Performance optimization for blockage
C-2
Chapter C: VMU settings
98-150471-A02
The simple time-out means that there is a fixed delay, plus the time needed to acquire
another satellite, before there is a chance of regaining good link performance.
If the signal is not sent to the modem, the system can in some cases remain linked and have
a higher uptime, but not provide a stable data connection. Such a link is of no value to the
subscriber. Not sending the blockage (TX-Mute) signal also extends the period of the poor-
quality link, as the VSAT modem is still relying on a time-out before switching to another
satellite, and the time-out may be constantly reset by the link coming and going.
Better blockage communication
A major disadvantage of this single signal is that if the VSAT modem has multiple satellites
to choose from, then, when selecting a new satellite, the VSAT modem is again relying on
the simple time-out. This continues until a satellite with no obstruction in the view from the
satellite terminal is selected. If the VSAT modem was able to receive information from the
ACU that the view towards the current satellite is blocked, it would be able to choose a
visible satellite much faster without the need to wait for multiple time-outs.
Minimum elevation angle
One safe way of getting optimum performance under the current conditions is to switch to
a satellite in view as fast as possible. This is done by having well defined satellite parameters
in the VSAT modem configuration. It is the VSAT modem that has the task of selecting the
correct satellite, and since the VSAT modem is only concerned about the satellite visibility
at the current geographic position, it is very important to enter the minimum elevation of a
satellite at which a stable link can be established.
It is often seen that the minimum elevation is set to 0 (zero). A setting of 0 is not only in
many cases below the usable limit of the satellite, but also a violation against ETSI EN 302
340, where a calculated minimum elevation ranges from 12° to 3° depending on power and
bandwidth must be ensured or FCC §25.205 which states the minimum elevation to be 5°.
See also the table in the section
Unless the vessel is operating in international waters with no other options, a minimum
elevation of 10° or higher is recommended.
Conclusion
Fewer but well-functioning satellites to choose from give better user performance than
having many satellites, which may have a longer uptime but do not provide a stable data
connection. In the end it is not the actual uptime the subscriber is concerned about, but it is
the uptime where the link gives a stable data connection.
SAILOR900HP.book Page 2 Monday, July 18, 2016 12:44 PM
