Imaging Spectroscopy

Imaging spectrometer

The Headwall Nano-Hyperspec is a compact imaging spectrometer designed specifically for UAV-based remote sensing applications. It utilizes CMOS (Complementary Metal-Oxide Semiconductor) technology, making it lightweight (0.52 kg) and compact, with dimensions of 76.2 mm x 76.2 mm x 119.9 mm. It captures radiance in the form of electromagnetic energy reflected by objects within the scene. The sensor operates as a push-broom imager, acquiring images line by line in the direction of flight, with each line recorded instantaneously. Each pixel row consists of 640 spatial pixels and 270 spectral bands, yielding a total of 172,800 pixel-wavelength combinations. Data are recorded at 12-bit, allowing for 4,096 levels of brightness, across the visible and near-infrared spectrum, ranging from 400 nm to 1000 nm, with a sampling interval of 2.2 nm and a full width at half maximum (FWHM) of 6 nm. 

UAV platform

The DJI Matrice 300 RTK (M300 RTK) is a commercial-grade quadcopter platform designed for industrial applications such as aerial surveying, inspection, search and rescue, and precision agriculture. Released in 2020, it is one of DJI’s flagship enterprise drones, recognized for its versatility, durability, and advanced capabilities. Equipped with Real-Time Kinematic (RTK) GPS technology, the M300 RTK offers centimeter-level positioning accuracy, essential for precise mapping. With a payload capacity of 2.7 kg, it supports the simultaneous mounting of multiple sensors and cameras, making it an exceptionally versatile tool for environmental monitoring.

Image processing 

The data processing workflow is divided into five stages: raw data in DN, sensor radiance, surface reflectance in sensor geometry, georeferenced and orthorectified images, and the final reflectance orthomosaic. Initially, DN values are converted to radiance values, representing the amount of radiation in physical units. Noise from the sensor is subtracted using a dark current file. Hyperspectral image files are converted from float to integer format for efficient processing and storage. Reflectance data is obtained using reference panels with known reflectance, calculated by the ratio of radiance reflected by the panel to all image pixels. Orthorectification and georeferencing map image pixels onto an absolute coordinate system, correcting distortions due to sensor and platform movement. The final step involves combining adjacent stripes into a single image mosaic.