Research On Wind Field Distribution Of Multi-rotor UAV And Its Influence On Spectral Information Acquisition Of Crop Canopy

With the popularization of using unmanned aerial vehicle（UAV）as remote sensing platform,UAV combined with spectral information acquisition and analysis has gradually become an effective way for monitoring agricultural conditions and assessing the growth of rice crops.The wind field generated by the multi-rotor UAV in low-altitude operation will cause the crops to be deformed,and may even affect the true spectral reflectance of the crop canopy.Rice and rape were chosen as the research object in this study.And a low-altitude UAV remote sensing simulation platform and a triple-direction wind field wireless sensor network system were built to explore the wind field distribution law of multi-rotor UAV and its influence on the collection of crop canopy spectral information.Combining with the multi-spectral image of rice canopy under different measurement points,the influence of the triple-direction wind field on the spectral information acquisition was analyzed through variance and regression analysis.The specific research contents and results were as follows:（1）A triple-direction wind field wireless sensor network system based on the ZigBee module with CC2530 chip was developed.Using the QT framework to design and develop the PC software"Anemometer"for collecting,real-time recording and storing wind speed data.The reliability test results of the system showed that the average relative error of the data is 1.05%,the average sampling frequency of a single sensor is 1.98 Hz,and the theoretical sampling frequency is 2 Hz.The stability of the system met with the test requirements,and the wind field data could be collected stably by using this system.The system can provide effective data support for the study of wind field distribution law of multi-rotor UAV and its influence on crop canopy spectral information collection.（2）In the linear array triple-direction wind field,for each sampling node,the change trend of wind speed in three directions was basically the same at different heights,but the Z-direction wind speed values were significantly larger than the X-direction and Y-direction wind speed values.The triple-direction wind speed at No.5sampling node is the largest among the 9 sampling nodes,and the trend is decremented from the center（No.5 sampling node）to both sides.The closer the location to the center,the higher wind speed value was,while the smaller the wind field width was.The wind speed of X and Y direction at the height of 50 cm above the ground were close to zero on a large scale,and only partial wind field distribution of smaller than 2 m/s had a width of about 1 m.The wind speed in the region of 0.3 to 0.6 m away from the No.5sampling node tends to be zero,and the range of the global wind field varies periodically from 0.9 m to 1.2 m.（3）Using the wind field data in low-speed area for linear regression analysis,the regression models of the Z-direction wind speed of the UAV wind field and the NDVI,RVI and difference_CCR of rice canopy were obtained.The goodness of fit R2 was 0.7820,0.7486 and 0.5268,respectively.The Z-direction wind speed and three indicators(NDVI,RVI and difference_CCR)were further analyzed by ANOVA.The Sig.values of three linear regression models(NDVI,RVI and difference _CCR)were 0.11574,0.13477and 0.27420,respectively.Yet the significance level was 0.05,so the effect of Z-direction wind speed on rice canopy NDVI,RVI and difference _CCR was not significant in the low-speed region of the wind field.（4）Univariate multivariate analysis of variance was performed on the rape canopy NDVI and RVI data under different hovering height and throttle ratio.The result showed that the h-Sig.values of ANOVA for both NDVI and RVI of rape canopy under different hovering height were more than 0.05.The p-Sig.value of ANOVA for NDVI and RVI of rape canopy under different throttle ratio were 0.008 and 0.021（smaller than 0.05）,respectively.Therefore,the hovering height of the UAV had no significant effect on the canopy NDVI and RVI of rape under any measuring node while the throttle ratio has a significant effect on the canopy NDVI and RVI of rape.The research showed that the triple-direction wind field wireless sensor network system developed in this study could collect wind field data efficiently and stably.Under the conditions of this study,the multi-rotor UAV wind field would have an influence on the acquisition of rape and rice canopy spectral information（NDVI,RVI）,which will ultimately affect the assessment of the agricultural information and the growth status of crops.The wind field and crop vegetation index model established in this study could provide data support for modified model of crop spectral information.So as to obtain more accurate agricultural information and provide further guidance for actual low-altitude remote sensing operations.The experimental results showed that the triple-direction wind field wireless sensor network system developed in this study could collect wind field data efficiently and stably.Based on the low-altitude UAV remote sensing simulation system and the F450 quadrotor,the spectral information（NDVI,RVI）collection of the rape and rice canopy woud get influenced by the multi-rotor UAV wind field under the condition of a hovering height in the range of 145 cm to 195 cm and a throttle ratio greater than 0.78.These influences,in turn,would affect the judgments on crop growth and agricultural information of decision makers.The wind field and crop vegetation index model established in this study could provide data support for the modified model of crop spectral information,so as to obtain more accurate agricultural information and provide scientific operation guidance for UAV-based crop growth monitoring.