|Paper title||Application of SI-Traceable Satellite (SITSat) Reflected Solar-band Spectrometers to the Global Space-based Inter-Calibration System (GSICS)|
|Form of presentation||Poster|
The Global Space-based Inter-Calibration System (GSICS) is an initiative of CGMS and WMO, which aims to ensure consistent accuracy among satellite observations worldwide for climate monitoring, weather forecasting, and environmental applications. To achieve this, algorithms have been developed to correct the calibration of various instruments to be consistent with community-defined reference instruments based on a series of inter-comparisons – either directly by the Simultaneous Nadir Overpass (SNO) or Ray-Matching approach – or indirectly using Pseudo Invariant Calibration Targets (PICTs), such as the Moon, desert sites or Deep Convective Cloud as transfer standards. In the former approach contemporary satellites are tied to current state-of-the-art reference instruments, while heritage satellites need to rely on older references. The invariant target approach relies on their characterisation by counterpart reference instruments and is typically applied in the Reflected Solar Band.
The 2020s will see the launch of a new type of satellite instrument, whose calibration will be directly traceable to SI standards on orbit, referred to here as SI-Traceable Satellites (SITSats). Examples include NASA’s CLARREO Pathfinder, ESA’s TRUTHS and FORUM, and the Chinese Space Agency’s LIBRA. The first of these will carry steerable VIS/NIR spectrometers, which will allow corresponding GSICS products to be tied to an absolute scale.
This presentation outlines two approaches being developed to exploit these SITSats. Firstly, by direct comparison of their observations with those of current GSICS reference instruments using Ray-Matching to ensure equivalent viewing conditions over simultaneous, collocated scenes. Secondly, by charactering the current Pseudo Invariant Calibration Targets including Deep Convective Clouds and desert sites, including their BRDF and spectral signature. The challenge of propagating uncertainties through the inter-calibration algorithms to achieve a full traceability chain will be discussed.
To optimise the benefits of such a SITSats requires GSICS to prioritise which reference instruments or PICTs are to be characterised, and close cooperation with the SITSat operators to ensure sufficient acquisitions are available to fully characterise them within the mission lifetime. Ultimately, tying GSICS products to an absolute scale would provide resilience against gaps between reference instruments and drifts in their calibrations outside their overlap periods, and allow construction of robust and harmonized current and historical data records from multiple satellite sources to build Fundamental Climate Data Records, as well as more uniform environmental retrievals in both space and time, thus improving inter-operability.