Methodology

QA4EO documentation provides a theoretical basis and practical guidance for implementing the principles of QA4EO, while case study examples describe implementations of QA4EO principles.

Confidence in data

‘...these FRM provide the maximum return on investment for a satellite mission by delivering, to users, the required confidence in data products, in the form of independent validation results and satellite measurement uncertainty estimation, over the entire end-to-end duration of a satellite mission.’

European Space Agency, ESA Earth Online

QA4EO and Metrology

QA4EO stops short of requiring SI-traceability in all circumstances, as it may not be viable for all applications and measurements. However, QA4EO guidelines are based on metrological concepts, such as using well-documented procedures, participation in comparisons, and uncertainty assessments.

Metrology, the science of measurement, is the discipline responsible for maintaining the SI and the associated system of measurement. The SI ensures measurements are stable over time and measurement standards are equivalent worldwide. These properties are achieved through the principles of metrological traceability: uncertainty analysis and comparison.

The terms Fiducial Reference Measurement (FRM), Fundamental Data Record (FDR), and Thematic Data Product (TDP) describe metrologically rigorous observations of specific relevance to space-based observations — terms increasingly used by the Earth observation community.

Definitions

FDR

A fundamental data record (FDR) is a record of sufficient duration for its application, of uncertainty-quantified sensor observations calibrated to physical units, and located in time and space, together with all ancillary and lower-level instrument data used to calibrate and locate the observations and to estimate uncertainty.

TDP

A thematic data product (TDP) is a record of sufficient duration for its application, of uncertainty-quantified retrieved values of a geophysical variable, along with all ancillary data used in retrieval and uncertainty estimation.

FRM

Fiducial reference measurements (FRM) are a suite of independent, fully characterised, and traceable sub-orbital measurements that follow the guidelines outlined by the GEO/CEOS Quality Assurance framework for Earth Observation (QA4EO) and have value for space-based observations.

Note the terms ‘Fundamental Climate Data Record’ (FCDR) and ‘Climate Data Record’ (CDR) are used for FDRs and TDPs respectively. These are typically of multi-decadal duration and come from a series of sensors that have been harmonised to a common reference and have value for climate studies. A definition of FRM, and more information about its implementation, is available on the CEOS CalVal Portal.

Steps to FDR / TDP/ FRM uncertainty budgets

Define measurand & measurement function

Considerations:

What is to be measured?
Assumptions?
How are measured values to be used?
What areas do individual measurements represent?
Are you calculating a quantity from measurements?
What is the measurement model?

Establish traceability with a diagram

Identify input quantities of the measurement function, considering:

Where does each term in the measurement model come from?
What sources of uncertainty are there?
- For each term
- With the form of the measurement model
Which type of diagram most helpfully describes the traceability and gives insight into the uncertainties?

Evaluate sources of uncertainty & fill out an effects table

Collect and quantify uncertainties per source, establishing:

Magnitude
Distribution
Sensitivity coefficient (converting uncertainty to measurand units)
Error correlation structures What are the error correlation dimensions?

Calculate the product and its uncertainty

Calculate and propagate the measured and its associated uncertainty, eg via CoMet Toolkit. For each source of uncertainty, establish:

Magnitude
Distribution
Sensitivity coefficient (converting uncertainty to measurand’s units)
Error correlation structures. What are the error correlation dimensions?

Store relevant information for future users

What information do today’s users need and how do we provide that?
- Simplified, summary information?
- Ensembles?
- Through tools like CoMet Toolkit?
What information do we need to store for long term data preservation purposes?
- Human readable
- Computer readable (digital effects tables)

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Methodology

Confidence in data

QA4EO and Metrology

Definitions

FDR

TDP

FRM

Steps to FDR / TDP/ FRM uncertainty budgets

Define measurand & measurement function

Establish traceability with a diagram

Evaluate sources of uncertainty & fill out an effects table

Calculate the product and its uncertainty

Store relevant information for future users

Glossary

Documents

Tools

Methodology

Confidence in datacheck-2

QA4EO and Metrology

Definitions

FDR

TDP

FRM

Steps to FDR / TDP/ FRM uncertainty budgets

Define measurand & measurement function​

Establish traceability with a diagram​

Evaluate sources of uncertainty & fill out an effects table​

Calculate the product and its uncertainty

Store relevant information for future users​

Glossary

Confidence in data

Define measurand & measurement function

Establish traceability with a diagram

Evaluate sources of uncertainty & fill out an effects table

Store relevant information for future users