This paper introduces a novel waveguide hybrid phased array (HPA) for mobile satellite communications operating in Ku-band. The array is low profile and low cost, adopting an electrical scan in ...elevation and mechanical scan in azimuth to track satellites. High directivity and wide bandwidth impedance is provided by a dual linear polarized waveguide radiating element that covers both Rx and Tx frequencies within the band. The vertical and horizontal polarizations are excited by direct feeding and slot coupled aperture feeding, respectively. The array structure of 32 × 12 elements is split into inclined and interlaced 12 subarrays 225 mm in height. The feed networks and duplexers are designed in unconventional waveguide sizes. In the system, there are 12 independent receiving (Rx) and transmitting (Tx) modules with phased shifter components, which realize beam and polarization tracking. The system is designed to track satellites by scanning 15°-75° (relative to the ground plane) electrically in the elevation and 0°-360° mechanically in the azimuth. The array has been successfully developed and measured. The maximal gain loss in scanning range is about 4.0 dB.
A compact circularly polarized non-resonant slotted waveguide antenna array is proposed with the aim of achieving wide-angle scanning, circular polarization, and low side-lobe levels. The designed ...antenna demonstrates a scanning range of +11° to +13° in the frequency domain and a beam scanning range of -45° to +45° in the phase domain. This design exhibits significant advantages for low-cost two-dimensional electronic scanning circularly polarized arrays. It employs a compact element that reduces the aperture area by 50% compared to traditional circular polarization cavities. Additionally, the staggered array method is employed to achieve an element spacing of 0.57λ within the azimuth plane. Isolation gaps were introduced into the array to enhance the circular polarization performance of non-resonant arrays. The Taylor synthesis method was employed to reduce the side-lobe levels. A prototype was designed, fabricated, and measured. The results indicate superior radiation efficiency, favorable VSWR levels, and an axis ratio maintenance below 3 dB across the scanning range. The proposed antenna and methodology effectively broaden the beam scanning angle of circularly polarized slotted waveguide array antennas.
Nitrogen and phosphorus resorption (NRE and PRE) is a critical nutrient conservation mechanism maintaining plant growth in already disturbed barren ecosystems. The complexity of plant nutrient ...resorption variations in long-term grazing regions is regulated by plant traits, nutritional utilization strategies, and soil conditions following changes in grazing patterns. Therefore, a detailed investigation into their underlying mechanism is still required. Here we investigated leaf nutrient concentration and resorption in dominant species Cleistogenes songorica (C. squarrosa) and Stipa breviflora (S. breviflora) response to 15-years continuous grazing (moderate and heavy grazing) in desert steppe. Moderate grazing enhanced green leaf N and P content in C. songorica and partially increased N content in S. breviflora. Heavy grazing consistently increased N content in C. songorica, but its P content as well as N and P content in S. breviflora were largely stable. Moderate grazing enhanced NRE but unaffected PRE in both S. breviflora and C. songorica. Heavy grazing reduced NRE and PRE in C. songorica. Although soil variables (nutrients and moisture) did not affect foliar nutrients, it's a key driver of nutrient resorption efficiency. Of all measured influence factors, soil moisture is the one most important and negatively correlated with NRE and PRE in S. breviflora. While it was not observed in C. songorica. In S. breviflora, its NRE was adversely linked with soil N, in addition, both NRE and PRE were positively associated with green leaf nutrients. Senesced leaf nutrients are the predominant factor influencing nutrient resorption efficiency in C. songorica, which were adversely associated. Overall, our results indicate significant variations in nutrient resorption efficiency patterns between the two dominant species due to divergent plant adaptation strategies to grazing and the local environment. The foliar nutritional status and soil conditions may play significant roles in regulating nutrient resorption in arid long-term grazing desert steppe.
Grazing exclusion using fences is a key policy being applied by the Chinese government to rehabilitate degraded grasslands on the Tibetan Plateau (TP) and elsewhere. However, there is a limited ...understanding of the effects of grazing exclusion on alpine ecosystem functions and services and its impacts on herders’ livelihoods. Our meta-analyses and questionnaire-based surveys revealed that grazing exclusion with fences was effective in promoting aboveground vegetation growth for up to four years in degraded alpine meadows and for up to eight years in the alpine steppes of the TP. Longer-term fencing did not bring any ecological and economic benefits. We also found that fencing hindered wildlife movement, increased grazing pressure in unfenced areas, lowered the satisfaction of herders, and rendered substantial financial costs to both regional and national governments. We recommend that traditional free grazing should be encouraged if applicable, short-term fencing (for 4–8 years) should be adopted in severely degraded grasslands, and fencing should be avoided in key wildlife habitat areas, especially the protected large mammal species.
It is predicted that climate change may increase the risk of local droughts, with severe consequences for agricultural practices.
Here we report the influence of drought on alfalfa yields and ...nutritional composition, based on artificially induced drought conditions during two field experiments. Two types of alfalfa cultivars were compared, Gold Queen and Suntory. The severity and timing of drought periods were varied, and the crop was harvested either early during flowering, or late at full bloom.
The obtained dry mass yields of Gold Queen were higher than Suntory, and the first was also more resistant to drought. Early harvest resulted in higher yields. Decreases in yields due to water shortage were observed with both cultivars, and the fraction of crude protein (CP) decreased as a result of drought stress; this fraction was higher in Gold Queen than in Suntory and higher in early harvest compared to late harvest. Severe drought late in spring had the highest effect on CP content. The fraction of fibre, split up into neutral detergent fibre (NDF) and acid detergent fibre (ADF) increased as a result of drought and was lower in early compared to late harvested plants. Suntory alfalfa produced higher fibre fractions than Gold Queen. The fraction of water-soluble carbohydrates (WSC) was least affected by drought. It was consistently higher in Gold Queen compared to Suntory alfalfa, and late harvest resulted in higher WSC content.
In combination, these results suggest that the nutritive value of alfalfa will likely decrease after a period of drought. These effects can be partly overcome by choosing the Gold Queen cultivar over Suntory, by targeted irrigation, in particular in late spring, and by harvesting at an earlier time.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Grasslands occupy 40% of the world’s land surface (excluding Antarctica and Greenland) and support diverse groups, from traditional extensive nomadic to intense livestock-production systems. ...Population pressures mean that many of these grasslands are in a degraded state, particularly in less-productive areas of developing countries, affecting not only productivity but also vital environmental services such as hydrology, biodiversity, and carbon cycles; livestock condition is often poor and household incomes are at or below poverty levels. The challenge is to optimize management practices that result in “win-win” outcomes for grasslands, the environment, and households. A case study is discussed from northwestern China, where it has been possible to reduce animal numbers considerably by using an energy-balance/market-based approach while improving household incomes, providing conditions within which grassland recovery is possible. This bottom-up approach was supported by informing and working with the six layers of government in China to build appropriate policies. Further policy implications are considered. Additional gains in grassland rehabilitation could be fostered through targeted environmental payment schemes. Other aspects of the livestock production system that can be modified are discussed. This work built a strategy that has implications for many other grassland areas around the world where common problems apply.
The response of natural vegetation to climate change is of global concern. In this research, changes in the spatial pattern of major terrestrial ecosystems from 1956 to 2006 in Inner Mongolia of ...China were analyzed with the Holdridge Life Zone (HLZ) model in a GIS environment, and net primary production (NPP) of natural vegetation was evaluated with the Synthetic model, to determine the effect of climate change on the ecosystem. The results showed that climate warming and drying strongly influenced ecosystems. Decreased precipitation and the subsequent increase in temperature and potential evapotranspiration caused a severe water deficiency, and hence decreased ecosystem productivity. Climate change also influenced the spatial distribution of HLZs. In particular, new HLZs began to appear, such as Warm temperate desert scrub in 1981 and Warm temperate thorn steppe in 2001. The relative area of desert (Cool temperate desert scrub, Warm temperate thorn steppe, Warm temperate desert scrub, Cool temperate desert and Warm temperate desert) increased by 50.2% over the last half century, whereas the relative area of forest (Boreal moist forest and Cool moist forest) decreased by 36.5%. Furthermore, the area of Cool temperate steppe has continuously decreased at a rate of 5.7% per decade; if the current rate of decrease continues, this HLZ could disappear in 173 years. The HLZs had a large shift range with the mean center of the relative life zones of desert shifting northeast, resulting a decrease in the steppe and forest area and an increase in the desert area. In general, a strong effect of climate change on ecosystems was indicated. Therefore, the important role of climate change must be integrated into rehabilitation strategies of ecosystem degradation of Inner Mongolia.
Nitrogen, as a limiting factor for net primary productivity in grassland ecosystems, is an important link in material cycles in grassland ecosystems. However, the nitrogen assimilation efficiency and ...mechanisms of grassland plants under grazing disturbance are still unclear. This study investigated Stipa breviflora desert steppe which had been grazed for 17 years and sampled the root system and leaf of the constructive species Stipa breviflora during the peak growing season under no grazing, light grazing, moderate grazing and heavy grazing treatments. The activities of enzymes related to nitrogen assimilation in roots and leaves were measured. Compared with no grazing, light grazing and moderate grazing significantly increased the activities of nitrate reductase (NR), glutamine synthetase (GS), glutamic oxaloacetic transaminase (GOT) and glutamic pyruvate transaminase (GPT) in leaves, and GS, GOT and GPT in roots of Stipa breviflora, while heavy grazing significantly decreased the activities of GS in leaves and NR in roots of Stipa breviflora. NR, GOT and GPT activities in leaves and roots of Stipa breviflora were positively correlated with nitrogen content, soluble protein, free amino acid and nitrate content. Grazing disturbance changed the activities of nitrogen assimilation related enzymes of grassland plants, and emphasized that light grazing and moderate grazing were beneficial for nitrogen assimilation by grassland plants. Therefore, establishing appropriate stocking rates is of great significance for material flows in this grassland ecosystem and for the stability and sustainable utilization of grassland resources.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Arbitrage trading is a common quantitative trading strategy that leverages the long-term cointegration relationships between multiple related assets to conduct spread trading for profit. ...Specifically, when the cointegration relationship between two or more related series holds, it utilizes the stability and mean-reverting characteristics of their cointegration relationship for spread trading. However, in real quantitative trading, determining the cointegration relationship based on the Engle-Granger two-step method imposes stringent conditions for the cointegration to hold, which can easily be disrupted by price fluctuations or trend characteristics presented by the linear combination, leading to the failure of the arbitrage strategy and significant losses. To address this issue, this article proposes an optimized strategy based on long-short-term memory (LSTM), termed Dynamic-LSTM Arb (DLA), which can classify the trend movements of linear combinations between multiple assets. It assists the Engle-Granger two-step method in determining cointegration relationships when clear upward or downward non-stationary trend characteristics emerge, avoiding frequent strategy switches that lead to losses and the invalidation of arbitrage strategies due to obvious trend characteristics. Additionally, in mean-reversion arbitrage trading, to determine the optimal trading boundary, we have designed an optimized algorithm that dynamically updates the trading boundaries. Training results indicate that our proposed optimization model can successfully filter out unprofitable trades. Through trading tests on a backtesting platform, a theoretical return of 23% was achieved over a 10-day futures trading period at a 1-min level, significantly outperforming the benchmark strategy and the returns of the CSI 300 Index during the same period.
•The stepwise linear regression is more suitable for estimating typical steppe AGB in northern China at the Sentinel scale.•Both traditional vegetation indices and soil-adjusted vegetation indices ...have similar effects on the estimation of typical steppe AGB in northern China.•Sentinel C-band SAR is not applicable for the estimation of typical steppe AGB in northern China.
Monitoring aboveground biomass (AGB) is crucial for assessing, managing, and utilizing grassland ecosystems. While the technical form of combining remote sensing and machine learning algorithms is widely used to estimate AGB at a regional scale, few studies have assessed and compared the performance of popular algorithms on the typical steppe in northern China. In this study, the northern Xilinhot, a representative area of typical steppe in China, was selected as the study area to compare the performance of six widely used machine learning algorithms for AGB estimation, namely stepwise linear regression (SLR), partial least square regression (PLS), principal component regression (PCR), random forest (RF), support vector machines (SVM), and k-nearest neighbors (KNN). Additionally, the study explored the modeling capability of multisource variables from Sentinel imagery and auxiliary data. The results showed that (1) considering the aspects of prediction accuracy, noise resistance, ease of operation, and transferability, the SLR algorithm is more suitable for estimating typical steppe AGB in northern China at the Sentinel scale. (2) Vegetation Indices (VI) play a significant role in the development of selected models, with significant contributions from both traditional and soil-adjusted indices. (3) Sentinel C-band synthetic aperture radar (SAR) is unsuitable for modeling typical steppe AGB. (4) Among the selected environmental factors, only clay content and soil pH are significantly linearly correlated with AGB, while elevation, precipitation, temperature, soil pH, and sand content are advantageous for RF prediction. This study can provide important technical references for the research on AGB in typical steppe in northern China.
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