Zenith Research Articles

Effect of Wavefront Distortion in Non-Kolmogorov Turbulence on the Performance of Multi-Beam Transmission Coherent Detection

This study aims to address the multi-beam transmission problem of optical wireless coherent communication systems under the influence of non-Kolmogorov turbulence. This paper establishes the mathematical model for mixing efficiency and BER of non-Kolmogorov turbulence wavefront distortion and multi-beam transmission coherent detection systems. The influence of factors such as spectral power-law index, zenith angle, and transmission distance on the communication performance of the system is analyzed, and an experimental system is built using adaptive optics equipment to test the proposed theory. Numerical analyses show that in a non-Kolmogorov turbulence environment, as the spectral power-law index, zenith angle, transmission distance, Fresnel zone, and pointing error attenuation increase, the mixing efficiency of the coherent detection system decreases and the BER increases; under the same conditions, the mixing efficiency and BER of the system are improved after increasing the number of transmission beams at the transmitting end. Experimental research shows that, for the coherent detection system after wavefront correction using adaptive optics, the wavefront PV and RMS variances are 1.68 μm2 and 0.05 μm2 when a single beam of light is transmitted at the transmitting end, and the wavefront PV and RMS variances are 0.23 μm2 and 0.01 μm2 when two beams are transmitted. It is verified that using multi-beam transmission wavefront superposition to suppress wavefront distortion and wavefront jitter can effectively improve the mixing efficiency and BER of the coherent detection system at the receiving end.

Airborne observations of the surface cloud radiative effect during different seasons over sea ice and open ocean in the Fram Strait

Abstract. This study analyses the cloud radiative effect (CRE) obtained from near-surface observations of three airborne campaigns in the Arctic north-west of Svalbard: Airborne measurements of radiative and turbulent FLUXes of energy and momentum in the Arctic boundary layer (AFLUX, March/April 2019), Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD, May/June 2017), and Multidisciplinary drifting Observatory for the Study of Arctic Climate – Airborne observations in the Central Arctic (MOSAiC-ACA, August/September 2020). The surface CRE quantifies the potential of clouds to modify the radiative energy budget at the surface and is calculated by combining broadband radiation measurements during low-level flight sections in mostly cloudy conditions with radiative transfer simulations of cloud-free conditions. The significance of surface albedo changes due to the presence of clouds is demonstrated, and this effect is considered in the cloud-free simulations. The observations are discussed with respect to differences of the CRE between sea ice and open-ocean surfaces and between the seasonally different campaigns. The results indicate that the CRE depends on cloud, illumination, surface, and thermodynamic properties. The solar and thermal-infrared (TIR) components of the CRE, CREsol and CRETIR, are analysed separately, as well as combined for the study of the total CRE (CREtot). The inter-campaign differences of CREsol are dominated by the seasonal cycle of the solar zenith angle, with the strongest cooling effect in summer. The lower surface albedo causes a stronger solar cooling effect over open ocean than over sea ice, which amounts to −259 W m−2 (−108 W m−2) and −65 W m−2 (−17 W m−2), respectively, during summer (spring). Independent of campaign and surface type, CRETIR is only weakly variable and shows values around 75 W m−2. In total, clouds show a negative CREtot over open ocean during all campaigns. In contrast, over sea ice, the positive CREtot suggests a warming effect of clouds at the surface, which neutralizes during mid-summer. Given the seasonal cycle of the sea ice distribution, these results imply that clouds in the Fram Strait region cool the surface during the sea ice minimum in late summer, while they warm the surface during the sea ice maximum in spring.

A new two-axis solar tracker based on the online optimization method: Experimental investigation and neural network modeling

This article presents a new two-axis solar tracker based on an online optimization algorithm so as to track the position of the sun without using its movement model. In this research, four well-known optimization algorithms are employed to find the two unknown parameters named azimuth and zenith angles, which determine the position of the sun. The magnitude of the sunray is considered as the cost function of all algorithms. Then, several experiments are carried out to find the best optimization algorithm with optimal population size, number of iterations, and also the best initialization method. Uniform initialization leads to faster convergence compared to random initialization. The results clearly show that the particle swarm optimization algorithm with a population size of 15 and 7 iterations using uniform initialization method has better performance than the other algorithms, with a convergence time of less than 40 s. The average fitness value or voltage received by the tracker is 2.4 Volts in this method, which is higher than other methods. TLBO also performs well with a population size of 15 and 7 iterations. Afterward, the artificial neural network with one hidden layer and 20 neurons is employed to predict these two parameters in each day and moment in a year in Shiraz city according to the experimental data extracted from PSO. Number of the day from January and the time are inputs and zenith and azimuth angles are considered the output of neural network modeling. The performance of the proposed ANN model is evaluated using regression plots, demonstrating a strong correlation between predicted and target outputs. Finally, the outcomes reveal the feasibility of using online optimization algorithms and neural network modeling in an effort to bypass the complex mathematical model of mechatronic systems and predict the movement of the sun automatically.

The cosmic ray energy spectrum measured with the Pierre Auger Observatory

In this contribution, we present the latest updates on the energy spectrum characterization obtained with data from the Pierre Auger Observatory. We present the measurement of the spectrum from 6×1015 eV up to above 1020 eV, obtained by combining five different methods. The exposure of over 80,000 km2 sr yr has been accumulated above the ankle region, making this the best estimate of the spectrum ever achieved in this energy range. The wide zenith angle range covered by the Pierre Auger Observatory and the enormous exposure also allow us to measure the spectrum in different regions of the sky. After a description of the techniques used to reconstruct the spectrum, the spectral features will be presented, and the astrophysical implications of the spectral shape will be briefly discussed.

Topographic effects amplify forest disturbances detected by yearly wide-time-window Landsat time series

ABSTRACT Widespread topographic effects in remote sensing images of mountainous regions with rugged terrain severely hinder satellite-based global land surface monitoring and change detection. However, topographic effects in Landsat time series (LTSs) disturbance detection remain highly debated and unquantified. In this study, we proposed a novel postprocessing approach to quantify the impact of topographic effects on long time-series (1989 − 2020) forest disturbance detection, taking the Hengduan Mountains Region (HDMR) of southwest China as an example. This approach applied a pixel-by-pixel simulation based on a semiempirical sun-canopy-sensor with a C-correction (SCS+C) model to identify and remove topography-induced disturbances from the LandTrendr-detected forest disturbances. Filtering of the detected forest disturbances with different patch sizes was conducted, and its effectiveness in removing topography-induced disturbances was quantitatively evaluated. The results showed that 10.43% of the total area of LandTrendr-detected forest disturbances was topography-induced. Removing topography-induced disturbances increased the traditional and area-adjusted overall accuracies (OAs) of the forest disturbance map by 3.50% and 0.62%. Topography-induced disturbances occurred mainly on northwest-facing (300°−360°) slopes with a gradient of 30°−55° and higher latitude and between the day of year (DOY) pairs (such as DOYs 90, 330, and 360) with a considerable disparity in solar azimuth and zenith angle. The pixel-by-pixel simulation approach outperformed spatial noise filtering with a minimum mapping unit (MMU), which considerably decreased the area percentage of topography-induced disturbances at the expense of decreasing the producer’s accuracy (PA) in detecting disturbed forests. This study highlighted the importance of removing topographic effects from the detected forest disturbances and shed new light on the promising potential of the postprocessing topographic correction method to replace the preprocessing topographic correction of the LTSs to map forest disturbances in mountainous regions globally.

Performance of FAST with an Ultra-Wide Bandwidth Receiver at 500–3300 MHz

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has been running for several years. A new ultra-wide bandwidth (UWB) receiver, simultaneously covering 500–3300 MHz, has been mounted in the FAST feed cabin and has passed a series of observational tests. The whole UWB band is separated into four independent bands. Each band has 1,048,576 channels in total, resulting in a spectral resolution of 1 kHz. At 500–3300 MHz, the antenna gain is around 14.3–7.7 K Jy−1, the aperture efficiency is around 0.56–0.30, the system temperature is around 88–130 K, and the half-power beamwidth is around 7.6′–1.6′. The measured standard deviation of pointing accuracy is better than ∼7.9″ when zenith angle is within 26.4°. The sensitivity and stability of the UWB receiver are confirmed to satisfy expectations through spectral observations, e.g., H i and OH. The FAST UWB receiver has already demonstrated good performance in capturing sensitive observations for various scientific goals.

An approach to track instrument calibration and produce consistent products with the version-8 total column ozone algorithm (V8TOZ)

Abstract. The Ozone Mapping and Profiler Suite (OMPS) has been on board the Suomi National Polar-orbiting Partnership (S-NPP) satellite since October 2011 and was followed by an OMPS on NOAA-20 (N20) in November 2017 as part of the US Joint Polar Satellite System (JPSS) program. The OMPS measurements are processed to yield various products of atmospheric composition data for near-real-time monitoring and offline study, including retrievals of total column ozone (TCO) and an ultraviolet-absorbing aerosol index (AI) based on the version-8 total ozone (V8TOZ) algorithm. With the implementation of changes to employ a broadband channel approach in the NOAA OMPS V8TOZ, the retrieved TCO and AI products have become more stable and consistent between S-NPP and N20. Two particular regions have been chosen for building soft-calibration adjustments for both OMPS S-NPP and N20, which force the V8TOZ retrievals to be in quite good agreement from both sensors with little change by season. However, bias analysis shows that some noticeable errors and differences still exist after soft-calibration, and those errors appear to be quite persistently associated with solar zenith angle (SZA) and satellite viewing angle (SVA) in the retrievals of TCO and AI for both OMPS S-NPP and N20. Comparisons of TCO and AI from NOAA OMPS retrievals with other products such as those from the Tropospheric Monitoring Instrument (TROPOMI) and the Earth Polychromatic Imaging Camera (EPIC) show that, although the sensor, algorithm, and solar spectra are different among them, the overall retrievals from those products are quite similar and consistent.

Rooftop Photovoltaic Energy Production Estimations in India Using Remotely Sensed Data and Methods

We investigate the possibility of estimating global horizontal irradiance (GHI) in parallel to photovoltaic (PV) power production in India using a radiative transfer model (RTM) called libRadtran fed with satellite information on the cloud and aerosol conditions. For the assessment of PV energy production, we exploited one year’s (January–December 2018) ground-based real-time measurements of solar irradiation GHI via silicon irradiance sensors (Si sensor), along with cloud optical thickness (COT). The data used in this method was taken from two different sources, which are EUMETSAT’s Meteosat Second Generation (MSG) and aerosol optical depth (AOD) from Copernicus Atmospheric Monitoring Services (CAMS). The COT and AOD are used as the main input parameters to the RTM along with other ones (such as solar zenith angle, Ångström exponent, single scattering albedo, etc.) in order to simulate the GHI under all sky, clear (no clouds), and clear-clean (no clouds and no aerosols) conditions. This enabled us to quantify the cloud modification factor (CMF) and aerosol modification factor (AMF), respectively. Subsequently, the whole simulation is compared with the actual recorded data at four solar power plants, i.e., Kazaria Thanagazi, Kazaria Ceramics, Chopanki, and Bhiwadi in the Alwar district of Rajasthan state, India. The maximum monthly average attenuation due to the clouds and aerosols are 24.4% and 11.3%, respectively. The energy and economic impact of clouds and aerosols are presented in terms of energy loss (EL) and financial loss (FL). We found that the maximum EL in the year 2018 due to clouds and aerosols were 458 kWh m−2 and 230 kWh m−2, respectively, observed at Thanagazi location. The results of this study highlight the capabilities of Earth observations (EO), in terms not only of accuracy but also resolution, in precise quantification of atmospheric effect parameters. Simulations of PV energy production using EO data and techniques are therefore useful for real-time estimates of PV energy outputs and can improve energy management and production inspection. Success in such important venture, energy management, and production inspections will become much easier and more effective.

Polarized radiative transfer in seawater-in-oil emulsions floated on seawater considering the impact of oil absorption on seawater droplet scattering.

Among various remote sensing approaches, optical polarization remote sensing shows great advantages in identifying oil-water emulsions in seawater and has become one of the most promising detection technologies. Herein, we focus on exploring the sensitivity of polarized radiative transfer properties for oil emulsion polarization detection to the influence factors of viewing angle, droplet volume fraction and radius, incident wavelength, and emulsion thickness. The radiative properties of seawater droplets dispersed in crude oil are calculated using the improved Lorenz-Mie theory considering the absorption of crude oil as the host medium, after which the reflected Stokes vector and the degree of linear polarization (DOLP) of seawater-in-oil emulsions floating on seawater are obtained using the spectral element method. By analyzing the calculation results of a 0° viewing azimuth angle, the detection wavelength and viewing zenith angles corresponding to the highest sensitivity of the DOLP to the above factors are significantly different; thus, quantitative remote sensing detection of the droplet volume fraction, droplet diameter, and emulsion thickness is possible. Exploring the sensitivity of polarized remote sensing signals for oil emulsion polarization detection to the above factors is a prerequisite for quantitative polarization detection of oil emulsions.

Performance analysis of the coherent FMCW photonic radar system under the influence of solar noise

The real-time high-resolution target detection in radar applications has increased the demand for photonic radar systems due to their higher bandwidth and faster processing capabilities over conventional microwave radar. Additionally, photonic-based radar technology can revolutionize the limited performance of conventional microwave radar caused by various atmospheric attenuations. This article presents a frequency-modulated continuous-wave photonic radar system using coherent detection. The performance of the proposed system is theoretically investigated under the effect of various real-time atmospheric weather attenuations and the influence of solar background noise, which is further verified on the simulation platform. This study is conducted under different atmospheric weather conditions, such as clear, haze, and fog, to highlight the practical limitation of free-space links in the presence of solar background noise. The detected signal at different target ranges with and without solar noise is measured and then compared in terms of signal-to-noise ratio. Furthermore, the impact of solar noise on the system performance when the Sun is under different sky conditions and zenithal angles is also analyzed. The results presented here provide insights into designing photonic radar systems for practical applications like autonomous vehicle radar systems, self-traffic control, and navigation.

Stability Analysis of Ice Crystal Orientation

Abstract The stability of ice crystal orientation is studied by modeling the airflow around ice crystals at moderate Reynolds number, where an ice crystal is approximated by a cylinder with three parameters: diameter D, length L, and zenith angle of the axis θ. In this paper, the torque acting on ice crystals is simulated at different θ first, and then a special θ with zero horizontal torque, denoted as θe, is sought as an equilibrium of ice crystal orientation. The equilibrium is classified into two kinds: stable and unstable. Ice crystals rotate to θe of stable equilibriums while deviating from θe of unstable ones once they are released into quiet air. Multiple equilibriums of ice crystal orientation are found via numerical simulations. A cylinder with D/L close to one has three equilibriums, two of which are stable (i.e., θe = 0° and 90°). A cylinder with D/L away from one has only two equilibriums, one of which is stable (i.e., either θe = 0° or 90°). In addition, an asymmetric cylinder has two, three, or five equilibriums, and their θe is sensitive to the distance between its geometrical center and its center of gravity. The sensitivity of θe to crystal asymmetry suggests large symmetric ice crystals tend to become asymmetric (or irregular) and subsequently oriented randomly. Significance Statement Ice crystal orientation impacts high-cloud reflectance and satellite-based observations of high clouds significantly. However, its laboratory and field observations look dissimilar: the percentage of horizontally oriented ice crystals (HOICs) observed in the laboratory is quite high, while in the field it is often low and varies greatly in space and time. The motivation for this study is to elucidate what causes the difference between the laboratory and field observations. The torque acting on ice crystals are computed by modeling the airflow around ice crystals, revealing the conditions for nonhorizontal orientations of ice crystals. In quiet air, an ice crystal is oriented either horizontally or vertically when its shape is close to sphere. When its shape is elongated in one direction, its orientation depends on its asymmetry in density and shape. The sensitivity of ice crystal orientation to ice crystal asymmetry explains the low percentage of HOICs in the field, because asymmetric ice crystals are common in clouds. As an application, this sensitivity together with the observed percentage of HOICs can be used to infer the processes of ice crystal growth in clouds, providing clues to better representation of ice crystals in weather and climate models.

Investigation on Absorption and Photocurrent in Silicon Absorber with Varied Pyramid Texture Angles by Ray Tracer

In this work, ray tracing is used to investigate the effects of pyramid texture angle towards light absorption and photocurrent in 250 μm-thick crystalline silicon (c-Si) absorber. Upright pyramids with texture angles of 10-50o are investigated. Planar c-Si absorber is used as a reference. When the pyramid angle increases, the broadband reflection reduces due to enhanced light scattering which leads to improved light absorption. At angle of 50o, the weighted average reflection (WAR) reduces to 14.7% and broadband light absorption increases. The optical path length enhancement increases to 12 at wavelength of 1100 nm. The reflection and photogenerated current density (Jg) exhibit an inverse relationship with increasing zenith angle. With increasing zenith angle, the reflection from the c-Si absorber increases and this results in lower light absorption and Jg. In the passivated emitter rear cell (PERC) solar cell, the planar solar cell exhibits short-circuit current density (Jsc) of 26 mA/cm2 with conversion efficiency of 13.6%. When both the pyramids and the silicon nitride (SiNx) anti-reflective coating (ARC) are incorporated on the solar cell, the Jsc increases to 39 mA/cm2 and conversion efficiency increases to 20.5%. This is attributed to the enhanced light-trapping and light-coupling effects in the device.

A new spectral index for estimation of wheat canopy chlorophyll density: considering background interference and view zenith angle effect

A new spectral index for estimation of wheat canopy chlorophyll density: considering background interference and view zenith angle effect

Analysis of Polarization Detector Performance Parameters on Polarization 3D Imaging Accuracy.

Three-dimensional (3D) reconstruction of objects using the polarization properties of diffuse light on the object surface has become a crucial technique. Due to the unique mapping relation between the degree of polarization of diffuse light and the zenith angle of the surface normal vector, polarization 3D reconstruction based on diffuse reflection theoretically has high accuracy. However, in practice, the accuracy of polarization 3D reconstruction is limited by the performance parameters of the polarization detector. Improper selection of performance parameters can result in large errors in the normal vector. In this paper, the mathematical models that relate the polarization 3D reconstruction errors to the detector performance parameters including polarizer extinction ratio, polarizer installation error, full well capacity and analog-to-digital (A2D) bit depth are established. At the same time, polarization detector parameters suitable for polarization 3D reconstruction are provided by the simulation. The performance parameters we recommend include an extinction ratio ≥ 200, an installation error ∈ [-1°, 1°], a full-well capacity ≥ 100 Ke-, and an A2D bit depth ≥ 12 bits. The models provided in this paper are of great significance for improving the accuracy of polarization 3D reconstruction.

Absorbing Aerosol Optical Properties and Radiative Effects on Near-Surface Photochemistry in East Asia

Absorbing aerosols have significant influences on tropospheric photochemistry and regional climate change. Here, the direct radiative effects of absorbing aerosols at the major AERONET sites in East Asia and corresponding impacts on near-surface photochemical processes were quantified by employing a radiation transfer model. The average annual aerosol optical depth (AOD) of sites in China, Korea, and Japan was 1.15, 1.02 and 0.94, respectively, and the corresponding proportion of absorbing aerosol optical depth (AAOD) was 8.61%, 6.69%, and 6.49%, respectively. The influence of absorbing aerosol on ultraviolet (UV) radiation mainly focused on UV-A band (315–400 nm). Under the influence of such radiative effect, the annual mean near-surface J[NO2] (J[O1D]) of sites in China, Korea, and Japan decreased by 16.95% (22.42%), 9.61% (13.55%), and 9.63% (13.79%), respectively. In Beijing–Tianjin–Hebei (BTH) and Yangtze River Delta (YRD) region, the annual average AOD was 1.48 and 1.29, and the AAOD was 0.14 and 0.13, respectively. The UV radiative forcing caused by aerosols dominated by black carbon (BC-dominated aerosols) on the surface was −3.19 and −2.98 W m−2, respectively, accounting for about 40% of the total aerosol radiative forcing, indicating that the reduction efficiency of BC-dominated aerosols on solar radiation was higher than that of other types of aerosols. The annual mean J[NO2] (J[O1D]) decreased by 14.90% (20.53%) and 13.71% (18.20%) due to the BC-dominated aerosols. The daily maximum photolysis rate usually occurred near noon due to the diurnal variation of solar zenith angle and, thus, the daily average photolysis rate decreased by 2–3% higher than that average during 10:00–14:00.

A new method to estimate aerosol radiative forcing on photosynthetically active radiation

A new method to estimate aerosol radiative forcing (ARF) on photosynthetically active radiation (PAR) is proposed using as input only the solar position and global irradiance measurements, available in many radiometric stations worldwide. The main contribution of this work is the proposal of a new and simple tool (max-kt method) based in the parameterization of the envelope of the relationship between clearness index (kt) and the solar position. To this aim, a 1-year database (2020) of cloud-free data acquired in a Southwest Mediterranean site was used for the proposal and two more years (2017 and 2018) were used to extend the results. The ARF values retrieved using the new method were compared with estimates calculated by two physical models widely employed in the literature, such as the SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer model) and an improved version of the CPCR2 (Code for Physical Computation of Radiation, 2 bands) model. The new method provided ARF values of the order of those provided by these physical models, especially with the SBDART model, confirming the validity of this new method. An ARF seasonal pattern was found with higher values in summer, (−30,7 ± 9,0) Wm−2 in August during 2017, (−40,1 ± 11,8) Wm−2 in June during 2018 and (−28,8 ± 7,7) Wm−2 in July during 2020 and minimum values in winter, (−8,1 ± 5,1) Wm−2 in January during 2017, (−5,7 ± 5,8) Wm−2 in December during 2018 and (−7,8 ± 5,4) Wm−2 in December during 2020. Moreover, a dependence on solar zenith angle (θz) was detected excepting during the year 2018, increasing ARF absolute values at θz from 0° to 45°-60° and decreasing to zero for the Sun near the horizon. This technique is very useful due to the difficulty of knowing all the inputs requested by the physical models.

Conceptual design of the optical system of the 6.5m wide field multiplexed survey telescope with excellent image quality

Astrophysics and cosmology in the coming decades urgently need a large field-of-view (FOV), highly multiplexed spectroscopic survey telescope satisfying challenging image quality and stability requirements. The 6.5 m MUltiplexed Survey Telescope (MUST) proposed by Tsinghua University will be constructed on the Saishiteng Mountain of Northwest China to improve the spectroscopic survey capability of ground-based optical telescopes. In this paper, we demonstrate the conceptual design of the optical system of MUST. MUST will adopt a 6.5 m primary mirror, a 2.45 m secondary mirror, and a multiple-element widefield corrector (WFC) to ensure excellent image quality with an 80% encircled energy size of image spots less than ~ 0.6 arcsec in diameter for the entire 3° FOV and the whole 50° zenith angle range. Thanks to its compact 6.5 m Ritchey-Chretien system and 20,000 optical fibers on its Cassegrain focus, MUST will carry out state-of-the-art wide-field spectroscopic surveys with efficiency ~ 19 times higher than the Dark Energy Spectroscopic Instrument (DESI) using a measure proposed by Ellis et al. Upon completion around 2029, MUST will be one of the world's most advanced wide-field spectroscopic survey telescopes and a new essential reference for the future development of wide-field survey telescopes. It will enable significant advances in many fields in astrophysics and cosmology.

Dual-Altitude Band Coverage for Spaceborne Optical Sensor with Field-of-View Constraint

Space-based optical monitoring systems have become promising options for space situational awareness with their advantages of observation range, duration, and quality. The dual-altitude band coverage evaluation of optical sensors is fundamental to designing and optimization of such systems. In this study, the dual-altitude band coverage evaluation of a spaceborne optical sensor with a cone field-of-view (FOV) constraint is addressed. Specifically, the target dual-altitude band region is discretized into crescent-shaped volume cells by introducing equidistant nodes along the radial and azimuth directions. When the size of each cell becomes sufficiently thin, the coverage of this cell can be approximated by the coverage of its centerline. Thus, the original dual-altitude band coverage problem is converted into a one-dimensional zenithal angular coverage problem. Then, considering geometric constraints on Earth’s occlusion and thermal background and the cone FOV constraints, we obtained the effective coverage of the azimuthal and zenithal angles of the target region by performing a comprehensive analysis of all 14 possible geometric cases. Alongside a pure coverage volume scoring method, a weighted coverage scoring method is introduced to take account of the nonuniform density distribution of space objects in altitude. Finally, the accuracy of the proposed method is validated by numerical examples.

Goniometer in the air: Enabling BRDF measurement of crop canopies using a cable-suspended plant phenotyping platform

The Bidirectional Reflectance Distribution Function (BRDF) quantifies the distribution of the spectral reflectance of a target surface at various viewing and illumination angles. In-field measurement of the BRDF of vegetation canopies improves the characterization of off-nadir measurements and informs radiative transfer models of canopy reflectance, where the Lambertian assumption does not hold. However, current field goniometers are unable to measure BRDF efficiently, especially for tall vegetation across the growing season because of the limitations of clearance, field accessibility, and flexibility of the sensor field of view. In this study, we explored the potential of using a large-scale cable-suspended field phenotyping system to quantify BRDF of the canopy reflectance at selected bands and Vegetation Indices (VIs) of maize and soybean canopies. The system performance benefited from the following conditions: no crop damage, full-season field accessibility of tall canopies, automatic measurement, and accurate positioning. Correlation analysis shows that a strong correlation exists among the reflectance, VIs, and the Green Pixel Fraction. The hemispheric distributions of the spectral reflectance at selected bands and VIs were quantified at multiple dates. Hot spots were observed in the backscatter direction at visible and near-infrared (NIR) bands at the largest sensor zenith angle around the Solar Principal Plane (SPP). In contrast, cold spots of the Normalized Difference Vegetation Index (NDVI) and its related VIs were observed in the backscatter direction around solar zenith angles. The row effect was found for the maize canopy at the NIR band and the Near-infrared reflectance of vegetation (NIRv). NDVI had the lowest anisotropy index values among investigated bands and VIs. This system could be further leveraged to generate rapid and detailed BRDF data at a high spatiotemporal resolution using multiple sensors.

A thermal radiation directionality correction method for the surface upward longwave radiation of geostationary satellite based on a time-evolving kernel-driven model

Thermal radiation directionality (TRD) characterizes the anisotropic signature of most surface targets in the thermal infrared domain. It causes significant uncertainties in estimating surface upward longwave radiation (SULR) from space observations. In this regard, kernel-driven models (KDMs) are suitable to remove TRD effects from remote sensing dataset as they are computationally efficient. However, KDMs requires simultaneous multi-angle observations as inputs to be well calibrated, which yields a difficulty with geostationary satellites as they can only provide a single-angle observation. To overcome this issue, we proposed a six-parameter time-evolving KDM that combines a four-parameter SULR diurnal variation model and a two-parameter TRD amplitude model to correct the TRD effect for single-angle estimated SULR dataset of geostationary satellites. The significant daytime TRD effect when solar zenith angle is within 60° can be effectively eliminated. The modeling accuracy of the time-evolving KDM is evaluated using a simulated SULR dataset generated by the 3D Discrete Anisotropic Radiative Transfer (DART) model; the TRD correction method based on the new time-evolving KDM is validated using a two-year single-angle estimated SULR dataset derived from data of the Advanced Baseline Imager (ABI) onboard Geostationary Operational Environmental Satellite-16 (GOES-16) against in situ measurements at 20 AmeriFlux sites. Results show that the proposed time-evolving KDM has a high accuracy with an R2 > 0.999 and a small RMSE = 1.5 W/m2; the TRD correction method based on the time-evolving KDM can greatly reduce the GOES-16 SULR uncertainty caused by the TRD effect with an RMSE decrease of 4.5 W/m2 (22.1%) and mean bias error decrease of 7.9 W/m2 (62.7%). Hence, the proposed TRD correction method is practically efficient for the operational TRD correction of SULR products generated from the geostationary satellites (e.g., GOES-16, FY-4A, Himawari-8, MSG).