BOREAS RSS-02 Extracted Reflectance Factors Derived from ASAS Imagery Summary Atmospherically-corrected bidirectional reflectance factor means for small homogeneous areas from several BOREAS sites were derived from multi-spectral, multi-angle imagery acquired by the Advanced Solidstate Array Spectroradiometer (ASAS) aboard the C-130 aircraft platform in 1994 and 1996. The data are stored in tabular ASCII files. Table of Contents * 1 Data Set Overview * 2 Investigator(s) * 3 Theory of Measurements * 4 Equipment * 5 Data Acquisition Methods * 6 Observations * 7 Data Description * 8 Data Organization * 9 Data Manipulations * 10 Errors * 11 Notes * 12 Application of the Data Set * 13 Future Modifications and Plans * 14 Software * 15 Data Access * 16 Output Products and Availability * 17 References * 18 Glossary of Terms * 19 List of Acronyms * 20 Document Information 1. Data Set Overview 1.1 Data Set Identification BOREAS RSS-02 Extracted Reflectance Factors Derived from ASAS Imagery 1.2 Data Set Introduction Atmospherically-corrected bidirectional reflectance factor means for small homogeneous areas from several BOREAS sites were derived from multi-spectral, multi-angle imagery acquired by the Advanced Solidstate Array Spectroradiometer (ASAS) aboard the C-130 aircraft platform in 1994 and 1996. At-ground reflectance factors (a mean value for small areas from the ASAS images) have been derived for the SSA_CAL (AVIRIS calibration site), and the SSA_Old Black Spruce, SSA_Old Aspen, SSA_Old Jack Pine, SSA_Young Jack Pine and SSA_Fen flux tower sites. 1.3 Objective/Purpose Derive at-surface reflectance factors from airborne multiangle reflected radiance data to study the bidirectional reflectance properties of boreal forest canopies. 1.4 Summary of Parameters ASAS measures at-sensor radiance of surfaces as a function of spectral wavelength, view geometry (combinations of view zenith angle, view azimuth angle, solar zenith angle, and solar azimuth angle), and altitude. For these data, mean surface reflectance factors have been derived from at-sensor radiances for small areas adjacent to scaffold towers at several flux tower sites and a soil calibration target. Also included are C-130 flight information, date and time of observations, viewing and solar geometry, image subset coordinates, and atmospheric conditions. 1.5 Discussion The main objectives of the Boreal Ecosystem Atmosphere Study (BOREAS) conducted in Canada throughout 1994 and 1996 were to improve process models which describe the exchanges of energy, water, carbon, and trace constituents between the boreal forest and the atmosphere, and to develop methods for applying the process models over large spatial scales using remote sensing and other integrative modeling techniques. The Remote Sensing Science Group, of which ASAS is a part, is responsible for developing linkages between optical and microwave remote sensing and boreal zone biophysical parameters at various scales (leaf, canopy, and regional) using measurements from field, aircraft and satellite sensors plus a range of radiative transfer models. Data tables described in this document were derived only from data acquired on May26, 1994; July 21, 1994; and July 20, 1996. ASAS at-sensor radiance image data are available for other dates; see the document for ASAS Level 1b image data. 1.6 Related Data Sets BOREAS RSS-01 PARABOLA Surface Reflectance and Transmittance Data BOREAS RSS-02 Level-1b ASAS Imagery: At-sensor Radiance in BSQ Format BOREAS RSS-03 Reflectance Measured from a Helicopter-Mounted Barnes MMR BOREAS RSS-03 Reflectance Measured from a Helicopter-Mounted SE-590 BOREAS RSS-18 Level-1B AVIRIS Imagery: At-sensor Radiance in BIL Format BOREAS RSS-19 Background Spectral Reflectance Data 2. Investigator(s) 2.1 Investigator(s) Name and Title Dr. James R. Irons 2.2 Title of Investigation Boreal forest bidirectional reflectances acquired by an airborne multispectral, multiangle imaging spectroradiometer 2.3 Contact Information Contact 1 --------- Dr. James R. Irons NASA/GSFC Greenbelt, MD 301-286-8978 James.R.Irons.1@gsfc.nasa.gov Contact 2 --------- Michael Bur NASA/GSFC Greenbelt, MD 301-286-8424 bur@gyrfalcon.gsfc.nasa.gov or Michael.J.Bur.1@gsfc.nasa.gov Contact 3 ------------- Jaime Nickeson Raytheon ITSS NASA Goddard Space Flight Center Greenbelt, Maryland Telephone: 301-286-3373 Fax: 301-286-0239 Jaime.Nickeson@gsfc.nasa.gov 3. Theory of Measurements ASAS is an airborne imaging spectroradiometer modified to point off-nadir by NASA/GSFC for the purpose of remotely observing directional anisotropy of solar radiance reflected from terrestrial surfaces. The instrument is capable of off- nadir pointing from approximately 70 degrees forward to 55 degrees aft along- track (in the direction of flight). As the aircraft approaches and approaches and passes the ground target, digital radiance measurements of the target are recorded for a discrete sequence of fore-to-aft view zenith angles within this range. The terms "tilt", "look", or "view" angles are used interchangeably when referring to the ASAS view zenith angles. For the BOREAS data collection flights, ASAS imaged most study sites at 8 different view zenith angles: +70, +60, +45, +26, nadir, -26, -45, -55 degrees. Imaging of sites at the 70-degree off-nadir view angle is problematic, and this particular angle may or may not be available in every data set. Data were acquired in 62 spectral bands ranging from 404-1023 nm with a spectral resolution of approximately 10 nm in each band. See sections below for further details. 4. Equipment: 4.1 Sensor/Instrument Description The ASAS instrument employs a cooled 1024x1024 element silicon charged-coupled- device (CCD) detector array to generate multispectral digital image data in a pushbroom mode. The first 324 rows of the CCD are masked. The next 186 rows are exposed to the output from the spectrometer. The final 516 rows are masked and used for readout of the array. Two of the rows under the mask collect smear data which are used to remove smear effects and dark current from the data. During the BOREAS missions the operating method of the array was to bin every 3 rows into one spectral band, which resulted in 62 spectral channels. In addition, every 2 detectors within each row were binned resulting in 512 pixels (per row or line) in the output image data (Level 1b). In this configuration the spectral band centers, which range from 404 to 1023 nm, are spaced at approximately 10 nm. Each spectral band has a full-width-half- maximum of approximately 10 nm. 4.1.1 Collection Environment The ASAS instrument is mounted on the underside of the platform aircraft fuselage with the sensor optics either slightly protruding into the slipstream or retracted into the fuselage pressure box, depending on the view angle. As the aircraft approaches the target site from a distance, the ASAS instrument is pointed forward-looking. A video camera bore-sighted with the ASAS optical head relays a picture to an onboard monitor screen at the ASAS operators' station. This enables the operator to identify the site and continue tracking it through a sequence of view angles as the aircraft proceeds on a flight line over the site. When the site comes into view on the forward point, the operator begins data acquisition. The sequence is timed such that the view is at nadir when the aircraft is over the site, and aft-looking views are taken after passing the site. Determining which views are forwardscatter or backscatter requires examination of the aircraft heading and the solar azimuth angle. During 1994 and 1996 BOREAS missions, multiangle data over the Flux Towers were usually acquired on 3 separate flights in azimuths parallel, perpendicular and oblique to the solar principal plane. As the platform aircraft flies forward, each row of 512 detector bins is electronically scanned to generate 62 spectral channels of digital image data in a pushbroom mode. The signals generated by the CCD detectors are sampled at a rate of 38 frame lines per second to produce the along-track dimension of the imagery (image lines). The sampled signal from each detector is digitized to 12 bits and the digital data are stored on a high-density S-VHS format tape using a buffered VLDS data recorder. 4.1.2 Source/Platform NASA/Ames Research Center C-130 Earth Resources Aircraft (1994) and NASA/Wallops Flight Facility C-130Q (1996). 4.1.3 Source/Platform Mission Objectives To collect multispectral, multiangle bidirectional reflectance data (acquired as at-sensor radiances) over a soil field and flux tower sites for study of boreal forest canopies, and to simulate Multi-Angle Imaging Spectro-Radiometer (MISR) data by obtaining measurements at MISR view angles. At-ground reflectance factors have been derived for some (but not all) datasets. 4.1.4 Key Variables ASAS measures at-sensor spectral radiance in the visible and near-infrared portion of the spectrum as a function of view geometry. At-ground spectral reflectance for (a) a soil field used as a calibration target and (b) small areas adjacent to the scaffold towers at Flux Tower sites have been derived using the atmospheric correction algorithm 6S (see section 17.1 for references). 4.1.5 Principles of Operation The ASAS optical head is mounted in an open port in the underside of the C-130 aircraft. A complex pointing mechanism incorporating a gimbal enables the sensor to view off-nadir, facilitating movement in the horizontal, vertical, rotational fore and aft, and yaw directions. As the aircraft approaches the target site from a distance, the ASAS instrument is pointed forward-looking. A video camera mounted adjacent to the ASAS optical head relays a picture to an onboard monitor screen at the ASAS operator's station. This enables the operator to identify the site and continue tracking it through a sequence of view angles as the aircraft proceeds on a flight line over the site. When the site comes into view on the first forward angle, the operator initiates data acquisition. The sequence is timed such that the view is at nadir when the aircraft is directly over the site, and aft-looking views are taken after passing the site. Yaw compensation can be performed by the operator (if necessary) to prevent the site from drifting out of the field of view. As the platform aircraft flies forward, each row of 1024x186 array elements are electronically scanned to generate 62 spectral channels of digital image data in a pushbroom mode. The signals generated by the CCD detectors are sampled at a rate of 38 frame lines per second to produce the along-track dimension of the imagery (image lines). The sampled signal from each detector is digitized to 12 bits and the digital data are stored on a high-density S-VHS format tape using a buffered VLDS data recorder. 4.1.6 Sensor/Instrument Measurement Geometry Radiation incident on the ASAS aperture is focused onto an entrance slit by an f/1.4 objective lens with a 57.2 mm focal length. The entrance slit is 50 um wide across-track, and 23 um wide along-track. The lens focuses incoming energy through the entrance slit into a 1:1 relay with an effective focal length of 76.3 mm in each half. In each half of the relay, a 90-degree mirror prism folds the optical path to create a compact optical head. A transmission grating ruled at 75 lines per mm and blazed at 530 nm is located between the two prisms to disperse the radiant energy into its wavelength spectrum, which is in turn directed by the second prism onto the 186 rows of the array in the focal plane, where the CCD is mounted. The instantaneous field-of-view (IFOV) of an ASAS pixel is a function of optics, detector dimensions, tilt angle (view angle), and aircraft altitude and attitude (pitch and roll). The optical system includes an f/1.4 objective lens with a 57.2 mm focal length, providing a 0.33 rad (19.3-degree) total angular across- track field-of-view. The individual angular resolution of the center detectors is 0.66 mrad across-track. The along-track field-of-view is 0.44 mrad. Each detector has dimensions of 19.0 micrometers spatially (across-track) and 19.0 micrometers spectrally, however with a binning factor of 2 in the spatial dimension and 3 in the spectral dimension, the resulting array pixel size is 38.0 micrometers in the spatial dimension and 57 micrometers in the spectral dimension. 4.1.7 Manufacturer of Sensor/Instrument The ASAS instrument evolved over a number of years. The original optics, built by TRW, were part of the Scanning Imaging Spectroradiometer (SIS) constructed in the early 1970s for NASA's Johnson Space Center. ASAS was created in 1981 when a charge-injection-device (CID) silicon detector array, made by GE, was incorporated with the optical system for a joint program involving NASA/JSC and the Naval Ocean Systems Center. In 1984, the sensor was transferred to NASA/GSFC, where the aircraft mounting bracket was modified for off-nadir pointing. In late 1991, the pointing mechanism was upgraded by NASA/GSFC to allow view angles of 70 degrees forward to 55 degrees aft, and to enable operator- controlled aircraft yaw compensation. In 1992, the CID was replaced with a Thomson CSF Model TH7896A (high speed version) charge-coupled-device (CCD) silicon detector array. BOREAS data were acquired with this CCD array. 4.2 Calibration Radiometric Calibration Radiometric calibration data for the BOREAS experiment were acquired from two primary calibration sources: 1) a 1.2 m diameter integrating hemisphere in the NASA/GSFC calibration laboratory, and 2) a 30-inch (.76 m) diameter portable hemisphere that is owned and operated by GSFC. The latter source was used for in-situ calibration data acquisition since it could be positioned directly under the aircraft-mounted instrument. The integrating hemisphere is operated and maintained by the Sensor Development and Characterization Branch at NASA/GSFC. Up to 12 levels of radiance can be provided for calibration by turning the internal tungsten filament lamps on or off. The hemisphere is calibrated on an absolute scale by comparison to the output from a National Institute of Standards and Technology traceable calibration lamp using a laboratory-based transfer spectroradiometer. In a calibration run, ASAS is exposed to a 12-level sequence of spectral radiance levels from the hemisphere. Dark current (the response of the instrument under conditions of no incident radiation) is also acquired. More detailed information about ASAS radiometric calibration can be found in the document for ASAS Level 1b images. See section 1.6. Spectral Calibration A McPherson Model 285 0.5 m double monochrometer serves as the spectral reference source for ASAS. Light from the monochrometer is collimated by a paraboloid mirror and directed to the ASAS optics. Instrument output is sampled every 0.5 nm. The band centers have been computed by determining the centroid of the area under the response curve for each band. Full-width-at-half-maximum (FWHM) was measured directly from the response curves. 4.2.1 Specifications ASAS spectral band centers and FWHM applicable to 1994 and 1996 BOREAS datasets are as follows: Band Center FWHM (nm) (nm) ---- ------ ---- 1 404.3 9.5 2 413.7 9.5 3 423.2 9.5 4 432.4 10.0 5 441.7 10.0 6 451.4 10.0 7 460.9 10.0 8 470.5 10.0 9 480.3 10.5 10 490.2 10.5 11 500.0 10.0 12 509.7 10.5 13 519.6 10.0 14 529.7 10.5 15 539.9 10.5 16 549.8 10.5 17 559.6 10.0 18 569.4 10.5 19 579.4 10.5 20 589.7 11.0 21 600.0 10.5 22 610.2 11.0 23 620.3 10.5 24 630.4 10.5 25 640.7 11.0 26 650.9 10.5 27 661.1 11.0 28 671.4 10.5 29 681.5 11.0 30 691.6 11.0 31 701.7 11.0 32 711.9 11.0 33 722.1 11.0 34 732.3 11.0 35 742.6 11.0 36 752.9 11.0 37 763.2 11.0 38 773.5 11.0 39 783.8 11.5 40 794.1 11.0 41 804.5 11.0 42 814.9 11.0 43 825.3 11.5 44 835.7 11.0 45 846.0 11.0 46 856.4 11.0 47 866.8 11.0 48 877.2 11.5 49 887.6 11.5 50 897.9 11.0 51 908.3 11.0 52 918.7 11.0 53 929.0 11.0 54 939.5 10.5 55 949.9 11.0 56 960.3 11.0 57 970.7 11.0 58 981.1 11.0 59 991.5 10.5 60 1001.9 10.5 61 1012.2 10.5 62 1022.7 10.5 4.2.1.1 Tolerance Information on spectral radiometric resolution factors and periodic horizontal striping (relevant to ASAS Level 1b images) is presented in this section in the document on ASAS Level 1b images. See section 1.6. 4.2.2 Frequency of Calibration In general, ASAS acquires radiometric calibration data at least twice for each mission, with one calibration set acquired prior to the mission, followed by a post-mission calibration after the instrument arrives back at GSFC. Radiometric calibration data were also acquired during each BOREAS field campaign using the portable integrating hemisphere described elsewhere in this document. For dates of calibration data used to calibrate BOREAS field data, see this section in the document for ASAS Level 1b images. Laboratory spectral calibrations of ASAS were performed both before and after the 1994 BOREAS field season. The spectral stability was also checked once in the middle of the field season using a portable helium neon laser. It has been determined that the spectral calibration results from October 13, 1994 are most appropriate for all 1994 and 1996 BOREAS data sets. 4.2.3 Other Calibration Information None. 5. Data Acquisition Methods The ASAS instrument is mounted on the underside of the platform aircraft fuselage with the sensor optics either slightly protruding into the slipstream or retracted into the fuselage pressure box, depending on the view angle. As the aircraft approaches the target site from a distance, the ASAS instrument is pointed forward-looking. A video camera bore-sighted with the ASAS optical head relays a picture to an onboard monitor screen at the ASAS operators' station. The operator identifies the site and tracks it through a sequence of view angles as the aircraft proceeds on a flight line over the site. When the site comes into view on the forward point, the operator begins data acquisition. The sequence is timed such that the view is at nadir when the aircraft is directly over the site, and aft-looking views are taken after passing the site. Determining which views are forwardscatter and backscatter requires examination of the aircraft heading and the solar azimuth angle, given in the ASAS ASCII header. During 1994 and 1996 BOREAS missions data were acquired as follows: Flux Towers at multiple view zenith angles on view azimuths parallel, perpendicular and oblique to the solar principal plane. As the platform aircraft flies forward, each row of 512 detector bins is electronically scanned to generate 62 spectral channels of digital image data in a pushbroom mode. Signals generated by the CCD detectors are sampled at a rate of 38 frame lines per second to produce the along-track dimension of the imagery (image lines). The sampled signal from each detector is digitized to 12 bits and the digital data are stored on a high-density S-VHS format tape using a buffered VLDS data recorder. 6. Observations 6.1 Data Notes None. 6.2 Field Notes ASAS operators do not make extensive notes about field conditions during missions. ASAS usually is not flown if atmospheric conditions are not sufficiently clear. Any observations noted by ASAS operators are made at altitude, and if considered pertinent to the data, are included in the ASAS header (of Level 1b images) COMMENTS field. 7. Data Description 7.1 Spatial Characteristics 7.1.1 Spatial Coverage At-ground reflectance factors (a mean value for small areas from the ASAS images) have been derived for the SSA_CAL (AVIRIS calibration site), and the SSA_Old Black Spruce, SSA_Old Aspen, SSA_Old Jack Pine, SSA_Young Jack Pine and SSA_Fen flux tower sites. At-sensor radiance images (ASAS Level 1b) are available for more sites in the SSA and for sites in the NSA. See the document on ASAS Level 1b images. Site Latitude Longitude ---------------- -------- --------- SSA_FEN 53.801 N 104.619 W SSA_OA 53.629 N 106.198 W SSA_OBS 53.988 N 105.119 W SSA_OJP 53.916 N 104.690 W SSA_YJP 53.876 N 104.647 W SSA_AVIRIS_CAL 53.24 N 105.69 W 7.1.2 Spatial Coverage Map Not available. 7.1.3 Spatial Resolution Across-track direction (x): ASAS spatial resolution in the x-direction is a function of the across-track field of view, view angle, and the altitude of the platform aircraft. Across- track pixels do not overlap. The across-track pixel size (in meters) is given in the header of each ASAS Level 1b image, however this information is not retained in the at-ground reflectance factor tables. Approximate ASAS pixel sizes are given below (the platform aircraft flew at slightly different altitudes in the two different years): Tilt angle 1994 x pixel size 1996 x pixel size (degrees) (meters) (meters) ---------- ----------------- ---------------- +70 10.7 9.0 +60 7.3 6.2 +45 5.2 4.4 +26 4.1 3.4 00 3.6 3.1 -26 4.1 3.4 -45 5.2 4.4 -55 6.4 5.4 Along-track direction (y): The along-track spatial resolution of an ASAS image pixel is more complicated. For a detailed explanation, see the document for ASAS Level 1b images. All ASAS datasets are oversampled in the along-track direction. This means that each image line somewhat overlaps the previous line, making the images appear more elongated than in reality. This frame or line overlap is not corrected for during operational processing. Essentially, the along-track pixel size is determined by the aircraft ground speed divided by the data frame rate, and this represents the smear distance portion of each pixel in the y-direction. For both 1994 and 1996 ASAS BOREAS data, this along-track (y) pixel size was approximately 3 m in all tilt angles. The ASAS multiangle images have not been georegistered, and though the sampled areas are selected carefully, they are not guaranteed to represent the exact same area in each image. 7.1.4 Projection Not applicable. 7.1.5 Grid Description Not applicable. 7.2 Temporal Characteristics 7.2.1 Temporal Coverage 7.2.2 Temporal Coverage Map Current coverage for at-ground reflectance factor tables: DATE SITE FLIGHT LINES --------- ------- ------------ 26-May-94 SSA_CAL 1 21-Jul-94 SSA_OBS 3 SSA_OA 3 SSA_OJP 3 SSA_YJP 1 SSA_FEN 1 20-Jul-96 SSA_FEN 1 7.2.3 Temporal Resolution ASAS site passes may vary slightly in time duration, depending on the length of the flight line and the aircraft speed. Typically one multiangle pass over a site has a time duration of about 5 minutes. At-ground reflectance factor tables have been generated for 3 dates only at this time: May 26, 1994; July 21, 1994; and July 20, 1996. 7.3 Data Characteristics Data characteristics are defined in the companion data definition file (asasrefl.def) 7.4 Sample Data Record Sample data format shown in the companion data definition file (asasrefl.def) 8. Data Organization 8.1 Data Granularity The smallest unit of data tracked by the BOREAS Information System (BORIS) was the data collected at a given site on a given date. 8.2 Data Format(s) The data files contain numerical and character fields of varying length separated by commas. The character fields are enclosed with single apostrophe marks. There are no spaces between the fields. Sample data format shown in the companion data definition file (asasrefl.def). 9. Data Manipulations 9.1 Formulae Not applicable. 9.1.1 Derivation Techniques and Algorithms See references for 6S (Section 17.1) 9.2 Data Processing Sequence 9.2.1 Processing Steps The processing of raw ASAS data to at-sensor radiances is explained in detail in the document for ASAS Level 1b images. Processing beyond the at-sensor radiances to produce the at-ground reflectance tables was as follows: 1. ASAS multiangle images were displayed using IDL (Interactive Data Language) software. 2. From each tilt angle image, roughly the same small image area was delineated using IDL software. 3. View angle information and radiance statistics were extracted for the sampled polygon in each tilt angle image. 4. 6S corrected the mean at-sensor spectral radiances for atmospheric effects and directly output table parameters. 9.2.2 Processing Changes Not applicable. 9.3 Calculations See 6S references (Section 17.1) 9.3.1 Special Corrections/Adjustments 9.3.2 Calculated Variables Calculated (derived) variables: at-ground radiance, at-ground reflectance factor 9.4 Graphs and Plots Not applicable. 10. Errors 10.1 Sources of Error Potential sources of uncertainty associated with ASAS spectral radiance and at- ground reflectance factors include the following: spectral radiance from the integrating hemisphere; spectral radiance from the portable hemisphere; transfer of spectral radiance to ASAS detector elements; spectral calibration of ASAS detector elements, and the atmospheric correction algorithm. Other factors such as polarization sensitivity, signal cross-talk between detectors, and stray light may contribute to the uncertainty, but these factors have not been evaluated. 10.2 Quality Assessment 10.2.1 Data Validation by Source ASAS Level 1b Image Data: During processing, frequency histograms of selected channels for each view angle are plotted and examined manually for anomalies. Images are also displayed and visually analyzed for target coverage, data dropouts, saturation and other potential problems. Derived Reflectance Factors: ASAS atmospherically-corrected reflectance factors are compared to reflectances acquired on the ground at the same sites when available; in most cases, ASAS reflectance factors have compared well to ground observations especially for the spectral range from 490 to 860 nm. 10.2.2 Confidence Level/Accuracy Judgement The uncertainty associated with ASAS spectral radiance values is approximately 6%. This number is the root-sum-square of the uncertainties contributed by the following factors: spectral radiance from the integrating hemisphere (5% uncertainty); transfer of spectral radiance to ASAS detector elements (2% uncertainty); and spectral calibration of the ASAS detector elements (1% uncertainty). The uncertainty associated with the radiance of the portable hemisphere has not been determined, however it is probably similar to that of the integrating hemisphere. Other factors such as polarization sensitivity, signal cross-talk between detectors, and stray light may contribute to the uncertainty, but these factors have not been evaluated. Uncertainty in the 6S atmospheric algorithm has not been evaluated. 10.2.3 Measurement Error for Parameters None given. 10.2.4 Additional Quality Assessments Spectral response curves for selected training areas are plotted and examined for known atmospheric absorption features. These plots are also compared to similar measurements made by other instruments, if data are available. 10.2.5 Data Verification by Data Center (For BORIS and ORNL DAAC Use) 11. Notes 11.1 Limitations of the Data The ASAS multiangle images have not been georegistered, and though the sampled areas are selected carefully, they are not guaranteed to represent the exact same area in each image. 11.2 Known Problems with the Data Image Data and derived products: (1) ASAS data acquired over the SSA calibration target (a soil field) was atmospherically corrected and compared to ground measurements. The results show ASAS to agree very well with the ground observations between 490 and 870 nm. Below 490 nm and above 870 nm, the ASAS response falls below the expected level. Extreme ASAS channels have much lower signal to noise ratios. (2) Though a specific sequence of view angles from +70 to -55 degrees was attempted for each flightline over the Flux Tower sites, not all look angles were achieved every time. Often the 70-degree off-nadir view missed the site or contained too much distortion for inclusion in the dataset. Other angles may be missing as well. Reflectance Factor Tables: (1) In general, atmospheric correction by 6S effectively removes the scattered path radiance in the visible, and diminishes the dips caused by gaseous absorption. However, a plot of reflectance factor vs. wavelength may show, for some datasets, that some dramatic dips and spikes still exist in the atmospherically-corrected data. These result from either overcorrection or undercorrection due to the difficulty of estimating the effect of absorption bands of varying widths. See the reference below by Brown de Colstoun (1995) for a discussion of this issue. 11.3 Usage Guidance At present, the reflectance factor tables originated from ASAS images which were not geo-rectified or geo-located. Geo-registered images may be available in the future. Contact ASAS staff or check the ASAS homepage (http://asas.gsfc.nasa.gov/) for updates on available data. Use special caution (or avoid) working with data from channels below 490 nm or above 870 nm (See Section 11.1). It is strongly suggested that you plot reflectance factor vs. spectral band number or band center and view each dataset for unusual dips and spikes. Affected bands should be avoided, and/or some smoothing algorithm should be applied to the data before use. 11.4 Other Relevant Information None. 12. Application of the Data Set None given. 13. Future Modifications and Plans None given. 14. Software 14.1 Software Description The Interactive Data Language (IDL) data analysis and visualization software package was used. 14.2 Software Access IDL is available from Research Systems, Inc. 2995 Wilderness Place Boulder, CO 80301 (303) 786-9900 http://www.rsinc.com/ 15. Data Access 15.1 Contact Information Ms. Beth Nelson NASA GSFC Greenbelt, MD (301) 286 4005 (301) 286 0239 (fax) beth@ltpmail.gsfc.nasa.gov 15.2 Data Center Identification See 15.1. 15.3 Procedures for Obtaining Data Users may place requests by telephone, electronic mail, or FAX. 15.4 Data Center Status/Plans The RSS-02 reflectance factor data are available from the EOSDIS ORNL DAAC (Earth Observing System Data and Information System) (Oak Ridge National Laboratory) (Distributed Active Archive Center). The BOREAS contact at ORNL is: ORNL DAAC User Services Oak Ridge National Laboratory Oak Ridge, TN (423) 241-3952 ornldaac@ornl.gov ornl@eos.nasa.gov 16. Output Products and Availability 16.1 Tape Products None. 16.2 Film Products None. 16.3 Other Products The data are available as tabular ASCII files. 17. References 17.1 Platform/Sensor/Instrument/Data Processing Documentation Kovalick, W. and Graham, D., 1991, ASAS Programmer's Manual, Hughes STX, Code 923, NASA/GSFC, Greenbelt, MD. (In-house document) Tanre, D., J.L. Deuze, M. Herman, R. Santer, and E. Vermote, Second simulation of the satellite signal in the solar spectrum - 6S code, [abs] in Proceedings of the 10th Annual Geoscience and Remote Sensing Symposium, vol. I, p. 187, IEEE Int. Geosci. and Remote Sens., New York, 1990. Vermote, E.F., D. Tanre, J.L. Deuze, M. Herman, and J.J. Morcrette, 1997. Second simulation of the satellite signal in the solar spectrum: an overview, IEE Trans. Geosci. Remote Sens. The 6S User Guide and code can be obtained via anonymous ftp on kratmos.gsfc.nasa.gov Information on ASAS data and the ASAS sensor can be obtained on the world-wide web at: http://asas.gsfc.nasa.gov/ 17.2 Journal Articles and Study Reports Many of the following articles describe data from the first generation CID array (prior to 1991). Radiometric resolution factors and spectral band centers differ among the various ASAS datasets. A more complete bibliography can be found in the documentation for ASAS Level 1b images. Abuelgasim, A. A. and A. Strahler (1994), Modeling bidirectional radiance measurements collected by the Advanced Solid-state Array Spectroradiometer (ASAS) over Oregon Transect conifer forests, Remote Sens. Environ. 47:261-275. Brown de Colstoun, E.C., C.L. Walthall, C.A. Russell, and J.R. Irons (1995), Estimating the fraction of absorbed photosynthetically active radiation (fAPAR) at FIFE with airborne bidirectional spectral reflectance data, J. Geophys. Res., Vol. 100:25,523-25,535. Deering, D. W., E. M. Middleton, J. R. Irons, B. L. Blad, E. A. Walter-Shea, C. J. Hays, C. L. Walthall, T. F. Eck, S. P. Ahmad, and B. P. Banerjee (1992), Prairie grassland bidirectional reflectances measured by different instruments at the FIFE site, J. Geophys. Res. 97:18887-18903. Guinness, E.A., R.A. Arvidson, J.R. Irons, and D.J. Harding (1991), Surface scattering properties estimated from modeling airborne multiple emission angle reflectance data, Geophys. Res. Letters 18(11):2051-2054. Hall, D.K., J.L. Foster, J.R. Irons, and P.W. Dabney (1993), Airborne bidirectional radiances of snow covered surfaces in Montana, USA, Annals of Glaciology 17:35-40. Irons, J.R., K.J. Ranson, D.L. Williams, R.R. Irish, and F.G. Huegel (1991), An off-nadir pointing imaging spectroradiometer for terrestrial ecosystem studies, IEEE Trans. on Geoscience and Remote Sensing 29(1):66-74. Lawrence, W. T., D. L. Williams, K. J. Ranson, J. R. Irons, and C. L. Walthall (1994), Comparative analysis of data acquired by three narrow-band airborne spectroradiometers over subboreal vegetation, Remote Sens. Environ. 47:204-215. Johnson, L. F. (1994), Multiple view zenith angle observations of reflectance from ponderosa pine stands, Int. J. Remote Sens. 15:3859-3865. Newcomer, J., D. Landis, S. Conrad, S. Curd, K. Huemmrich, D. Knapp, A. Morrell, J. Nickeson, A. Papagno, D. Rinker, R. Strub, T. Twine, F. Hall, and P. Sellers, eds. 2000. Collected Data of The Boreal Ecosystem-Atmosphere Study. NASA. CD- ROM. Ranson, K. J., J. R. Irons, and D. L. Williams (1994), Multispectral bidirectional reflectance of northern forest canopies with the Advanced Solid- state Array Spectroradiometer (ASAS), Remote Sens. Env. 47:276-289. Russell, C. A., C. L. Walthall, J. R. Irons, and E. C. Brown de Colstoun (1995), Comparison of airborne and surface spectral bidirectional reflectance factors, spectral hemispherical reflectance and spectral vegetation indices, J. Geophys. Res., Vol. 100:25,509-25,522. Russell, C. A., J. R. Irons, and P. W. Dabney (1997). Bidirectional reflectance of selected BOREAS sites from multiangle airborne data, J. Geophys. Res. 102:29,505-29516. Schaaf, C. B., and A. H. Strahler (1994), Validation of bidirectional and hemispherical reflectances from a geometric-optical model using ASAS imagery and pyranometer measurements of a spruce forest, Remote Sens. Env. 49:138-144. Sellers, P., F. Hall. 1994. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1994-3.0, NASA BOREAS Report (EXPLAN 94). Sellers, P., F. Hall, H. Margolis, B. Kelly, D. Baldocchi, G. den Hartog, J. Cihlar, M.G. Ryan, B. Goodison, P. Crill, K.J. Ranson, D. Lettenmaier, and D.E. Wickland. 1995. The boreal ecosystem-atmosphere study (BOREAS): an overview and early results from the 1994 field year. Bulletin of the American Meteorological Society. 76(9):1549-1577. Sellers, P., F. Hall, K.F. Huemmrich. 1996. Boreal Ecosystem-Atmosphere Study: 1994 Operations. NASA BOREAS Report (OPS DOC 94). Sellers, P., F. Hall. 1996. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1996-2.0, NASA BOREAS Report (EXPLAN 96). Sellers, P., F. Hall, K.F. Huemmrich. 1997. Boreal Ecosystem-Atmosphere Study: 1996 Operations. NASA BOREAS Report (OPS DOC 96). Sellers, P.J., F.G. Hall, R.D. Kelly, A. Black, D. Baldocchi, J. Berry, M. Ryan, K.J. Ranson, P.M. Crill, D.P. Lettenmaier, H. Margolis, J. Cihlar, J. Newcomer, D. Fitzjarrald, P.G. Jarvis, S.T. Gower, D. Halliwell, D. Williams, B. Goodison, D.E. Wickland, and F.E. Guertin. (1997). "BOREAS in 1997: Experiment Overview, Scientific Results and Future Directions", Journal of Geophysical Research (JGR), BOREAS Special Issue, 102(D24), Dec. 1997, pp. 28731-28770. CDROM COLLECTIONS with ASAS data Angelici, G.L., J.W. Skiles, and L.Z. Popovici (1992), OTTER: Oregon Transect Ecosytem Research Project, Collected Data, Volume 1, Version 1, Satellite, aircraft and ground measurements, CD-ROM USA_NASA_PLDS_OT_0001, NASA/ARC. Arvidson, R.E., Dale-Bannister, M.A., Guinness, E.A., Slavney, S.H., and Stein, T.C., 1991. Archive of Geologic Remote Sensing Field Experiment, Data-Release 1.0, CD-ROM Volume USA_NASA_PDS_GR_001, NASA Planetary Data System, JPL, Pasadena, CA. Strebel, D.E., D.R. Landis, J.A. Newcomer, B.W. Meeson, P.A. Agbu, and J.M.P. McManus (1992), Collected Data of the First ISLSCP Field Experiment, Volume 4: ASAS & PBMR Imagery 1987 & 1989, CD-ROM USA_NASA_PLDS_FIFE_0004, NASA/GSFC. ASAS reflectance factor tables for selected sites can be found in Volume 1: Surface Observations and Non-Image Data Sets (under Grab Bag category). 17.3 Archive/DBMS Usage Documentation None. 18. Glossary of Terms None. 19. List of Acronyms AGL - Above Ground Level ASAS - Advanced Solid-state Array Spectroradiometer BOREAS - BOReal Ecosystem-Atmosphere Study BORIS - BOReas Information System DAAC - Distributed Active Archive Center EOS - Earth Observing System EOSDIS - EOS Data and Information System FFC-T - Focused Field Campaign - Thaw FWHM - Full Width Half Maximum GSFC - Goddard Space Flight Center HDR - Header IFC - Intensive Field Campaign IFOV - Instantaneous Field-of-View NASA - National Aeronautics and Space Administration ORNL - Oak Ridge National Laboratory SPP - Solar Principal Plane URL - Uniform Resource Locator 20. Document Information 20.1 Document Revision Date Created: 30-Jun-1997 Revised: 05-Feb-1999 20.2 Document Review Date(s) BORIS Review: 14-Jan-1998 Science Review: 25-Feb-1998 20.3 Document ID 20.4 Citation When using these data, please include the following acknowledgment as well as citations of relevant ASAS papers in Section 17.2: The ASAS data were provided by James R. Irons and Philip W. Dabney (NASA Goddard Space Flight Center) If using data from the BOREAS CD-ROM series, also reference the data as: Irons, James R. and M. Bur ,"Boreal forest bidirectional reflectances acquired by an airborne multispectral, multiangle imaging spectroradiometer." in Collected Data of The Boreal Ecosystem-Atmosphere Study. Eds. J. Newcomer, D. Landis, S. Conrad, S. Curd, K. Huemmrich, D. Knapp, A.Morrell, J. Nickeson, A. Papagno, D. Rinker, R. Strub, T. Twine, F. Hall, and P. Sellers. CD-ROM. NASA, 2000. Also, cite the BOREAS CD-ROM set as: Newcomer, J., D. Landis, S. Conrad, S. Curd, K. Huemmrich, D. Knapp, A. Morrell, J. Nickeson, A. Papagno, D. Rinker, R. Strub, T. Twine, F. Hall, and P. Sellers, eds. Collected Data of The Boreal Ecosystem-Atmosphere Study. CD-ROM. NASA, 2000. 20.5 Document Curator 20.6 Document URL BRDF ASAS reflectance RSS02_ASAS_ReflFact.doc 03/03/99