Hemisphere GPS V104n User Manual page 33

Chapter 3: Operation
the tilt sensor, defines the search space as a wedge-shaped location (see
Figure 3-2).
Figure 3-2: V104n's gyro aiding
The gyro aiding accurately smoothes the heading output and the rate of
turn. It provides an accurate substitute heading for a short period depending
on the roll and pitch of the vessel, ideally seeing the system through to
reacquisition. The gyro provides an alternate source of heading, accurate to
within 1º per minute for up to three minutes, in times of GPS loss for either
antenna. If the outage lasts longer than three minutes, the gyro will have
drifted too far and the V104n begins outputting null fields in the heading
output messages. There is no user control over the timeout period of the
gyro.
Calibration, which is set at the factory, is required for the gyro to remove
latency from the heading solution as well as provide backup heading when
GPS is blocked. The receiver will calibrate itself after running for a while but
it may be important to follow the manual calibration instructions if you want
to guarantee performance quickly after powering up the receiver.
With the gyro enabled, the gyro is also used to update the post HTAU
smoothed heading output from the GPS heading computation. This means
that if the HTAU value is increased while gyro aiding is enabled, there will be
no lag in heading output due to vehicle maneuvers. Hemisphere GNSS' GPS
Technical Reference includes information on setting an appropriate HTAU
value for the application.
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