Furuno Felcom 250 Service Manual page 333

1. Tracking
When the system is in tracking status, the Antenna unit will also control the AZ, EL, and
Cross EL axes by itself so that the system will always be facing the satellite.
The AZ and Cross EL axes are controlled by calculating the phase differences between
the signals received by the Antenna elements on the right and the left. The EL axis is
controlled by the Clinometer (FB-1500) and the Rate Gyro (FB-1250).
Reference)
In normal cases for when the system is located at Furuno HQ at Nishinomiya, for the
APAC satellite under the condition of EL: 45 degrees, the Signal strength for the
status monitor screen will be 62 dB/Hz (regional beam) when communication is not
established, and will be 68 dB/Hz (spot beam) when communication is established.
If the signal strength goes below "50 dB/Hz" when communication is not established,
the system will identify the status as blocking status, and will change its mode to
search mode.
** The global beam signal is used for tracking.**
Fig. 11.2.2 Antenna unit block diagram
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11.2 Antenna Unit: FB-1500/1250