Standards and guides

Various standards and guides exist to facilitate quality, standards-based observations of the various solar and terrestrial irradiance parameters. The International Organization for Standardization released a number of documents in the 1990s related to the design and operation of irradiance measurement instrumentation. A more recent, widely adopted reference is the Guide to Meteorological Instruments and Methods of Observation produced by the World Meteorological Organization, which specifies the performance characteristics recommended for instruments to undertake various observational roles.

A summary of the operating characteristics of pyranometers has been reproduced from this guide in Table Pyranometer-class below. Similar tables along with supporting text are included in the document for other instruments.

Table Pyranometer-class  Operating requirements for pyranometers of various performance classes*
Characteristic High Quality Good Quality Moderate Quality
Response time (95 per cent response) < 15 s < 30 s < 60 s
Zero offset
(a) Response to 200 W m-2 net thermal radiation (ventilated) 7 W m-2 15 W m-2 30 W m-2
(b) Response to 5 K h-1 change in ambient temperature 2 W m-2 4 W m-2 8 W m-2
Resolution (smallest detectable change) 1 W m-2 5 W m-2 10 W m-2
Stability (change per year, percentage of full scale) 0.8 1.5 3.0
Directional response for beam radiation (the range of errors caused by assuming that the normal incidence responsivity is valid for all directions when measuring, from any direction, a beam radiation whose normal incidence irradiance is 1 000 W m-2) 10 W m-2 20 W m-2 30 W m-2
Temperature response (percentage maximum error due to any change of ambient temperature within an interval of 50 K) 2 4 8
Non-linearity (percentage deviation from the responsivity at 500 W m-2 due to any change of irradiance within the range 100 to 1 000 W m-2) 0.5 1 3
Spectral sensitivity (percentage deviation of the product of spectral absorptance and spectral transmittance from the corresponding mean within the range 300 to 3 000 nm) 2 5 10
Tilt response (percentage deviation from the responsivity at 0° tilt (horizontal) due to change in tilt from 0° to 90° at 1 000 W m-2) 0.5 2 5
Achievable uncertainty (95 per cent confidence level):
(a) Hourly totals 3% 8% 20%
(b) Daily totals 2% 5% 10%

Performance class

High Quality
Suitable for use as a working standard; maintainable only at stations with special facilities and staff.
Good quality
Acceptable for network operations.
Moderate quality
Suitable for low-cost networks where moderate to low performance is acceptable.

*Reproduced from Guide to Meteorological Instruments and Methods of Observation, WMO

While instrumentation is an important component, and often the prime focus of solar resource monitoring, adoption of standards and best practice is required at all stages of the process if traceable, quality solar data are to result. Guides and/or standards have been produced which cover areas including site selection and instrument exposure, observational practices, calibration and maintenance, production, maintenance and distribution of metadata, and the estimation of the uncertainty association with the observational data.

References