1. Laboratory Panel and Radiometer Calibration 28 July 2011, IGARSS, Vancouver Andrew Deadman 1 , Nigel Fox 1 , Irina Behnert 1 D Griffith 2 1 National Physical Laboratory (NPL), United Kingdom 2 Council for Scientific and Industrial Research (CSIR), South Africa

6. 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)

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 comparison Environmental 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 Comparison 2009 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 Problem with the performance of one radiometer. No absolute calibration but were able to establish bias between instruments

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 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.

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

20. Reasons for difference in panel reflectance 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

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

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-field panel calibration – expected result NPL laboratory calibration In situ calibrations Reported calibration value

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

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

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.