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

1. Laboratory Panel and Radiometer Calibration 28 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

2. Laboratory Panel and RadiometerCalibration - Outline • Background - why? • Details of the calibration of the radiometers and reflectance panels • Results • Conclusions

3. 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.

4. Laboratory Panel and RadiometerCalibration There were three different comparisons: • Laboratory radiometer comparison • Laboratory reflectance panel comparison • In-field reflectance panel comparison

5. Laboratory Panel and RadiometerCalibration Radiometer Comparison

6. Laboratory Radiometer Comparison NPL provided a radiance source called TSARS (Transfer Standard Absolute Radiance Source) 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.

7. Laboratory Radiometer comparison- Results First TSARS measurement – 15 August 2010

8. Laboratory Radiometer comparison- Results Second TSARS measurement – 26 August 2010

9. Laboratory Radiometer comparison- Results Difference between two TSARS measurements. Similar trend for most participants.

10. 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.

11. Laboratory Radiometer Comparison2009 TSARS calibration Data from 2009 – difference between before and after field measurements. Shows better TSARS performance

12. Laboratory Radiometer comparison- 2010 Results Difference between measurement of TSARS over the two days No absolute calibration but were able to establish bias between instruments Problem with the performance of one radiometer.

13. Laboratory Panel and RadiometerCalibration Laboratory Reflectance Panel Comparison

14. Laboratory Panel Comparison 45/0 geometry Stable illumination conditions Same source and detector

15. 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

16. 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.

17. Laboratory Panel Comparison- Results Results of laboratory panel comparisons. Calibrated reflectance factor values.

18. Laboratory Panel Comparison- Results Difference in panel reflectance factor values before and after the field campaign. Good agreement. No offsets in data or panel ageing affects. Differences less than the estimated calibration uncertainty of ± 1%.

19. Laboratory Panel Comparison- Results Comparison of laboratory calibration results and reported reflectance

20. Reasons for difference in panelreflectance factor values Diffuse illumination, 8° view – 8°/hemispherical geometry Reported calibration Laboratory calibration Bidirectional illumination 45°, nadir view - 45/0 geometry Reflectance panels are not perfectly lambertian – reflectance factor greater than unity at low sun zenith angles

21. 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

22. Laboratory Panel and RadiometerCalibration In-field Reflectance Panel Comparison

23. 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.

24. Laboratory & In-fieldpanel calibration – expected result In situ calibrations NPL laboratory calibration Reported calibration value

25. Laboratory & In-fieldpanel calibration – expected result In situ calibrations NPL laboratory calibration Reported calibration value

26. Reasons for difference in panelreflectance factor values Diffuse illumination, 8º view – 8°/hemispherical geometry Reported calibration Laboratory calibration Bidirectional illumination 45º, nadir view - 45/0 geometry Diffuse and Direct illumination Sun zenith & irradiance variability In field calibration

27. In-field panel comparison 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.

28. 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.