difference between the means of the performance error taken over two time intervals within different observation periods
NOTE 1 This is expressed by:
NOTE 2 Where the time intervals and are separated by a time interval DtPRE.
NOTE 3 The durations of Dt1 and Dt2 are sufficiently long to average out short term contributions. Ideally they have the same duration. See annex A.1.4 for further discussion of the choice of Dt1, Dt2, DtPRE
NOTE 4 The two intervals Dt1 and Dt2 are within different observation periods
NOTE 5 The mathematical definitions of the PDE and PRE indices are identical. The difference is in the use: PDE is used to quantify the drift in the performance error during a long observation, while PRE is used to quantify the accuracy to which it is possible to repeat an observation at a later time.