Accurate radiometry from space: an essential tool for climate studies

Abstract

The Earth's climate is undoubtedly changing; however, the time scale, consequences and causal attribution remain the subject of significant debate and uncertainty. Detection of subtle indicators from a background of natural variability requires measurements over a time base of decades. This places severe demands on the instrumentation used, requiring measurements of sufficient accuracy and sensitivity that can allow reliable judgements to be made decades apart. The International System of Units (SI) and the network of National Metrology Institutes were developed to address such requirements. However, ensuring and maintaining SI traceability of sufficient accuracy in instruments orbiting the Earth presents a significant new challenge to the metrology community. This paper highlights some key measurands and applications driving the uncertainty demand of the climate community in the solar reflective domain, e.g. solar irradiances and reflectances/radiances of the Earth. It discusses how meeting these uncertainties facilitate significant improvement in the forecasting abilities of climate models. After discussing the current state of the art, it describes a new satellite mission, called TRUTHS, which enables, for the first time, high-accuracy SI traceability to be established in orbit. The direct use of a 'primary standard' and replication of the terrestrial traceability chain extends the SI into space, in effect realizing a 'metrology laboratory in space'.

The Spectral Signature of Recent Climate Change

“…Indeed, we would contend that there is currently no hyperspectral instrument in space that possesses the level of in-orbit, SI traceable calibration or sampling characteristics needed to provide a benchmark record of the true climate state. Efforts are ongoing to rectify this situation via, for example, the CLARREO [81••] and Traceable Radiometry Underpinning Solar and Terrestrial Studies (TRUTHS) initiatives [82]. A CLARREO Pathfinder mission is scheduled for launch in 2020 on-board the International Space Station with the goal to demonstrate the target in-orbit calibration accuracy across the RSW domain, and efforts continue to secure funding for the full mission (currently scheduled for launch no earlier than 2023).…”

The impact of large scale ionospheric structure on radio occultation retrievals

“…We then show the results of our study to quantify residual bias using simulation. Our analysis should be useful to establishing SI-traceability in the presence of retrieval bias due to the ionosphere, at least for effects caused by large-scale ionospheric structure (see Fox et al, 2011 and references therein for a discussion of accuracy and SI-traceability).…”

Establishing a metrology lab in space to meet the demands of climate change

“…The calibration method for the TRUTHS Earth Imager (EI) has simplified from that originally described in [10] and is now a three step process. The SI primary standard 'traceable' cryogenic radiometer (CR) power measurement capability is transferred via a monochromatic based source to a simple transfer radiometer (TR) and from there, using the same source to provide a known radiance field to over-fill the entrance aperture of the EI via a Lambertian diffuser plate.…”

“…A full description of the instrumentation employed on TRUTHS is beyond the scope of this paper but a brief summary of the key instrument is included here to aid the in the outline description of the novel calibration method, more details can be found elsewhere [10,11].…”

“…The Cryogenic solar absolute radiometer (CSAR) is the heart of the TRUTHS mission [10], providing the means to establish SI traceability in orbit. CSAR (and laboratory cryogenic radiometers) are based on the principle of 'electrical substitution', where optical power incident on a black (spectrally neutral) absorbing surface will cause a rise in temperature of the disk, which is sensed by a thermometer.…”