Comtech EF Data Vipersat SLM-5650A, Installation And Operation Manual

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SLM-5650A Satellite Modem
Revision 10
Appendix B B–23 MN-SLM-5650A
B.4.1.2 Plesiochronous
The stability of station reference clocks is normally 1 x 10-12 (derived from a cesium standard).
While the stability is exceptionally high, the two clocks are not in synchronization with each other
and will eventually pass by each other.
The clock used for the TX signal is passed over the satellite, but will not be used at the RX earth
station where a national network derives its time locally. A buffer will fill up with data using the
clock from the satellite and will empty using the local clock. The object of the buffer is to ensure
that the buffer overflows or underflows at regular, determinable intervals (typically every 40 days).
The buffer depth required (from center to end) would be:
Minimum slip period (seconds) * [stability of far end (transmit)
clock + stability of local clock]
For example:
Far end (transmit) clock stability 1 x 10 -9
Local (buffer) clock 1 x 10 -11
Minimum clock slip 40 days
Buffer depth = (40*24*60*60) (1*10 -9 + 1*10 -11 ) = 3.49 ms
Because the buffer will either fill or empty (depending on the frequency relationship of the two
clocks), the total buffer depth will be:
2*3.49ms = 6.98ms