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The GPS steering functions are not guaranteed to work with all GPS systems. Each
manufacturer of GPS equipment puts his own spin on how to assemble the data on the
NMEA data bus. Sometimes the data on the bus will not conform to the needs of the
autopilot. The autopilot expects to see, at least, the NMEA data sentences $GPRMB and
$GPRMC at 4800 baud. These sentences are the minimum recommended data to be
transmitted when there is an active way point. Some GPS systems do not conform to this
NMEA specification.
Almost all GPS systems truncate the crosstrack error data to 0.01 NM resolution, this
means that the autopilot cannot be expected to stay on track any closer than 60 to 120 feet
from the course line.
There is a large discrepancy between manufacturers in how they warn the user when the
navigation fix is compromised. Some systems alert the NMEA bus listener immediately
upon position fix loss but wait 30 seconds to a minute to warn the operator on the GPS
display. This is disconcerting to the user because his autopilot drops out (The autopilot
stops using the GPS steering command immediately upon receiving a warning from the
navigator.) of GPS steering and there is often no indication on the GPS display that
anything is wrong with the GPS. Some manufacturers wait a 30 seconds to alert the user
via the display and
the devices on the NMEA data bus when the position fix is broken.
The worst performance in all the GPS units we have tested occurs when a satellite is
occluded by the horizon; we have seen steering errors on the order of several hundred
feet in these situations.
The autopilot cannot fix steering errors that are GPS generated. Keep an eye on where
you are going when you couple the GPS to the autopilot.
When the autopilot is taking steering commands from the GPS, the GPS LED will be lit
solid. If the autopilot detects a GPS problem it will blink this LED and sound the
warning horn. Press any button to extinguish the blinking LED, and cancel the warning
horn.
GPS Steering Patterns
North must be set in order for any of the GPS shing patterns to work correctly. It also needs to be done
if any GPS steering or radar overlay functions are going to be used.
Verify NMEA Connections:
Verify that the NMEA connections for the GPS are functioning. Turn on the GPS. With
the Autopilot in [Standby], press and release [Setup] button on the handheld. Press and light up the number 4 and the
number 8 LED’s [Code 48]. If the [up arrow] LED lights when you hold down the [GPS]/ [Select Load] button, the
autopilot does not acknowledge the validity of the GPS data.
To set North:
To run a GPS course requires that the autopilot compass is in agreement with the GPS’s magnetic map. You need to set
North with the pilot in standby mode.
If you have a GPS connected
to the autopilot and $GPRMC is valid and correctly formatted > Set North by running in
STBY with code 48 selected, run your boat at planning speed, at any constant heading - press and hold the GPS [Select
Load] button, and then press and release the [Deckmount]. Your pilot will power down after North is set.