What changes a measurement
Objective
Describe how the atmospheric state, the illumination and viewing geometry and the sensor's own spectral response shape what a satellite measures, and how much uncertainty each of them contributes.
Observed in real data See the results How stages are defined
A top-of-atmosphere reading is never a direct view of the surface. Aerosols and water vapour scatter and absorb light, the sun and view angles change the path it travels, and each sensor integrates the spectrum through its own spectral response function. These effects act at once, vary from scene to scene, and leave no label on the data saying which contributed what.
Every later step rests on this one. If the effects that shape a reading are described wrongly, every comparison built on them inherits the error, however precise the result appears.
We model each effect explicitly and compare predicted with observed measurements: first on designed test data where the right answer is known, then on real observations taken under clearly different conditions, including different sun and viewing angles, drier and wetter atmospheres, and lunar observations, which a sensor makes with no atmosphere in the path.
Predicted and observed measurements agree within the stated uncertainty across clearly different conditions, and each effect is traced to a cause that predicts how it varies.
Each result is dated and published on its own page with its data, conditions and limits. Results that went against the objective are listed here with the rest.
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What shapes a measurement, rechecked on a like-for-like basis
Viewing geometry is now seen at more than one site on a like-for-like basis, though the result is still preliminary, and the sensor-response models handle a real measurement consistently. Which cause explains each site's remaining offset is the subject of the P02 research area. The atmospheric comparison still marks a boundary: a passed-through prior cannot be treated as evidence that RefCal retrieved the atmosphere independently.
Full result, data and credits -> -
What shapes a measurement, checked against real satellite data
The viewing-geometry and sensor-response parts of RefCal's measurement model are grounded in real evidence, not only designed test cases, and their conditions are recorded. The atmospheric comparison shows a boundary instead: a passed-through prior cannot be treated as evidence that RefCal retrieved the atmosphere independently.
Full result, data and credits ->
Established work this area builds on. Results from the programme are published separately, with their data and limits.
- Evaluation of measurement data: Guide to the expression of uncertainty in measurement (JCGM 100:2008) Joint Committee for Guides in Metrology, BIPM The reference framework for stating measurement uncertainty.
- Determining uncertainties FIDUCEO project, University of Reading How individual effects in a satellite measurement, and their uncertainties, are described and traced.
Questions about this work?
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