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This paper demonstrates the ability to generate quantitative remote sensing products by means of an Unmanned Aerial Vehicle (UAV) equipped with one unaltered and one near infrared-modified Commercial Off-The-Shelf (COTS) camera. Radiometrically calibrated orthomosaics were generated for 17 dates, from which digital numbers (DNs) were corrected to surface reflectance and to Normalized Difference Vegetation Index (NDVI). Validation against ground measurements showed that 84-90% of the variation in the ground reflectance and 95-96% of the variation in the ground NDVI could be explained by the UAV-retrieved reflectance and NDVI respectively. Comparisons against Landsat 8 data showed relationships of 0.73≤R 2 ≥0.84 for reflectance and 0.86≤R 2 ≥0.89 for NDVI. It was not possible to generate a fully consistent time series of reflectance, due to variable illumination conditions during acquisition on some dates. However, the calculation of NDVI resulted in a more stable UAV time series, which was consistent with a Landsat series of NDVI extracted over a deciduous and evergreen woodland. The results confirm that COTS cameras, following calibration, can yield accurate reflectance estimates (under stable within-flight illumination conditions), and that consistent NDVI time series can be acquired in very variable illumination conditions. Such methods have significant potential in providing flexible, low-cost approaches to vegetation monitoring at fine spatial resolution and for usercontrolled revisit periods.

ABSTRACT:Commercial off-the-shelf (COTS) digital cameras on-board unmanned aerial vehicles (UAVs) have the potential to be used as multispectral imaging systems; however, their spectral sensitivity is usually unknown and needs to be either measured or estimated. This paper details a step by step methodology for identifying the spectral sensitivity of modified (to be response to near infra-red wavelengths) and un-modified COTS digital cameras, showing the results of its application for three different models of camera. Six digital still cameras, which are being used as imaging systems on-board different UAVs, were selected to have their spectral sensitivities measured by a monochromator. Each camera was exposed to monochromatic light ranging from 370 nm to 1100 nm in 10 nm steps, with images of each step recorded in RAW format. The RAW images were converted linearly into TIFF images using DCRaw, an open-source program, before being batch processed through ImageJ (also open-source), which calculated the mean and standard deviation values from each of the red-green-blue (RGB) channels over a fixed central region within each image. These mean values were then related to the relative spectral radiance from the monochromator and its integrating sphere, in order to obtain the relative spectral response (RSR) for each of the cameras colour channels. It was found that different un-modified camera models present very different RSR in some channels, and one of the modified cameras showed a response that was unexpected. This highlights the need to determine the RSR of a camera before using it for any quantitative studies.