World Standard Group
(Picture courtesy of PMOD/WRC)

Accurate measurements of total solar irradiance (TSI) are crucial for good climate models and the evaluation of solar cells. NPL and the Physikalisch-Meteorologisches Observatorium Davos / the World Radiation Centre of WMO (PMOD/WRC) are working on a new instrument in order to improve the accuracy of the current standard by a factor of 10.

The Sun and the energy from it is the driving force of our planet, not only does it provide our dominant source of heat (~ 1.36 kWm^-2) but it is also responsible for weather. Most of the Sun’s impact upon us, as indicated above, requires no action from mankind and similarly cannot be influenced by us.  However, in recent years there has been increasing interest in improving the efficiency of our use of this natural energy source e.g. photovoltaics for electricity production as this makes no direct contribution to “global warming”.

The Sun’s output is often considered to be invariant (the solar constant), however this is not true.  The averaged variation is relatively small (~ 0.1 %) on an 11 yr cycle (indicated from measurements over the last 30 yrs), although with occasional short-term bursts much larger than this.  These short-term bursts, the results of solar storms, can destroy satellites, communications, and electrical power distribution on Earth.  Although variations of 0.1 % are small and have only a small impact on the Earths systems, and is thus not considered to be a dominant contributor to current climate change, it is known that the Sun has more significant variations on timescales longer than this.  From 1900 to 1950 the Sun warmed a little and during this period it did play a significant role in global warming (although the scale of change was much smaller than today) and back in the 17th century, “the mini-ice age” was believed to result from a 0.3 % cooling of the Sun. 

Figure 1: Measurements of total solar
irradiance from space over the last 30 years

These are some of the reasons why it is crucial to make long-time based measurements of the Sun with accuracies of < 0.1 %.  Although not achieving as yet these uncertainty levels, scientists have been monitoring the “total solar irradiance” (TSI) on the ground for more than a 100 yrs and from space for ~ 30 yrs.  The accuracies required for such measurements are probably the most demanding of any optical radiometric measurement, and the tendency has until recently been to emphasise stability and reproducibility, in order to monitor change, rather than necessarily absolute accuracy, see for example figure 1.  In this figure the relatively large biases between instruments (top set of curves) are removed (and somewhat ignored) through the normalisation process used to generate a time-series.  A similar approach has been used for terrestrial based measurements through the establishment and use of the World Radiometric Reference (WRR) of the World Meteorological Organisation (WMO). 

Such approaches whilst arguably valid for monitoring change can be considered of high risk when large unknown instrumental biases exist.  For example any failure to have an overlapping data set in space (mission delay, instrument failure …) will result in a catastrophic loss of the full 30 yr plus total solar irradiance (TSI) data set for climate studies.  Maintained traceability to SI has long been recognised as the only reliable route to establish accurate measurements for both short and long term requirements.  

NPL and the Physikalisch-Meteorologisches Observatorium Davos / the World Radiation Centre of WMO  (PMOD/WRC) are working to improve the above situation and establish an improved reference standard for TSI for both terrestrial and space based applications directly traceable to the International System of Units (SI). It will be 10 times more accurate than the current standard significantly improving our knowledge and confidence for global climate research.

The World Radiation Reference (WRR) is internationally recognised as the current terrestrial based standard. It was established at the PMOD in 1975 and ensures that solar irradiance measurements are consistent worldwide. This is achieved through formal comparisons organised by PMOD/WRC every 5 yrs in Davos, Switzerland, home of PMOD. At the last international comparison (IPC X) there were more than 100 participants from around the globe, all simultaneously viewing the Sun from the Swiss mountains to establish traceability to the WRR. However, the WRR is not an ideal standard. It is based on taking the mean result from a group of individual radiometers, the World Standard Group (WSG). This group was established more than 25 years ago. Since the lifetime of these devices are limited (some of the original set have already failed), there is the need for a more robust replacement and one truly based on SI. Although the WRR has been shown to be stable over time with uncertainties of < 0.1 % and the IPC disseminate its scale with uncertainties of < 0.05 % its absolute accuracy to SI is only estimated to be ~ 0.3 %.  

NPL and PMOD/WRC started to improve the traceability of the WRR through comparisons to the NPL primary standard cryogenic radiometer. This technology, pioneered by NPL, has been recognised as the most accurate means of measuring optical radiation and is the primary standard of choice at most of the world’s National Metrology Institutes.  The first comparison between the SI radiometric scale (as maintained by NPL) and the WRR was made as early as 1991. This exercise has been repeated twice since then.

Unfortunately the design of the cryogenic radiometer does not allow a direct comparison to be made, since it measures radiant power as opposed to irradiance and the operating power levels differ by a factor of 10. Thus a novel but simple measurement set-up was devised as shown below.

Comparison of the Solar and the SI Radiometric Scale

Measurement Setup for the Comparison of the Solar and the SI Radiometric Scale

The measurement sequence is as follows:

1. The Reference Trap is calibrated against the NPL cryogenic radiometer.
2. The ratio between the Reference Trap and the Monitor Trap is determined, while the optical power incident at the Reference Trap position is ~ 2 mW.
3. The power incident at the Reference Trap position is increased to ~ 20 mW.
4. A radiometer of the WRC (in this case called: “PMO6 Radiometer”) replaces the Reference Trap and the ratio to the Monitor trap is measured. The beam underfills the aperture of the radiometer.

The results of these measurements suggest that the WRR and the SI scale are identical for all practical purposes, but do rely on a number of assumptions, in terms of spectral range and indirect measurements associated with irradiance e.g. geometry, diffraction and scattering.

The next logical step is therefore to build an instrument that uses the technology of the more accurate SI primary standard cryogenic radiometer, but that is optically designed to measure total solar irradiance.  This is the objective of a new project to design and build CSAR (Cryogenic Solar Absolute Radiometer) as a long-term replacement for the WSG and the WRR. The project will build on expertise of the PMOD/WRC in space and terrestrial radiometers and that of NPL in cryogenic radiometry and previous design concepts of CSAR.

The CSAR although initially deployed on the ground will be designed with space flight in mind and is the core instrument of a space based mission proposal called TRUTHS.

For further information, please contact: Rainer Winkler