Toward a Climate and Calibration Observatory in space: NASA CLARREO Pathfinder and ESA TRUTHS
Authors: Fox Nigel , Shea Yolanda , Fehr Thorsten , Gary Fleming , Lukashin Constantine , Pilewskie Peter , Remedios John , Smith Paul 
Published: March 2021

&lt;p&gt;The number, range and criticality of applications of Earth viewing optical sensors is increasing rapidly.&amp;#160; Not only from national/international space agencies but also through the launch of commercial constellations such as those of planet and the concept of Analysis Ready Data (ARD) reducing the skill needed for utilisation of the data.&amp;#160; However, no one organisation can provide all the tools necessary, and the need for a coordinated holistic earth observing system has never been greater. Achieving this vision has led to international initiatives coordinated by bodies such as the Committee on Earth Observation Satellites (CEOS and Global Space Inter-Calibration System (GISCS) of WMO to establish strategies to facilitate interoperability and the understanding and removal of bias through post-launch Calibration and Validation.&amp;#160;&lt;/p&gt;&lt;p&gt;In parallel, the societal challenge resulting from climate change has been a major stimulus for significantly improved accuracy and trust of satellite data. Instrumental biases and uncertainty must be sufficiently small to minimise the multi-decadal timescales needed to detect small trends and attribute their cause, enabling them to become unequivocally accepted as evidence.&amp;#160;&lt;/p&gt;&lt;p&gt;Although there have been many advances in the pre-flight SI-traceable calibration of optical sensors, in the last decade, unpredictable degradation in performance from both launch and operational environment remains a major difficulty. &amp;#160;Even with on-board calibration systems, uncertainties of less than a few percent are rarely achieved and maintained and the evidential link to SI-traceability is weak. For many climate observations the target uncertainty needs to be improved ten-fold.&amp;#160;&lt;/p&gt;&lt;p&gt;However, this decade will hopefully see the launch of two missions providing spectrally resolved observations of the Earth at optical wavelengths, CLARREO Pathfinder on the International Space Station from NASA [1] and TRUTHS from ESA [2] to change this paradigm.&amp;#160; Both payloads are explicitly designed to achieve uncertainties close to the ideal observing system, commensurate with the needs of climate, with robust SI-Traceability evidenced in space.&amp;#160; Not only can they make high accuracy climate quality observations of the Earth and in the case of TRUTHS also the Sun, but they will also transfer their SI-traceable uncertainty to other sensors.&amp;#160; In this way creating the concept of a &amp;#8216;metrology laboratory in space&amp;#8217;, providing a &amp;#8216;gold standard&amp;#8217; reference to anchor and improve the calibration of other sensors. The two missions achieve their traceability in orbit through differing methods but will use synergistic approaches for establishing in-flight cross-calibrations.&amp;#160; This paper will describe these strategies and illustrate the benefit through examples where improved accuracy has the most impact on the Earth observing system.&lt;/p&gt;&lt;p&gt;The complementarity and international value of these missions has ensured a strong partnership during early development phases of the full CLARREO mission and that of the NPL conceived TRUTHS. Following a proposal by the UK Space Agency &amp;#160;and subsequent adoption into the ESA EarthWatch program this partnership is further strengthened with the ESA team and a vision that together the two missions can lay the foundation of a framework for a future sustainable international climate and calibration observatory to the benefit of the global Earth Observing community.&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;[1]&amp;#160; https://clarreo-pathfinder.larc.nasa.gov/&lt;/p&gt;&lt;p&gt;[2] https://www.npl.co.uk/earth-observation/truths&lt;/p&gt;