CALIBRATION OF THE SOLAR ULTRAVIOLET SPECTRAL IRRADIANCE MONITOR (SUSIM) ON ATLAS-2

Uncertainty in the calibration of the Solar Ultraviolet Spectral Irrad iance Monitor (SUSIM) produces an uncertainty in the measurement of so lar irradiance. The three primary sources of uncertainty in the irradi ance during the ATLAS-2 mission are from wavelength determination, irr adiance standard, and the in-flight ageing correction. The wavelength determination leads to an uncertainty in the irradiance in both the ca libration data and the solar observations. The first effect is importa nt only at short wavelengths. The second effect is important wherever the solar spectrum is steep but can be eliminated by summing over spec tral features. The uncertainty in the irradiance standard is about 2%. The ageing correction produces a wavelength-dependent uncertainty whi ch is typically 1% to 2%. The total lo uncertainty in the ATLAS-2 SUSI M 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.