Comtech EF Data SDM-650B - Manual (Page 88)

Configuration SDM-650B Satellite Modem
3–46 Rev. 5
3.3.1.1 Buffer Size
The depth of the receive buffer will depend upon four parameters:
• Doppler shift caused by satellite
• Stability of each clock (plesiochronous/Doppler operation only)
• Frame/multiframe length of multiplexed data format
• Allowable time between clock slips
3.3.1.2 Doppler
A geostationary satellite with a 24-hour orbit should be positioned directly over the
equator. The exact inclination of the satellite (relative to the equator) is influenced by the
gravity of the earth, moon, and sun. Solar wind and station keeping motors are required
to maintain the satellite’s orbital position.
When viewed from the earth, the satellite’s orbit appears to be an ellipse around the
planet. However, the orbit changes to a figure 8 shape as the angle of inclination
increases.
The satellite’s orbit can result in a peak-to-peak altitude variation of 0.2% (85 km). A
newly launched satellite’s orbit will be typically
± 0.1 ° (150 km). The total effect will be
172 km relative to the nominal 42,164 km radius.
The variation in propagation delay will typically be 1.15 ms (up to the satellite and back
down), depending upon the location of the earth station to the satellite. The 2 ms delay is
used for most commercial satellites.
Since station keeping involves using fuel in the motors, the lifetime of the satellite can be
extended. The satellite’s age can be prolonged by allowing the satellite to drift into a
wider figure 8 orbit and using the motors less.
The older satellites will be found to be in a more inclined orbit with the station keeping
varying in latitude by as much as
± 4.0 °. The total effect of the inclined orbit may result
in a typical variation in path delay of 35 ms.