G3.03    Missing generic and specific standards for co-location criteria in validation work

Gap detailed description

Different validation exercises on the same ECV/instrument combinations are often performed using different (sub-optimal) co-location criteria, ranging for instance from fixed maxima imposed on spatial and temporal distance (e.g. Ohyama et al. 2013, Dils et al. 2014, Hubert et al. 2015), over criteria based on the state/dynamics of the atmosphere (e.g. Wunch et al. 2011 ) or on representativeness area’s derived from models (e.g. Oshchepkov et al. 2012), to airmass matching techniques that take into account the actual 3D/4D sensitivity of each measurement (e.g. Lambert et al. 1997,1999, Balis et al. 2007).  This makes an intercomparison of the validation results difficult and it limits optimal use of the ground-based networks. To ensure reliable and traceable validation results, as required in operational validation work, community-agreed standards for co-location criteria should therefore be developed and published. Moreover, the optimal co-location strategy depends heavily on specifics such as user requirements, network coverage, instrument properties, atmospheric regimes etc. and standards should thus be diversified accordingly. As such, resolution of this gap depends to a large extent on a corresponding effort regarding gap G3.02.

Activities within GAIA-CLIM related to this gap

No attempt will be made within GAIA-CLIM to produce an authoritative document on this matter, but work within task T3.2 will contribute to the evaluation of different co-location criteria (cfr. gap G3.02), i.e. the foundation of any future recommendation or standard regarding criteria to be adopted in an (operational) validation system. Some discussion on common co-location criteria is included in D3.2 (Section 4.1).

Gap remedy

Remedy #1

Specific remedy proposed

The publication aimed for as a remedy to gap G3.02 could conclude with a set of clear recommendations, which then need to be advertised to and adopted by the different stakeholders (validation teams, space agencies defining validation requirements etc.).  

Measurable outcome of success

Success is achieved when an authoritative document exists which is referred to in publications on validation results and in upcoming validation protocols.

Achievable outcomes

The technological effort on top of that required to address gap G3.02 is very small. Dissemination among and acceptance by the key stakeholders is more challenging and can probably best be achieved in the context of overarching frameworks such as the CEOS Working Group on Calibration & Validation (WGCV). The financial cost should be very low.


This remedy directly addresses the gap. It will provide stakeholders with a traceable, authoritative reference on which to base their validation requirements and protocols regarding co-location criteria. It will also facilitate meta-analysis of different validation studies without  the need to take into account differences in results due to differences in the impact of co-location mismatch on the results. 


The writing of these recommendations requires only a small amount of time (of the order of a few months, if results from G3.02 are available). However, for these recommendations to be widely adopted may require several years.

Gap risks to non-resolution


Identified future risk / impact

Probability of occurrence if gap not remedied

Downstream impacts on ability to deliver high quality services to science / industry / society

Difficulty in inter-comparing the results of validation work on related products.


Difficulty for the end user to choose the product most suited  for his needs. Consequently also sub-optimal use of both the EO and non-satellite data sets.

Sub-optimal use of the non-satellite reference measurements.


Sub-optimal characterisation of the data quality, hampering the full exploitation of the capabilities of the EO system.


Work package: