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ComNav Admiral P3 Installation & Operation How Autopilots Work
Document PN 29010075 V3.1 - 18 -
Autopilot Operation: Following a Track – NAV Mode
So far, we have seen how an autopilot operates in AUTO mode: a new desired Heading must
be selected on the Control Head each time you wish to change direction. This is handy for
short trips or when you know the direct course to your objective.
But for longer trips, which might involve a number of legs at different headings, it would be
useful to be able to have your autopilot steer your boat along all the legs, in sequence.
ComNav autopilots allow you to do just that, with NAV mode.
Before using NAV mode:, the autopilot must be connected to an external source of navigation
information (commands & data) – such as a chart plotter, or a Navigation program running on
a PC. A source of position data is required, too; it may be built into the external Navigation
System, or it may be another device or system: a GPS receiver (such as a ComNav Vector
GPS Compass), a LORAN C receiver, etc.1
ComNav Autopilots implement the industry-standard NMEA 0183 protocol for reception &
transmission of navigation information. Fortunately, almost all of today’s Navigation Systems
& Devices do comply with NMEA 0183, so interfacing those systems & devices to a ComNav
autopilot is relatively simple.
A long passage will consist of a series of waypoints, which are specific locations on the
Earth’s surface (hopefully on the water!) defined by their Latitude & Longitude. These
waypoints are entered into the external Navigation System by the user. That system
determines the current location of the boat (from the source of position data), and then
calculates the course that must be steered in order to reach the next waypoint.
When the Autopilot is switched into NAV mode, it begins to look for some specific navigation
information from the Navigation System: the course to steer to the next waypoint, and the
Cross-Track error (which is the distance from the boat to the desired Track from the last
waypoint to the next one). Using these two pieces of information, the autopilot steers the
boat on the shortest possible course to the next waypoint.
Crosswinds and/or currents are compensated for automatically each time the Navigation
System updates the cross-track error. This is why NAV mode is the answer to the “track slip”
problem that can occur in AUTO mode, when a crosswind/current exists.
When a waypoint is reached, the Navigation System sends the SPU the next set of
navigation information – for the next waypoint – and the SPU then turns the boat to head to
that next waypoint.
When the last waypoint is reached, the boat typically (depending on how the Navigation
System is set up) will orbit that waypoint, until the autopilot either is switched out of NAV
mode or is given new navigation information by the user, via the Navigation System.
Because the boat will turn automatically turn – without input from the user – at each waypoint,
when planning the trip it is crucial that the user verify that there will be sufficient room to
execute the turn at each waypoint.
The autopilot can be set to alert he user – by activating an alarm – as the boat nears a
waypoint.
1 … all such external devices are in general referred to in this manual with the generic term Navigation System.
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