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GPS Steering
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 waypoint. Some GPS systems do not conform to this NMEA specication.
Almost all GPS systems truncate the cross track 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. There is a large discrepancy between
manufacturers in how they warn the user when the navigation x is compromised. Some systems alert the NMEA
bus listener immediately upon position x 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 manufactures wait 30 seconds to
alert the user via the display and the devices on the NMEA’S data bus when the position x 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 x 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 (Select Load) LED will be lit solid. If the
autopilot detects a GPS problem it will blink this LED. Press any button to extinguish the blinking LED.
Set North
To run a GPS course requires that the autopilot compass is in agreement with the GPS’s course estimate. You need to set
North with the pilot in standby mode. Do not set North except in calm sea’s and un-accelerated conditions. You should
be running at a reasonably high speed, it won’t hurt to be running on plane with your main engine. With your GPS
turned on and setup code 48 selected, press and hold the GPS (Select Load) button.
If the Up Arrow LED illuminates, this means that your GPS connection is not sending course information to the
autopilot correctly. You must point your boat north and then press and release the Deckmount switch to set compass
north.
If the Up Arrow LED does not illuminate, press and release the Deckmount switch to set compass north. In this case
you do not need to be pointing north-you only need to be going in a straight line.
Your pilot will power down after north has been set.
If you press and release the GPS (Select Load) button when the autopilot is in heading hold and the GPS does not have
an active route, the pilot will maintain the GPS course over ground instead of the magnetic heading. Be aware that the
course over ground calculated by the GPS is erratic at low speeds.
All the special function and turn buttons will continue to behave as if you are in normal heading hold. If you use any of
the normal turning buttons, go to waypoint and course over ground hold are terminated.
Course Overground Hold
GPS