Uncertainty in the calibration of the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) produces an uncertainty in the measurement of solar irradiance. The three primary sources of uncertainty in the irradiance during the ATLAS-2 mission are from wavelength determination, irradiance standard, and the in-flight ageing correction. The wavelength determination leads to an uncertainty in the irradiance in both the calibration data and the solar observations. The first effect is important only at short wavelengths. The second effect is important wherever the solar spectrum is steep but can be eliminated by summing over spectral features. The uncertainty in the irradiance standard is about 2 %. The ageing correction produces a wavelength-dependent uncertainty which is typically 1% to 2%. The total 1σ uncertainty in the ATLAS-2 SUSIM irradiance measurement (130 nm to 410 nm) is 2% to 4%. For the ATLAS-3 mission, the performance of the instrument has been enhanced by an improved wavelength determination, a new D2 lamp and power supply, and the addition of a filter wheel in place of a fixed entrance filter.
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