of R2 > 0.99 with Al-HVL for the given QI values. Conclusions It is possible to obtain Bw more accurately by using QI specified by the measured Al-HVL.

In this article, a novel method of wavelength measurement with a spectral resolution of 64 attometers (am) based on Rayleigh speckle obtained from a single-mode fiber (SMF) is proposed. A commercial standard SMF is used to provide Rayleigh backscattering signals and measured by an optical frequency-domain reflectometry (OFDR) based system. The Rayleigh backscattering in SMF is a disordered process and provides high-sensitivity wavelength-dependent speckles. In the experiments, by extracting the Rayleigh speckles from a 2-km SMF, we achieve a 64-am spectral resolution (standard deviation) with a measurement refresh rate of 3.3 kHz. Compared with some similar disordered media based wavemeter schemes, the wavelength information can be directly read out from the Rayleigh speckles without using complex algorithms, since the Rayleigh speckle is a 1-D image and shows a linear relationship with the wavelength change, which makes this scheme simple and prospective for high spectral resolution applications.

The main factors causing the occurrence of systematic errors in measuring the backscatter patterns of targets when using irradiator assemblies in collimator type radar measuring complexes are considered. It is shown that the implementation of a quasimonostatic measurement scheme with a bistatic angle determined by spatial separation of the phase centers of irradiators assembly is the most significant factor in the experimental determination of the backscattering characteristics of reflectors with highlighted directed properties, to which the monostatic-bistatic equivalence theorem is not applicable. To quantify the influence of this factor, it is proposed to use indicators characterizing the decrease of the maximum and average recorded cross-section of trihedral corner reflectors within the main lobe of the backscatter diagram, determined by the half power level. The listed indicators do not take into account the non-flatness of the phase surface of the probe field, however, they can be considered as reasonably justified lower bounds for systematic measurement errors for each irradiators assembly. Using the mathematical modeling, mono- and bistatic cross-section diagrams of trihedral corner reflectors of various wave sizes with different face shapes were calculated, error estimation for their backscattering diagrams were determined using a set of irradiator assemblies. Ratios are obtained for estimating of width of the lobe of the bistatic scattering indicatrix at the level of –3 dB of trihedral corner reflectors with triangular, radial, and square face shapes, and a recommendation on the rational choice of the working frequency of the assembly when implementing large-scale electrodynamic modeling is made.

Based on composite rough surface and stochastic multiscale models, an improved two-scale model (TSM) is proposed. Different from the classical TSM, the proposed method adopts the integral equation method (IEM) to replace the small perturbation method (SPM) for calculating the contribution from small-scale roughness. Kirchhoff approximation (KA) is still kept in the model to calculate the contribution from large-scale roughness. Because the IEM has wider application range than the SPM, the proposed model exhibits a higher accuracy and has a wider application range in terms of sea surface roughness compared with the classical TSM. Usually, the classical TSM is sensitive to the cutoff wavenumber. The proposed model adopts an adaptive cutoff wavenumber instead of choosing a cutoff wavenumber in the classical TSM. The effectiveness of the proposed model is demonstrated through comparisons with the simulation results of multilevel fast multipole algorithm (MLFMA) and the measured results. Different roughness at different frequencies is analyzed as well.

A way of estimating the intensity of the structure characteristic of fluctuations of the refractive index $$C_{n}^{2}$$ based on data from a two-channel turbulent aerosol lidar operating on the backscatter enhancement (BSE) effect is shown. It is proposed to use V.V. Vorob’ev’s approximation which, for the case of homogeneous turbulence, determines the dependence of $$C_{n}^{2}$$ on the ratio of echo signals. This technique is based on experimental data which suggest that the BSE effect appears in a relatively small region of the space near the scattering volume. Results of sounding for a horizontal path are presented.

The visibility of the underwater image is reduced due to the backscattering and signal attenuation, especially in highly turbid media. In this paper, we propose a polarization-based underwater image restoration method using logarithmic transformation and dark channel. Logarithmic transformation is first performed while maintaining the degree of polarization of the scene to increase the details of the images. Then we calculate the backscatter value through dark channel to estimate the degree of polarization of backscatter. The restored image can be obtained by the polarimetric recovery method. The experimental results illustrate the capabilities of the proposed method. Document code: A Article ID: 1673-1905(2020)02-0149-5

In this article, we present a multipolarimetric estimation approach for two model-based vegetation structure parameters (shape A and orientation distribution ψ of the main canopy elements). The approach is based on a reduced observation set of three incoherent (no phase information) polarimetric backscatter intensities (|S HH | 2 , |S HV | 2 , and |S VV | 2 ) combined with a two-parameter (A P and ψ) discrete scatterer model of vegetation. The objective is to understand whether this confined set of observations contains enough information to estimate the two vegetation structure parameters from the L-band radar signals. In order to disentangle soil and vegetation scattering influences on these signals and ultimately perform a vegetation only retrieval of vegetation shape A and orientation distribution ψ, we use the subpixel spatial heterogeneity expressed by the covariation of co- and cross-polarized backscatter Γ PP-PQ of the neighboring cells and assume it is indicative for the amount of a vegetation-only co-to-cross-polarized backscatter ratio μ PP-PQ . The ratio-based retrieval approach enables a relative (no absolute backscatter) estimation of the vegetation structure parameters which is more robust compared to retrievals with absolute terms. The application of the developed algorithm on global L-band Soil Moisture Active Passive (SMAP) radar data acquired from April to July 2015 indicates the potential and limitations of estimating these two parameters when no fully polarimetric data are available. A focus study on six different regions of interest, spanning land cover from barren land to tropical rainforest, shows a steady increase in orientation distribution toward randomly oriented volumes and a continuous decrease in shape arriving at dipoles for tropical vegetation. A comparison with independent data sets of vegetation height and above-ground biomass confirms this consistent and meaningful retrieval of A P and ψ. The retrieved shapes and orientation distributions represent the main vegetation elements matching the literature results from model-based decompositions of fully polarimetric L-band data at the SMAP spatial resolution. Based on our findings, A P and ψ can be directly applied for parameterizing the vegetation scattering component of model-based polarimetric decompositions. This should facilitate decomposition into ground and vegetation scattering components and improve the retrieval of soil parameters (moisture and roughness) under vegetation. This work was supported with the MIT-Germany Seed Fund “Global Water Cycle and Environmental Monitoring using Active and Passive Satellite-based Microwave Instruments” and with the MIT-Belgium UCL Seed Fund “Early Detection of Plant Water Stress Using Remote Sensing”. C. Montzka is supported by the European Commission Horizon 2020 Program that funded the ERA-PLANET/GEOEssential (Grant Agreement no. 689443) project. M. Piles received partial funding through project RTI2018-096765-A-100 (MCIU/AEI/FEDER, UE).

This letter proposes a simple and effective method to remove haze from a single input foggy or underwater image. The algorithm is originated from the physically valid description of the transmission of the scattering light during the volumetric scattering medium, interpreting the formation of the haze with the layered transmission model. We establish the explicit expression of the transmittance with respect to statistics of the back scattered light in front of the scene. The parameters for the evaluation of the transmittance can be estimated by the second order statistics of the back scattered light both with and without the scene. Since the transmittance map is evaluated for each RGB channel respectively, the color recovery by the proposed method is wavelength dependent and physically valid. Experimental results on both terrestrial and underwater images are given, which show the effectiveness and advantage of the proposed method.

Persistent scatterer (PS) techniques are a set of important time-series tools for interferometric synthetic aperture radar (InSAR) that enable deformation analysis in highly decorrelated terrain. Detailed knowledge of the statistics of persistent scatterers in InSAR images is critical for the design of better techniques that will enable both the extraction of deformation in traditionally difficult regions as well as develop a better understanding of how performance of these algorithms relates to important system parameters. In this article, we characterize the backscatter statistics of both persistent and distributed scatterers over wavelength using data from X -band (COSMO-SkyMed), C -band (Sentinel-1), and L -band (ALOS) sensors. We show that popular distributions that have previously been used to fit SAR backscatter can effectively capture the returns from both PS and clutter, with the $\text {G}^0$ distribution being the most applicable across wavelength and scatterer type. Thus, our work paves the way for improved detection algorithms to be designed based on these distributions and also builds an initial foundation for developing a greater theoretical understanding of PS statistics.

The surface wave investigation and monitoring (SWIM) aboard the China-France Oceanography Satellite (CFOSAT), a pioneer conically scanning wave spectrometer, was successfully launched on October 29, 2018. Its innovative configuration composed of one nadir and five rotating near-nadir beams is designed to simultaneously observe the directional wave spectrum at a global scale. In this study, we systematically implement the spectral analysis of the radar backscattering with the periodogram technique to obtain the fluctuation spectrum for each azimuth direction. The 2-D fluctuation spectrum of the three spectral beams ( θ=6∘ , 8°, and 10°) combines all the azimuth directions within one entire rotation of 360°. The case study demonstrates that the wave features (peak wavelength and direction) are roughly consistent between the estimated fluctuation spectrum and the collocated WaveWatch III wave slope spectrum. A marked up-to-downwave asymmetry of the fluctuation spectrum with larger spectral level in the upwave direction for all the three spectral beams is observed. A ratio is defined between the fluctuation spectrum within the [0°, 180°] sector relative to the [180°, 360°] sector. Statistics display that this ratio is greater than 1 when it denotes the up-to-downwave ratio and smaller than 1 for the down-to-upwave ratio. This observed spectrum asymmetry is linked to the asymmetric modulation from upwind to downwind. In addition, we employ such finding to help remove the 180° wave direction ambiguity from a practical point of view. Preliminary results of the direction ambiguity removal display a bias of 41.3°, 40.6°, and 36.7° for the beams. The 10° beam shows slightly better performance compared to the other two beams in terms of bias and standard deviation. This shall lay a strong basis for the operational implementation of such algorithm to resolve the direction ambiguity.

Dust orientation has been an ongoing investigation in recent years. Its potential proof will be a paradigm shift for dust remote sensing, invalidating the currently used simplifications of randomly oriented particles. Vertically resolved measurements of dust orientation can be acquired with a polarization lidar designed to target the off-diagonal elements of the backscatter matrix which are nonzero only when the particles are oriented. Building on previous studies, we constructed a lidar system emitting linearly and elliptically polarized light at 1064 nm and detecting the linear and circular polarization of the backscattered light. Its measurements provide direct flags of dust orientation, as well as more detailed information of the particle microphysics. The system also has the capability to acquire measurements at varying viewing angles. Moreover, in order to achieve good signal-to-noise ratio in short measurement times, the system is equipped with two laser sources emitting in an interleaved fashion and two telescopes for detecting the backscattered light from both lasers. Herein we provide a description of the optical and mechanical parts of this new lidar system, the scientific and technical objectives of its design, and the calibration methodologies tailored for the measurements of oriented dust particles. We also provide the first, preliminary measurements of the system during a dust-free day. The work presented does not include the detection of oriented dust (or other oriented particles), and therefore the instrument has not been tested fully in this objective.

We theoretically explore the possibility to detect weak localization of light in a hot atomic vapor, where one usually expects the fast thermal motion of the atoms to destroy any interference in multiple scattering. To this end, we compute the coherent backscattering peak, assuming high temperature and taking into account the quantum level structure of the atomic scatterers. It is found that the decoherence due to thermal motion can be partially counterbalanced by working at large laser detuning and using small atomic cells with an elongated geometry. Under these conditions, our estimates suggest that weak localization in a hot vapor should be within reach of experimental detection., Comment: 11 pages, 5 figures

Structural and material symmetries can be engineered to create unique electromagnetic effects including backscattering-immune propagation. In this work we investigate the scattering properties of a class of structures with combined rotational and dual symmetries, i.e., self-dual structures. We demonstrate how self-duality will be manifested through different forms of zero backscattering in periodic and non-periodic structures.

The eVe dual-laser/dual-telescope lidar system is briefly given here, focusing on the optical and mechanical parts of system’s emission and receiver units. The compact design of linear/circular emission unit along with the linear/circular analyser in the receiver unit, allows eVe to simultaneously reproduce the operation of the ALADIN lidar on board Aeolus as well as the operation of a traditional ground-based polarisation lidar system with linear emission. As such, eVe lidar aims to provide: (a) ground reference measurements for the validation of the Aeolus L2A aerosol products, and (b) the atmospheric conditions for which linear polarisation lidar systems can be considered for Aeolus L2A validation, by identifying any possible biases arisen from the different polarisation state in the emission between ALADIN and these systems, and the detection of only the co-polar component of the returned signal from ALADIN for the L2A products retrieval. In addition, a brief description is given concerning the polarisation calibration techniques that are applied in the system, as well as the developed software for the analysis of the collected signals and the retrieval of the optical products. More specifically, the system’s dual configuration enables the retrieval of the optical properties of particle backscatter and extinction coefficients originating from the two different polarisation states of the emission, the linear and circular depolarisation ratios, as well as the direct calculation of the Aeolus like backscatter coefficient, i.e., the backscatter coefficient that Aeolus would measure from ground. Two cases, one with slightly-depolarising particles and one with moderately-depolarising particles, were selected from the first conducted measurements of eVe in Athens, in order to give a glimpse of the system’s capabilities. In the slightly depolarising scene, the Aeolus like backscatter coefficient agrees well with the actual backscatter coefficient, which is also true when non-depolarising particles are present. The agreement however fades out for strongly depolarising scenes, where an underestimation of ~17 % of the Aeolus like backscatter coefficient is observed when moderately-depolarising particles are probed.

On the base of laboratory experiments on co- and cross-polarized microwave signal scattering on a wavy water surface, the dependences of NRCS on wind speed and friction velocity at high wind speeds were obtained. It is shown that the cross-polarized NRCS demonstrates sensitivity to the wind speed when its value is more than 20 m/s, in contrast to the co-polarized NRCS. Based on the analysis of the Doppler spectra, it was suggested that the backscattered signal is formed mainly on wave breakers. This assumption was verified by analyzing the dependence of the scattered signal power at both polarizations on the area of the white-cap coverage, which revealed their direct dependence. Based on the phenomenological approach used in statistical physics, a parametrization of the dependence of the white-cap coverage fraction on the wind friction velocity was proposed. This parametrization was based on the use of the universal Gibbs method, the central concept of which is a canonical ensemble or an ensemble of a thermodynamic system states which are in a weak thermal contact with a "thermostat". In this case, the atmospheric boundary layer acts as a thermostat, and the entire ensemble of states of the sea surface, including breakers, is the canonical ensemble. Based on this parametrization, the GMF was proposed to retrieve the wind speed and wind friction velocity for wind speeds above 40 m/s, which also considering the angular dependence of the NRCS.

The paper is focused on investigation of microwave backscattering from wind waves on a clean water surface. Field experiments were carried out in the coastal zone of the Black Sea using dual co-polarized Doppler X-band scatterometer and a three-band Doppler dual co-polarized radar (X-, С-, S-bands). The radar incidence angles were about 50 - 60 degrees, the wind changed in a wide range of speeds. We assumed that microwave backscattering at VV and HH polarizations is composed by a Bragg (polarized) component associated with Bragg waves and a non-polarized component (NBR). Analysis of Doppler spectra of NBR allowed us to remove the effect of strong wave breaking (overturning wave crests) from the time series and to study the backscatter associated only with dm-scale waves. Measurements of wind waves with a wire gauge were carried out simultaneously with the radar monitoring. It is shown that the velocities of non-Bragg scatterers not associated with strong wave breaking in X-, С-, S-bands correspond to the velocities of short dm waves and weakly depend on radar wavelength. The speeds of the scatterers in X-, С-, S-bands associated with overturning wave crests are also close to each other (within the measurement error). The intensity of NBR in X-, С-, S-bands grows with wind speed as well as with the intensity of dm-waves measured by the wire gauge. Strong suppression of NBR and simultaneously measured decrease of short dm-wave intensity are demonstrated, thus confirming the assumption that the intensity of the NBR in X-, С-, S-bands is determined by dm waves.

Recent MagLIF experiments at the NIF have measured the energy coupling and propagation of a ~35 kJ laser pulse in 1 cm-long gas pipe targets with hydrocarbon gas fills and pre- imposed magnetic fields