Upload
grssieee
View
331
Download
0
Embed Size (px)
Citation preview
Laboratory Panel and Radiometer Calibration28 July 2011, IGARSS, Vancouver
Andrew Deadman1, Nigel Fox1, Irina Behnert1
D Griffith2
1 National Physical Laboratory (NPL), United Kingdom
2 Council for Scientific and Industrial Research (CSIR), South Africa
Laboratory Panel and RadiometerCalibration - Outline
Background - why?
Details of the calibration of the radiometers and reflectance panels
Results
Conclusions
Laboratory Panel and RadiometerCalibration
Why do it?
Establish differences in primary calibrations of instruments and reflectance panels used in vicarious calibration.
Different routes of traceability; different supplier of instrument, independent test facilities, different methodologies and measurement geometries.
Laboratory Panel and RadiometerCalibration
There were three different comparisons:
Laboratory radiometer comparison
Laboratory reflectance panel comparison
In-field reflectance panel comparison
Laboratory Panel and RadiometerCalibration
Radiometer Comparison
Laboratory Radiometer Comparison
Measured before and after the field measurements. The difference used to check the stability check of the calibration set-up and also the performance of the radiometers.
NPL provided a radiance source called TSARS (Transfer Standard Absolute Radiance Source)
Laboratory Radiometer comparison- Results
15 Aug 2010
-20
-15
-10
-5
0
5
10
350 550 750 950 1150 1350 1550 1750 1950 2150 2350
Wavelength/nm
Per
cent
age
diffe
ren
ce to
mea
n
GEDAIFJH
First TSARS measurement – 15 August 2010
Laboratory Radiometer comparison- Results
26 Aug 2010
-20
-15
-10
-5
0
5
10
350 550 750 950 1150 1350 1550 1750 1950 2150 2350
Wavelength/nm
Pe
rcen
tag
e d
iffe
ren
ce
to
me
an
G
E
D
A
I
F
J
H
Second TSARS measurement – 26 August 2010
Laboratory Radiometer comparison- Results
Difference between two TSARS measurements. Similar trend for most participants.
25
26
27
28
29
30
31
32
33
34
35
36
Tem
pera
ture
(°C
)
0
10
20
30
40
50
60
Rel
ativ
e H
umid
ity (%
)
Temperature
Humidity
Laboratory Radiometer comparisonEnvironmental conditions
First TSARS calibration 15 Aug 2010
Second TSARS calibration 26 Aug 2010
Environmental conditions outside normal operating conditions for TSARS so absolute calibration was not possible.
Laboratory Radiometer Comparison2009 TSARS calibration
Data from 2009 – difference between before and after field measurements. Shows better TSARS performance
Laboratory Radiometer comparison- 2010 Results
Difference between measurement of TSARS over the two days
Problem with the performance of one radiometer.
No absolute calibration but were able to establish bias between instruments
Laboratory Panel and RadiometerCalibration
Laboratory ReflectancePanel Comparison
Laboratory Panel Comparison
45/0 geometry
Stable illumination conditions
Same source and detector
Laboratory Panel Comparison
Each panel was measured by comparison with an NPL calibrated reference panel.
Measured before and after field measurements. Test stability of panels and robustness of the set-up
Laboratory Panel Comparison
Set up tested at NPL before measurements in Turkey.
Type A uncertainty < ± 0.2% for most wavelengths of interest.
Confidence in set-up.
Laboratory Panel Comparison- Results
Results of laboratory panel comparisons. Calibrated reflectance factor values.
Laboratory Panel Comparison- Results
Good agreement. No offsets in data or panel ageing affects.
Differences less than the estimated calibration uncertainty of ± 1%.
Difference in panel reflectance factor values before and after the field campaign.
Laboratory Panel Comparison- Results
Reflectance factor at 500 nm Panel 8°/hemispherical -
reported value 45/0 – laboratory
comparison B 0.949 0.966 D 0.992 0.998 E 0.990 1.004 H 0.990 1.002 I 0.988 1.007 J 0.991 1.009
Comparison of laboratory calibration results and reported reflectance
Reasons for difference in panelreflectance factor values
Laboratory calibration
Reported calibration
Bidirectional illumination 45°, nadir view - 45/0 geometry
Diffuse illumination, 8° view – 8°/hemispherical geometry
Reflectance panels are not perfectly lambertian – reflectance factor greater than unity at low sun zenith angles
Laboratory Panel Comparison- Results
Some participants had a goniometric calibration of their panel, so a direct comparison is possible.
Very good agreement considering three independent methods used traceable to three national standards laboratories
Laboratory Panel and RadiometerCalibration
In-field ReflectancePanel Comparison
In-field Panel Comparison
Repeat of laboratory exercise, using the sun as the source.
Sun zenith angle between 30° and 35°.
Each panel compared to NPL reference panel.
Laboratory & In-fieldpanel calibration – expected result
NPL laboratory calibration
In situ calibrations
Reported calibration value
Laboratory & In-fieldpanel calibration – expected result
NPL laboratory calibration
In situ calibrations
Reported calibration value
Reasons for difference in panelreflectance factor values
Laboratory calibration
In field calibration
Reported calibration
Bidirectional illumination 45º, nadir view - 45/0 geometry
Diffuse and Direct illuminationSun zenith & irradiance variability
Diffuse illumination, 8º view – 8°/hemispherical geometry
In-field panel comparison
08:40 08:50 09:00 09:10 09:20 09:30 09:40 09:50 10:00-10
-8
-6
-4
-2
0
2
4
6
8
Time (UTC)
GH
I V
aria
tion
(%)
GHI Variation as Percentage : 21-Aug-2010
380 nm
440 nm 500 nm
675 nm
761 nm
870 nm
935 nm1020 nm
1124 nm
Shows deviation of solar irradiance from mean over the period of in-field panel comparison.
Corresponds well with AERONET, although there are short term variations not captured by AERONET.
Data courtesy of CSIR.
Laboratory Panel and RadiometerCalibration
Conclusions:
Although full calibration of the radiometers was not possible, were able to establish relative bias.
Illumination conditions should be measured and monitored at same time as measurement of target.
Minimum requirement should be frequent measurement of reflectance panel.
Bi-directional, goniometric, calibration of reflectance panel or look-up table to correct 8°/hemispherical reflectance values.