Filter
Top Products
SLM-5650 Satellite Modem Revision 2
Configuration MN/SLM5650.IOM
3–18
3.5 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) 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 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.