Day 4

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Paper title Optimising the design of SITSats for on-orbit reference calibration of Earth-viewing sensors
  1. Madeline Stedman National Physical Laboratory Speaker
  2. Samuel E. Hunt National Physical Laboratory NPL
  3. Jacob Fahy National Physical Laboratory
  4. Nigel Fox National Physical Laboratory
Form of presentation Poster
  • B1. Calibration, validation and data quality, FRM
    • B1.01 SI-Traceable Satellites - a Gold Standard for Climate and Intercalibration
Abstract text Society is becoming increasingly dependent on remotely sensed observations of the Earth to assess its health, help manage resources, monitor food security, and inform on climate change. Comprehensive global monitoring is required to support this, necessitating the use of data from the many different available sources. For datasets to be interoperable in this way, measurement biases between them must be reconciled. This is particularly critical when considering the demanding requirements of climate observation – where long time series from multiple satellites are required.

Typically, this is achieved by on-orbit calibration against common reference sites and/or other satellites, however, there often remain challenges when interpreting such results. In particular, the degree of confidence in the resultant uncertainties and their traceability to SI is not always adequate or transparent. The next generation of satellites, where high-accuracy on-board SI-traceability is embedded into the design, so-called SITSats, can therefore help to address this issue by becoming “gold standard” calibration references. This includes the ESA TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio- Studies) mission, which will make hyperspectral observations from visible to short wave infrared with a target uncertainty of 0.3 % (k = 2).

To date, uncertainty budgets associated with intercalibration have been dominated by the uncertainty of the reference sensor. However, the unprecedented high accuracy that will be achieved by TRUTHS, and other SITSats, means that the reference sensor will no longer be the dominant source of uncertainty. The accuracy of cross-calibration will instead be ultimately limited by the inability to correct for differences between the sensor observations in comparison, e.g., spectral response, viewing geometry differences. The work presented here aims to assess the impact of these differences on the accuracy of intercalibration achievable using TRUTHS as a ‘reference sensor’ and evaluate to what extent these can be limited through appropriate design specifications on the mission.

A series of detailed sensitivity analyses have been performed to evaluate how the intercalibration uncertainty for TRUTHS and a given target sensor can be best minimised, based on potential TRUTHS design specifications. This includes the impact of TRUTHS’ bandwidth and spectral sampling definition, which was studied using a radiative transfer model to investigate how well TRUTHS can reconstruct target sensor bands for comparison. A similar simulation-based approach is used to evaluate the sensitivity of intercalibration to TRUTHS’ spatial resolution. Target sensor (Sentinel-2 MSI) images are resampled as a proxy to simulate TRUTHS images at a range of spatial resolutions. The ability of TRUTHS to reconstruct target sensor images is then assessed by resampling the simulated-TRUTHS image back to the spatial resolution of the target sensor. These simulation studies were carried out for a range of sites, including CEOS desert pseudo-invariant calibration site (PICS) Libya-4, representing the types of scenes that are used as targets in the sensor intercalibration process. Target sensors simulated in these studies include the widely used sensors Sentinel-2 MSI, Sentinel-3 OLCI and Suomi-NPP VIIRS, as they are representative of many of the types of sensors TRUTHS will be used to calibrate.