Spatial and temporal sampling of ecosystem optical properties with the UniSpec-DC


Courtesy of PP Systems

Satellite sensors (e.g. MODIS) provide large spatial patterns with regional or global coverage (Chen, 1999). However, the coarse resolution of satellite sensors are sometimes problematic, particularly for patchy landscapes (Turner et al., 2003). To properly validate and interpret satellite imagery, additional ground remotely sensed and biophysical data are needed. The UniSpec-DC (PP Systems, Amesbury, MA, USA) along with a mobile tram system can make a repeatable continuous measurement on the ecosystem optical properties. It can offer the ground level remotely sensed signal to validate the satellite measurement (Cheng et al., in press).

A research group directed by Dr. John A. Gamon at California State University has conducted long term studies at Sky Oaks Biological Field Station (run by San Diego State University). They have used a UniSpec-DC system along with a mobile tram system (Fig. 1) in collecting remotely sensed data for over 6 years. An automated tram system (based on the NOAA ATDD tram design, Baldocchi and Vogl 1996) was installed near the eddy tower. This system allowed researchers to collect spectral reflectance repeatedly over the diurnal (for studying solar angle effect on vegetation indices) and seasonal cycle (for studying the seasonal effects on the ecosystem processes). The tram was 100 m long (a dimension within the eddy covariance footprints). The tram track was mounted just above the top of the vegetation and the distance from the ground varies based on the topography. A dual channel spectrometer (UniSpec-DC) mounted in a motorized cart ran along the track and measured spectral reflectance at exactly 1 m intervals using a triggering system, in connection to an auxiliary port of the UniSpec-DC system with a DB9 straight connection. A fiber optic cable used as a foreoptic for the spectrometer was mounted on a boom at the side of the cart. With a field of view restrictor, this fiber provided an approximately 20° angle (adjustable as needed). This provided a view footprint of approximately 0.3 m at the top of the canopy and 1-2 meters at the ground surface (depending on the tram height, which varied with topography).

The spectrometers have a nominal spectral range from approximately 305 nm to 1135 nm with approximately 3 nm nominal bandwidth (10 nm full width, half max). Thus, for each measurement, the UniSpec-DC software program (UniSync 2.5) automatically collected 256 data points covering the entire spectral range for both upwelling and downwelling channels. This instrument allows measurements to be conducted under any weather conditions since the upwelling channel measures incoming solar irradiance while the downwelling channel measured reflected radiance. The spectral reflectance is calculated using a reference scan over a well leveled lambertian 99% reflectance panel (Spectralon, Labsphere Inc., North Sutton, NH, USA). The ratio of target scan and reference scan at earch wavelength corrected by sky condition is the reflectance at the wavelength.

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