GPS receiver and chartplotter
Navigation
The GPS works out where you are from satellite signals; the chartplotter shows that position on an electronic chart with your course, speed and routes. On some boats they're one box; on others a separate GPS antenna feeds the plotter, VHF, autopilot and AIS.
How it works
A GPS receiver times signals from several satellites to find latitude and longitude, then works out speed and course over ground (COG) from recent positions. That's why COG is unreliable when you're stopped or turning slowly, while a compass shows heading directly. The plotter draws your position on the chart and can pass it to other gear by NMEA 0183 or NMEA 2000. A receiver that has been off for weeks, or moved far, needs a few minutes for its first fix.
Using it
- Give a new or long-stored unit a clear view of the sky and a few minutes to find itself; one maker says 3-5 minutes after it has lost its stored satellite data or had a software update.
- Know heading from course over ground: heading is where the bow points, COG is where you're actually going. Current and leeway make them differ, most of all at slow speed.
- Make sure positions you enter or plot are on the chart's datum. NOAA charts use NAD 83/WGS 84, and NOAA warns that positions on older datums such as NAD 27 are significantly offset.
- Keep a backup for when GPS or the plotter fails: another device with charts, or paper charts and a way to plot on them.
- If GPS behaves oddly, such as a wrong date, jumping positions or sudden loss of fix, report it to the Coast Guard Navigation Center, as GPS.gov asks users to do.
Looking after it
- Keep plotter and GPS software up to date; GPS.gov says devices that weren't designed for the 2019 GPS week-number rollover needed software or firmware patches.
- Keep an internal-antenna unit where it can see the sky, or fit an external antenna if it can't.
- Check the external GPS antenna's mount, cable and connectors for damage and corrosion.
- Update charts regularly (see Electronic charts).
Troubleshooting
GPS won't get a fix (on one maker's plotters, a flashing question mark over the boat)
- Leave it powered and still for several minutes, especially after a software update or long storage.
- With an internal antenna, move the unit or the plotter to where it can see the sky; if there's no good spot, fit an external antenna.
- With an external antenna, check its cable and connectors, or check that it appears in the plotter's NMEA 2000 device list.
- If more than one GPS source is available, select a different one in the plotter's settings.
- Update the software. If there's still no fix, check for official notices of GPS outages or interference in your area.
Wrong date or time, or strange behaviour, on an older GPS
- Older receivers may mishandle the GPS week-number rollover (the last one was April 6, 2019; the next for older-signal receivers is November 20, 2038).
- Check the maker's site for a firmware update.
- If there's no fix available, the receiver may need replacing; report the problem to the maker and to NAVCEN.
Boat position on the chart is offset from where you really are
- Check the datum used for any positions you entered and for the chart itself.
- Check which GPS source the plotter is using.
- Remember that some chart areas rest on old surveys; slow down and use eyes, depth sounder and radar.
Buying a boat that has one
- Note the plotter's age and whether the maker still offers software and chart updates for it.
- Check the date and time shown are correct; a wrong date can mean an old receiver never got a week-number rollover fix.
- Any Loran-C receiver aboard is useless: the Coast Guard began shutting Loran-C down on February 8, 2010.
- Find out which unit supplies position to the VHF, AIS and autopilot. Replacing that unit means reconnecting all of them.
- Inspect the external GPS antenna and cable, which are often as old as the plotter.
Sources (6)
support.garmin.com · gps.gov · federalregister.gov · nauticalcharts.noaa.gov · ritchienavigation.com · www8.garmin.com
General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.