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7.4.12
Linear Polarization Optimization
It is critical that the linearly polarized satellite ground antenna align its polarization vector closely
with that of the satellite. Any mismatch between satellite and terminal antennas will reduce signal
power on the forward and return links. Even a small misalignment of the polarization vector will
significantly increase cross-polarized interference and further degrade the communications link
between satellite and terminal. Both of these potential performance shortfalls are addressed and
minimized by the AAQ.
The satellite pointing solution implemented by the AAQ and described earlier, address the angle of
the satellite’s linear polarization vector in addition to Azimuth and Elevation pointing angles. All
three of these angles depend on the same factors, including the pitch and roll of the antenna’s
pedestal. Prior to acquisition of the satellite, all of the satellite pointing and polarization angles are
calculated based on the initial, relatively inaccurate estimate of pedestal heading. The antenna’s
linear polarization is configured based on this initial value, which is more than adequate for
acquisition and peaking purposes.
Once the AAQ has successfully acquired and peaked on the target satellite (or the SATTANT
satellite), the pedestal heading is updated with an extremely accurate value. Without this final
adjustment, the satellite terminal’s performance would be degraded and the resulting cross-
polarized interference on the return link could be a significant issue for the satellite operator and
other users of the system.
7.4.13
Tracking on the Signal (Optional)
The vast majority of communications satellites in geosynchronous orbit are essentially stationary
relative to any point on the Earth’s surface. Occasional station keeping maneuvers keep these
satellites within a very small “box” centered on their assigned orbital location. In most applications,
there is no reason for the AvL antenna system to move (change antenna pointing) once the satellite
has been acquired and the peaking process is complete.
There are exceptions to this general rule and the AAQ is equipped to deal effectively with them.
One exception involves large aperture antennas (> 2.4 meters) operating at Ka-band. Antennas of
this size have very narrow “beam widths” at 30 GHz and they suffer a measurable loss in antenna
gain due to pointing error as the geostationary satellite drifts around in its small box. Since these
antennas are typically used in small gateways for many remote terminals, the entire network suffers
when the gateway antenna is not performing optimally. In this case, the gateway antenna is usually
required to track the small motion of the satellite and maintain the antenna gain very close to peak
level.
