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SPECTRAL  REFLECTANCE MEASUREMENT  METHODOLOGIES FOR  TUZ  GOLU  FIELD  CAMPAIGN   Y. Boucher ,  F. Viallefont,  A. Deadma...
Outline <ul><li>Introduction </li></ul><ul><li>Principles and instrumentations used for the field reflectance measurements...
Introduction <ul><li>Vicarious calibration  = comparison of the  radiance  measured on-orbit  and the  radiance  propagate...
Principle of field reflectance mesurement (1) <ul><li>Aim  : Vicarious calibration of multispectral satellite sensors  </l...
Principle of field reflectance mesurement (2) Possible compensation of irradiance temporal variation:    Measurement on t...
Principle of field reflectance mesurement (3) <ul><li>Possible compensation of BRDF effects : </li></ul><ul><li>Usually, t...
Instrumentation for spectral reflectance measurements of Tuz Gölü site: Sensors <ul><li>CMA/NSMC, CSIR, GISTDA, KARI, ONER...
Instrumentation for spectral reflectance measurements of Tuz Gölü site: Reference panels <ul><li>All the  reference panels...
Site description and spatial sampling of ground-based reflectance measurements <ul><li>Problem :  </li></ul><ul><li>Area  ...
Sampling strategies <ul><li>Challenges: </li></ul><ul><li>Giving a good assessment of the mean reflectance of a (large) ar...
Spaced out sampling averaging several measurements on each point (1) <ul><li>Regular grid with spaced out sampling points ...
Spaced out sampling averaging several measurements on each point (2) <ul><li>A similar strategy had also been used by: </l...
Spaced out sampling averaging several measurements on each point (2) <ul><li>A similar strategy had also been used by: </l...
Spaced out sampling averaging several measurements on each point (2) <ul><li>A similar strategy had also been used by: </l...
Spaced out sampling averaging several measurements on each point (2) <ul><li>A similar strategy had also been used by: </l...
Spaced out sampling with local variability measurements <ul><li>As previous strategy, uses a  regular grid  with spaced ou...
In-motion sampling (1) <ul><li>The “ In-motion” technique  was developed by University of Arizona and South Dakota State U...
In-motion sampling (2) <ul><li>A similar strategy had also been used by: </li></ul><ul><li>CSIR : for each transect: optim...
In-motion sampling (2) <ul><li>A similar strategy had also been used by: </li></ul><ul><li>CSIR: for each transect: optimi...
Results comparison (1) <ul><li>All teams pre-process the data, and supplied to NPL the reflectance factor R(  ) with the ...
Results comparison (2) <ul><li>Possible reasons of the differences:  </li></ul><ul><ul><li>Measurements bias and uncertain...
Conclusion <ul><li>Ground-based reflectance measurements of Tuz Gölü site has been made by 9 teams </li></ul><ul><li>Simil...
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TH4.T05.3_IGARSS-2011-3233-BOUCHER.ppt

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TH4.T05.3_IGARSS-2011-3233-BOUCHER.ppt

  1. 2. SPECTRAL REFLECTANCE MEASUREMENT METHODOLOGIES FOR TUZ GOLU FIELD CAMPAIGN Y. Boucher , F. Viallefont, A. Deadman, N. Fox , I. Behnert, D. Griffith, P. Harris, D. Helder, E. Knaeps, L. Leigh, Y. Li, H. Ozen, F. Ponzoni, S. Sterckx
  2. 3. Outline <ul><li>Introduction </li></ul><ul><li>Principles and instrumentations used for the field reflectance measurements </li></ul><ul><li>Measurement spatial sampling strategies </li></ul><ul><li>Results and conclusion </li></ul>
  3. 4. Introduction <ul><li>Vicarious calibration = comparison of the radiance measured on-orbit and the radiance propagated from the optical or radiative properties of the measured reference test site </li></ul><ul><li>Radiance-based method : field upwelling radiance measurement at the time of the satellite acquisition + propagtion </li></ul><ul><li>Reflectance-based method : field reflectance measurement (same illumination conditions) + meas/model of the irradiance + propagation </li></ul><ul><li>Advantages : more accurate and detailed site characterization </li></ul><ul><li>One of the objective of 2010 CEOS Key comparison: Tuz Gölu campaign, was to compare different methods of field reflectance characterization for satellite calibration. </li></ul>
  4. 5. Principle of field reflectance mesurement (1) <ul><li>Aim : Vicarious calibration of multispectral satellite sensors </li></ul><ul><li> Measurements of r adiance and reflectance meas. have to be spectrally resolved </li></ul><ul><li> Ground measurements done with a s pectroradiometer </li></ul><ul><li>Measurement of the reflectance is relative to a reference: </li></ul><ul><li>Meas. of the radiance of the surface of interest at t 1 </li></ul><ul><li>Meas. of the radiance the reference at t 2 </li></ul><ul><li>R ground = R ref x S ground (t1) / S ref (t2) </li></ul>Meas. on the surface at t 1 Meas. on the reference at t 2
  5. 6. Principle of field reflectance mesurement (2) Possible compensation of irradiance temporal variation:  Measurement on the reference before and after measurement on the surface R ground = R ref x S ground (t 2 ) / S ref-mean with S ref-mean = [S ref (t 1 ) + S ref (t 3 ) ] / 2 Meas. on the surface at t 2 Meas. on the reference at t 1 Meas. on the reference at t 3
  6. 7. Principle of field reflectance mesurement (3) <ul><li>Possible compensation of BRDF effects : </li></ul><ul><li>Usually, the reference panel reflectance factor (calibration) R ref = directional (8°)- hemispherical reflectance </li></ul><ul><li>The reflectance factor R surface of the ground and of the panel measured outdoors = bidirectional reflectance + hemispherical-directional reflectance </li></ul><ul><li>( because of the direct + diffuse illumination conditions) </li></ul><ul><li>But the reference panel (like spectralon) is nearly lambertian but not exactly </li></ul><ul><li>BRDF correction on the reference panel reflectance R ref is suitable, according to the solar zenith angle and the viewing zenith angle, for applying the equation: </li></ul><ul><li>R ground = R ref (  sun ,  v ) x S ground / S ref </li></ul>R D-H ref 8° IFOV R surface
  7. 8. Instrumentation for spectral reflectance measurements of Tuz Gölü site: Sensors <ul><li>CMA/NSMC, CSIR, GISTDA, KARI, ONERA, SDSU, TUBITAK and VITO used an ASD Fielspec spectroradiometer , calibrated in radiance </li></ul><ul><li>INPE used a CIMEL 313 field multispectral radiometer </li></ul>FOV: 10° 5 bands (filter wheel) Calibrated in radiance 8° FOV  10-20 cm (depending on h) h 1.8-2.5µm 1-1.8µm 0.35-1µm Spectral domain InGaAs InGaAs CCD Detector 6.5nm 8.5nm 3nm Spectral resolution 2nm resamp 1nm 2nm resamp 1nm 1.4nm resamp 1nm Spectral sampling Sample: 10 spectra averaged Dark current: 25 spectra averaged Measure-ments 550 450 650 850 1500 nm
  8. 9. Instrumentation for spectral reflectance measurements of Tuz Gölü site: Reference panels <ul><li>All the reference panels were spectralon ® from Labsphere, </li></ul><ul><li>except one of NPL ( Gigahertz Optik ) </li></ul><ul><li>and the one of CMA ( Hefei Institute of Physical Science ) </li></ul><ul><li>Reference panel were calibrated : </li></ul><ul><ul><li>Directional-hemispherical reflectance, by Labsphere (CSIR, KARI, Tübitak) </li></ul></ul><ul><ul><li>Directional-hemispherical reflectance, by themthelves (NPL, Onera, INPR) </li></ul></ul><ul><ul><li>BRDF (SDSU, CMA) </li></ul></ul>
  9. 10. Site description and spatial sampling of ground-based reflectance measurements <ul><li>Problem : </li></ul><ul><li>Area wide enough for satellite calibration </li></ul><ul><li>Small enough to be measured on the ground in a reasonable time </li></ul><ul><li>Which sampling strategy for ground-based reflectance measurement? </li></ul>AVNIR-2 acquisition image of Tuz Gölü site on 15 Aug 2010
  10. 11. Sampling strategies <ul><li>Challenges: </li></ul><ul><li>Giving a good assessment of the mean reflectance of a (large) area </li></ul><ul><li>Giving variations around this mean </li></ul><ul><li>Do all the measurements in less than one hour if possible </li></ul><ul><li>3 sampling strategies: </li></ul><ul><li>Spaced out sampling averaging several measurements over one point </li></ul><ul><li>Space out sampling with estimation of the local spatial variability </li></ul><ul><li>In-motion sampling </li></ul>etc etc etc 10 10 10 10
  11. 12. Spaced out sampling averaging several measurements on each point (1) <ul><li>Regular grid with spaced out sampling points </li></ul><ul><li>Several measurements are averaged for each sampling point </li></ul><ul><li> allows evaluation of the type A uncertainty (repeatability) </li></ul>Strategy used by Onera for 300m x 100m areas 21 sampling points. Time of measurement: 1 hour For each sampling point: Optimization, Ref.x5, ground meas.x10, ref.x5 40 m 40 m 40 m 40 m 40 m 40 m 40 m 20m 30m 20m 20m 30m etc 10 10 10 10
  12. 13. Spaced out sampling averaging several measurements on each point (2) <ul><li>A similar strategy had also been used by: </li></ul><ul><li>CSIR : for each point: optimisation, 5 references, 5 ground measurements </li></ul>CSIR sampling for M3 300m x 100m area
  13. 14. Spaced out sampling averaging several measurements on each point (2) <ul><li>A similar strategy had also been used by: </li></ul><ul><li>CSIR: for each point optimisation, 5 references, 5 ground measurements </li></ul><ul><li>INPE : 1h30 for ~30 points, for each: 5 references, 5 ground measurements </li></ul>INPE sampling for one 300m x 100m area
  14. 15. Spaced out sampling averaging several measurements on each point (2) <ul><li>A similar strategy had also been used by: </li></ul><ul><li>CSIR: for each point optimisation, 5 references, 5 ground measurements </li></ul><ul><li>INPE: 1h30 for 29-30 points, for each 5 references, 5 ground measurements </li></ul><ul><li>Tübitak Uzay : 15 to 30 sampling points, ref. every 10-15min, 10 meas. averaged for ref and ground </li></ul>Tübitak Uzay's sampling for one 300m x 100 m area
  15. 16. Spaced out sampling averaging several measurements on each point (2) <ul><li>A similar strategy had also been used by: </li></ul><ul><li>CSIR: for each point optimisation, 5 references, 5 ground measurements </li></ul><ul><li>INPE: 1h30 for 29-30 points, for each 5 references, 5 ground measurements </li></ul><ul><li>Tübitak Uzay: 15 to 30 sampling points, ref. every 10-15min, 10 scans averaged for ref and ground </li></ul><ul><li>CMA : 11 sampling points in 1h30min, for each point: 2 ref. and 5 ground meas. </li></ul>CMA's sampling for the M9 1km x 1km area
  16. 17. Spaced out sampling with local variability measurements <ul><li>As previous strategy, uses a regular grid with spaced out sampling points </li></ul><ul><li>For each point of the grid, several meas . are locally recorded over different points near the nominal location </li></ul>Strategy used by VITO : for each sampling point: 5 ref., 4 points (4 meas. per point), 5 ref. Drawback : time consuming etc VITO's sampling strategy for the 300 m x 100 m areas (25 points) Sun Local measurements for each sampling point VITO's sampling strategy for the M9 1km x 1km area (11 points)
  17. 18. In-motion sampling (1) <ul><li>The “ In-motion” technique was developed by University of Arizona and South Dakota State University (SDSU). The ASD spectrometer operator is moving continuously and measuring every few seconds (  few meters). </li></ul><ul><li>Ref. panel is measured at the beginning and the end of the transect. </li></ul>Advantage : high number of measurements are done in a short time Strategy used by SDSU : 20 meas. continuously acquired / 2.5m, averaged in one sample. 100 samples / row. 300m x 100m area measured in 30 minutes North Ref. panel meas. 8° FOV ~10cm x 2.5m
  18. 19. In-motion sampling (2) <ul><li>A similar strategy had also been used by: </li></ul><ul><li>CSIR : for each transect: optimisation, 5 ref, continuous measurements, 5 ref at the end </li></ul>CSIR in-motion sampling for M3 300m x 100m area
  19. 20. In-motion sampling (2) <ul><li>A similar strategy had also been used by: </li></ul><ul><li>CSIR: for each transect: optimisation, 5 ref, continuous measurements, 5 ref at the end </li></ul><ul><li>Onera for M9 1km x 1km area: 10 meas. continuously acquired / 15m-20m (10s), averaged in one sample. ~50 samples / transect. </li></ul>Onera in-motion sampling for M9 1km x 1km area
  20. 21. Results comparison (1) <ul><li>All teams pre-process the data, and supplied to NPL the reflectance factor R(  ) with the associated standard uncertainty Type A and Type B </li></ul><ul><li>NPL re-calculated the RF of the site, as a weighted mean of the individual sampling points values and the associated standard uncertainty to this weighted RF value </li></ul><ul><li>The results are still preliminary , since some data of some teams have to be recalculated. </li></ul><ul><li>The plotted RF had been measured at different dates (17 to 25 August 2010) – for similar sun zenith angles </li></ul><ul><li>Good agreement (+/- 0.02) </li></ul>M4 300m x 300m area reported reflectance
  21. 22. Results comparison (2) <ul><li>Possible reasons of the differences: </li></ul><ul><ul><li>Measurements bias and uncertainties, </li></ul></ul><ul><ul><li>Temporal variability of the reflectance (solar zenith angle difference 30° to 40°, soil humitity content) </li></ul></ul><ul><ul><li>Slight directional effects on ref. panel </li></ul></ul><ul><ul><li>Spatial variability </li></ul></ul><ul><li>Results on M9 1km x 1km site: </li></ul><ul><ul><li>(Less) good agreement (+/- 0.05 in the visible, +/- 0.03 in the SWIR </li></ul></ul><ul><ul><li>Site bigger  higher sensitivity to spatial variability; </li></ul></ul><ul><ul><li>Longer time of measurement  different sun Zenith angles </li></ul></ul>M9 1km x 1km area reported reflectance
  22. 23. Conclusion <ul><li>Ground-based reflectance measurements of Tuz Gölü site has been made by 9 teams </li></ul><ul><li>Similarities: </li></ul><ul><ul><li>Instrumentation (ASD fieldspec, except INPE) </li></ul></ul><ul><ul><li>Preliminary intercalibration (spectroradiometers and reference panels) </li></ul></ul><ul><ul><li>&quot;Best practise sharing&quot; before the experiment (Tuz Gölü 2009) </li></ul></ul><ul><li>Differences: </li></ul><ul><ul><li>3 different sampling strategies: spaced out sampling, spaced out sampling with local variability, in motion sampling </li></ul></ul><ul><ul><li>Non simultaneity of the measurements </li></ul></ul><ul><li>Promissing results: </li></ul><ul><ul><li>Good agreement between the measurements made by the different teams </li></ul></ul><ul><ul><li>Some reprocessing in progress… </li></ul></ul>

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