Title: Absolute Radiometric Measurements in Space, ARMS
Author: David Pollard
Affil: National Physical Laboratory, Queens Road, Teddington, Middlesex, UK.
Email: david.pollard@npl.co.uk
Authors: Nigel Fox, John Martin, Eric Usadi.
Abstract: Over recent decades, spacecraft observations of total solar irradiance (TSI) have shown variations of 0.1% between solar maxima and minima and of up to 0.2% over shorter timescales. However, the data sets obtained by different spacecraft have shown significant discrepancies, which have historically been removed by normalising the data and applying other corrections. A proposed new spacecraft instrument is described, that will allow absolute radiometric measurements of TSI and solar spectral irradiance (SSI) for the first time with an accuracy of <0.01% and <0.1% respectively over a timescale of up to 10 years. ARMS will be a suite of instruments based around a cryogenic solar absolute radiometer (CSAR), which is of a similar design to the hardware used for the realisation of primary radiometric scales. This will allow a long-term, consistent observation of TSI and SSI within the necessary uncertainties to be made. The flight of such an instrument would also bring primary standard traceability into the space environment, enabling the Sun itself to be calibrated as a secondary standard, which can then be used to calibrate Earth observation instruments. The instrument could be configured for operation on a dedicated or shared platform.