Careful data analysis showed that the primary Dobson instrument, which is regularly
calibrated by the Langley plot method at Hawaii, has a long-term stability of better than 1% [Komhyr
5
et al, 1989b]. The calibration of the individual Dobson spectrophotometers by comparison with the
standard instrument introduces an additional uncertainty, which is also expected to be less than ±1%
as documented in GAW reports