Raymarine Autohelm ST7000 Installation Instructions Manual page 49

Waypoint Advance
If your navigation receiver transmits valid
waypoint number and bearing to waypoint NMEA
headers (see table) it is possible to advance from
one waypoint to the next by simply pressing the
Track button.
As the vessel passes the target waypoint the
navigation receiver should select, manually or
automatically, the next target waypoint. The
ST7000 will detect and display the bearing to the
new waypoint whilst sounding an alarm to indicate
waypoint arrival.
Note: While the waypoint advance alarm is
sounding, track control is stopped and the ST7000
will maintain the current heading,
Once it is considered safe to turn onto the new
track press the Track button once. This will cancel
the waypoint arrival alarm and steer the boat
towards the next waypoint.
Limitations
Although there is no need to fully understand the
details of the track keeping algorithm, it is very
important to understand its limitations to obtain the
best performance from the Track control. The
most significant of these limitations is imposed if
NMEA 0180 cross track error data is transmitted
by the Radio Navigation Receiver. This data is
restricted to +0.30nm, which means that even if
the vessel were 5 miles to starboard of track, the
transmitted data would still be 0.30nm,
Attempts to engage Track control beyond the
0.30nm limit will lead to excessive overshoots and
can result in the vessel circling. For this reason
the alarm code is displayed (see operating
handbook) whenever the cross track error
exceeds 0.30nm The requirement to remain within
0.30nm of track also limits the maximum allowable
angular error between the track course and the
vessel's heading. If the angular error is too great,
the Track control will be unable to cancel it within
the 0.30nm limit leading to the problems outlined
above.
The NMEA 0183 format transmits cross track
error data up to 9.99nm and enables the Track
control to operate with larger cross track errors.
However, the alarm code will still be
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displayed in case there are navigational hazards
close to the intended track.
Low Speed Operation
Operating the 'rack control at low speeds requires
more care as the effect of tidal streams is far more
significant than at higher speeds. In general
terms, providing the tidal flow is less than 35% of
the vessel speed no noticeable difference will
occur, in the performance of the Track control.
However, extra care should be taken to ensure
that the vessel is as close as possible to track,
and that the direction made good over the ground
is as close as possible to the direction of the next
waypoint before engaging Track control. Under
these circumstances positive positional checks at
regular intervals are vital especially if navigational
hazards are close.
Dodges
Full control remains available from all control units
when the autopilot is in Track control. Dodges are
accomplished by simply selecting the desired
course change on the Autohelm keypad. Once the
hazard has been avoided the course change
selected for the dodge manoeuvre should be
cancelled by selecting an equal course change in
the opposite direction. Provided the vessel
remains within 0.1nm of track there is no need to
steer back towards the track.
Safety
Passage making in Track control removes the
chores of compensation for wind and tidal drift and
will aid precise navigation. It is most important
however to maintain an accurate log with regular
plots and to verify the computed position read
from the Radio Navigation Receiver with a dead
reckoned position from recording the average
course steered and the distance logged. In open
water such plots should be at least hourly and
more frequent in confined waters or when
potential hazards are near.
Local variations in radio signal quality and
changes in the tidal stream will produce deviations
from the desired track. When setting