SLM-3650 Satellite Modem
Revision 3
Operation
MN/SLM3650.IOM
3–98
3.7.1.2 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 transmit signal is passed over the satellite, but will not be used at
the receive 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) will 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 x 24 x 60 x 60) x (1 x 10
-9
+ 1 x 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 x 3.49 ms = 6.98 ms.
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