To maintain the accuracy of Dobson spectrophotometers in the WMO/GAW ozone observing network, it is important to ensure traceability through intercomparison and calibration with the world standard and the regional standard Dobson spectrophotometers. The national standard instruments are calibrated by these regional standards for each at the Regional Dobson Calibration Centres (RDCCs).
    JMA maintains and manages a reginal standard instrument for Dobson spectrophotometer in Asia (RA-II) at the Aerological Observatory in Tsukuba. This regional standard instrument is calibrated through comparison with the WMO world standard owned by NOAA once every three years. The world standard instrument itself is calibrated in Mauna Loa, Hawaii by absolute assay using the Langley method. Table 1 shows all past intercomparisons between the regional and world standards.