A new TOMS requirement is to measure total ozone trends of 1% per decade. The ozone calibration depends on knowledge of diffuser plate reflectance and spectrometer wavelength changes. Absolute diffuser reflectance changes are now measured with a reflectance calibration assembly, containing a UV Hg-phosphor lamp. Three diffusers are flown to permit frequent solar calibrations and infer relative diffuser changes when exposed at different rates. Finally, an ambiguity in the wavelength monitor was corrected. These three changes to the original Nimbus-7 TOMS design are expected to produce 0.5% ozone trend data, based on pre launch test results.