For centuries, the number of sunspots has been observed to vary on an 11 year cycle. Measurements during the last two solar cycles have shown that sunspot numbers and the magnitude of solar UV light are roughly correlated. Solar UV light can only be accurately measured from outside the Earth's atmosphere because this is where most of it is absorbed. The most practical way to observe the sun in the UV over an entire 11 year solar cycle is through a satellite-based instrument.
SUSIM UARS makes measurements over its 115-410 nm wavelength range daily at 1 and 5 nm resolutions and weekly at 0.15 nm resolution. It is hoped through careful and accurate calibrations made both before and during flight that the calibration of the instrument can be maintained to an absolute accuracy of 6% and a relative accuracy of 2% for the duration of a solar cycle.