Site considerations
98-175666-C
Chapter 2: Installation
2-4
Ship motion and offset from the shipʼs 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 shipʼs 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 shipʼs motion centre (h max)
Minimum
roll period
Maximum ADU mounting height (h max)
Full performance
Potential risk of damage
4 s
12 m
16 m
6 s
27 m
35 m
8 s
48 m
62 m
10 s
75 m
98 m
Table 2-2: Maximum distance from the shipʼs motion center versus shipʼs roll
period
h max
Note
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.
