OK here is an extract from FCOM A320
EPE (RATE or THRESHOLD)
REMARK
IRS/GPS
(FOM˛ + 100˛) in meters
FOM = Figure of Merit of GPS
If above 0.28 NM the GPS position is rejected.
IRS/DME/DME
Tends towards 0.28 NM
EPE decreases from initial value to 0.28 Nm.
IRS/VOR/DME
0.1 NM + 0.05 X DME DIST minimum : 0.28 NM
EPE increases or decreases as the distance between the a/c and the VOR/DME.
IRS ONLY
+ 8 NM/h for the first 21 min.
+ 2 NM/h after
EPE increases continuously
sorry once again for the lack of clarity before