MERIS Radiometric and Spectral Calibration

1
MERIS Radiometric and Spectral Calibration J-P
Huot, ESA-ESTEC, Noordwijk, the
Netherlands XVII Ocean Optics Calibration
Workshop Fremantle 30th October 2004
2
MERIS Instrument Design
3
MERIS Bands
15 Bands
4
MERIS Spatial Resolutions
5
MERIS Radiometric Calibration
On-ground Characterization
BRDF at 410 nm for four illumination conditions
Diffuser viewing geometry
6
MERIS on-ground Characterization
Diffuser BRDF measurement Inter-comparison
Inter-comparison with NASA, agreement within
measurement accuracy (1)
7
Instrument Gain Azimuth dependence
27.38 deg vs 22.61 deg(4.7 Degrees)
35.18 deg vs 27.38 deg (7.8 Degrees)
24.23 deg vs 22.61 deg (1.6 Degrees)
Diffuser illumination azimuth angles
Diffuser BRDF model agrees with the on-orbit
verification to 0.5.
8
MERIS Instrument Degradation
Diffuser illumination
Degradation is lt 3 after more than 2 years in
space
Degradation Model based on the SeaWifs model
(Barnes et al.)
9
MERIS Diffuser Aging
Dif.-1 100 min Dif.-2 10 min
Diffuser Degradation is lt0.5 after 100 min
of Solar exposition
Diffuser Degradation process appears linear
10
MERIS Vicarious Calibration
Ocean (CNES)
Error bars correspond to standard deviation of
results

• Incredibly good consistency between Rayleigh and
  Glitter
• All results within 1.5 of MERIS official
  calibration
• best result ever seen with these methods
• No significant variation with time

11
MERIS Vicarious Calibration
SIMBADA Ocean (LOA)

• 23 independant pixels from 14 scenes
• AOT max 0.15
• within 3 hours from satellite overpass
• Case 2 waters rejected

12
MERIS Vicarious Calibration
Dark Water (LISE)
13
MERIS Radiometric Calibration
Conclusions

• MERIS Radiometric Scale is on-board diffuser BRDF
  
• Instrument Degradation (aging) is lt 3
• Diffuser Degradation (aging) is lt 0.5
• Vicarious Calibration compares to 2 (over
  water)

14
MERIS Spectral Calibration
MERIS Band Programmability
15
Erbium Doped Diffuser
MERIS Spectral Calibration
Pink Diffuser Measurements
Erbium absorption spectrum
Band settings
16
MERIS Spectral Calibration
Erbium Doped Diffuser Results
Method Determine the position of the absorption
peak in pixel number, with correction for
Air-Vacuum changes (Edlen)
17
MERIS Spectral Calibration
Oxygen O2A
Natural target Measurements
Oxygen O2A absorption spectrum MERIS spectral
response overlay
Band settings
18
MERIS Spectral Calibration
Oxygen O2A Results

• 2 Methods
• A pressure minimization (LISE)
• Minimizes the in surface pressure
• retrieved from 7 O2A-campaign bands
• Spectrum-matching (FUB)
• Neural net trained on a large number
• of radiative transfer computations

When using O2A results for absolute spectral
Calibration, great care should be taken in the
selection of the scene to be analyzed as the
absorption spectrum is very sensitive to the
underlying surface and air-mass seen.
19
MERIS Spectral Calibration
Fraunhofer Lines
White diffuser measurement
Examples of Fraunhofer absorption spectrum With
MERIS spectral response overlay
Band settings (3 configurations)
20
MERIS Spectral Calibration
Fraunhofer Lines Results
Results all Fraunhofer lines (O2)
Line 2 Raw data 5-cameras, 3 Fov
Method A spectrum-matching, with correction for
Air-Vacuum (Edlen)
21
Spectral Calibration Results
All Spectral Calibration Results
22
MERIS Spectral Model
Deviation from Linear
Offset Slope
Residual Error
Simple instrument model where k and l stand for
the spatial and spectral co-ordinates of a given
detector respectively , the mean dispersion law
mainly linear is a polynomial of order 3 (best
fit), and, the across-track variation term, is a
linear fit of the data at 395, 656 and 671nm
expressed relative to its mean value
(Model vs Cal Data)
23
MERIS Spectral Calibration
Conclusions

• Spectral Calibration based on
• 2 Erbium absorption lines
• 5 Fraunhofer absorption lines
• 1 Oxygen (O2A) absorption lines
• Used to derived a simple Instrument model
• Model compares to measurements to 0.1nm

24
MERIS Instrument Calibration
Conclusions

• MERIS Radiometric Scale is based on diffuser BRDF
  
• Instrument Degradation (aging) is lt 3
• Diffuser Degradation (aging) is lt 0.5
• Vicarious Calibration compares to 2 (over
  water)
• Spectral Calibration based on
• 2 Erbium absorption lines
• 5 Fraunhofer absorption lines
• 1 Oxygen (O2A) absorption lines
• Model compares to measurements to 0.1nm
• Geolocation accuracy lt 500m .. (Improving to 100
  m in 2005)

25
MERIS Solar Spectrum
26
MERIS In-Band Solar Irradiance
27
MERIS Smile Correction
Effect of smile on overlapping pixels at camera
interface
MERIS radiometric calibration accounts for the
instrument spectral model on a pixel basis. The
solar terrestrial irradiance used in the
determination of the instrument gains differs on
a pixel basis, which will lead to large
radiance jumps at camera interfaces. The largest
contribution comes from the in-band solar
irradiance however, even when corrected, (i.e
reflectance), a colored signal such as Rayleigh
scattering can yield a difference of 1 for a 1nm
Inter-camera jump in the Blue end of the
spectrum.
28
MERIS Smile Correction
Overlapping pixels agree following smile
correction
29
Smile Correction Validation

• Smile correction includes
• Conversion to reflectance using an
• in-band solar irradiance per pixel,
• Linear interpolation between adjacent
• bands, to the nominal wavelength
• for which the L2 LUTs are computed

Central Wavelength Band-2
Chlorophyll concentrations before (left) and
after (right) smile correction