  The MEPED instruments on board the NOAA POES andMetOp satellites have been
continuously measuring energetic particles in the magnetosphere since 1978.
However, degradation of the proton detectors over time leads to an increase in
the energy thresholds of the instrument and imposes great challenges to studies
of long-term variability in the near-Earth space environment as well as a
general quantification of the proton fluxes. By comparing monthly mean
accumulated integral flux from a new and an old satellite at the same magnetic
local time (MLT) and time period, we estimate the change in energy thresholds.
The first 12 monthly energy spectra of the new satellite are used as a
reference, and the derived monthly correction factors over a year for an old
satellite show a small spread, indicating a robust calibration procedure. The
method enables us to determine for the first time the correction factors also
for the highest-energy channels of the proton detector. In addition, we make
use of the newest satellite in orbit (MetOp-01) to find correction factors for
2013 for the NOAA 17 and MetOp-02 satellites. Without taking into account the
level of degradation, the proton data from one satellite cannot be used
quantitatively for more than 2 to 3 years after launch. As the electron
detectors are vulnerable to contamination from energetic protons, the corrected
proton measurements will be of value for electron flux measurements too. Thus,
the correction factors ensure the correctness of both the proton and electron
measurements.
