Reference Instruments and their Traceability
EUMETSAT: Tim Hewison (Chair), Marianne König, Ruediger Lang
NOAA: Fangfang Yu, Fred Wu, Bob Iacovazzi
NASA: Jack Xiong, Dave Young, Kurt Thome, Dave Doelling
NIST: Rüdiger Kessel, Dave Walker
CMA: Yuan Li, Lin Chen, Yong Rhang, Mi Liao, Ronghua Wu
CNES: Denis Blumstein, Bertrand Fougnie
JMA: Hiromi Owada
Jack Xiong had been invited to this web meeting to provide a thorough overview of the MODIS calibration system and its suitability for use as a reference for the inter-calibration of the solar band channels of other satellite instruments. This made it clear that a great deal of effort has been put into both pre-launch characterisation and analysis of in-flight data. The stability demonstrated over ~10 years gives us confidence that MODIS can provide a good radiometric reference for inter-calibration of channels in the solar band. Of particular interest to this meeting was the error budget Jack presented breaking down numerous contributions from different parts of the calibration system to the overall uncertainty.This suggested overall uncertainties on reflectivity is typically ~1.57% for most channels in the solar band.
Ruediger Lang reported on comparisons of AVHRR and GOME2 on Metop-A that had been done by Rutherford Appleton Labs (UK). He showed these comparison were valuable to identify shifts in the geolocation of GOME2 data, which were needed to improve the correlation of these instruments. This method is now under further development at EUMETSAT for instrument monitoring purposes. It was suggested that GOME2 could be a valuable research tool for GSICS to allow the investigation of any anomolies found in the inter-calibration of solar channels against an operational reference. For example, the hyperspectral sampling of GOME2 could be exploited to investigate potential changes to the Spectral Response Function of other instruments.
Fred Wu and Rüdiger Kessel provided different points of view on the concept of traceability on practical and theoretical basis. These were followed up by Dave Walker, who reported on NIST's programme on microwave radiometry. This include the development of an SI-based standard black body target and a standard reference radiometer. He stated their intention to produce a 'Best Practice' document, similar to NISTIR7637, to "aid designers/builders in standardizing procedures and analysis". He has also proposed a workshop on this subject aiming to work towards SI traceability for microwave radiometry, which is likely to take place in summer 2011 in the USA.
Due to time constraints, Dave Young's presentation on the use of CLARREO as a reference instrument for inter-calibration of solar band channels was postponed until the next web meeting
In the discussions, it was suggested that GSICS adopt MODIS as a solar channel reference for now, with the intention to migrate to CLARREO when it becomes available. Further discussions, including the method of transferring to a new reference instrument will be followed-up at the next web meeting
- 04 May 2010