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

Page: 292 / 516

SLM-5650A Satellite Modem
Revision 10
Appendix B B–22 MN-SLM-5650A
B.4.1 Buffer Size
The depth of the receive buffer will depend on four parameters:
1. Doppler shift caused by satellite.
2. Stability of each clock (plesiochronous/Doppler operation).
3. Frame/Multiframe length of multiplexed data format.
4. Allowable time between clock slips.
B.4.1.1 Doppler
A geostationary satellite should be positioned directly over the equator and orbit with 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 an 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.