Sailor Inmarsat B, Workshop Manual

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2 SYSTEM DESCRIPTION Inmarsat B
PAGE 2-4 9936
2.1.3 ANTENNA BEHAVIOUR DURING START-UP SEQUENCE
When the complete system, or just the ADE, has been reset or switched off and on, the ADE will initialize
and search for the satellite. The behaviour of the antenna during this process is described in this chapter.
1. Initialization
A few seconds after resetting, the horizontal axis will go to the bottom stop position. A knocking
sound will be heard for a few seconds. After this the axis will go up to the optical zero sensor.
Then the elevation axis will initialize in the same manner. Also from this axis a knocking sound
will be heard when it is in the bottom position. When the elevation axis has returned to horizontal
position, the cross-elevation axis will begin to move towards horizontal position. A high-fre-
quency switch-mode sound can be heard when the cross-elevation axis moves. The antenna will
be ready after 2-3 minutes.
If the transceiver has received the position of the vessel, the fast search will begin.
2. Fast search
The elevation axis will move up to the calculated elevation angle of the chosen satellite, and the
azimuth will turn clockwise 360°. This rotation will last about 1-2 minutes. After the search, the
azimuth will go to the position with the highest signal level. This rotation will last less than 1
minute. Here a fine search will be performed.
3. Fine search
The fine search is a cross-shaped search, the centre of which will be in the expected direction of
the satellite. The angular speed of this search is lower than the speed of the fast search. First
the elevation axis will search vertically from 20° below the expected satellite position to 20°
above this position. Then it will search horizontally from 15° to the left of the expected position
to 15° to the right of it. Finally it will move back to the azimuth and elevation angles where the
highest signal levels were measured. The fine search will last 1-2 minutes.
The total start-up sequence will last 5-6 minutes.
2.1.4 ANTENNA BEHAVIOUR DURING GLOBAL SEARCH
If the modem cannot achieve synchronisation on the received NCSC signal within 5 seconds after the fast
search, a global search will start. In this search mode the azimuth will rotate slowly 360° clockwise, and
at the same time the elevation axis will move up and down in a zigzag shape. The elevation top of this zigzag
is 80° above the horizon and the elevation bottom is 5° above the horizon. After this the satellite dish is
turned to the direction where the highest signal level was measured. A ± 25° horizontal and vertical fine
search is performed around the direction whre the highest signal level was measured.
The total global search will last between 6-7 minutes.
2.1.5 ANTENNA BEHAVIOUR DURING REGION SHIFT SEARCH
The behaviour of the antenna during a region shift search depends on whether the position of the vessel
is keyed in manually or received directly from a GPS. In the following description, the values in square
brackets are those of a position keyed in manually, the other values are with a functional GPS connected
to the system.
If for some reason the signal between the satellite system and the ship is blocked by eg. a smokestack
or an other object on board or ashore, it can be necessary to change to an other region. Blocking objects
can cause the signal level to drop to a value where the system is still receiving from the satellite, but where
the quality of the signal is too poor to perform communication. This error may occur if for instance several
calls (Ship to shore) fail, or if only two LEDs are illuminated on the handset. In such cases the transceiver
can be shifted to an other region, in which case a region shift search is performed.