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, pp. 875-878, College Park, Maryland.