Simrad HS50 - Manual (Page 49)

OPERATING INSTRUCTIONS
20221081 / F
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4.5 Operation
The HS50 will normally output signals on the serial lines
without any involvement from the user. However, the following
should be taken into consideration during operation:
Height aided GPS position
In periods with weak satellite geometry, the position can be
made more accurate by using height aiding function in the
General submenu. Refer General, page 28.
The height aiding improves the solution by using the knowledge
that a vessel at sea has only small short-term variations in height
caused by heave. The long-term variations caused by tide are
taken care of in the system. The height value used, as input to
the GPS solution is a low-pass filtered value of the measured
height.
A start value for the low-pass filter should be specified for use at
system start-up. The value to use is the height of the Sensor unit
above the WGS-84 ellipsoid when the sea level is at its mean
value. This height can be found in the primary Navigation/POS
display, where the HGT value is the GPS height. Refer
Navigation/POS display, page 36. The value should be
monitored for some minutes, and the average value should be
used.
Note! The height difference between mean sea level and the WGS-84
ellipsoid can be significant. Values between +/- 100 meters can
be encountered.
For normal operation, height aiding should be set to FILTER or
OFF. When FILTER or FIXED is selected, an initial height
has to be entered.
Height aiding is active only with input of differential GPS.