Surface directional radiances and reflectances were measured with a helicopter mounted Spectron Engineering (SE)-590 spectrometer, at a relatively high spectral resolution of 5 to 15 nm over the visible and near infrared (400-1000 nm).
The NASA Bell UH-1B helicopter optical remote sensing system supported a data acquisition system consisting of a bore-sighted SE-590; a color video camera; and a 35-mm camera. Controller units for all the optical devices are rack- mounted inside the helicopter and are wired such that a single switch closure triggers all devices. The switch closure also activates an audible tone which is recorded on one of the two audio tracks of a Beta-format video recording system. The other audio track of the VCR was used to record cabin intercom conversations among the helicopter crew. If one desires to examine site conditions in greater detail, the higher resolution 35-mm still photography can be reviewed. Note that the 35-mm camera was not carried during all field campaigns.
When mounted on the helicopter, the SE-590 optical head is equipped with a lens having a 1-degree FOV. Given a nominal data acquisition altitude of 300 m, spectral irradiance from the ground surface area of approximately 5 m in diameter is recorded at nadir. An operator controlled pointing mechanism was used to provide off-nadir measurements. Measurements for each set containing 30-50 scans were made over a 5-10 minute period using the helicopter mounted SE-590 instruments. The duplicate set of the instrument measured radiance from a barium sulfate panel on the ground at one minute interval.
Errors associated with the measurements can occur due to orientation of the SE-590. The angle of attack of the helicopter varies during flight. The motion of the helicopter cabin beneath the main rotor blades is analogous to a pendulum swinging. There has been no quantitative inquiry into this effect as only very small (estimated at 1 to 2 degrees) off-nadir-induced errors have been estimated in extreme cases. The instrument operator can normally wait for this motion to stop or can anticipate the position of the helicopter before triggering the instruments. The helicopter roll,pitch and yaw are dependent on atmospheric conditions, engine performance and aircraft crew fatigue. Off-nadir observations are especially questionable under less-than-optimal atmospheric conditions. There is no atmospheric correction in these data. The shadowing caused by the SE-590 and the helicopter in measuring the "hot spot" area is another source of error. Variable cloud cover could be an error source with reflectance factors since the incoming radiation measurements were not made simultaneously with the surface measurements. Differences in irradiance between the reference panel location and the site being measured are an additional source of error (spatial distribution of atmospheric properties affecting irradiance).
There is also some debate on the amount of atmosphere in the pathlength between the helicopter and the surface. This has not been resolved and is under investigation at the time of this writing. The results anticipated from this experiment will enable development and validation of the integrated model to usefully characterize the ecosystem dynamics of the boreal forest under a variety of conditions.
Spatial Coverage - The FED study site is located 56 km north of Bangor, Maine in Penobscot County (45 deg. 12'N, 68 deg. 44'W). The area is within the 7000 ha Northern Experimental Forest (NEF) owned by International Paper. The helicopter was used to collect data over the sites listed below. Following is a list of the preselected targets. These sites are listed in priority order.
Site Latitude Longitude
1. Tower/"spruce" site 45 degree 12.27'N 68 degree 44.34'W (tower)
45 degree 12.40'N 68 degree 44.42'W (spruce)
2. "Hemlock" site 45 degree 13.28'N 68 degree 44.29'W
3. Esker site 45 degree 13.00'N 68 degree 42.34'W (mixed
hardwood)
4. Bog site 45 degree 12.30'N 68 degree 42.30'W
(just south of "cal" site)