XPS has redundant XUV Photometers (XPs) in order to have a working photometer for daily measurements and a lower duty cycle (reference) photometer for degradation analysis of the working photometer. The reference photometer will be used for a solar measurement every week; therefore, the cadence of the reference photometer is once per 105 measurements by the daily channel assuming 15 orbits per day.
XPS, being inherently stable, has shown no degradation except for the Lyman-alpha photometer with about 10% per year.
As a validation of the reference XPS photometers, an underflight calibration experiment will be flown about once a year. The cadence of the underflight experiment is once per 52 measurements by the reference photometers, which is similar to the cadence of the reference photometers relative to the daily photometers. The underflight calibration experiment is the prototype TIMED SEE instruments that were built in 1997-1998. This transfer of calibration includes both the relative change uncertainty (about 5%) plus the absolute irradiance uncertainty (10%); consequently, its uncertainty is larger at about 11%.
Any other solar XUV irradiance measurements, such as from SOHO and TIMED SEE, are included in the XPS validation program.