The SI-Traceable Space-based Climate Observing System Workshop (SITSCOS) was hosted by the National Physical Laboratory in London, UK, 9-11 September 2019 and sponsored by the UK Space Agency. The workshop was organized under the auspices of the Global Space-based Inter-Calibration System (GSICS) and the Committee on Earth Observation Satellites - Working Group on Calibration and Validation (CEOS-WGCV). The international workshop included about 100 attendees including users, satellite instrument designer/builders, metrologists, and space agencies with expertise across a wide range of applications and technologies.

This presentation introduces the Workshop Report, which integrates information from the last decade of progress on laboratory metrology, SI traceability of satellite instruments, GISCS inter-calibration, CEOS Cal/Val activities, climate science accuracy requirements, and analysis of the economic value of more accurate climate change observations. The report addresses not only climate observations but also the advantages of improved SI traceability for other space-based applications such as weather prediction/analysis/re-analysis, land & ocean surface imaging, microwave imaging, and sounding.

The Workshop Report summarizes the results of the workshop in a form that can be easily understood and used by the research community as well as space agency program managers and other related organizations. It includes a Summary Report, authored by the workshop’s Science Organizing Committee, which provides a high-level overview of the workshop and its conclusions as a concise, standalone document. The remainder of the report is broken into several sections for easy access, organized by application, remote sensing spectral region, and type of measurement. The Workshop Report is available along with the presentations at https://calvalportal.ceos.org/sitscos-ws and is complemented by a special issue of the journal Remote Sensing.

The report includes a summary of published climate observing system accuracy requirements and compares current capabilities to those requirements. The workshop found that current climate observation accuracy is typically 2 to 10 times less than the SI traceability required to both survive observation gaps as well as to verify in-orbit calibration drifts over time. The workshop concluded that advances in ground-based calibration metrology as well as new approaches to fly reference spectrometers in orbit offer the ability to bridge the gap between current and needed capabilities in the near future.

Furthermore, reference SI-Traceable Satellite (SITSat) spectrometers capable of in-orbit calibration transfer through inter-calibration were recommended by the workshop as the least expensive and most robust method to achieve climate change calibration capability in orbit. The first of these SITSats are planned for launch this decade, including NASA’s CLARREO Pathfinder, ESA’s TRUTHS and FORUM, and the Chinese Space Agency’s LIBRA.

Finally, the presentation summarizes a set of recommendations from the Workshop Report to improve and maintain SI traceability in orbit – not only for the reflected solar and infrared (current SITSats), but to achieve and further examine these capabilities for passive microwave as well as active satellite instruments.

Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS) – A ‘gold standard’ reference for an integrated space-based observing system for environment and climate action
Nigel Fox1, Thorsten Fehr2, Paul Green1, Sam Hunt1, Andrea Marini2, Kyle Palmer3
1National Physical Laboratory, Hampton Rd, Teddington, Middx, TW11 0LW, UK
2ESTEC, European Space Agency, Noordwijk, Netherlands
3Airbus Defence and Space, Gunnels Wood Rd, Stevenage, SG1 2AS, UK

The number, range and criticality of applications of Earth viewing optical sensors is increasing rapidly. 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. 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.

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.

Current efforts to address the climate emergency similarly need trustworthy comprehensive data in the near term to assess mitigation actions. The range of satellites launched to support these actions must be consistent and interoperable to avoid debate, confusion and ultimately excuses for inaction. In the longer-term we need to have benchmarks of the state of the planet from which we can assess progress in as short a time-scale as possible.

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. 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.

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. 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. In this way heralding the start of the era of SITSats (SI Traceable Satellites) and the requests of the international community [3, 4, 5] can start to be addressed.

TRUTHS is a UK-led mission currently under development by ESA within its EarthWatch program based on a concept conceived at the UK National Metrology Institute, NPL, some 20 years ago. The mission is explicitly designed not only to embed high accuracy SI-traceability on-board but also to ensure that the methods and sources of uncertainty are transparent and evidenced throughout the whole processing chain, input photon to delivered radiance/irradiance. TRUTHS will make spectrally and spatially resolved measurements of incoming solar and earth/moon reflected radiation from the UV (320 nm) to SWIR (2400 nm) with an uncertainty goal of 0.3% (k=2).

In addition to establishing benchmark observations of the radiation state of the planet for climate, its unprecedented SI-traceable uncertainty can be transferred to other sensors through in-orbit reference calibration. In this way creating the concept of a ‘metrology laboratory in space’, providing a ‘gold standard’ reference to anchor and improve the calibration of other sensors. Full details of the mission and its operations are presented in a dedicated session. However, this paper provides a summary of the satellite and payload together with the means to evidence SI traceability and transfer this to other satellites. This presentation will emphasise the role and value of SITSats in a future global space-based climate observing system, and the necessary complementarity with other elements of the Earth observing system e.g. Fiducial Reference Measurements (FRMs) used for validation etc.

References
[1] https://clarreo-pathfinder.larc.nasa.gov/
[2] https://www.npl.co.uk/earth-observation/truths
[3] Strategy Towards an Architecture for Climate Monit... | E-Library (wmo.int)
[4] GCOS 200 ‘implementation plan’ doc_num.php (wmo.int)
[5] http://calvalportal.ceos.org/report-and-actions

The need for flying an on-orbit infrared SI reference sensor as soon as possible is presented. Basically, the higher, proven accuracy of such a sensor will allow subtle climate changes to be resolved and assessed much sooner. The flight of a single, high quality, reference sensor of the type defined for the NASA Climate Absolute Radiance and Reflectivity Observatory (CLARREO) program would allow the international complement of operational IR sounders to be used to establish an initial climate benchmark for future mission comparisons.

We are advocating for the flight of the laboratory-proven Absolute Radiance Interferometer (ARI) developed for NASA in support of the CLARREO program. Unfortunately, the NASA CLARREO program in the US is currently only committed to a Solar Pathfinder. While we are strongly supportive of the Forum IR mission in Europe and the LIBRA mission in China that would include the IR, the importance of this mission and the metrology perspective argue for multiple implementations. The highest practical accuracy is needed soon for monitoring international progress towards achieving the purpose of the Paris Agreement and other long-term goals.

CLARREO IR spectrometer requirements for the emission spectrum (3.7-50 microns) have been met by the UW-SSEC ARI Engineering Model, demonstrating better than 0.1 K 3-sigma brightness temperature measurement accuracy (Taylor et al., Remote Sens. 2020, 12, 1915; doi:10.3390/rs12121915 ). A key aspect of the ARI instrument is the On-orbit Verification and Test System (OVTS) for verifying its accuracy by reference to International Standards (SI). The OVTS includes an On-orbit Absolute Radiance Standard (OARS), a high emissivity cavity blackbody that can be operated over a wide range of temperatures to directly verify ARI calibration. The OARS uses 3 small phase change cells to establish its fundamental temperature scale to better than 10 mK on orbit. A broad-band heated-halo source is also provided for monitoring its cavity spectral emissivity on-orbit. Further, a Quantum Cascade Laser (QCL) is used by the OVTS to monitor the ARI spectral line shape and the emissivity of its calibration blackbody relative to that of the OARS.

After intercalibration with a single ARI flown in a pure polar or 50 degree precessing obit, like ISS, the near-term international fleet of operational temperature and water vapor sounders in sun-synchronous obit (0930, 1330, 1730) will provide the global coverage required for establishing a climate benchmark. We expect that ARI will serve as the ultimate IR reference sought for future Global Space-based Inter-Calibration System (GSICS) activities.

This work presents the latest calibration results for the Copernicus Sentinel-6A ‘Michael Freilich’, Sentinel-3A , -3B
and the Jason-3 radar altimeters as determined by the Permanent Facility for Altimetry Calibration
(PFAC) in west Crete and Gavdos, Greece. Radar altimeters are to provide operational measurements
for sea surface height, significant wave height and wind speed over oceans. To maintain Fiducial
Reference Measurement (FRM) status, the stability and quality of altimetry products need to be
continuously monitored throughout the operational phase of each altimeter. External and independent
calibration and validation facilities provide an objective assessment of the altimeter’s performance by
comparing satellite observations with ground-truth and in-situ measurements and infrastructures.
Three independent methods are employed in the PFAC: Range calibration using two transponders,
sea-surface calibration relying upon sea-surface Cal/Val sites, and crossover analysis. Procedures to
determine FRM uncertainties for Cal/Val results have been demonstrated for each calibration. Diverse calibration results by various techniques, infrastructure and settings are presented.

Climate data records derived from satellite observations provide unique information for climate monitoring and research.
However, individual instruments on a specific satellite only operate over a limited time.
Thus, providing a long climate data record requires the combination of measured values from several similar satellite sensors.
A simple combination of the observations of the different sensors would result in a temporally inconsistent data record.
For historical sensors, the sensor behaviour in orbit can be different from its behaviour during pre-launch testing and more scientific value can be derived from considering the time series as a whole.

Here we consider harmonisation as a process which obtains new calibration coefficients for each sensor by
comparing the output of one satellite to a more radiometrically accurate sensor used as a reference using appropriate match-ups, such as simultaneous overpasses.
Those match-ups would ideally be populated with SI traceable measurements but they do not exist in all spectral ranges and not for the past.
When we perform a comparison of two sensors using match-ups we must take into account the fact that those sensors are not observing exactly the same radiance.
This is in part due to uncertainties in the collocation process itself and due to differences in the spectral response functions of the two instruments, even when nominally observing the same spectral band.
We do not aim to correct for spectral response function differences, but to reconcile the calibration of different sensors given their estimated spectral response function differences.

The harmonisation consists of a gradient based minimisation to determine the optimal coefficients with a
subsequent Hessian computation and matrix inversion to get the full posterior uncertainty covariance matrix.
All derivative code used is generated by Automatic Differentiation, allowing for quick and easy development, extension, and maintenance.

We present the harmonisation framework that establishes calibration coefficients for several sensors simultaneously and rigorously with respect to their uncertainties and covariances.
We show results of the harmonisation of microwave humidity sounders MHS, MWHS-1, MWHS-2, and ATMS for three different spectral bands.

In addition to direct match-ups, which are available at high latitude over cold surfaces only, we also use indirect collocations based on sensors aboard geostationary satellites and on NWP-model based double-differences between sensors.
This increases the number of match-ups in particular over warmer regions of the globe and allows to exploit the full range of radiations.
SI traceable reference measurements in space could be integrated into the harmonisation framework that is general enough to work in different spectral ranges.