Intercomparison of
Proba-V observations
to simulations over
the Libya-4 site
M. Bouvet – ESA/ESTEC
Proba-V QWG#2
ESA UNCLASSIFIED – For Official Use
Outline
•
Proba-V observations vs. ProbaV simulated observations (on the MERIS
radiometric scale) over Libya-4
•
Proba-V observations vs. simulated VGT-2 observations over Libya-4
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 2
Proba-V observations vs. Proba-V
simulations on the MERIS
radiometric scale
ESA UNCLASSIFIED – For Official Use
Approach (from QWG#1)
•
We start from: a hyperspectral TOA reflectance model based on the
Monte Carlo MYSTIC radiative transfer code
•
A pre-defined atmosphere that requires O3 and WV as input
•
A surface BRDF model with 4 free parameters.
•
‘Calibration’ of TOA model on MERIS = inversion of the 4 BRDF free
parameters that best explain the MERIS observations at each spectral
band.
•
The inverted BRDF models obtained in each MERIS spectral band are
spectrally interpolated using a sand BRF measurement database =>
hyperspectral surface BRDF model
•
TOA observations from Proba-V can be simulated using the
hyperspectral BRDF model and the same pre-defined atmosphere + the
Proba-V spectral responses
See Bouvet M., Radiometric comparison of multispectral imagers over a pseudo-invariant calibration
site using a reference radiometric model, Remote Sensing of Environment 140 (2014) 141–154
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 7
MERIS observations vs. simulations
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 11
BRDF inversion result: surface BRDF
model
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 13
Spectral validity of the model: a sand
spectral database
83 BRF sand spectra measured on samples collected at the Simpson
desert (Australia), Muleshoe (Texas, USA) and in the Namibian desert.
Illumination is at 45 degrees and measurement at nadir (courtesy J. E.
Bullard and K. H. White )
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 15
MERIS, Proba-V (Center/Left/Right)
and VGT-2 RSR
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 17
Applying the hyperspectral TOA
model to MERIS full archive
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 18
Application of the hyperspectral TOA
model to simulate various sensor
observations
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 19
•
Bias between model and 10
years of MERIS data for all
spectral bands < 1%
•
RMSE between model and
MERIS < 1.5 % outside
absorption bands (not
shown here)
•
VGT-2 radiometric scale
about 4 % lower than MERIS
radiometric scale
Proba-V observations vs. ProbaV simulated
observations on the MERIS radiometric scale
PROBA-V ICP modifications => XCEL spreadsheet
ftp://ftp.estec.esa.int/pub/xe/anonymous/ProbaV_vs_MERIS_and_VGT2_results.html
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 22
Proba-V SRD requirements
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 23
Proba-V observations vs. Proba-V
simulations on MERIS radiometric
scale
Standard
deviation
Nb
95% CI
Mean of
associated
acquisitions
assuming
Nb
(rho_measto
Band name TMA name
discarded
normal
acquisitions
rho_sim)/r
(rho_measby 3-sigma
distribution
ho_sim in %
rho_sim)/r
filtering
in %
ho_sim in
%
BLUE
LEFT
172
3
-1.52
0.25
1.63
BLUE
CENTER
96
6
-0.22
0.33
1.62
BLUE
RIGHT
150
13
-2.18
0.13
0.77
RED
LEFT
169
7
-1.82
0.15
1
RED
CENTER
98
4
-0.37
0.27
1.34
RED
RIGHT
149
14
-0.98
0.13
0.82
NIR
LEFT
169
7
-2.81
0.32
2.06
NIR
CENTER
95
7
-1.55
0.33
1.6
NIR
RIGHT
154
9
-2.18
0.28
1.74
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 24
1-sigma CI
95% CI
associated
Temporal assuming
to each
linear trend normal
simulation
in %/year distribution
of MERIS in
in %/year
%
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.41
-1.55
0.18
-0.58
-1.69
-0.42
-1.17
-1.2
-1.3
0.43
0.48
0.22
0.26
0.32
0.23
0.53
0.53
0.45
Proba-V observations vs. Proba-V
simulations on MERIS radiometric
scale
The comparison of PV observations and PV simulations indicates that:
•
Assuming MERIS 2% absolute radiometry uncertainty is true (so far not
demonstrated otherwise), the absolute accuracy requirement < 5 % (VI-PER220) is met for all VIS-NIR bands.
•
On average for all bands and TMA in the VNIR, Proba-V radiometry is about 1.5
% below MERIS radiometry
•
Stability < 3 % (VI-PER-240) over 6 months is met although statistically
trends up to about 1.7 % / year are found but these are explained partially by
the operational changes to the calibration
•
Interband < 3 % (VI-PER-250) is also met if we restrict the interband to the
VNIR bands
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 25
Proba-V observations vs. Proba-V
simulations on MERIS radiometric
scale
•
Remaining issues (of the model?):
•
NIR oscillation due to residual error in water vapour absorption
modelling?
•
What about the SWIR?
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 26
Proba-V observations vs. VGT-2
simulations
ESA UNCLASSIFIED – For Official Use
Proba-V observations vs. VGT-2
simulations: the approach
•
The reference TOA model outputs VGT-2 like observations over the
Libya-4 site (in 4 spectral bands only)
•
The approach is similar to the one used previously: a TOA model with 4
free parameters (surface BRDF) is fitted to the VGT-2 observations in
the 2006-2009 period.
BUT
•
There is no attempt to further account for the differences in RSR
between VGT-2 and Proba-V (i.e., no spectral interpolation of the
obtained surface BRDF models in the VGT-2 bands)
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 28
Performance of the VGT-2 TOA model
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 29
Proba-V observations vs. VGT-2
simulations
ftp://ftp.estec.esa.int/pub/xe/anonymous/ProbaV_vs_MERIS_and_VGT2_results.html
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 30
Proba-V observations vs. VGT-2
simulations: synthesis of results
Band name
TMA name
BLUE
BLUE
BLUE
RED
RED
RED
NIR
NIR
NIR
SWIR1
SWIR1
SWIR1
SWIR2
SWIR2
SWIR2
SWIR3
SWIR3
SWIR3
LEFT
CENTER
RIGHT
LEFT
CENTER
RIGHT
LEFT
CENTER
RIGHT
LEFT
CENTER
RIGHT
LEFT
CENTER
RIGHT
LEFT
CENTER
RIGHT
Standard
95% CI
1-sigma CI
95% CI
Nb
Mean of
deviation
assuming
associated to
Temporal
assuming
Nb
acquisitions
(rho_measassociated to
normal
each
linear trend in
normal
acquisitions discarded by 3- rho_sim)/rho_
(rho_measdistribution in
simulation of
%/year
distribution in
sigma filtering
sim in %
rho_sim)/rho_
%
VGT-2 in %
%/year
sim in %
172
3
1.99
0.27
1.79
2
0.71
0.47
96
6
4.86
0.33
1.64
2
-1.21
0.53
148
15
2.14
0.17
1.05
2
0.39
0.3
170
6
1.4
0.18
1.15
2
-0.77
0.28
97
5
2.33
0.28
1.39
2
-1.65
0.35
148
15
1.79
0.16
0.98
2
-0.55
0.27
167
9
0.42
0.31
2
2
-1.25
0.51
94
8
2.41
0.33
1.61
2
-1.37
0.52
153
10
3.01
0.31
1.93
2
-1.6
0.48
96
4
-1.29
0.32
1.56
2
-1.7
0.44
43
3
0.64
0.44
1.43
2
-1.07
0.68
41
2
0.02
0.54
1.74
2
-1.15
0.93
66
3
0.2
0.31
1.27
2
-1.78
0.38
36
2
0.61
0.55
1.65
2
-0.74
0.96
68
2
-0.97
0.33
1.36
2
-1.26
0.47
50
3
0.37
0.41
1.44
2
-1.16
0.6
47
1
0.62
0.64
2.19
2
-0.53
1.16
86
7
-3.86
0.17
0.79
2
0.38
0.32
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 31
Proba-V observations vs. VGT-2
simulations: synthesis of results
The comparison of PV observations and VGT-2 simulations indicates that:
•
Mean relative difference between PV and the simulated VGT-2 in VNIR spectral
bands is +2.3 %. It was -1.5% between PV observations and PV simulations based
on the MERIS radiometric scale.
•
Mean difference PV and the simulated VGT-2 is on average over the SWIR
detectors ~0%
•
Interband < 3 % (VI-PER-250) is not met if we now take VGT-2 as reference for
interband. BLUE/CENTER and SWIR3/RIGHT appear to be outliers.
•
The stability requirement < 3 % (VI-PER-240) over 6 months is met
•
Remaining issues (of the model?) => Larger than expected seasonal oscillations in
the SWIR possibly due again to a modeling of the water vapour absorption?
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 32
Next steps
•
Continue the monitoring of Proba-V over Libya-4
•
Use Proba-V calibration data over ocean and apply the Rayleigh, Glint and
Desert methodologies implemented in ESA/DIMITRI radiometric tool.
Estimated start time: end 2015
ESA Presentation | Marc Bouvet | ESRIN| 28/10/2015 | Slide 33