Laboratory Panel and Radiometer Calibration 28 July 2011, IGARSS, Vancouver

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.


There were three different comparisons:

Laboratory radiometer comparison

Laboratory reflectance panel comparison

In-field reflectance panel comparison


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

renc

e to

mea

n

GEDAIFJH

First TSARS measurement – 15 August 2010


26 Aug 2010

-20

-15

-10

-5

0

5

10

350 550 750 950 1150 1350 1550 1750 1950 2150 2350

Wavelength/nm

Per

cen

tag

e d

iffe

ren

ce t

o m

ean

G

E

D

A

I

F

J

H

Second TSARS measurement – 26 August 2010


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 Reflectance

Panel Comparison

Laboratory Panel Comparison

45/0 geometry

Stable illumination conditions

Same source and detector


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


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.


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.


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


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


In-field Reflectance

Panel 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 RadiometerCalibrationConclusions:

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.

Documents

Laboratory Panel and Radiometer Calibration 28 July 2011, IGARSS, Vancouver