**Assessing the Operation Parameters of a Low-altitude UAV for the Collection of NDVI Values Over a Paddy Rice Field**

#### **Rui Jiang 1,2,3,4, Pei Wang 1,3,4, Yan Xu <sup>1</sup> , Zhiyan Zhou 1,3,4,\*, Xiwen Luo 1,3,4, Yubin Lan 3,5 , Genping Zhao <sup>6</sup> , Arturo Sanchez-Azofeifa <sup>2</sup> and Kati Laakso <sup>2</sup>**


Received: 5 May 2020; Accepted: 4 June 2020; Published: 8 June 2020

**Abstract:** Unmanned aerial vehicle (UAV) remote sensing platforms allow for normalized difference vegetation index (NDVI) values to be mapped with a relatively high resolution, therefore enabling an unforeseeable ability to evaluate the influence of the operation parameters on the quality of the thus acquired data. In order to better understand the effects of these parameters, we made a comprehensive evaluation on the effects of the solar zenith angle (SZA), the time of day (TOD), the flight altitude (FA) and the growth level of paddy rice at a pixel-scale on UAV-acquired NDVI values. Our results show that: (1) there was an inverse relationship between the FA (≤100 m) and the mean NDVI values, (2) TOD and SZA had a greater impact on UAV–NDVIs than the FA and the growth level; (3) Better growth levels of rice—measured using the NDVI—could reduce the effects of the FA, TOD and SZA. We expect that our results could be used to better plan flight campaigns that aim to collect NDVI values over paddy rice fields.

**Keywords:** NDVI; solar zenith angle; flight altitude; time of day; operating parameters; UAV
