Comtech EF Data SDM-300A - Manual (Page 232)

SDM-300A Satellite Modem Revision 6
Buffering SDM300A.IOM
8–4
8.2 Buffer Size
Refer to Table 8-1.
The depth of the receive buffer will depend on four parameters:
• Doppler shift caused by satellite
• Stability of each clock (plesiochronous/Doppler operation)
• Frame/Multiframe length of multiplexed data format
• Allowable time between clock slips
8.3 Doppler
A geostationary satellite should be positioned directly over the equator and orbit
with a duration of 24 hours. In practice, the exact inclination of the satellite
(relative to the equator) is influenced by the earth, moon, and sun’s gravity, as
well as solar wind. Station keeping motors are required to maintain the orbital
position.
When viewed from the earth, the satellite appears to prescribe and ellipse in
space, degrading to a “figure 8” as the angle of inclination increases.
The orbit of the satellite can result in a peak-to-peak altitude variation of
± 2%
(85 km), while the station keeping of a newly launched satellite will typically be ±
0.1 ° (150 km). The total effect will be 172 km relative to the nominal 42,164 km
radius.
Depending upon the location of the earth station relative to the satellite, the
variation in propagation delay will typically be 1.15 ms (up to satellite and back
down), therefore a buffer depth of 2 ms is sufficient to cope with most
commercial satellites.
Since station keeping involves using fuel in the motors, the “lifetime” of the
satellite can be extended by allowing the satellite to drift into a wider “figure 8”
and using the motor less often.
The older satellites will be found in a more inclined orbit with the station keeping
varying in latitude by as much as
± 4°. The total effect of the inclined orbit may
result in a typical variation in path delay of 35 ms.