BOREAS RSS-20 POLDER Helicopter-Mounted Measurements of Surface BRDF Summary This data set contains measurements of surface BRDF made by the POLDER instrument over several sites (pine, spruce, fen) of the BOREAS study areas during 1994. Single-point BRDF values were acquired from NASA’s WFF helicopter. A related data set collected from the C-130 platform is available as is POLDER imagery acquired from the C-130. 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-20 POLDER Helicopter-Mounted Measurements of Surface BRDF 1.2 Data Set Introduction The POLarization and Directionality of Earth Reflectances (POLDER) instrument measures Bidirectional Reflectance Distribution Function (BRDF) and Bidirectional Polarization Distribution Function (BPDF) of terrestrial surfaces in several visible and near-infrared spectral bands. The instrument scanned several surface types (pine, spruce, fen, and others) in the BOReal Ecosystem- Atmosphere Study (BOREAS) study areas during the Intensive Field Campaigns (IFCs) in 1994. Single-point BRDF measurements were acquired either from the C- 130 aircraft or the helicopter. POLDER images acquired from the C-130 are also available for illustration purposes. 1.3 Objective/Purpose The objective of the investigation was to characterize the bidirectional reflectance properties of different cover types in boreal forests over several seasons (IFC-1, -2, and -3). This characterization can then be used to retrieve biophysical parameters such as Leaf Area Index (LAI), chlorophyll content, and structural canopy parameters, either through the use of semi-empirical relations between reflectances and biophysical parameters, or through the inversion of a BRDF radiative transfer model. The overall goal is to establish methodologies for monitoring the ecological state of the boreal forest using remote sensing techniques. 1.4 Summary of Parameters Surface bidirectional reflectances derived from low-altitude helicopter-borne POLDER measurements over sites of opportunity (no atmospheric correction). 1.5 Discussion The POLDER instrument measures surface reflectance as a function of wavelength and observation geometry. This data set is comprised of individual site measurements of surface BRDF made by the POLDER instrument over several surface types (pine, spruce, fen) in the BOREAS study areas, acquired during the 1994 IFCs. 1.6 Related Data Sets BOREAS RSS-01 PARABOLA SSA 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-11 Ground Network of Sun Photometer Measurements BOREAS RSS-20 POLDER C-130 Measurements of Surface BRDF 2. Investigator(s) 2.1 Investigator(s) Name and Title Dr. Marc Leroy, Dr. François-Marie Bréon, Patrice Bicheron, Olivier Hautecoeur 2.2 Title of Investigation Estimation of Photosynthetic Capacity using POLDER Polarization 2.3 Contact Information Contact 1 ------------- Dr. Marc Leroy Centre d'Etudes Spatiales de la Biosphère (CESBIO) Toulouse, France +33 5 61 55 85 14 +33 5 61 55 85 00 (fax) Marc.Leroy@cesbio.cnes.fr Contact 2 ---------------- Dr. François-Marie Bréon Laboratoire de Modélisation du climat et de l'Environnement Gif sur Yvette, France +33 1 69 08 94 55 +33 1 69 08 77 16 (fax) fmbreon@cea.fr Contact 3 -------------- Jaime Nickeson Raytheon ITSS NASA/GSFC Greenbelt, MD (301) 286-3373 (301) 286-0239 (fax) Jaime.Nickeson@gsfc.nasa.gov 3. Theory of Measurements POLDER is an optical sensor designed to observe the surface reflectance in visible and near-infrared bands. The main characteristic of the POLDER instrument is that it can observe an area from multiple directions. POLDER has a wide field-of-view (FOV) lens with ± 51° along-track and ± 43° cross-track viewing, and a charge-coupled device (CCD) array detector to collect images. Two principles of operation should be distinguished during the BOREAS experiment. When POLDER was mounted on the helicopter, the purpose was to collect data over the target at a low altitude, typically 300 m. One image acquired directly over a homogeneous surface provides the BRDF of the experimental site. From the National Aeronautics and Space Administration’s (NASA) Ames Research Center (ARC) C-130 aircraft, at high altitude, typically 5500 m, the surface cannot be considered homogeneous. POLDER's capacity to observe an area from various view angles allows for measurement of the complete BRDF with the successive images acquired along different flight axes over the experimental site. 4. Equipment 4.1 Sensor/Instrument Description 4.1.1 Collection Environment It is mandatory to operate POLDER in totally clear sky conditions, so that the distribution of irradiance does not change from one measurement to another, and so that calculation of reflectances in absolute units from radiances is possible. 4.1.2 Source/Platform During IFC-1 and IFC-2, the POLDER instrument was installed alternatively on the NASA C-130 aircraft or the NASA Wallops Flight Facility (WFF) helicopter. During IFC-3, the instrument was deployed only on the helicopter. The data described in this document was collected from the WFF helicopter platform. 4.1.3 Source/Platform Mission Objectives The POLDER mission objective was to collect multiangle and multispectral bidirectional reflectance data over flux tower and auxiliary sites to study the boreal forest canopy. 4.1.4 Key Variables POLDER measures multispectral radiance in the visible and near infrared domain as a function of solar and view geometry. 4.1.5 Principles of Operation The POLDER optical system consists of a telecentric lens, a filter wheel, and a CCD array as a detector. The light is almost vertically incident on the filter wheel after passing the telecentric lens. The CCD array (288 x 384 elements) can collect 2-D images. The filter wheel contains 10 slots for spectral filters and polarizers. The first channel is reserved for dark current measurement while the others allow measurements in five spectral bands (443, 550, 670, 864, and 910 nm). Two spectral bands (443 and 864 nm) are associated with three polarized filters oriented by steps of 60°. A 10-channel image, corresponding to the 10 positions of the filter wheel, is collected within 3 seconds, and this acquisition is repeated every 10 seconds. The POLDER optical system was installed on a horizontal stabilized platform developed by J.E. Kalshoven at NASA Goddard Space Flight Center (GSFC). A vertical gyroscope is integral to the platform and provides the feedback for control. The system kept the POLDER optical axis aligned with respect to true nadir, regardless of the helicopter attitude. On the other hand, the heading and yaw were the same as that of the helicopter. The typical flight altitude was 300 m. During a flight, the helicopter flew from site to site and stabilized itself for a few minutes over each target. These were mostly the BOREAS tower sites and auxiliary sites, but also some targets of opportunity that looked homogeneous within the instrument FOV. 4.1.6 Sensor/Instrument Measurement Geometry The long axis of the CCD array was set parallel to the aircraft longitudinal axis. An inclinometer was used to record the initial bias between the optical axis and true nadir. 4.1.7 Manufacturer of Sensor/Instrument The instrument was designed and manufactured by: Laboratoire d'Optique Atmosphérique (LOA) 59655 Villeneuve d’Ascq Cedex Lille, France. 4.2 Calibration Radiometric calibration data were acquired at LOA by J.-Y. Balois before and after the BOREAS experiment (11-May-1994, 24-Oct-1994) using a calibrated integration sphere. The whole exit port of the integration sphere is used to derive the equalization coefficients (see definition in Section 9.2.1.1). For absolute calibration, the exit port is reduced by a diaphragm to illuminate only a small circular area in the center of the CCD array. Readings of 15 x 15 pixel windows are corrected for dark current and averaged to obtain the absolute calibration coefficients (see Section 9.2.1.1). Other calibration experiments were made during the BOREAS experiment using a 30- inch (0.76-m) diameter portable hemisphere that is owned and operated by NASA’s Goddard Space Flight Center (GSFC). This portable hemisphere was made available to RSS-20 by Brian Markham and John Schaffer. The calibration of POLDER was performed at the Prince Albert airport when POLDER was installed in C-130 aircraft on 27-May-1994 and 21-Jul-1994. There is a good agreement between the LOA calibration and the first insitu calibration. The second insitu calibration shows discrepancies greater than 10% for all channels. The reasons for such discrepancies are currently unknown. 4.2.1 Specifications The general specifications of calibration accuracy were 5% absolute accuracy, 3% interband relative calibration accuracy, and 2% multitemporal relative calibration accuracy. 4.2.1.1 Tolerance A general rise of the sensitivity was noted between the two calibration experiments made at LOA (11-May-1994, 24-Oct-1994): 8% in the blue (443 nm), 3.5% in the green (550 nm) and in the red (670 nm), 5.5% for the 864 nm channel, and 5% for the 910-nm channel. For subsequent processing, mean coefficients obtained at LOA are used. 4.2.2 Frequency of Calibration The instrument is generally calibrated once before an experimental campaign and once after the campaign. Calibration was performed at LOA on 11-May-1994 and 24-Oct-1994. On-site calibration was performed on 27-May-1994 and 21-Jul-1994. 4.2.3 Other Calibration Information Knowing the spectral radiance at the outport of the sphere or the hemisphere, the sensitivity of the various filters, and the spectral value of the solar exoatmospheric irradiance, the normalized radiance is computed using: where, L : spectral radiance (Wm-2sr-1µm-1) as a function of wavelength (??i) S : spectral sensitivity as a function of wavelength E : spectral exoatmospheric solar irradiance (Wm-2µm-1) as a function of wavelength The normalized radiance is used (see Section 9.2.1) to derive the absolute calibration coefficient . 5. Data Acquisition Methods During a helicopter flight, the POLDER instrument operated continuously. In data analysis, sites were first selected that were homogeneous within the POLDER FOV. This selection was obtained by looking at the symmetry of the measurement with respect to the principal plane. Each pixel in the image corresponds to a different viewing geometry for a surface that is assumed to be invariant. Thus, the image simultaneously gives a representation of the complete directional signature. The measurements corresponding to the principal plane and the perpendicular plane have been extracted from the image data. 6. Observations 6.1 Data Notes None given. 6.2 Field Notes None given. 7. Data Description 7.1 Spatial Characteristics 7.1.1 Spatial Coverage The following are North American Datum of 1983 (NAD83) coordinates of locations that were visited: BORIS West North UTM UTM UTM Site Grid ID Longitude Latitude Easting Northing Zone ----- -------- --------- -------- ------- -------- ---- NSA OBS T3R8T 98.48139 55.88007 532462.9 6192628.0 14 NSA YJP T8S9T 98.28706 55.89575 544583.9 6194706.9 14 Spruce #1 ------ no coordinates given ------------ Aspen ------ no coordinates given ------------ Bare Soil ------ no coordinates given ------------ Oat ------ no coordinates given ------------ Grated Field ------ no coordinates given ------------ Spruce #2 G2L7S 104.63784 53.90349 523823.6 5972622.0 13 Spruce #3 ------ no coordinates given ------------ NSA Fen F0L9T 98.42072 55.91481 536207.9 6196749.6 14 Jack Pine G7K8P 104.63784 53.90349 523823.6 5972622.0 13 Pasture ------ no coordinates given ------------ Harvested Field ------ no coordinates given ------------ Wheat ------ no coordinates given ------------ 7.1.2 Spatial Coverage Map Not available. 7.1.3 Spatial Resolution The pixel size is about 2 m when POLDER collects data from the helicopter at a 300-m altitude. 7.1.4 Projection Not applicable. 7.1.5 Grid Description Not applicable. 7.2 Temporal Characteristics 7.2.1 Temporal Coverage POLDER data were collected on one of two platforms during the three 1994 IFCs. Dates are indicated in Section 7.2.2. Most experiments took place in the morning, except 16-Sep, G7K8P. 7.2.2 Temporal Coverage Map BOREAS Site Ops Grid IFC-1 IFC-2 IFC-3 ---- ---------- ----- ----- ----- Black Spruce T3R8T 06/07 Young Jack Pine T8S9T 06/07 Spruce 1 06/07 Aspen 09/06 Bare Soil 09/12 Oat 09/12 Grated Field 09/12 Spruce 2 G2L7S 09/16 Spruce 3 09/16 Fen F0L9T 09/16 Jack Pine G7K8P 09/16 Pasture 09/17 Harvested Field 09/17 Wheat 09/17 7.2.3 Temporal Resolution See Section 7.2.1. 7.3 Data Characteristics Data characteristics are defined in the companion data definition file (rss20helo.def). 7.4 Sample Data Record Sample data format shown in the companion data definition file (rss20helo.def). 8. Data Organization 8.1 Data Granularity All of the BRDF data are in one file. 8.2 Data Format(s) The data files contain American Standard Code for Information Interchange (ASCII) 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 records are shown in the companion data definition file (rss20helo.def). 9. Data Manipulations 9.1 Formulae See Section 9.2. 9.1.1 Derivation Techniques and Algorithms See Section 9.2. 9.2 Data Processing Sequence 9.2.1 Processing Steps 9.2.1.1 Level 1 Images The raw radiometric data are digital numbers noted , where i, j are indices of pixel location on the CCD matrix, k is the wavelength, and a is the polarizer number for spectral bands comprising three polarizers. For the other spectral bands, a is meaningless. The processing from level 0 to level 1 data consists of the transformation of raw data into data proportional to normalized radiances , according to the equation: with reference exposure time, used in calibration : 100 ms t exposure time during operation average of line j of dark current calibration coefficient relative sensitivity (high and low frequency) of instrumental (optics + CCD) transmission. It is normalized such that the local average of at the matrix center equals 1. sensitivity of absolute calibration to CCD temperature CCD temperature during calibration T CCD temperature in operation is a digital number proportional, for the channels without polarizers, to the observed normalized radiance, i.e., with observed radiance (Wm-2sr-1µm-1) for pixel i, j in band k exoatmospheric solar irradiance in band k (Wm-2µm-1). For polarized bands, the aircraft displacement between successive channels acquisition must be taken into account to obtain a normalized spectral radiance from the three polarized channels where (x,y) are surface coordinates that refer to CCD pixels coordinates (i,j) in each of the polarized channels viewing the same ground point (x,y). The level 1 images provide data that for each band are equal to the right-hand side of the two previous equations. They are essentially normalized radiances. 9.2.1.2 BRDF Over Tower Sites Radiance to Reflectance: The radiance is converted to reflectance according to where is the solar zenith angle. Image Coregistration (for C-130 data): The POLDER electronics subsystem recorded the aircraft position and attitude parameters during the flights. The initial bias between the inertial reference system and the POLDER optical axis was measured before each flight. Attitude and location data should be sufficient to correct POLDER images for geometry. However, the uncertainty in the aircraft position was too large. A ground control point technique was therefore used to fine-tune the geometric multi- image registration of the whole set. To derive the BRDF over tower sites, a simple translation was made on geocoded images. The reflectance measurements were then averaged on a 5 x 5 pixel window (175 x 175 m2) around each tower site to minimize residual misregistration effects. 9.2.2 Processing Changes None. 9.3 Calculations 9.3.1 Special Corrections/Adjustments None. 9.3.2 Calculated Variables Radiance and reflectance were calculated. 9.4 Graphs and Plots None. 10. Errors 10.1 Sources of Error For images and BRDF data, there is some uncertainty in the absolute calibration coefficient, as illustrated by the calibration tables shown above. For the BRDF data, an additional source of error results from image registration. In the processing, it is assumed that the position of the site is the same for all images of the sequence, which can induce a error in the location of less than 1 pixel. These errors are lessened with the spatial averaging procedure. The smoothing aspect of the BRDF data tends to show that the misregistration errors are not critical. 10.2 Quality Assessment 10.2.1 Data Validation by Source The POLDER data have been tested against the four-scale BRDF reflectance model (Leblanc et al., 1997) as well as against the PARABOLA data and the DART 3-D BRDF model (Gastellu-Etchegorry et al., 1997). 10.2.2 Confidence Level/Accuracy Judgment The uncertainty associated with POLDER spectral reflectances values, taking into account only error in the absolute calibration coefficient, is approximately less than 0.005 for the visible channels and 0.01 for the near-infrared channel. The confidence level in these measurements is good because of their reproducibility for different axes during the same flight. 10.2.3 Measurement Error for Parameters Not available. 10.2.4 Additional Quality Assessments The directional reflectances obtained with POLDER data corrected from atmospheric effects for the flux tower or auxiliary sites can be compared to similar data made by other instruments. 10.2.5 Data Verification by Data Center BORIS staff has looked at some of the POLDER imagery from the C-130. It appears that there are some registration problems between bands in some of the imagery. 11. Notes 11.1 Limitations of the Data None. 11.2 Known Problems with the Data None. 11.3 Usage Guidance Non applicable. 11.4 Other Relevant Information None. 12. Application of the Data Set The data set could be used for BRDF model inversion and verification of BRDF direct models. 13. Future Modifications and Plans None. 14. Software 14.1 Software Description None given. 14.2 Software Access Raw data and processing software might be available upon request. Contact one of the individuals listed in Section 2.3. 15. Data Access 15.1 Contact Information Ms. Beth Nelson BOREAS Data Manager NASA GSFC Greenbelt, MD (301) 286-4005 (301) 286-0239 (fax) Elizabeth.Nelson@gsfc.nasa.gov 15.2 Data Center Identification See Section 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-20 POLDER helicopter data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). 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 American Standard Code for Information Interchange (ASCII) files. 17. References 17.1 Platform/Sensor/Instrument/Data Processing Documentation None. 17.2 Journal Articles and Study Reports Bréon, F.M., V. Vanderbilt, M. Leroy, P. Bicheron, C.L. Walthall, and J.E. Kalshoven. 1997. Evidence of hot spot directional signature from airborne POLDER measurements. IEEE Transactions on Geoscience and Remote Sensing. vol. 35, pp. 479-484. Bicheron, P., M. Leroy, O. Hautecoeur, and F.M. Bréon. 1997. Enhanced discrimination of boreal forest covers from airborne directional POLDER data. JGR, BOREAS special issue (in press). Deschamps, P.Y., F.M. Bréon, M. Leroy, A. Podaire, A. Bricaud, J.C. Buriez, and G. Sèze. 1994. The POLDER mission: Instrument characteristics and scientific objectives. IEEE Transactions on Geoscience and Remote Sensing, 32, pp. 598-615. Leblanc, S.G., P. Bicheron, J.M. Chen, M. Leroy, and J. Cihlar. 1997. Investigation of directional reflectance in boreal forests with an improved 4- scale model and airborne POLDER data. IEEE Transactions on Geoscience and Remote Sensing (submitted 1997). Leroy, M. and F.M. Bréon. 1996. Surface reflectance angular signatures from airborne POLDER data. Remote Sensing of Environment, 57, pp. 97-107. Leroy, M., P. Bicheron, and O. Hautecoeur. 1997. An algorithm of LAI and FAPAR retrieval to be used with spaceborne POLDER/ADEOS data. In: 7th International Symposium - Physical Measurements and Signatures in Remote Sensing. Courchevel, France. Gastellu-Etchegorry J.P., P. Guillevic, F. Zagolski, V. Demarez, V. Trichon, D. Deering, and M. Leroy. 1997. Modelling BRDF and radiation regime of boreal and tropical forest. Remote Sensing of Environment (submitted).. Sellers, P.and F. Hall. 1994. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1994-3.0, NASA BOREAS Report (EXPLAN 94). Sellers, P.and F. Hall. 1996. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1996-2.0, NASA BOREAS Report (EXPLAN 96). Sellers,P.J., F.G. Hall, R.D. Kelly, A. Black, D. Baldocchi, J. Berry, H. Margolis, M. Ryan, J. Ranson, P.M. Crill, D.P. Lettenmeier, J. Cihlar, J. Newcomer, D. Halliwell, D. Fitzjarrald, P.G. Jarvis, S.T. Gower, D. Williams, B. Goodison, D.E. Wickland, and F.E. Guertin. 1997. BOREAS in 1997: Scientific results, experiment overview and future directions. BOREAS Special Issue, JGR-Atmospheres, 102:28,731-28,770, No. D24.. Sellers, P., F. Hall, and K.F. Huemmrich. 1996. Boreal Ecosystem-Atmosphere Study: 1994 Operations. NASA BOREAS Report (OPS DOC 94). Sellers, P., F. Hall, and K.F. Huemmrich. 1997. Boreal Ecosystem-Atmosphere Study: 1996 Operations. NASA BOREAS Report (OPS DOC 96). 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. 17.3 Archive/DBMS Usage Documentation None. 18. Glossary of Terms None. 19. List of Acronyms 6S - Second Simulation of the Satellite signal in the Solar Spectrum ARC - Ames Research Center ASCII - American Standard Code for Information Interchange BOREAS - BOReal Ecosystem-Atmosphere Study BORIS - BOREAS Information System BRDF - Bidirectional Reflectance Distribution Function BPDF - Bidirectional Polarization Distribution Function CCD - Change Coupled Device DAAC - Distributed Active Archive Center EOS - Earth Observing System EOSDIS - EOS Data and Information System FOV - Field of View GSFC - Goddard Space Flight Center IFC - Intensive Field Campaign LAI - Leaf Area Index LOA - Laboratoire d'Optique Atmospherique NASA - National Aeronautics and Space Administration NSA - Northern Study Area OA - Old Aspen OBS - Old Black Spruce OJP - Old Jack Pine ORNL - Oak Ridge National Laboratory PANP - Prince Albert National Park POLDER - POLarization and Directionality of Earth’s Reflectances SSA - Southern Study Area URL - Uniform Resource Locator UTM - Universal Transverse Mercator WFF - Wallops Flight Facility YJP - Young Jack Pine 20. Document Information 20.1 Document Revision Date Written: 12-Sep-1996. Updated: 08-Feb-1999 20.2 Document Review Date(s) BORIS Review: 1-Oct-1997 Science Review: 20.3 Document ID 20.4 Citation When using these data, please include the following acknowledgement as well as citations of relevant papers in Section 17.2: Acknowledge Marc Leroy and Patrice Bicheron (CESBIO, Toulouse) and François- Marie Bréon (LMCE, Saclay) for providing the POLDER data. Thank LOA (Lille) for providing the POLDER instrument. If using data from the BOREAS CD-ROM series, also reference the data as: Leroy, Marc and F. Bréon, "Estimation of Photosynthetic Capacity using POLDER Polarization." 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 Keywords: POLDER Bidirectional Reflectance Aircraft Sensors Polarized Reflectance RSS20_POLDER_Helo.doc 03/03/99