Low Sun Elevation Research Articles

Light Transfers Through a Koch Shape Cloud

AbstractModeling radiative transfer in a 3D cloudy atmosphere is critical to climate projections. A recently developed fast 3D radiation parameterization scheme gains some success in quantifying horizontal radiative transfer through cloud sides using cloud area fraction. Based on 3D Monte Carlo simulations of radiative transfer through an idealized single‐layer cloud with Koch‐shaped fractal geometry edges, here we show that radiative energy transport through cloud sides correlates more significantly with cloud area fraction than with cloud perimeter length. The results exemplify the importance of accounting for the horizontal radiative energy exchanges between cloud‐free and cloudy regions with cloud area fraction. Results from additional sensitivity simulations show that increased cloud vertical extent often enhances cloud‐side sunlight leak more significantly than cloud‐side sunlight interception. At low sun elevations, cloud‐side sunlight interception is enhanced more than cloud‐side sunlight leak does with the increase of cloud mass.

Estimation of mean radiant temperature across diverse outdoor spaces: A comparative study of different modeling approaches

The mean radiant temperature (Tmrt) is an important environmental parameter that affects the thermal comfort of human beings. However, both the measurement and estimation of Tmrt have some inherent challenges. This study evaluated the performance of five methods in estimating Tmrt, using data at 670 locations across 14 representative urban forms in Hong Kong. The evaluated methods include the customized globe thermometer method, recalibrated globe thermometer method, SOLWEIG simulation method, regression model, and neural network model. Values calculated from the integral radiation method were used as references for comparison. Results indicate that the customized and recalibrated globe thermometer methods and the SOLWEIG model consistently underestimate Tmrt throughout most of the day, with substantial errors observed at low sun elevations and sunlit sites. The regression model provides a moderate fit to the data. The deep neural network (DNN) model yields the highest estimation accuracy, with an R2 of 0.878 and a root mean square error (RMSE) of 1.92 °C. To assess the generalizability of the DNN model, an additional dataset from Singapore is employed, including hourly meteorological data from 28 measurement stations over a two-year period. The DNN model demonstrates strong consistency between modeled Tmrt and the reference across most of the sites, affirming its effectiveness in estimating Tmrt in complex urban environments.

Building integrated innovative non-imaging concentrating solar collector for clean cooking under low sun elevation

This paper aims to report the building-integrated innovative non-imaging solar thermal collector (BISTC) for cooking, which is constructed and tested under field conditions in two configurations. A ray tracing analysis enables an understanding of the optics of the BISTC.The results show that BISTC attained a temperature in the range of 110–124 °C under low sun elevation and ensures the potential for better cooking performance under high sun elevation. The mean values of the Cooker Opto-thermal Ratio (COR) for BISTC1 and BISTC2 are 0.109 and 0.116 ((m2-°C)/W), respectively. Typical theoretical and experimental values of maximum achievable load temperature (Tfmax) are 129.8 °C and 119 °C and that for reference time (τR) are 77 and 76 min, respectively, for BISTC1. The typical theoretical and experimental values of Tfmax are 137.5 °C and 124.7 °C and that for τR are 80.7 and 75 min, respectively, for BISTC2.This work provides vital information about the opto-thermal behavior of the non-tracking type solar cooker design, i.e., BISTC, which can lead to the creation of similar systems for regular or building-integrated solar cooking. Thus, the BISTC is one of the promising technologies to be promoted with several future-optimized designs to fulfill sustainable development goals.

Estimating leaf moisture content at global scale from passive microwave satellite observations of vegetation optical depth

Abstract. The moisture content of vegetation canopies controls various ecosystem processes such as plant productivity, transpiration, mortality, and flammability. Leaf moisture content (here defined as the ratio of leaf water mass to leaf dry biomass, or live-fuel moisture content, LFMC) is a vegetation property that is frequently used to estimate flammability and the danger of fire occurrence and spread, and is widely measured at field sites around the globe. LFMC can be retrieved from satellite observations in the visible and infrared domain of the electromagnetic spectrum, which is however hampered by frequent cloud cover or low sun elevation angles. As an alternative, vegetation water content can be estimated from satellite observations in the microwave domain. For example, studies at local and regional scales have demonstrated the link between LFMC and vegetation optical depth (VOD) from passive microwave satellite observations. VOD describes the attenuation of microwaves in the vegetation layer. However, neither were the relations between VOD and LFMC investigated at large or global scales nor has VOD been used to estimate LFMC. Here we aim to estimate LFMC from VOD at large scales, i.e. at coarse spatial resolution, globally, and at daily time steps over past decadal timescales. Therefore, our objectives are: (1) to investigate the relation between VOD from different frequencies and LFMC derived from optical sensors and a global database of LFMC site measurements; (2) to test different model structures to estimate LFMC from VOD; and (3) to apply the best-performing model to estimate LFMC at global scales. Our results show that VOD is medium to highly correlated with LFMC in areas with medium to high coverage of short vegetation (grasslands, croplands, shrublands). Forested areas show on average weak correlations, but the variability in correlations is high. A logistic regression model that uses VOD and additionally leaf area index as predictor to account for canopy biomass reaches the highest performance in estimating LFMC. Applying this model to global VOD and LAI observations allows estimating LFMC globally over decadal time series at daily temporal sampling. The derived estimates of LFMC can be used to assess large-scale patterns and temporal changes in vegetation water status, drought conditions, and fire dynamics.

The effect of partial loads on the performance of a funnel solar cooker

Several solar cooker designs have been proposed over the last decades. The funnel cooker is a well-known model, representative of the category of panel-type solar cookers. From the empirical experience of real practice, it is known that the use of partial loads has an impact on the performance of the cooker. However, this effect has not yet been investigated rigorously. This work aims to fill this lack. Extensive experimental work was performed to determine the effect of partial loads on a funnel cooker thermal performance. Tests were conducted on two identical funnel cookers, in Malaga, Spain, with low sun elevation. Cookers were tracked only azimuthally. Experimental protocol was based on ASAE 580.1 Standard for better replicability. Results showed that the standardised power drops by about 15% of the original value when the water volumetric load fraction drops by 25%, for both cooker operations, with and without glass enclosure. This important reduction is explained on how the fill level of the cooking vessel affects its function as a thermal radiation receiver. Results from experimental tests were correlated into a simple formula of practical interest. Finally, a new cooking vessel design, that improves performance at partial loads, i.e., a 25.4% increase in cooker power, was proposed and tested.

Analysis of boulders population around a young crater using very high resolution image of Orbiter High Resolution Camera (OHRC) on board Chandrayaan-2 mission

The Orbiter High Resolution Camera (OHRC) is a very high spatial resolution panchromatic camera (0.45–0.70 μm) on-board Chandrayaan-2 orbiter. Its spatial resolution of 0.25 m from 100 km altitude is highest among all lunar orbiter missions. A simple crater with substantial boulder population was observed in an OHRC image of a region near Boguslawsky E crater. Boulders are distinctly seen in this image because of high spatial resolution of 0.28 m and low sun elevation angle (6°) which enhanced the boulders' shadows. We have identified and mapped >2000 boulders around this young un-named simple crater (74.9216° S, 54.5148° E). It is observed that the OHRC is capable of extending the lower limit of size for identifiable boulders below 1 m. The distributions of mapped boulders are studied and compared with previous studies. It was found that the coefficient values estimated by fitting power laws to various distributions, such as size-frequency, size-range, etc., are well within the ranges reported in literature for craters distributed on lunar surface around the landing sites. Weibull distribution was also fit to the data, and the fitting coefficients were compared with the values obtained in similar studies. The crater age was estimated to be in the range of 50‐90 Ma using empirical relations, and comparison with areal density of other craters near lunar landing sites. This study also provides a glimpse of the low-light imaging capability of the OHRC showing inside the shadow regions, which were illuminated by reflected light from adjoining areas.

Performance of solar funnel cookers using intermediate temperature test load under low sun elevation

Solar funnel cookers can be designed as attractive and affordable low-cost devices accessible to people in all continents of the world regardless of their walk-in life. The present work is aimed to investigate the applicability of funnel cookers to attain the temperature in excess of the boiling point of water. It widens their acceptability for cooking and opens new opportunities for technological developments in such designs of solar cookers. Two identical designs of funnel cookers, FC1 and FC2, have been tested with appropriately sized identical cooking pots and glass enclosure to serve the purpose. Cooker FC2 is tested with a glass enclosure only, while a glass enclosure is not used in FC1. Glycerine is used as a test load. Cooker Opto-thermal Ratio (COR) as a thermal performance parameter and overall cooking efficiency are used to compare the performance of the two funnel cookers. It is shown that the temperature of the test load in the cooker FC2 can reach 140 to 150 °C. So, this funnel cooker design can be preferred for cooking food at a relatively high temperature over the boiling point of water. The experimental results show that: i) the mean values of the Cooker Opto-thermal Ratio for cookers FC2 and FC1 are estimated to be 0.157 and 0.110 (m2°C)/W, respectively, and ii) the values of the overall cooking efficiency for cookers FC2 and FC1 are estimated to be 11.8% and 10.2%, respectively.

An Economic Analysis of the Shading Effects of Transmission Lines on Photovoltaic Power Plant Investment Decisions: A Case Study.

In today’s photovoltaic (PV) power plants, traditional crystalline PV modules are the prevalent technology, which is highly susceptible to partial shading due to the risk of irreversible damage. Therefore, it is advisable to explore potential construction sites for objects that might cause shading, including high-voltage transmission towers, whose shading effects can be significant due to their height. By means of innovative simulation, using a model, validated with actual data, this study endeavored to deliver novel information related to the problems of shading by high-voltage transmission lines. In the context of Hungary, it examined the risk factors, technical and economic aspects, and possible solutions important for PV projects. It provides new insight, much needed also at the international level, considering the fact that the extent of the shadows cast by conductors on the surface at low Sun elevations is not known at present and neither are the shading characteristics of conductors between two transmission towers, depending on their height, in winter, when the Sun is low. An added practical benefit of the study is that its technical and economic approaches and the software solutions are all based on the practice of PV system design and construction. Related to the investigated issues, this can facilitate the formulation of the technical and economic aspects of suitable PV power plant building strategies in Hungary.

Characteristics of phase bias from CNES and its application in multi-frequency and multi-GNSS precise point positioning with ambiguity resolution

While precise point positioning ambiguity resolution (PPP AR) is a valuable tool of the multi-constellation global navigation satellite system (multi-GNSS), phase biases are critical to implement PPP AR. Multi-frequency phase biases and satellite attitude files are provided freely by Centre National d’Etudes Spatiales (CNES), which are estimated based on GeoForschungs Zentrum (GFZ) satellite rapid orbit and clock products. However, the temporal characteristics of these phase biases and their positioning performance in the multi-frequency and multi-GNSS PPP AR have not been investigated yet, especially in the low sun elevation and satellite maneuver period. We introduce the transformation between multi-frequency phase biases and integer recovery clock model. In this transformation, inter-frequency clock biases (IFCBs) and inconsistencies in satellite attitude model errors between GFZ and CNES products are well considered. Experiments with GPS/Galileo/BeiDou observations from 34 stations were performed in static and kinematic modes, and the multi-frequency phase residuals were analyzed in the low sun elevation. Our results show that the impact of IFCBs and inconsistencies in satellite attitude errors could be mitigated at the user ends by using phase biases and satellite attitude files. Under the condition of satellite reverse yaw maneuvers, the performance of kinematic PPP without phase biases or deleting maneuvering satellites would be degraded significantly until the end of satellite observation arc or the next reverse yaw maneuver occurs. By applying phase biases with PPP AR, the positioning accuracy could be improved by 34.4%, 23.1%, and 37.4% in the east (E), north (N), and up (U) directions, respectively. Therefore, we suggested that PPP users should apply phase biases and satellite attitude files when using the GFZ rapid orbit and clock products, especially for satellite maneuvers and low sun elevation.

Experimental determination of the standardised power of a solar funnel cooker for low sun elevations

Over the past few decades, many efficient solar cooker designs have evolved. It is important to measure their performance and communicate this information to researchers, manufacturers and users around the world. Funnel type solar cookers are simple, effective and have multiple advantages in practical use, but there is limited experimental data about them in the academic literature. This work provides performance data for a well-known design of funnel cooker. When possible, the experimental protocol was based on the ASAE S580.1 Standard. The following procedures were added to improve the quality and reliability of the measured data: i) two identical funnel cookers were tested simultaneously, side by side, to control for weather variables, ii) a large number of observations were made iii) confidence and prediction intervals were calculated for the main figures of merit iv) cooled water was used to extend the range of valid observations. Tests were conducted in Malaga, Spain, at latitude 36.9° N, between November 2019, and February 2020, a period with low sun elevation. Azimuthal solar tracking was applied. Two different pot lids were compared to investigate their influence on the standardised cooker power obtained.

Parameterizing anisotropic reflectance of snow surfaces from airborne digital camera observations in Antarctica

Abstract. The surface reflection of solar radiation comprises an important boundary condition for solar radiative transfer simulations. In polar regions above snow surfaces, the surface reflection is particularly anisotropic due to low Sun elevations and the highly anisotropic scattering phase function of the snow crystals. The characterization of this surface reflection anisotropy is essential for satellite remote sensing over both the Arctic and Antarctica. To quantify the angular snow reflection properties, the hemispherical-directional reflectance factor (HDRF) of snow surfaces was derived from airborne measurements in Antarctica during austral summer in 2013/14. For this purpose, a digital 180∘ fish-eye camera (green channel, 490–585 nm wavelength band) was used. The HDRF was measured for different surface roughness conditions, optical-equivalent snow grain sizes, and solar zenith angles. The airborne observations covered an area of around 1000 km × 1000 km in the vicinity of Kohnen Station (75∘0′ S, 0∘4′ E) at the outer part of the East Antarctic Plateau. The observations include regions with higher (coastal areas) and lower (inner Antarctica) precipitation amounts and frequencies. The digital camera provided upward, angular-dependent radiance measurements from the lower hemisphere. The comparison of the measured HDRF derived for smooth and rough snow surfaces (sastrugi) showed significant differences, which are superimposed on the diurnal cycle. By inverting a semi-empirical kernel-driven bidirectional reflectance distribution function (BRDF) model, the measured HDRF of snow surfaces was parameterized as a function of solar zenith angle, surface roughness, and optical-equivalent snow grain size. This allows a direct comparison of the HDRF measurements with the BRDF derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite product MCD43. For the analyzed cases, MODIS observations (545–565 nm wavelength band) generally underestimated the anisotropy of the surface reflection. The largest deviations were found for the volumetric model weight fvol (average underestimation by a factor of 10). These deviations are likely linked to short-term changes in snow properties.

Little auks under the midnight sun: diel activity rhythm of a small diving seabird during the Arctic summer

Many animal species exhibit a diel, 24-hr pattern of activity, which is steered by timing cues, with the daily light–dark cycle considered the most powerful. This cue, however, is reduced in polar zones under continuous daylight conditions associated with the midnight sun. The rhythm of animal behaviour under such conditions is poorly understood. Here, we examine periodicity and patterns of daily activity (colony attendance and foraging) in a High-Arctic seabird, the little auk (Alle alle). We demonstrated a regular rhythm of colony attendance at the population level, with birds being the most abundant in the colony during hours of relatively low sun elevation. This pattern is likely to be associated with predation pressure that may be perceived by birds as lower during hours with low sun elevation, because of better predator detectability. Regarding rhythms at an individual level, however, we found the most common periodicity to be 23.2 hr (range from 19.9 hr to 30.8 hr) but no clear pattern of daily colony attendance of individuals. Such a flexibility in daily rhythms indicates that individuals may become arrhythmic in regard to the 24-hr environmental cycle, despite regularities observed at the population level. Finally, we compared males and females in terms of daily activity patterns but we did not find significant sex differences.

Little auks under the midnight sun: diel activity rhythm of a small diving seabird during the Arctic summer

Many animal species exhibit a diel, 24-hr pattern of activity, which is steered by timing cues, with the daily light–dark cycle considered the most powerful. This cue, however, is reduced in polar zones under continuous daylight conditions associated with the midnight sun. The rhythm of animal behaviour under such conditions is poorly understood. Here, we examine periodicity and patterns of daily activity (colony attendance and foraging) in a High-Arctic seabird, the little auk (Alle alle). We demonstrated a regular rhythm of colony attendance at the population level, with birds being the most abundant in the colony during hours of relatively low sun elevation. This pattern is likely to be associated with predation pressure that may be perceived by birds as lower during hours with low sun elevation, because of better predator detectability. Regarding rhythms at an individual level, however, we found the most common periodicity to be 23.2 hr (range from 19.9 hr to 30.8 hr) but no clear pattern of daily colony attendance of individuals. Such a flexibility in daily rhythms indicates that individuals may become arrhythmic in regard to the 24-hr environmental cycle, despite regularities observed at the population level. Finally, we compared males and females in terms of daily activity patterns but we did not find significant sex differences.

Modeling mean radiant temperature in outdoor spaces, A comparative numerical simulation and validation study

The parameter governing outdoor human thermal comfort (HTC) on warm, clear-sky days is radiation. Its effect on HTC is accounted for by mean radiant temperature (Tmrt). While Tmrt differences owing to different measurement methods are well established, the impact of different computational approaches have not been systematically evaluated. This study assesses the performance of three microclimate models in their ability to estimate Tmrt values in complex urban environments. The evaluated models are RayMan Pro, SOLWEIG and ENVI-met. The model evaluation encompasses both the comparison of modeled Tmrt values with those derived from observations and model intercomparisons with analyses extending to several radiation terms and parameters that comprise or explain the resultant Tmrt. Results indicate that the models systematically underestimate nighttime Tmrt. SOLWEIG and ENVI-met tend to overestimate Tmrt during prolonged periods of shade and underestimate when the sites are sunlit. RayMan underestimates Tmrt values during most part of the day. The largest Tmrt errors occur at low sun elevations in all three models, mainly as a result of underestimated longwave emitted and shortwave reflected radiation fluxes by the adjacent facades. These errors indicate room for improvement with regards to surface temperature estimation and shortwave reflected radiation calculations in the models.

Theoretical study on energy performance of a stratospheric solar aircraft with optimum Λ-shaped rotatable wing

The sun-tracking rotatable wing can be used to improve energy performance of solar aircraft at low sun elevation angles. This work mainly focuses on studying the energy performance of a symmetric Λ-shaped rotatable wing solar aircraft through developing net energy optimization model, which considers the coupling of additional solar energy conversion and extra energy consumption. The derived energy conversion models reveal that, only when the morphing angle is larger than the sun elevation angle, the Λ-shaped solar aircraft is capable of achieving more energy conversion than that of planar wing. The optimum flight attitude control is adjusting its yawing angle to make wingspan axis coincide with the horizontal projection of sunlight. The numerical results of energy performance demonstrate that, with solar cell efficiency of 0.3, the optimum Λ-shaped solar aircraft can achieve 27 kWh more net energy nearby 55°N in winter, corresponding to more than 8 hours increase of flight endurance. Meanwhile, the yearly maximum perpetual flight date at 65°N increases by 34 days. In addition, the Λ-shaped solar aircraft is more effective to improve energy performance with higher conversion efficiency of solar cell. For the solar cell efficiency of 0.45, the net energy is more than 45 kWh and the increment of maximum flight latitude is 10.7° on winter solstice. The results indicate that the optimum Λ-shaped rotatable wing is an effective method to improve energy performance of solar aircrafts.

Satellite-derived mapping of kelp distribution and water optics in the glacier impacted Yendegaia Fjord (Beagle Channel, Southern Chilean Patagonia)

Giant kelp Macrocystis pyrifera is a brown alga with extensive global distribution, however, recent evidence suggests that its dynamics presents high degree of regional variability. In southern Chilean fjord region, largely unexplored kelp forests are currently being threatened by global change and human impacts. High-resolution satellite (Sentinel-2) imagery was used to describe temporal and spatial distribution patterns of kelp beds in Yendegaia Fjord (Beagle Channel) using Spectral Mixture Analysis (SMA), and to characterize water optical gradients of this habitat strongly influenced by river runoff from a melting glacier. The suitability of SMA for kelp classification was contrasted with other vegetation indices (NDVI, EVI, FAI). Validation was made using drone aerial photographs of kelp canopies. Different analysis tools resulted in up to 35% difference in kelp coverage estimation. The overall accuracy (66–82%) of kelp classification followed an order FAI < EVI < NDVI < SMA. Omission error of SMA and lower coincidence with vegetation indices occurred in pixels with low kelp pixel abundance (<0.50). Based on SMA, the lowest kelp abundance was observed in the river mouth with high turbidity, increasing towards the Beagle Channel. The highest kelp abundance was observed in late summer, but otherwise no clear seasonal patterns could be observed. Water turbidity presented both spatial and seasonal variation. Strong particle sedimentation (leading to light attenuation, interference with remote detection of kelps, and even to their detachment due to substrate quality) and tidal fluctuations in glacier-impacted fjord-type environments can be identified as key features affecting both the kelp population dynamics as well as their remote sensing. Also, low sun elevation at high latitudes in mid winter produces uncertainties in image analyses. In all, the remote sensing approach used in the present study can be regarded as a useful tool to map and monitor kelps forests from a remote region.

Effect of solar cell efficiency and flight condition on optimal flight control and energy performance for Z-shaped wing stratospheric solar aircraft

The parameters of solar cell efficiency, flight date of year, latitude and sun elevation angle are important to energy conversion of solar cells, which affect optimal flight control and energy performance of Z-shaped solar aircraft. This work is mainly focusing on studying the effect of varying these parameters on optimum attitude planning, optimum wing morphing control and maximum net energy input for Z-shaped solar aircraft. This study shows that, with higher solar cell efficiency, the solar aircraft prefers to more daylight hours configured as Z-shaped wing, larger morphing angle and larger angle between projection of sunlight and wingspan axis, which enhances to achieve more net power input. For the middle and high latitudes in winter with low sun elevation angles, the Z-shaped solar aircraft tends to large morphing angles and small angles between projection of sunlight and wingspan axis. For the low and middle latitudes of the other seasons with high sun elevation angles, it prefers to small morphing angles to make its central wing totally expose to the sunlight, and orient flight attitude to make the projection of sunlight coincide with wingspan axis. Moreover, the research on the impact of solar cell efficiency shows that, compared with the planar wing, the optimal Z-shaped wing is more effective to improve energy margin and extend perpetual flight days with the same increase of solar cell efficiency. The results can provide valuable engineering reference on optimum wing morphing control and optimum attitude planning for Z-shaped stratospheric solar aircraft.

Decoupling light harvesting, electron transport and carbon fixation during prolonged darkness supports rapid recovery upon re-illumination in the Arctic diatom Chaetoceros neogracilis

During winter in the Arctic marine ecosystem, diatoms have to survive long periods of darkness caused by low sun elevations and the presence of sea ice covered by snow. To better understand how diatoms survive in the dark, we subjected cultures of the Arctic diatom Chaetoceros neogracilis to a prolonged period of darkness (1 month) and to light resupply. Chaetoceros neogracilis was not able to grow in the dark but cell biovolume remained constant after 1 month in darkness. Rapid resumption of photosynthesis and growth recovery was also found when the cells were transferred back to light at four different light levels ranging from 5 to 154 µmol photon m−2 s−1. This demonstrates the remarkable ability of this species to re-initiate growth over a wide range of irradiances even after a prolonged period in the dark with no apparent lag period or impact on survival. Such recovery was possible because C. neogracilis cells preserved their Chl a content and their light absorption capabilities. Carbon fixation capacity was down-regulated (ninefold dark decrease in $$P_{\text{m}}^{\text{C}}$$ ) much more than was the photochemistry in PSII (2.3-fold dark decrease in ETRm). Rubisco content, which remained unchanged after one month in the dark, was not responsible for the decrease in $$P_{\text{m}}^{\text{C}}$$ . The decrease in PSII activity was partially related to the induction of sustained non-photochemical quenching (NPQ) as we observed an increase in diatoxanthin content after one month in the dark.

Energy optimization and investigation for Z-shaped sun-tracking morphing-wing solar-powered UAV

To achieve perpetual flight, solar-powered UAVs (SPUAVs) need to convert sufficient solar energy into electricity during daylight flight. However, perpetual flight at middle and high latitude regions in winter days still poses great challenges for gaining enough solar energy due to the low sun elevation angle and short daylight duration. Among measures for flight endurance enhancement, tracking the sun through changing geometry of wing and orienting flight direction may be an effective design scheme for gaining more solar energy. In this paper, the energy optimization method for Z-shaped sun-tracking morphing-wing SPUAV is developed and used to investigate the advantages on energy performance of the optimal Z-shaped wing over the conventional planar wing. Energy optimization for a whole year's flight shows that the optimal Z-shaped wing is capable of achieving longer flight endurance, improving perpetual flight latitude and extending perpetual flight date efficiently compared with the planar wing, especially for the middle and high latitude regions during winter days. The investigation on implications of conversion efficiency of solar cells indicates that, with higher conversion efficiency of solar cells, the optimal Z-shaped wing is more effective to collect more net energy, retain more energy margin and achieve perpetual flight at higher latitude. The Z-shaped sun-tracking morphing-wing design combined with the energy optimization method proposed is helpful to increase flight endurance and improve perpetual flight latitude for SPUAVs.

Yield Estimates by a Two-Step Approach Using Hyperspectral Methods in Grasslands at High Latitudes

Ruminant fodder production in agricultural lands in latitudes above the Arctic Circle is constrained by short and hectic growing seasons with a 24-hour photoperiod and low growth temperatures. The use of remote sensing to measure crop production at high latitudes is hindered by intrinsic challenges, such as a low sun elevation angle and a coastal climate with high humidity, which influences the spectral signatures of the sampled vegetation. We used a portable spectrometer (ASD FieldSpec 3) to assess spectra of grass crops and found that when applying multivariate models to the hyperspectral datasets, results show significant predictability of yields (R2 &gt; 0.55, root mean squared error (RMSE) &lt; 180), even when captured under sub-optimal conditions. These results are consistent both in the full spectral range of the spectrometer (350–2500 nm) and in the 350–900 nm spectral range, which is a region more robust against air moisture. Sentinel-2A simulations resulted in moderately robust models that could be used in qualitative assessments of field productivity. In addition, simulation of the upcoming hyperspectral EnMap satellite bands showed its potential applicability to measure yields in northern latitudes both in the full spectral range of the satellite (420–2450 nm) with similar performance as the Sentinel-2A satellite and in the 420–900 nm range with a comparable reliability to the portable spectrometer. The combination of EnMap and Sentinel-2A to detect fields with low productivity and portable spectrometers to identify the fields or specific regions of fields with the lowest production can help optimize the management of fodder production in high latitudes.