GRWG IR Sub-Group Web Meeting 2016-06-21

GSICS Web Meeting on Reference Instrument Traceability Report


  1. Tim Hewison (EUMETSAT) - Introduction and Report Template
  2. Dave Tobin (SSEC) - Review of CrIS Uncertainty paper ( Tobin, 2013), as example for Error Budget section
  3. Tim Hewison (EUMETSAT) - Setting up an inter-comparison database
  4. Larrabee Strow (UMD) - Spectral conversion method - TBC
  5. All - Discussion - Agree authors and timeline
  6. All - Application to other spectral bands?


This is an important GSICS initiative, as it supports the choice of hyperspectral reference instruments (IASI, CrIS and AIRS) to inter-calibrate channels in the thermal infrared, and the use of Metop-A/IASI as the anchor/primary/benchmark* reference.

This report will first review each reference instruments' error budgets and consider the traceability of their in-flight calibration to absolute (SI) and community reference standards. The use of pre-launch test results will be considered to validate these uncertainty analyses.

It will then consolidate the results from numerous in-flight comparisons of hyperspectral reference instruments by different authors, which may include:

a) Polar SNOs

b) Quasi-SNOs

c) GEO-LEO Double Differencing

d) NWP Bias Monitoring double-differencing

e) Aircraft Double-Differences

f) Massive Means (regional or global averages)

It is not intended to generate new results. Rather, the results of existing comparisons will be expressed in a common way, allowing their direct comparison. In particular, common ranges will be used for the dates, diurnal coverage, spectral bands (both hyperspectral and broad-band averages), viewing angle, and, most importantly, scene radiance (or brightness temperature). This analysis will include common treatment of the uncertainties in each of the inter-comparison techniques, which is critical when assessing whether their results are statistically consistent.

It is expected that similar reports will be generated in future to support the traceability of GSICS products for other spectral bands by reviewing the calibration uncertainty of candidate reference instruments. So even if you cannot be involved in this particular activity, you are encouraged to attend and consider how it could be applied to GSICS references in other spectral bands.

The discussion was based on a template for the report and a strawman spreadsheet for compiling the results of the various inter-comparisons.


Chair: Tim Hewison

CNES: Denis Jouglet, Elsa Jacquette

EUMETSAT: Tim Hewison, Dorothée Coppens, Viju John

JMA: Arata Okuyama, Masaya Takahashi

KMA: Minju Gu, Hyeji Yang

NOAA: Fred Wu, Fangfang Yu, Likun Wang, Manik Bali, Xingming Liang

SSEC: Dave Tobin

UMBC: Larrabee Strow (and chickens)


The proposed structure of the report was agreed, with the addition of a sub-section in the introduction to address the need for continuous monitoring of the reference instruments' calibration. Additional sub-sub-sections were also identified to briefly address a) radiometric noise, b) spectral calibration and c) geometric factors (navigation accuracy etc) in the error budget - although it was recognised that these need not be treated in a fully rigorous approach, given their negligible impact on the inter-calibration products [for a) and b)] and the difficulty of assessment [for c)].

The contributor authors to each sub-section were identified - either as firm, or tentative.

The spectral resolution of the comparisons was discussed at length and different spectral conversion methods described. It was felt that 10cm-1 bins would be sufficient.
It seems the most difficult issue is dealing with AIRS' gap channels. It was agreed that further discussion on this topic is needed, so another web meeting will be set up to discuss this in mid-August 2016.

Action GRWG.20160621.1: Tim Hewison (EUMETSAT) to check with NIST/NPL and confirm the recommended coverage factor to be used for error budgets and comparisons.
- Action completed 2016-08-03, with the following response from Emma Woolliams (NPL) - and agreed by Dave Walker (NIST):

The uncertainty analysis should all be performed with standard uncertainties. Any uncertainty budget (table) should definitely be full of standard uncertainties. The adding in quadrature (applying the Law of Propagation of Uncertainties) must be done with standard uncertainties. But the final result may be quoted as an expanded uncertainty. In which case the k value must be provided and if it’s not 2, the number of degrees of freedom should be provided too. That means that other people can divide by the right number when including your uncertainty analysis into their budgets.

Action GRWG.20160621.2: Denis Jouglet (CNES) to distribute spectral averaging coefficients and documentation describing their application by early July.
- Action completed 2016-06-21 - See slide sent by email.

Action GRWG.20160621.3: Dave Tobin (SSEC) to attempt to populate comparison database with SNO results for IASI-AIRS comparisons by next web meeting.
Topic revision: r9 - 31 Aug 2016, TimHewison
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