# VENuS-Derived NDVI and REIP at Different View Azimuth Angles
###### Abstract  
The bidirectional reflectance distribution function (BRDF) is crucial in determining the quantity of reflected light on the earth's surface as a function of solar and view angles (i.e., azimuth and zenith angles). The Vegetation and ENvironment monitoring Micro-Satellite (VENuS) provides a unique opportunity to acquire data from the same site, with the same sensor, with almost constant solar and view zenith angles from two (or more) view azimuth angles. The present study was aimed at exploring the view angles' effect on the stability of the values of albedo and of two vegetation indices (VIs): the normalized difference vegetation index (NDVI) and the red-edge inflection point (REIP). These products were calculated over three polygons representing urban and cultivated areas in April, June, and September 2018, under a minimal time difference of less than two minutes. Arithmetic differences of VIs and a change vector analysis (CVA) were performed. The results show that in urban areas, there was no difference between the VIs, whereas in the well-developed field crop canopy, the REIP was less affected by the view azimuth angle than the NDVI. Results suggest that REIP is a more appropriate index than NDVI for field crop studies and monitoring. This conclusion can be applied in a constellation of satellites that monitor ground features simultaneously but from different view azimuth angles. vegetation indices; change vector analysis; precision agriculture; BRDF; albedo 1  
M.T.T. seconds of maneuvering, while the camera was in standby mode, the satellite measured the five tiles of the east strip (i.e., E01-E05) in a backward view mode for 20 s. Finally, the 10 tiles of the south strip (i.e., S01-S10) were covered in 30 s in the backward view mode. The time needed to acquire all 27 tile images over the Israeli scientific sites was 161 s. According to the spatial coverage of these strips, there are some overlapping zones between the west and the south strips. The overlapping images can be used to explore the effect of the view azimuth angle on the spectral reflectance acquired from space as one component of the bidirectional reflectance distribution function (BRDF) [2]. The differences between the other three components, i.e., view zenith angle, solar azimuth angle, and solar zenith angle, are assumed to be minimal and, therefore, neglected. The view angle and sun elevation, along with their combination, are well known and have been explored from various spaceborne platforms and observation heights [3; 4; 5]. To the best of our knowledge, the VEN\(\upmu\)S satellite is the only publicly available platform that allows data collection from the same site, with the same sensor, from two (or more) view azimuth angles within minutes. The current study was aimed at exploring the view angle's effect on the stability of the values of vegetation indices (VIs) and albedo due to changes in the view angle under the minimum time gap. It was hypothesized that the view angle would affect VI values and that this effect would vary between ground features. The two selected VIs were the normalized difference vegetation index (NDVI) [6], which is easily available and frequently applied, and the red-edge inflection point (REIP), which is unique to sensors with four red-edge bands (e.g., VEN\(\upmu\)S and Sentinel-2 [7]). The REIP showed better sensitivity to several vegetation properties, such as leaf area index (LAI) [8] and chlorophyll and nitrogen contents [9]. The albedo characterizes the brightness of ground features affected by the BRDF, which was used as a reference for each VI.