SI-traceable data in space
The TRUTHS mission is unique in establishing high-accuracy, SI-traceable data in-flight, enabling a 'calibration laboratory in space'. The mission is one of the first examples of a new class of missions, SI-traceable satellites (SITSats). The satellite will carry two main instruments: the Hyperspectral Imaging Spectrometer (HIS) and the Cryogenic Solar Absolute Radiometer (CSAR) in addition to its novel Onboard Calibration System (OBCS). Together, these instruments will provide global hyperspectral (320 nm to 2400 nm) measurements of unprecedented accuracy of:
- Top-of-Atmosphere Earth spectral radiance (0.3% k=2)
- Solar irradiance, both total and spectrally resolved (0.02% and 0.3% respectively)
- Lunar spectral irradiance (0.3%)
TRUTHS will make continuous measurements of the Earth’s incoming (solar) radiation and outgoing (reflected) radiation in the solar reflective domain up to 10X more accurately than current satellites. This will enhance our ability to estimate the Earth’s radiation budget by an order of magnitude. An accurate knowledge of these energy exchanges is fundamental to understanding and monitoring change, including changes made as a result of climate mitigation.
At the heart of the TRUTHS calibration capability is the primary standard Cryogenic Solar Absolute Radiometer (CSAR) which can mimic, in space, the same SI-traceable calibration capability and associated measurement uncertainty achievable in metrology laboratories on Earth.
Prototype of the TRUTHS Cryogenic Solar Absolute Radiometer (CSAR)
Alongside the CSAR, the hyperspectral imager (HIS) will make spectrally and spatially resolved measurements of the ocean and land surfaces to support studies of the carbon cycle and underpin improvements in observations to support agriculture, pollution monitoring and land cover change.
‘Gold-standard’ calibration reference
Another key objective of the mission is to act as an SI-traceable ‘gold-standard’ calibration reference, facilitating an upgrade in the performance of the global Earth observing system. Its data will provide the anchoring point to many of reference calibration methodologies endorsed by Committee on Earth Observation Satellites (CEOS) and Global Space-based Inter-Calibration System (GSICS) including:
- Simultaneous Nadir Overpasses (SNOs), where two satellite sensors observe approximately the same location at approximately the same time, providing the opportunity to transfer (the orbit of TRUTHS allows these to take place regularly over different parts of the globe)
- Using dedicated Earth-based reference standard sites endorsed by CEOS such as deserts and RadCalNet
- Improved characterisation of the Moon as a reference site.
Learn more about how TRUTHS works in the animation below:
The mission specification has been designed to optimise cross-calibration performance, through the spectral and spatial sampling, and a novel 90-degree precessing orbit to maximise SNO opportunities with key target missions.
In addition to establishing benchmark measurements of the Earth itself, the TRUTHS mission plans to establish a set of internationally-accepted SI-traceable reference targets, including the Sun, Moon and the Earth’s deserts. These could be used to provide calibrations for EO data collected by other satellites. Such reference targets are highly stable, and many have been observed by previous satellites, so could be used to back-correct archived data.
TRUTHS is a complementary mission to other satellite-based Earth observation missions providing a more complete spectrally resolved picture of the Earth’s radiation imbalance which drives global warming. Alongside the FORUM and IASI-NG missions TRUTHS will enable a complete picture of the Earth’s radiation imbalance. In partnership with its NASA CLARREO Pathfinder sister mission, due for launch on the International Space Station, and the proposed Chinese Libra mission, TRUTHS will initiate a new epoch of SI traceability from space and a new satellite class, the so called SITSats.
Find out more:
SI traceability in-flight