The Solar Radiation Measurement Reference: WRR
There is no laboratory artifact for direct calibration of broadband solar radiometers. Instead, the Sun itself is used as a source. A group of specialized ACR instruments (first mentioned in Sect. 2) defines the reference, and the solar radiation scale. These radiometers are a sophisticated type of pyrheliometer. They match the temperature rise induced by absorption of sunlight by a cavity with a precision aperture to the temperature rise induced by an electrical current through the cavity walls while the cavity is blocked from the Sun. The precision aperture is normally not protected by a window, and therefore is sensitive to all wavelengths of the incident spectrum. (The WRR ACRs are only operated under favorable sky conditions; they are stored in an adjacent room at all other times.) The temperature rise in each case is measured as the voltage output of thermocouples in thermal contact with the cavity. The area of the precision aperture, heating current, and thermocouple voltages are “absolute” measurements used to compute the equivalent electrical and solar optical power density, thus the adjective “absolute” in the reference radiometer name (Kendall 1970; Willson 1973).
The World Radiometric Reference (WRR) is the measurement reference standard of irradiance for solar radiometry. The WRR was introduced to ensure world-wide homogeneity of solar radiation measurements. The WRR was originally determined from the weighted mean of the measurements of a group of 15 ACRs which were fully characterized. It has an estimated precision of 0.1%, accuracy of 0.3%, and stability of better than 0.01% peryear (Frohlich 1991). This determination establishes an experimental radiometric scale, which has been verified to closely correspond to the absolute definition of the International System of Units (SI) irradiance scale (Romero etal. 1991, 1995).
The World Meteorological Organization (WMO) introduced the mandatory use of WRR in its status in 1979, as a replacement for the older International Pyrhe - liometric Scale of 1956 (IPS56). This scale change also meant that older irradiance data had to be increased by 2.2% for consistency. The WRR is now realized by a group of ACRs called the World Standard Group (WSG). At the moment, the WSG is composed of 6 reference instruments: PMO-2, PMO-5, CROM-2L, PACRAD-3, TMI-67814 and HF-18748, operated by the World Radiation Center at the Physikalish-Meteorologisches Observatorium Davos (PMOD/WRC) in Davos, Switzerland (WMO 1983, 1996). This setup is shown in Fig. 1.5.
Every five years, an International Pyrheliometer Comparison (IPC) is held at the PMOD/WRC to transfer the WRR to the participating national reference instruments. The IPC is intended for the calibration of the ACRs from the Regional Radiation Centers of the six WMO regions. Thus the WSG/WRR is the “stated reference” that is the basis for traceability of solar measurements, through comparisons to the WSG. The procedures for the implementation of the transfer of WRR from the WSG are described elsewhere (Reda et al. 1996).
A slight degradation of uncertainty (from 0.3 to 0.35%) results from the transfer of WRR to regional ACRs during an IPC. More generally, the 0.3% uncertainty in the WRR is the highest definitive accuracy that can be achieved for a direct beam
Fig. 1.5 The WRR group of reference radiometers in normal operation at the World Radiation Center in Davos, Switzerland. (Photo courtesy PMOD/WRC)
measurement alone. The uncertainty in any other solar radiation measurement system other than the WSG must be greater. There are no internationally accepted references for the total global or diffuse sky radiation components independent of the WRR.