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SPECTRAL REFLECTANCEMEASUREMENT 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

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Outline

Introduction

Principles and instrumentations used for the field reflectance measurements

Measurement spatial sampling strategies

Results and conclusion

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Introduction

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

Radiance-based method: field upwelling radiance measurement at the time of the satellite acquisition + propagtion

Reflectance-based method: field reflectance measurement (same illumination conditions) + meas/model of the irradiance + propagationAdvantages: more accurate and detailed site characterization

One of the objective of 2010 CEOS Key comparison: Tuz Gölu campaign, was to compare different methods of field reflectance characterization for satellite calibration.

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Principle of field reflectance mesurement (1)

Aim : Vicarious calibration of multispectral satellite sensors

→ Measurements of radiance and reflectance meas. have to be spectrally resolved

→ Ground measurements done with a spectroradiometer

Meas. on the surface at t1

Meas. on the reference at t2

Measurement of the reflectance is relative to a reference:

o Meas. of the radiance of the surface of interest at t1

o Meas. of the radiance the reference at t2

Rground = Rref x Sground(t1) / Sref(t2)

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Possible compensation of irradiance temporal variation:

→ Measurement on the reference before and after measurement on the surface

Rground = Rref x Sground(t2) / Sref-mean with Sref-mean = [Sref(t1) + Sref(t3) ] / 2

Principle of field reflectance mesurement (2)

Meas. on the surface at t2

Meas. on the reference at

t1

Meas. on the reference at

t3

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Possible compensation of BRDF effects:- Usually, the reference panel reflectance factor (calibration)

Rref = directional (8°)- hemispherical reflectance

- The reflectance factor Rsurface of the ground and of the panel measured outdoors = bidirectional reflectance + hemispherical-directional reflectance (because of the direct + diffuse illumination conditions)

- But the reference panel (like spectralon) is nearly lambertian but not exactly

Principle of field reflectance mesurement (3)

RD-Href

8°

IFOV

Rsurface

BRDF correction on the reference panel reflectance Rref is suitable, according to the solar zenith angle and the viewing zenith angle, for applying the equation:

Rground = Rref (θsun,θv) x Sground / Sref

BRF spectralon theta_v = 0° λ=633nm

0,70

0,75

0,80

0,85

0,90

0,95

1,00

1,05

0 10 20 30 40 50 60 70 80 90théta_s

BR

F

0

2040

6080

90100

120140

160180

average

ϕ_v - phi_s

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Instrumentation for spectral reflectance measurements of Tuz Gölü site: Sensors

CMA/NSMC, CSIR, GISTDA, KARI, ONERA, SDSU, TUBITAK and VITO used an ASD Fielspec spectroradiometer, calibrated in radiance

8° FOV

∅ 10-20 cm (depending on h)

h

1.8-2.5µm1-1.8µm0.35-1µmSpectral domain

InGaAsInGaAsCCDDetector

6.5nm8.5nm3nmSpectral resolution

2nm

resamp 1nm

2nm

resamp 1nm

1.4nm

resamp 1nm

Spectral sampling

Sample: 10 spectra averaged

Dark current: 25 spectra averaged

Measure-ments

INPE used a CIMEL 313 field multispectral radiometer

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

Wavelenght (nm)

Band_1

Band_2

Band_3

Band_4

Band_5

550450 650 850 1500 nm

FOV: 10°5 bands (filter wheel)Calibrated in radiance

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Instrumentation for spectral reflectance measurements of Tuz Gölü site: Reference panels

All the reference panels were spectralon® from Labsphere, except one of NPL (Gigahertz Optik) and the one of CMA (Hefei Institute of Physical Science)

Reference panel were calibrated: • Directional-hemispherical reflectance, by Labsphere (CSIR, KARI, Tübitak)• Directional-hemispherical reflectance, by themthelves (NPL, Onera, INPR)• BRDF (SDSU, CMA)

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Site description and spatial sampling of ground-based reflectance measurements

AVNIR-2 acquisition image of Tuz Gölü site on 15 Aug 2010

Problem: Area wide enough for satellite calibration Small enough to be measured on the ground in a

reasonable time Which sampling strategy for ground-based

reflectance measurement?

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Sampling strategies

Challenges: Giving a good assessment of the mean reflectance of a (large) area Giving variations around this mean Do all the measurements in less than one hour if possible

3 sampling strategies: Spaced out sampling averaging several

measurements over one point

Space out sampling with estimation of the local spatial variability

In-motion sampling

etc

etc

etc10 10 10 10

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Spaced out sampling averaging several measurements on each point (1)

Regular grid with spaced out sampling points Several measurements are averaged for each sampling point

→ allows evaluation of the type A uncertainty (repeatability)

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

RréfPS

PSP

iref

igroundigroundR ×=

)(

)()(

∑=

=10

12 ),()(

jijgroundiground tPSPS ∑∑

==

⋅+⋅=5

135

15

115

1 ),(),()(j

ijrefj

ijrefiref tPStPSPS

( ) ( ))55(²

²

10²

²2

+⋅+

⋅=

ref

ref

ground

ground

repeat S

S

S

Su

σσ

40 m40 m40 m40 m40 m40 m40 m20m

30m

20m

20m

30m

40 m40 m40 m40 m40 m40 m40 m20m

30m

20m

20m

30m40 m40 m40 m40 m40 m40 m40 m40 m40 m40 m40 m40 m40 m40 m20m20m

30m30m

20m20m

20m20m

30m30m

etc10 10 10 10

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Spaced out sampling averaging several measurements on each point (2)

A similar strategy had also been used by: CSIR: for each point: optimisation, 5 references, 5 ground measurements

CSIR sampling for M3 300m x 100m area

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Spaced out sampling averaging several measurements on each point (2)

A similar strategy had also been used by: CSIR: for each point optimisation, 5 references, 5 ground measurements INPE: 1h30 for ~30 points, for each: 5 references, 5 ground measurements

INPE sampling for one 300m x 100m area

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Spaced out sampling averaging several measurements on each point (2)

A similar strategy had also been used by: CSIR: for each point optimisation, 5 references, 5 ground measurements INPE: 1h30 for 29-30 points, for each 5 references, 5 ground measurements Tübitak Uzay: 15 to 30 sampling points, ref. every 10-15min, 10 meas.

averaged for ref and ground

Tübitak Uzay's sampling for one 300m x 100 m area

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Spaced out sampling averaging several measurements on each point (2)

A similar strategy had also been used by: CSIR: for each point optimisation, 5 references, 5 ground measurements INPE: 1h30 for 29-30 points, for each 5 references, 5 ground measurements Tübitak Uzay: 15 to 30 sampling points, ref. every 10-15min, 10 scans

averaged for ref and ground CMA: 11 sampling points in 1h30min, for each point: 2 ref. and 5 ground meas.

CMA's sampling for the M9 1km x 1km area

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Spaced out sampling with local variability measurements

As previous strategy, uses a regular grid with spaced out sampling pointsFor each point of the grid, several meas. are locally recorded over different points near the nominal location

etc

Strategy used by VITO: for each sampling point: 5 ref., 4 points (4 meas. per point), 5 ref.

VITO's sampling strategy for the 300 m x 100 m areas (25 points)

Su

Local measurements for each sampling point

VITO's sampling strategy for the M9 1km x 1km area

(11 points) Drawback: time consuming

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In-motion sampling (1)

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).

Ref. panel is measured at the beginning and the end of the transect.

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

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In-motion sampling (2)

A similar strategy had also been used by: CSIR: for each transect: optimisation, 5 ref, continuous measurements, 5 ref at

the end

CSIR in-motion sampling for M3 300m x 100m area

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In-motion sampling (2)

A similar strategy had also been used by: CSIR: for each transect: optimisation, 5 ref, continuous measurements, 5 ref at

the end Onera for M9 1km x 1km area: 10 meas. continuously acquired / 15m-20m

(10s), averaged in one sample. ~50 samples / transect.

Onera in-motion sampling for M9 1km x 1km area

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Results comparison (1)

All teams pre-process the data, and supplied to NPL the reflectance factor R(λ) with the associated standard uncertainty Type A and Type B

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

The results are still preliminary, since some data of some teams have to be recalculated.

The plotted RF had been measured at different dates (17 to 25 August 2010) – for similar sun zenith angles

Good agreement (+/- 0.02)

M4 300m x 300m area reported reflectance

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Results comparison (2)

Results on M9 1km x 1km site: (Less) good agreement (+/- 0.05 in

the visible, +/- 0.03 in the SWIR Site bigger higher sensitivity to

spatial variability; Longer time of measurement

different sun Zenith angles

M9 1km x 1km area reported reflectance

Possible reasons of the differences: Measurements bias and uncertainties, Temporal variability of the reflectance (solar zenith angle difference

30° to 40°, soil humitity content) Slight directional effects on ref. panel Spatial variability

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Conclusion

Ground-based reflectance measurements of Tuz Gölü site has been made by 9 teams

Similarities: Instrumentation (ASD fieldspec, except INPE) Preliminary intercalibration (spectroradiometers and reference

panels) "Best practise sharing" before the experiment (Tuz Gölü 2009)

Differences: 3 different sampling strategies: spaced out sampling, spaced

out sampling with local variability, in motion sampling Non simultaneity of the measurements

Promissing results: Good agreement between the measurements made by the

different teams Some reprocessing in progress…