Multi-Channel Optical Receiver for Ground-Based Topographic Hyperspectral Remote Sensing

Abstract

Receiver design is integral to the development of a new remote sensor. An effective receiver delivers backscattered light to the detector while optimizing the signal-to-noise ratio at the desired wavelengths. Towards the goal of effective receiver design, a multi-channel optical receiver was developed to collect range-resolved, backscattered energy for simultaneous hyperspectral and differential absorption spectrometry (LAS) measurements. The receiver is part of a new, ground-based, multi-mode lidar instrument for remote characterization of soil properties. The instrument, referred to as the soil observation laser absorption spectrometer (SOLAS), was described previously in the literature. A detailed description of the multi-channel receiver of the SOLAS is presented herein. The hyperspectral channel receives light across the visible near-infrared (VNIR) to shortwave infrared (SWIR) spectrum (350–2500 nm), while the LAS channel was optimized for detection in a narrower portion of the near-infrared range (820–850 nm). The range-dependent field of view for each channel is presented and compared with the beam evolution of the SOLAS instrument transmitter. Laboratory-based testing of each of the receiver channels was performed to determine the effectiveness of the receiver. Based on reflectance spectra collected for four soil types, at distances of 20, 35, and 60 m from the receiver, reliable hyperspectral measurements were gathered, independent of the range to the target. Increased levels of noise were observed at the edges of the VNIR and SWIR detector ranges, which were attributed to the lack of sensitivity of the instrument in these regions. The suitability of the receiver design, for the collection of both hyperspectral and LAS measurements at close-ranges, is documented herein. Future development of the instrument will enable the combination of long-range, ground-based hyperspectral measurements with the LAS measurements to correct for absorption, due to atmospheric water vapor. The envisioned application for the instrument includes the rapid characterization of bare or vegetated soils and minerals, such as are present in mine faces and tailings, or unstable slopes.

Validation of a ground-based telescope-assisted hyperspectral remote sensor for soil measurements

“…The resulting reflectance values were then plotted as a function of wavelength. While the proximal range measurements did not require additional corrections, the SOLAS telescope-assisted measurements collected indoors under artificial illumination 12 were splice corrected 14 if an offset was observed at the transition wavelengths between the three instrument detector ranges (VNIR, SWIR 1, and SWIR 2). For the measurements collected outdoors under solar illumination, the 1900-nm long-path water vapor absorption band interfered with the aforementioned splice correction procedure at the 1800-nm detector transition.…”

“…The range-dependent D FOV of the SOLAS receiver was previously described in more detail. 12 Each specimen was 25 cm in diameter and was prepared dry. The same five soil types that were tested proximally were tested at a range of 40 m. The target was inclined 29 deg relative to the ground, while the SOLAS receiver was declined 15 deg for an effective incidence Laptop ASD FieldSpec 4 spectroradiometer Multichannel receiver relay 8 Viewfinder Fig.…”

“…The noise statistics for typical telescope-assisted measurements were presented previously; 12 however, the noise was not previously characterized across the entire VNIR to SWIR wavelength range. Therefore, for completeness, the noise-equivalent radiance for the ASD FieldSpec 4 instrument is plotted as a function of wavelength in Fig.…”

“…The development of the SOLAS prototype was described previously in the literature. 11,12 In its current form, the SOLAS utilizes a portable, tripod-mounted telescope that collimates reflected light through an optical receiver into a high radiometric-resolution spectroradiometer [Analytical Spectral Devices (ASD) FieldSpec 4 Hi-Res; Malvern Panalytical Ltd., Malvern, United Kingdom]. Light is detected on 2151 continuous bands from the ultraviolet to SWIR wavelengths.…”

Remote Sensing Applied in Forest Management to Optimize Ecosystem Services: Advances in Research

“…This method is used in scientific research to determine the status of a species or a geographical area in terms of conservation [132]. In this sense, various study applications have begun to be implemented, such as the AudioMoth case, with the aim of designing techniques to monitor anthropogenic disturbances, which may be linked to illegal hunting of wildlife [133].…”