As seen in Figure 1
, the CSAC instability at one-second averaging time is sy(t =1)˜10-10
and improves with averaging time as 1/t1/2
, typical of atomic clocks. At longer times, here t >1000
seconds, the CSAC performance degrades due to temperature sensitivity and long-term drift. On the other hand, the GPS signal is relatively unstable at shorter averaging times, sy(t =1)˜10-8
, but improves as 1/t
such that it has superior stability at longer averaging times. To optimize the system performance on all time scales, the CSAC should be disciplined to GPS with a loop time constant equal to the averaging time at which the two noise processes intersect. From Figure 1
, the two lines intersect at ˜3000 seconds and thus this should be chosen as the loop time constant. An example of the combined stability of GPS and CSAC can be seen in the ADEV chart below where the CSAC was optimally disciplined with a time constant of 3000 seconds.