TRUTHS: A small satellite mission to enable a space-based climate observing system

Lead Organisation: National Physical Laboratory (NPL)
Partners: Airbus DS & University of Reading

The establishment of an observational climate benchmark data set of sufficient accuracy to enable the unequivocal detection of climate change with the ability to constrain and test climate forecast models on a decadal time scale is one of the key challenges laid down by the international climate science community. The UK led TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio- Studies) and its US sister, CLARREO (Climate Absolute Reflectance and Refractivity Observatory) are mission concepts proposed to address this exacting issue. TRUTHS’ primary goal is to provide benchmark measurements of both incoming (solar) and outgoing (reflected solar) radiation with sufficient spectral resolution and accuracy to detect the subtle changes in as short a timescale as possible (~12 yrs) – limited by natural variability of the climate system. In summary, measuring global spectrally resolved (5-10 nm) Earth radiances, continuously sampled (spectrally and spatially) with GIFOV of ~50 m from 320 – 2350 nm and the corresponding solar spectral irradiances both with uncertainties to SI units of <0.3%.

Although not selected in the ESA EE8 call, TRUTHS received a strong recommendation of support and encouragement that an early implementation should be explored as part of a wider international collaborative effort. In support of that goal investment from CEOI_ST and elsewhere has been made in the intervening period to optimise and where possible simplify the mission concept and undergo prototyping to raise the TRL of the key technologies towards 5/6.  Although some of the prototyping is ongoing as part of existing project developments improvements to the mission implementation concept has led to a reduction in the number of instruments from five to two without loss of performance or science delivery.  In a similar manner, integration on to an appropriately sized and equipped satellite platform together with matched ground segment has been studied and costed to enable the mission to be considered ready for implementation, with all elements potentially deliverable from the UK but also from a range of organisations across Europe.

Since 2010, climate and the concept of climate services has become of increased importance to the UK and Europe as a whole. The ESA CCI programme and the EU C3S service all demonstrating renewed vigour in addressing climate related issues with Earth Observation data central to the providing the necessary data and information. This is reinforced in the new ESA science strategy and consequently expected to be reflected in the EE9 call, with a focus on both societal and science benefits.

The UK has embarked on a strategy to take a leadership role in this domain building on the strong climate science expertise (e.g Met office, University of Reading, Leicester, NCEO etc) together with the breadth of cal/val and data QA knowledge from both the primary level such as NPL & RAL through to operational delivery through organisations like Telespazio Vega and underpinning this with investments in infrastructure at Harwell such as CEMS. A mission such as TRUTHS led from the UK will play to these strengths and facilitate opportunities for UK in its chosen areas of strength.

Although climate remains at the forefront of the TRUTHS missions objectives, it has become clear that there is significant interest in the cross-cutting value that the mission brings through reference calibration i.e. its ability to upgrade the radiometric performance of sensors like some of the Sentinels towards climate quality. Similarly, the level 1 hyperspectral data from the mission can also be convolved to address a range of EO applications in a similar manner to EnMAP only with significantly higher accuracy.

This project will specifically look to expand and strengthen the science case and benefits of TRUTHS accuracy and flexibility through analysing its impact on a range of EO application areas. This will be further strengthened in readiness for EE9 by expanding the EE8 international science team to encompass a broad range of nationalities and complimentary expertise in appropriate science disciplines. The team will also continue to explore opportunities to partner with other international space agencies as part of a global project.

The 21st Conference of the Parties (COP 21) exemplified the intense political, scientific and public debate about climate change. Observing the changing Earth is at the heart of ESA’s Living Planet Programme. Measurements made in support of this programme must be universally accepted, robust and of sufficient accuracy to ensure stability of long-term records. TRUTHS provides this missing assurance by directly anchoring the space-based Earth observing (EO) systems to the internationally-accepted physical standards of the SI measurement system, thus guaranteeing the temporal consistency of data, even beyond the lifetime of individual systems. This will enhance performance and ensure consistency and coherence of data from the instruments on-board the Sentinels and allow the upgrade of Copernicus and the global EO system to create a foundation for an “operational climate observing system”.

As an Earth Explorer Mission, TRUTHS will be the scientific forerunner of a paradigm shift in how the Earth is observed. It will be a standards laboratory in space, providing SI-traceable measurements of unequivocal accuracy.

TRUTHS will establish a fiducial data set of incoming and outgoing solar radiation with sufficient spatial and spectral resolution and unprecedented accuracy. This will:

  • Provide an SI-traceable benchmark of the state of the planet’s shortwave (SW) radiative balance, forcings and feedbacks from which human induced climate trends can be detected in as short a timescale as possible, limited only by natural variability.
  • Facilitate an upgrade in performance and functionality of current and future (e.g. Sentinels) and some heritage Earth observing systems to meet the specific needs of climate through an SI-traceable anchored reference calibration in orbit, ensuring robust coherence and interoperability. This is the ‘lynch pin’ needed to establish an ‘integrated Earth observing system’ and associated Climate Data Records (CDRs). TRUTHS will also serve as an enabler for the growth of next-generation ‘micro-satellites’ by providing a reference calibration for sensors too small for robust calibration systems of their own.
  • Deliver data of sufficient quality and flexibility to test and improve the retrieval of solar reflective Essential Climate Variables (ECVs), (particularly the carbon cycle on land and ocean) and other operational applications and services, following an Earth-system-science approach to mission design and utilisation.
  • Provide a robust SI-traceable anchor to address the continued debate and uncertainty regarding the impact of solar radiation (spectral and total) on the atmosphere and, consequently, climate in the near and medium term.
  • Initiate a space-based climate observing system as the European contribution to a coordinated international effort, underpinning CEOS and GEOSS.
  • Be a science-led mission ready to transition to deliver sustainable long term observations operationally.

With a high spectral resolution sensor, TRUTHS observes climate-relevant processes related to the atmosphere, oceans, land and cryosphere and provides new insight for societal understanding of the grand challenges: food, water, natural resources, health and disaster management. Many of the science challenges outlined in the ESA strategy lie in the interfaces between disciplines and are driven by uncertainty in our understanding of the Earth system. TRUTHS provides the data to test and constrain climate models and retrieval algorithms upgrading the capability of not only satellite systems but also surface-based observing systems. In this way it builds on and enhances the current research programme through injecting new and improved data into proven methods.

The IPCC concludes that the mix of natural variability and anthropogenic effects on decadal timescales is far from fully understood or measured, requiring significant improvements in accuracy. Unequivocal attribution and quantification of subtle fingerprint indicators from this noisy background are fundamental to our ability to predict climate reliably and to adopt appropriate mitigation/adaptation strategies. The uncertainty in climate prediction lies in the complexity of the models, our inadequate understanding of the Earth system, its feedback mechanisms and the relatively poor quality of available data against which to test predictions on the necessary decadal timescales. TRUTHS accuracy has been optimised to be within 20 % of the perfect observing system; allowing detection of trends in key climate parameters, from the clutter of natural variability, in as short a time as possible. For example it will detect a trend in Cloud Radiative Forcing, the dominant and most uncertain climate feedback, in half the time of other sensors. TRUTHS will address these challenges through two complementary but distinct approaches: It will establish a fiducial data set which can then

  • be used as a directly sampling of climate signals
  • improve the performance in other observing systems through reference calibrations