Doppler; Table C-21. Minimum Doppler/Plesiochronous Buffer Capacity Requirements; C.13 Doppler - Comtech EF Data SDM-300A Installation And Operation Manual

SDM-300A Satellite Modem
C.13

Doppler

Refer to Table C-21.
A geo-stationary satellite should be positioned directly over the Equator and orbit with a
duration of 24 hours. In practice, the Earth, Moon, and Sun's gravity, as well as solar
wind influence the exact inclination of the satellite (relative to the Equator). 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, allowing the satellite to drift into
a wider "figure 8" can extend the "lifetime" of the satellite, using the motor less often.
The older satellite will be found in a more inclined orbit with the station keeping various
in latitude by as much as ± 4°. The total effect of the inclined orbit may result in a typical
variation inpath delay of 35 ms.

Table C-21. Minimum Doppler/Plesiochronous Buffer Capacity Requirements

Satellite Orbit Inclination
Notes:
1.
2.
3.
Rev. 4
(Degrees)
0.1 (Nomnal)
0.5
1.0
1.5
2.0
2.5
3.0
4.0
The above minimum buffer capacities include a factor of two to allow the buffer to start in the
center and then drift in either direction. In the Master/Slave case, where the TX timing at one end
of the link is derived from the demodulator and used to TX back to either the station which
originated the timing signal or to a different station, another factor of two has been applied to the
Doppler contribution to account for the passage of the timing signal twice through the satellite.
Adding the buffer requirements due to satellite delay variations (Doppler) and that due to the
interfacing of different national clocks (plesiochronous) derived the above capacities.
Actual buffer capacity is likely to be larger due to the need to slip primary order streams on a
multiframe basis.
Buffer Capacity (ms)
For Various Circuit Configuration
Master/Master
1.5
2.5
3.9
5.5
6.9
8.5
9.9
12.9
Specifications
Master/Slave
2.4
4.4
7.4
10.4
13.2
16.4
19.2
25.2
C–57