Instrumentation and approach for unattended year round tower based measurements of spectral reflectance

Phyto-pigments play an important role in plant physiological processes and primary production. Because different groups of pigments absorb light in distinctive ways, spectroscopy can be used as a tool for phyto-pigment quantification. While technical restrictions have prevented the use of remote sensing for pigment detection in the past, the advent of fine spectral resolution radiometers has offered opportunities to detect leaf-pigment concentrations at a range of scales from portable spectro-radiometers to airborne instruments. This paper describes the development of a fully automated spectral data collection system composed of a radiometer, a motor driven probe, and a datalogger mounted on a tower to measure year round spectral reflectance under different view and sun angles. The instrument features a motor driven probe pointing at the canopy which is able to sample spectra in a near 360° view around the tower by completing a full rotation every 15 min. The view zenith angle is adjustable and currently set to 62°; one scan is completed within 3 s and the probe is forwarded about 11.5° every 30 s. Simultaneous measurement of solar irradiance and reflectance facilitates sampling under various sky conditions. The height of the tower is 45 m, and the sensor is located about 10 m above the canopy. Over a 6-month test period, nearly 2 million scans have been collected and compiled at a field site, where ongoing eddy covariance (EC) flux measurements are being undertaken as part of the Fluxnet-Canada research network.