Ultralight SE590 and Video Data
OTTER Data Description
INVESTIGATOR:
Richard W. McCreight, Research Scientist, Co-director
Center for Airborne Environmental Analysis
College of Forestry
Peavy Hall 035
Oregon State University,
Corvallis, Oregon, 97331
CONTACTS:
Richard H. Waring, Professor, Director
Center for Airborne Environmental Analysis
College of Forestry
Peavy Hall 029
Oregon State University,
Corvallis, Oregon, 97331
REQUESTED ACKNOWLEDGMENT:
Co-author if measurements are used in primary analysis, otherwise
citation or acknowledgment is acceptable.
INTRODUCTION:
During the OTTER project, ultralight aircraft provided low-
altitude reflectance and video records of the research sites.
These data allowed ground measurements to be scaled to satellite
observations.
Previous remote sensing studies have employed truck-mounted booms
and helicopters to provide near-surface spectral measurements
(Bauer et al.,1981, Williams et al., 1984; Ripple et al., 1987). Light
aircraft have provided near-surface measurements in cases where boom
operations were not feasible or helicopters proved too expensive
(Foran and Pickup 1984; Howard and Barton 1973).
Ultralight aircraft were introduced as part of the OTTER project
in 1989 (McCreight and Waring, 1990). They were operated in
accordance with Federal Aviation Flight rules as public research
aircraft. Research flights were performed by McCreight, a certified
pilot with over 2400 flight hours experience. The addition of a
ballistic parachute, designed to recover the aircraft and crew in
the event of an emergency, provided an added margin of safety.
EQUIPMENT:
During 1989 and 1990, ultralight observations were made with a
Quicksilver Model MXLI. This aircraft has a single seat with a
45 hp engine and a 125 kg load capacity. The aircraft is configured
with 3-axis control surfaces to ensure adequate stability of the
platform in turbulent weather. By 1990 increased remote sensing
instrument loads were approaching the payload capacity of the
aircraft. Therefore, during the winter of 1990-91 a "third-
generation" ultralight, the Quicksilver Model GT500 was
acquired. The increased wing size and 65 hp engine of the
GT500 significantly improved platform performance, with a
payload of 195 kg.
Remote sensing instruments
Remote sensing instruments were mounted to the airframe near
the center of gravity of the aircraft, adjacent to the
pilot's seat and with an unobstructed view of the ground
below the aircraft. Upon this mount, three or four sensors
were attached, depending on research objectives.
Video camera.
At all times, a Sony Model TR5 8mm video camera recorded the
surface features measured by the SE590 spectroradiometer.
The standard optics of the video camera provide an
instantaneous field-of-view from 3 cm at 100 m altitude to 5
m at 1000 m altitude. In addition, the camera's color
(red/green/blue) separation of the recorded signal gives an
approximation of visible wavelength landscape spectral
patterns at relatively high spatial resolution. Over 230,000
images were recorded by this system in a two-hour tape.
Numerical analyses of the images were done by linking
the video camera to a computer system via a Matrox graphics
board and the Resource Analysis Software (Decision Images,
Inc. 1989). Individual frames of the video imagery were
selected visually and then electronically transferred to the
computer system. The image processing system digitizes the
red, green, and blue spectral components of the color video-
image separately. This provides a three-component spectral
data base which can then be subjected to spectral
classification.
The video camera documented SE590 spectroradiometer coverage
along the flight-line with ancillary data such as altitude,
air speed, direction, and weather observations recorded on
the sound-track. Time, recorded on both the video and
spectral output, served as a flight index that allowed
precise cross-referencing between data sets.
Reflectance spectroradiometer.
The Spectron Engineering SE590 visible and near-infrared
spectroradiometer provided continuous reflectance spectra
between 380 and 1100 nm at a nominal 10 nm spectral
resolution. Outfitted with a one degree field-of-view lens,
the ground resolution of this instrument was approximately
4.5 m at 300 m altitude. The instrument employs a linear
array of detectors with a spectral dispersing grating in
front of the array to provide rapid acquisition of the
continuous spectra, a critical factor in flight operations.
Considerable experience has been accumulated with the SE590
instrument (Williams et al. 1984; Petzold and Goward 1988)
and its performance is relatively well understood.
The SE590 system was linked, through an RS-232 cable system
to a Toshiba portable computer (model 1200 HD). This
computer sits in the lap of the pilot/scientist and provides
direct control of measurement acquisition and recording (on
the hard disk). The software that accomplishes these
operations was provided by D. Williams and M. Kim, NASA
Goddard Space Flight Center and was tested extensively
(Williams and Walthall 1990).
CALIBRATION:
The SE590 spectroradiometer was recalibrated annually for
both spectral and radiometric sensitivity. The instrument
was shipped to the NASA Goddard Space Flight Center, where F.
Wood and M. Kim, under support from D. Williams, carried out
the required measurements on the GSFC integrating sphere and
monocrometers. In addition, during 1990, J. Dungan, NASA
Ames Research Center, carried out spectroradiometric
assessments of the sensors at the field sites in Oregon. In
general, the instruments performed nominally within
specification. The OSU SE590 sufferred minor damage in
August of 1990 and, as a result, shifted 10 nm from its
calibration. The instrument was returned to the manufacturer
for repair.
Spectral reflectance, the radiance reflected from the test
surface ratioed against reflectance of incoming solar
radiation off of a standard halon panel, served to
characterize the general spectral properties of each scene.
This measurement normalizes the observations for variations
in incident radiation and atmospheric conditions. Such a
reference is essential to compare measurements over time
(Spanner 1989; Williams et al., 1990; Ripple et al., 1987).
By making measurements of incident solar radiation with the
SE590 at 300 m interval between 1200 m and 300 m, atmospheric
effects were recorded and removed. The SE590
spectroradiometer recorded data while the aircraft was in
level flight oriented at a 45 degree angle to the solar plane.
These measurements provided an estimate of the most important
atmospheric effects due to both small particle (aerosol)
scattering and water vapor absorption. The incident solar
radiation at the surface at the time of the over-flight was
approximated by extrapolating in-flight calibration
measurements as follows:
S = R1-(R2-R1)
(1)
where,
S = incident solar radiation estimated at the surface,
R1= halon panel reference at 300 m altitude, and
R2= halon panel reference at 600 m altitude.
Estimates of solar irradiance at the surface were ratioed to
target radiance values to calculate reflectance factors.
PROCEDURE:
The ultralight flight schedule was coordinated with intensive
ground measurements and flights of NASA aircraft. Ultralight
flights were carried out in June, August, and October of 1990
and in June-July, 1991. Flights were made when skies were
clear at solar zenith angles above 50o and winds were less
than 50 km/hr. Flights were usually made between 0800 and
1600 hr (solar time). At nadir, video and spectroradiometer
data were collected from an altitude of 300 m at a flight
speed of approximately 55 km/hr. Spectral data were recorded
along parallel flight lines oriented nearly perpendicular to
the solar plane. Generally, 20 to 70 SE590 observations were
acquired over each study site and replicated over time.
Remote sensing data were reviewed immediately upon landing to
ensure adequate data quality and coverage. In the
laboratory, spectral data were geo-referenced and indexed
using the video flight record. Observations made over
roads, clouds, or during aircraft maneuvers were removed
before calculating site-averaged reflectance values.
ERRORS:
1. Surface reflectances: The relative importance of
accounting for atmospheric attenuation of radiation can be
observed by comparing the Cascade Head reflectance
measurements (sites 1A and 1OG) to western sites. The large
drop in recorded spectral reflectance centered at 950 nm, for
the Cascade Head site, is the result of of water vapor in the
300 m of atmosphere below the aircraft. Due to rapid changes
in cloud cover over the sites, it was not possible to obtain
complete altitude profile measurements needed to extrapolate
the airborne spectral measurements to the ground.
2. Instrument performance: An inspection of the data signal
to noise ratio of the SE590 spectroradiometer indicates a
significant amount of noise in wavelengths below 400 nm and
above 1000 nm. Nonlinear shifts in spectral sensitivity of 3
to 10 nm were also noted for the instrument over the period
of the OTTER study. Annual calibrations were necessary to
compensate for variations in the instrument spectral
sensitivity.
NOTES:
Site-averaged reflectance spectra for sites 2 to 6 were
calculated from October, 1990 coverage. The Cascade sites
were measured in July, 1991. No attempt was made to
normalized reflectance spectra for background illumination
(i.e., sunlit vs shadow area). Video images representing the
central third of each study site were acquired from 300 m
above the ground in July, 1991. Video coveerage from an
altitude of 100 m and 1200 m is also represented for site 2.
REFERENCES:
Bauer, M. E., C. S. T. Daughtry, and V. C. Vanderbilt. 1981.
Spectral-agronomic relations of corn, soybeans and wheat
canopies. LARS Purdue University, West Lafayette, Indiana,
Tech. Report 091281.
Foran, B. D., and G. Pickup. 1984. Relationship of
aircraft radiometric measurements to bare ground on semi-arid
desert landscapes in central Australia. Australian Rangeland
Journal 6 : 59-68.
Howard, J. A., and I. J. Barton. 1973. Instrumentation for
remote sensing solar radiation from light aircraft. Applied
Optics 12 : 2472-2476.
McCreight, R. W., and Waring, R. H. 1990. An ultralight
system for environmental monitoring. Airborne Geoscience
Newsletter, 90-3:9.
Ripple, B., R. W. McCreight, A. Long, and B. Barnet. 1987.
Spectral reflectance patterns of some key Cascade west slope
vegetation types, In: Proceedings of the National Remote
Sensing and Photogrammetric Engineering Symposium, Anchorage,
Alaska.
Williams, D. L., S. N. Goward, and C. L. Walthall. 1984.
Collection of in situ forest canopy spectra using a
helicopter: A discussion and preliminary results. In: 10th
International Symposium on Machine Processing of Remotely
Sensed Data. IEEE, pp. 94-106, Purdue University, West
Lafayette, Indiana.
Williams, D. L. and C. L. Walthall. 1990. Helicopter-based
multispectal data collection over the northern experimental
forest: Preliminary results from the 1989 field season. In:
10th Annual International Geoscience and Remote Sensing
Symposium, Institute for Electrical and Electronic Engineers,
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