Calibration of ground-based instruments

Regular calibration of field equipment is important to help ensure operation within acceptable measurement tolerances. Calibration will assist in reducing instrument-related systematic observation errors, thus lowering the associated uncertainty of the data. Greater confidence in the interpretation and application of data from various sources can be achieved when instruments have been calibrated against a traceable reference.

The Bureau uses matched sensors for diffuse and global pyranometry, and instruments are chosen such that there is a 95% confidence that there will be < 1% change in sensitivity over 12 months due to sensor degradation. However, pyranometer sensitivity may change with time and exposure to radiation, mainly due to the deterioration of the sensor. Consequently, a routine calibration process is an integral component of the Bureau's high quality surface solar observation program.

Calibration process
Figure Calibration. The calibration process followed by the Bureau of Meteorology for its solar irradiance measurements

The flow diagram in Figure Calibration illustrates the stages of the calibration process through which the Bureau of Meteorology solar measurements are traceable to the World Radiometric Reference and the International System of Units (SI).

  1. World Radiometric Reference - WRR
    • The WRR is the measurement standard of the SI unit of irradiance
    • The estimated uncertainty of the WRR is 0.3%
  2. World Standard Group - WSG
    • The WSG is a set of absolute cavity radiometers maintained by the World Radiation Center (WRR) and used to determine the WRR.
    • Every five years the WRR undertakes an International Pyrheliometer Comparison (IPC) to transfer the WRR to the participating pyrheliometers in order to ensure world-wide homogeneity of solar radiation measurements. Results of the 2011 IPC are available in WMO IOM Report #108.
    • While the IPC is primarily intended for the calibration of absolute radiometers from the Regional Radiation Centers of the six World Meteorological Organization (WMO) regions, other participants may also attend.
    • The Bureau of Meteorology represents Region V of the WMO, covering the south-west Pacific.
    • The Australian standard is defined by comparison with the WSG at the IPC. The instrumental uncertainty is estimated to be less than 0.5%.
  3. Calibration of Bureau of Meteorology instruments
    • The Bureau maintains field calibration kits, with each kit comprising two pyrheliometers and one pyrgeometer (for calibration of the long-wave irradiance measurement instrument).
    • A program of on-site pyranometer calibration is undertaken, during which the global and diffuse pyranometers are rotated in purpose (i.e. the global pyranometer is replaced by the diffuse pyranometer, and vice versa).
    • Within the current network a site which forms part of the BSRN/GCOS* is visited every six to twelve months, and every two years for other sites. The primary purpose of these visits is to calibrate the pyrheliometers, pyrgeometers and the data acquisition systems as per the BSRN/CGOS protocols.
      • * Baseline Surface Radiation Network / Global Climate Observing System
    • Calibration of the pyranometer equipment is achieved using the "Alternate Method" (Forgan, 1996) that only requires the routinely corrected data; that is, observations made every day as part of the solar monitoring program can be used to ensure the pyranometers are in calibration. Between visits the pyranometer being used to measure global irradiance operates over a wide range of conditions. These data are compared against the corresponding calculations of global irradiance determined from the pyrheliometer and diffuse irradiance pyranometer. An analysis of the difference between the irradiance data is undertaken to adjust the calibration of the global pyranometer if required. At the next visit, the two pyranometers are swapped, and the pyranometer previously used to measure global irradiance becomes the calibrated diffuse pyranometer. As a result, the sum of pyrheliometer direct irradiance measurements and those from the well-calibrated diffuse pyranometer provide the most accurate measurement of the global irradiance.
    • The solar observing system achieves a 95% uncertainty < (greater of 3% or 15 Wm-2)

Typical on-site calibration sequence

Solar irradiance instruments

  1. Instruments are connected in parallel to local and calibration kit data systems
  2. Direct measuring station pyrheliometer compared to 2 calibration pyrheliometers (for >90 minutes, 1 Hz sampling)
  3. Global and diffuse pyranometers swapped in function
  4. Station pyrheliometer again compared to the 2 calibration pyrheliometers (for >90 minutes, 1 Hz sampling)

Long-wave irradiance pyrgeometer

  1. Confirm that the cloud base is higher than 3 km
  2. Undertake a 90 minute comparison post sunset with the station and calibration pyrgeometers

Collected data from the short and long wave irradiance instruments are then analysed, and if the comparisons are within tolerance the calibration is deemed complete. The data system is verified by parallel voltage and resistance measurements.

References