Improving confidence in climate change forecasts

TRUTHS is a proposed satellite mission to support climate adaptation by establishing a space-based climate and calibration observing system facilitating improved confidence in climate change forecasts.

The proposed Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS) mission offers a novel approach to the provision of key scientific data with unprecedented radiometric accuracy for Earth observation (EO) and solar studies and, in particular, for decadal climate change. The small satellite consists of a hyper-spectral imager measuring spectrally-resolved incoming and reflected solar radiation (320 to 2350 nm) at high spatial resolution anchored by an innovative on-board calibration system.

TRUTHS provides 'benchmark' measurements of key climate 'radiative forcing feedbacks' such as clouds and albedo with uncertainties small enough that future change, from a background of natural variability, can be detected in as short a time as possible. It will allow& rigorous testing of model forecasts to support decision-making.

Uniquely, TRUTHS can also upgrade the performance of the Earth observing system as a whole: satellite and ground, through the provision of 'reference calibrations' of high SI traceable accuracy. This facilitates the upgrade of existing and planned 'operationally' focused missions to become the building blocks of a global climate observatory. In essence, TRUTHS becomes a 'standards laboratory in space'.

TRUTHS is a concept conceived and designed at NPL, the UK's national measurement institute, in conjunction with UK space industry and has wide support in the international science community.

The need for TRUTHS

The need for such a mission has been specifically highlighted by the United Nations GCOS (Global Climate Observing System), WMO GSICS (Global Satellite Inter-Calibration System) committees and Committee on Earth Observation (CEOS). The US Academy of sciences and NASA has taken action to develop a strong scientific argument and a mission called 'CLARREO (Climate Absolute Radiance and Refractivity Observatory)'. TRUTHS is complementary to CLARREO and' through partnership' would establish the first elements of an 'international calibration and climate constellation', with TRUTHS delivering traceability for the solar reflective spectral domain.

Calibrating TRUTHS

TRUTHS is the first satellite mission to calibrate its observations directly to an SI 'primary standard' in orbit. This overcomes the usual uncertainties associated with drifts of sensor gain by using an electrical rather than an optical standard as the basis of its calibration and one which is a direct mimic of those used terrestrially.

The hyperspectral imagery will also make nadir and multi-angular spectrally-resolved measurements of the ocean and land surfaces. This will support studies of the carbon cycle, and underpin improvements in observations which support agriculture or pollution monitoring. It will also provide data to improve atmospheric radiative transfer codes by anchoring boundary conditions at the top of the atmosphere. TRUTHS provides a 'gold standard' to underpin a new era in EO, not only from traditional space agencies, but also emerging 'new space' led by the commercial sector. It enables traceability and with it improved interoperability and trust in the observations of the Earth from space.

The range of instruments flown as part of the payload will also provide accurate input data to improve atmospheric radiative transfer codes by anchoring boundary conditions, through simultaneous measurements of aerosols, particulates and radiances at various heights. It will also make multi-angular spectrally resolved measurements of the ocean and land surfaces to support studies of the carbon cycle. In this way TRUTHS will provide the basis to significantly improve the performance and accuracy of all EO optical missions, whether they are global, operational or have focused specific scientific objectives.

TRUTHS has many strategies for providing 'reference calibrations'. These include using dedicated Earth based reference standard sites such a those of CEOS, near simultaneous observations of 'targets' at orbital cross-overs (the orbit of TRUTHS allows these to take place regularly over different parts of the globe) and through calibration of the Moon.

These reference calibrations not only improve synergy between missions, by reducing uncertainties due to different calibration biases, but also offer cost reductions for future missions by reducing the demands for on-board calibration systems. These improvements are essential for strategies such as GEO, the EU Copernicus programme, and the Paris Agreement to ensure that data from constellations of micro/nano-sats can be integrated seamlessly into the emerging 'big data' analytical systems that will fuel the growth in services and information content demanded by society.

TRUTHS mission goals

  • Measure the incoming solar and reflected (Earth/Moon) radiation with sufficient accuracy and spectral resolution to allow decadal climate change detection, or 10 times more accurately
  • Be the first satellite to calibrate itself – directly traceable to SI units via a primary standard – in orbit.
  • Provide absolute high accuracy traceable calibration for other EO missions
  • Be the global nadir hyperspectral observations of the Earth's reflectance that can provide unprecedented insight into the health and sustainability of the planet

Assessments of climate change depend on data collected over decades so that very small, often subtle, changes are large enough to be reliably detected. Scores of EO satellites are used to provide inputs and developments of models that advise policy-makers. There has to be complete confidence that any change is real and not caused by changes in instrument performance or calibration. Satellite surveys are not only used for climate, but more readily to quantify the interactions of complex environmental factors, such as the atmosphere, the Sun, oceans, pollution, volcanoes, vegetation and deforestation.

We are moving into an era of increased dependence on timely information, and satellite observations should provide enough information to support national and international legislation. However, in most cases their evidence can be challenged through lack of traceability and in many cases insufficient accuracy.

Whilst there is now no doubt that the Earth's climate is changing and near global consensus that mankind is playing a major role, the timescale and nature of its impact remains uncertain. The last IPCC report highlights this uncertainty, showing that the ensemble of climate models forecast a range of potential temperature rises of the Earth from 2 °C to 10 °C by 2100. The vastly different consequences associated with this wide range is clearly unacceptable to society. For policy-makers to accept and act upon the data with any degree of confidence, it must be referenced to a traceable and internationally accepted standard and have significantly narrower range in potential outcomes. Traceable measurements of unprecedented accuracy, would be provided by the proposed TRUTHS calibration satellite. Allowing the models to be constrained and tested means that decisions on their likely validity can be made in less than half the time of current observing systems.

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 Earth 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. In addition, use will be made of Simultaneous Nadir Overpass (SNO) where the Earth viewing imager of TRUTHS can simultaneously, or within a few minutes, view the same part of the Earth as another in-flight satellite. The TRUTHS mission is unique in establishing high accuracy SI traceable data in-flight – a 'calibration laboratory in space'.

The impact

TRUTHS is a proposed satellite mission able to record data related to the Earth's climate to accuracies a factor of ten better than current sensors. It will monitor key climate indicators such as the amount of cloud, albedo or solar radiation at a level that allows trends to be detected in a third of the time required compared with current instruments.

In addition to providing highly-accurate climate data itself, TRUTHS will act as an in-orbit measurement laboratory which is able to upgrade the performance of much of the world's optical-based EO systems. By using an on-board primary standard, an instrument called a 'Cryogenic Solar Absolute Radiometer', and performing reference spectrally-resolved calibrations of other in-flight sensors through near simultaneous observations of the same target, it can transfer its calibration accuracy to them. This not only upgrades other sensors for climate, but also enables new micro/nano satellites to be integrated with other EO systems to provide a seamless supply of reliable data for commercial and public services.

TRUTHS represents a step change in climate monitoring with its performance limited by natural variability. It will provide invaluable benchmark data on the status of the Earth's climate, allowing scientists to identify trends in the shortest time possible. It will also improve our understanding of carbon and water cycles, crucial in providing the robust evidence needed to enable concerted international action on climate change.


Establishing a climate and calibration observatory in space


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