  The Medium Energy Proton and Electron Detector (MEPED) on board the National
Oceanic and Atmospheric Administration Polar Orbiting Environmental Satellites
(NOAA POES) is known to degrade with time. In recent years a lot of effort has
been put into calibrating the degraded proton detectors. We make use of
previous work and show that the degradation of the detectors can be attributed
to the radiation dose of each individual instrument. However, the effectiveness
of the radiation in degrading the detector is modulated when it is weighted by
the mean $\textit{ap}$ index, increasing the degradation rate in periods with
high geomagnetic activity, and decreasing it through periods of low activity.
When taking $\textit{ap}$ and the radiation dose into account, we find that the
degradation rate is independent of spacecraft and detector pointing direction.
We have developed a model to estimate the correction factor for all the MEPED
detectors as a function of accumulated corrected flux and the $\textit{ap}$
index. We apply the routine to NOAA POES spacecraft starting with NOAA-15,
including the European satellites MetOp-02 and MetOp-01, and estimate
correction factors.
