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SLM-7650 Satellite Modem
Theory of Operation
As the satellite distance decreases, the frequency of the signal being received at the earth
station will be at a slightly higher rate. If the satellite distance increases, then the receive
frequency at the earth station will be at a lower rate. The effects of the rate changes can
be nullified by the receive buffer. The buffer will fill up and overflow if the buffer clock
is slower than the recovered clock from the satellite. The buffer will empty if the buffer
clock is faster than the recovered clock from the satellite. The buffer will recenter when
the buffer overflows or underflows. An overflow or underflow of the receive buffer will
cause a loss of the data that is in the buffer. Proper configuration of the buffer will
minimize the adverse affects on the overall communication network. A loss of bit count
integrity (BCI) will not cause the buffer to recenter. The buffer will recenter if the
decoder looses lock.
The time between clock slips can be calculated as follows:
T= B/2/ ∆
Where:
•
•
•
The buffer size is divided in half because the buffer starts at the 50% fill state.
The following tables are used to determine buffer size.
•
•
Parameter
Maximum variations
Maximum rate of variations
Note: The maximum variations are based on the peak-to-peak uplink plus downlink delay
and the maximum rate of variations include the uplink and downlink.
B = Total Buffer
size
∆ = Difference in frequency
T = Time in seconds
Table 4-7 will provide the delay variations expected for an inclined orbit satellite.
Table 4-8 gives some recommended buffer sizes based on the examples that
follow.
Table 4-7. Delay Variations for Inclined Orbit Satellites
0.5
1.1
40
Table 4-8. Recommended Buffer Size
Satellite Orbit
Inclination
(Nominal)
0.1
0.5
1.0
1.5
2.0
2.5
3.0
Inclination in Degrees
1.0
1.5
2.0
1.8
2.6
3.3
67
94
121
Buffer size in milliseconds
Example 1
Example 2
Doppler +
Doppler only
Plesiochronous
1.5
2.5
3.9
5.5
10.4
6.9
13.2
8.5
16.4
9.9
19.2
4–34
MN/SLM7650.IOM
2.5
3.0
Units
4.1
4.8
milliseconds
148
175
nsec/sec
2.4
4.4
7.4
Revision 4
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