Centre National d'Etudes Spatiales
SCA _RAD_08
Activity : CAL/VAL
TRO-34-NT2786-CNES
Level A1/A2 comparison
Prepared by :
CNES Team
Verified by :
N Karouche
Contents
1. OBJECTIVE ....................................................................................................................................1
2. METHODS.......................................................................................................................................2
3. PRODUCTS USED .........................................................................................................................2
4. RESULTS .........................................................................................................................................3
4.1. MS MODE ORBIT NUMBER 3775 ..............................................................................................3
4.1.1. IMAGE......................................................................................................................................... 3
4.1.2. RADIOMETRIC ERROR ......................................................................................................... 4
4.1.3. ABSOLUTE RADIOMETRIC ERROR ON CHANNEL 5 .................................................... 5
4.2. MS MODE ORBIT NUMBER 5061 ..............................................................................................7
4.3. STATISTICAL APPROACH ........................................................................................................7
5. CONCLUSION ................................................................................................................................8
1. OBJECTIVE
The two main Channels are Channel 2 (solar channel, 0.2-4µm) and Channel 3 (total channel, 0.2200µm). Channel 2 is dedicated to measure solar radiance (Lsw). Channel 3 measures total radiance
(Ltot). Long wave radiance (Llw) is computed by the difference between total channel and solar
channel : Llw = Ltot – A’ Lsw
A’ represents the difference of sensibility in the SW domain, between Channel 2 and Channel 3.
To compute Llw, it is absolutely necessary that Channel 2 (Lsw) and Channel 3 (Ltot) aim exactly at
the same location. The difference of location between the two channels is called registration.
In-flight Scarab registration performances are very close to the requirements for L1A1 product.
However, the small difference of location between Channel 2 and Channel 3 can be reduced by
resampling the Channel 3 over the Channel 2. This is done for the L1A2 product.
Resampling can bring artefacts, however, as Scarab spatial sampling respects Shannon theorem,
these artefacts should be negligible.
2/8
The object of this study is to compare L1A1 and L1A2 products, in term of radiometry, and to see if
L1A2 is significantly better than L1A1.
In nominal mode, Channel 2 (solar filter) and Channel 3 (no filter) images are too different to be
compared.
But with MS mode, both channels have the same solar filter. If they aim exactly at the same
location, they should give almost exactly the same result.
If not, this can be assign to several factors:
+A’ factor (the proportional factor between Channel 2 and 3). This factor is easily determined in
MS mode.
+A’ spectral sensibility. A’ is not constant and depends on spectral signature of the scene.
+Registration (both channels don’t aim exactly at the same location).
By comparing Channel 2 and 3 in MS mode, we can assess all these elements.
2. METHODS
Lsw2 represents the luminance measured by Channel 2. As Channel 2 has a filter, it is a Short Wave
(sw) luminance.
Lsw3 represents the luminance measured by Channel 3. As Channel 3 has a filter, in this MS mode,
it is also Short Wave (sw) luminance.
1/
Calculate the A’ factor for this image in MS mode.
A’MS = mean(Lsw3) / mean (Lsw2)
2/
Calculate C5 Channel
L5 = Lsw3 – Lsw2*A’MS
In this case, C5 is not the infrared channel. C5 should be equal to 0, as Channel 2 and Channel 3 are
in solar mode.
L5 can be calculated for level A1 and level A2 products.
Standard deviation of L5 measures directly the error mainly due to registration.
3. PRODUCTS USED
See table
3/8
4. RESULTS
4.1.
4.1.1.
MS MODE ORBIT NUMBER 3775
IMAGE
C2 channel
C3 channel
C5 from level A1
C5 from level A2
4/8
Zoom :
Madagascar
Lake Malawi / Tanganika
Channel 3
Madagascar coast
Channel 5 from level A1
Madagascar coast
Channel 5 from level A2
On level A2, Madagascar coast is almost fainted.
4.1.2.
RADIOMETRIC ERROR
For each image, we can compute standard deviation, min and max of the error of the Channel 5. We
can also compute the mean value of the original scene Channel 2.
Standart
deviation
Min
Max
Mean
W/m²/sr
W/m²/sr
W/m²/sr
W/m²/sr
C2
48
C5_A1
0,57
-3,8
7,8
C5_A2
0,41
-2,9
7,6
This table shows that resampling reduce the RMS error by a factor 1.4. The maximum error is
slightly decreased by this treatment.
For this image, the radiometric error can be computed with:
5/8
e A1 =
e A2 =
σ C 5 _ A1
L2sw
σ C 5 _ A2
L2sw
= 1 .1 %
= 0.85%
It means that the final error on the luminance of the Channel 5, due to registration, will be roughly
0.85% of the luminance of Channel 2. As the mean luminance (for the day part of this image) is
about 48 W/m²/sr, it means that the mean error on the Channel 5 will be 0.4W/m²/sr.
+ eA1
__ eA2
This radiometric error can be evaluated for each luminance level of L2 :
Roughly, the radiometric error follow the law:
For L1A1 :
0.4% at 220 W/m²/sr to 1.2% at 20 W/m²/sr
e = 0.4% + 1.2%/200*(220-L2sw )
For L1A2 :
0.3% at 220 W/m²/sr to 0.9% at 20 W/m²/sr
e = 0.3% + 0.6%/200*(220-L2sw )
4.1.3.
ABSOLUTE RADIOMETRIC ERROR ON CHANNEL 5
Long wave radiance (Llw) is computed by the difference between total channel and solar channel for
nominal image: Llw = Ltot – A’ Lsw
6/8
The radiometric error that we should have on Llw, due to registration, can be evaluated on MS
image by:
dLlw d ( L3sw − A' ms L2sw )
=
Llw
Llw
In MS mode, Channel 3 has a solar filter. It is not possible to calculate exactly Llw. But this value
can be estimated with the luminance Lir of Channel 4 (infrared window) with polynomial
interpolation. This polynomial P can be calculated on night scene, where Channel 3 receives only
thermal radiance.
Llw = P(Lir)
The radiometric error can be estimated by :
e' A1 ( Llw _ estimated ) =
e' A 2 ( Llw _ estimated ) =
σ C 5 _ A1 ( Llw _ estimated )
Llw _ estimated
σ C 5 _ A 2 ( Llw _ estimated )
Llw _ estimated
High level of luminance for Llw corresponds to hot temperature and lower luminance for Lsw.
Low level of luminance for Llw corresponds to cold temperature and higher luminance for Lsw
(white clouds).
7/8
The black line represents the radiometric budget. L1A2 product generates good margins.
4.2.
MS MODE ORBIT NUMBER 5061
4.3.
STATISTICAL APPROACH
Level A1 product
Orbit
315
329
809
1007
1303
1304
2277
2986
3380
mean
MIN
W/m²/sr
-6,2
-3,9
-7,1
-6,3
-4,3
-3,0
-5,6
-1,4
-4,3
MAX
W/m²/sr
5,5
4,1
5,5
7,2
3,6
2,5
7,9
1,1
4,2
STD
<C3-AC2>
W/m²/sr
0,72
0,64
0,76
0,88
0,34
0,35
0,44
0,20
0,48
<C3>
W/m²/sr
66
50
65
82
43
55
50
19
45
STD<C3>
(%)
1,1
1,3
1,2
1,1
0,8
0,6
0,9
1,0
1,1
1.0
MAX
W/m²/sr
2,9
2,2
4,3
4,3
3,7
2,0
7,1
1,1
3,5
STD
<C3-AC2>
W/m²/sr
0,51
0,43
0,52
0,62
0,25
0,27
0,34
0,16
0,34
<C3>
W/m²/sr
66
50
65
82
43
55
50
19
45
STD<C3>
(%)
0,8
0,9
0,8
0,8
0,6
0,5
0,7
0,8
0,8
0.73
Level A2 product
Orbit
315
329
809
1007
1303
1304
2277
2986
3380
mean
MIN
W/m²/sr
-4,2
-2,6
-4,0
-5,2
-2,7
-3,0
-3,9
-1,0
-3,4
This study confirms the figure found on orbit 3775.
8/8
5. CONCLUSION
Here is the absolute radiometric budget for Channel 5 L1A2.
The registration figure is updated with the analysis presented here.
Bright
clouds
cold
Hot / bright scene
Night scene
250 SW + 50 LW
210 SW + 130
LW (20°)
0 SW + 80 LW
0,21%
0,11%
0,14%
0.12%
0.12%
0.12%
1%
0.3%
0%
1.5%
0.55%
0%
0,4%
0,40%
0,40%
@1σ
1.9%
0.8%
0.45%
Budget @1σ
W/m²/sr
0.95
1.0
0.35
W/m²/sr
W/m²/sr
W/m²/sr
L1A2
Instrumental
noise
Random
Calibration
CALM
Bias
A’ factor
Random
0.2%
Registration
Random
e = 0.3% +
0.6%/200*(220L2sw )
Location
Random
Budget
%