GPS troubleshooting can lead to many things that can cause accuracy issues with a boat GPS, and you will need to systematically work through them. Don’t simply blame software issues or it’s a satellite problem, most problems are at the boat end and can be rectified. The following troubleshooting information should get you through most problems.
The GPS system may be down, or a satellite may be shut down. Check your navigation information source for news of outages. The Horizontal Dilution of Position (HDOP) may simply be excessive due to poor satellite geometry in your location. With sequential receivers, loss of signal may be a problem in heavy sea states. This causes rapid antenna movement, also significant vertical altitude changes on waves, all have an effect. For fishermen in inland river areas, satellite shadowing can occur from forest and timber to steep cliff and bank faces. So review your situation and consider the effects.
Errors that are not large but consistently outside normal accuracy levels are due to a number of sources. The signal may be subject to an excessive amount of atmospheric disturbances, such as periods of extensive solar flare activity. The aerial connections and part of the installation may have degraded, so check the entire system. Make sure aerial is vertical and not partially pushed over in fixed installations. In many handheld cases they are just lying down somewhere, and it is best to get proper bracket and keep them vertical.
GPS troubleshooting worst case is not position fix. This is often caused by a total loss of a satellite view or when a satellite goes out of service. Another common cause is the aerial being pushed over to horizontal, so check that it is vertical in fixed installations. Handheld users should make sure that the unit is in a clear position with aerial vertical. Aerial damage after being struck by equipment is a cause of a sudden fix loss. Check all cables and connections. If these show no defects, a check of all initialization parameters may be necessary; if those check out, the receiver and aerial may require manufacturer servicing.
This error can be caused by power supply problems. If you are having problems than the first thing is to check whether this coincides with trolling motor or outboard run periods, or diesel and gasoline motor run periods. Ignition systems are a common cause as are faulty alternators. Radiated interference is also another possibility, often from other radio equipment such as a mobile phone or even a VHF radio.
Another quite common cause of data corruption is that caused by “fingers.” The dreaded finger fault is very common. Questions to ask, nicely of course, as the usual response will be “I didn’t do anything, I didn’t touch anything, its not my fault”. Has another person unfamiliar with operating the GPS altered configuration parameters such as time settings or altitude etc? It is really very common. It is good practise to have the basic operating instructions on a plasticised card, as manuals don’t last very long on small boats. More useful information on fishing and boats.