Site considerations
98-175666-A Chapter 2: Installation 2-4
Ship motion and offset from the ship’s motion centre
When installing the ADU you must consider the mounting height carefully. The higher up
the ADU is mounted, the higher is the linear g force applied to the ADU. The g force also
depends on the roll period of the ship, see Table 2-2. If the g force applied is too high,
performance and ADU signal stabilization may be reduced and eventually the ADU may be
damaged.
Even though it is recommended to mount the ADU high, keep the distance between the
ADU and the ship’s motion center as short as possible.
2.2.2 Obstructions (ADU shadowing)
The ADU beam is approximately 1 m in diameter for the first 30 m from the ADU. Beyond
30 m the beam gradually widens so that it is approximately 5 m in diameter at 100 m
distance. This beam expansion continues with increasing distance. Any obstructions, such
as masts, funnels, bridge house etc. within this field can cause signal degradation or signal
loss.
For optimum performance adhere to the following guidelines:
1. Place the ADU so that it has as much free line-of-sight to the satellite as possible
without any structures in the beam through one full 360 degrees turn of the vessel.
2. Do not place the ADU close to large objects that may block the signal.
Figure 2-1: Maximum distance from the ship’s motion centre (h max)
Minimum
roll period
Maximum ADU mounting height (h max)
Full performance Potential risk of damage
4s 12m 16m
6s 27m 35m
8s 48m 62m
10 s 75 m 98 m
Table 2-2: Maximum distance from the ship’s motion center versus ship’s roll period
Due to the short wavelength at Ka band and the narrow beam width of the ADU
even a 6 mm steel wire placed within 50 m inside the beam can causes
signal degradation.
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