Cryogenic Solar Absolute Radiometer (CSAR)

Project Description

Determining the Total Solar Irradiance (TSI) is a long-standing goal in climate research. However, none of the currently available TSI measurements are directly traceable to an SI radiometric standard, the World Radiometric Reference (WRR). The WRR is defined from the average of a group of absolute ambient temperature cavity pyrheliometers, the World Standard Group (WSG). By comparing the WRR to the SI, it was recently shown that the originally stated WRR uncertainty of 0.3% is still valid. This relatively large uncertainty stems from the different ranges of operation of the two types of standards making it difficult to compare the two. While the WRR is designed to measure broad-band radiation of about 1 kWm^-2.

The SI-unit for radiant power (in Watts) is normally realised by national metrology institutes by using cryogenic radiometers and international comparisons are periodically carried out to ensure a worldwide uniform measurement system, the measurement uncertainty of cryogenic radiometer being below 0.01%.

The goal of the project is to build a new instrument based on cryogenic technology and being capable of measuring direct solar irradiance with a smaller uncertainty than the current standard instruments of the WSG. In carrying out this task, significant consideration will be given to the potential use of such a radiometer in space. In space, the CSAR would not only provide measurements of TSI, but also serve as a reference standard to allow the in-flight calibration of Earth viewing imaging radiometers as well.

Progress Report 2022-02-27

The aim of the project is to develop an instrument that will be able to serve as a future primary standard for the ground-based measurement of Solar Irradiance; this instrument is called Cryogenic Solar Absolute Radiometer (CSAR). The long-term objective is to replace the current artefact-based standard (World Standard Group, WSG) with an instrument that is exclusively based on fundamental physical principles, and that can therefore operate as a true SI standard. The aim is also to reduce the uncertainty of the current standard (0.3%) by approximately a factor of ten.

Following the International Pyrheliometer Comparison (IPC) in 2015, CSAR has continued to take measurements alongside the WSG, thus building a solid record of comparison between CSAR and the current primary standard. There is no significant activity to report since that comparison.

Martin Dury (NPL) has taken over the project coordination from Rainer Winkler (NPL).

Progress Report 2020-06-09

There has not been any activity in 2019.

Progress Report 2019-05-23

Progress Report 2018-01-23

Progress Report 2017-01-11

Following the International Pyrheliometer Comparison (IPC) in 2015 (see last project report), CSAR has continued to take measurements alongside the WSG, thus building a solid record of comparison between CSAR and the current primary standard.

Progress Report 2016-01-21

The aim of the project is to develop an instrument that will be able to serve as a future primary standard for the ground-based measurement of Solar Irradiance; this instrument is called Cryogenic Solar Absolute Radiometer (CSAR). The long-term objective is to replace the current artefact-based standard (World Standard Group, WSG) with an instrument that is exclusively based on fundamental physical principles, and that can therefore operate as a true SI standard. The aim is also to reduce the uncertainty of the current standard (0.3%) by approximately a factor of ten.
The project is on schedule. Significant improvements have been made to CSAR: the thermal stability of the detector has been improved by a factor of ten and more robust superconductors have been tested and implemented. CSAR was delivered to the World Radiation Center well in advance of the International Pyrheliometer Comparison (IPC) in 2015, which allowed extensive in-situtesting, optimisation and preparation for the comparison exercise on the solar tracker.In the IPC, a difference was found between the current standard and CSAR, with the WSG measuring 0.26% higher than CSAR; this is consistent with a number of other investigations into the difference between WSG and SI. The overall estiamted uncertainty achieved for the CSAR measurements (including window transmittance measurement) was 0.04% (k=1), and the overall uncertainty of the comparison of CSAR and the WSG was 0.05% (k=1).

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