The limitations of GPS

All navigators must have a good understanding of the limitations of GPS to use it safely, and whilst it is an enormous help there are also potential pit falls that can lead to disaster for anyone who ignores the basic precautions.

Set up functions

There are some adjustments that can be made to GPS receivers that can cause problems.

  • Variation and Deviation can both be set automatically. Normally GPS bearing are given in degrees true, but sets can be adjusted to allow for variation and deviation, if the navigator does not know or has not noticed this can produce large errors.
  • The offset for different datums can be pre set in the receiver. If it is set to the wrong datum (crossing from the UK to France), there will be an error.
  • The offset may have been entered manually and the navigator may not know, or it may have been entered incorrectly.

All of the above can happen easily on a charter boat, the previous charterer may have set things differently to the way you normally do, or their children may have been playing with the adjustments and left them in a dangerous settings. I once picked up a boat and when we set course from the Needles Channel by the Isle of Wight to Alderney the direction to the waypoint was obviously in error. I eventually found that the variation was pre-set to 15 degrees east!

Datum errors

The wrong datum may be set up.

One of the first times I used a chart plotter was leaving the Beaulieu River in thick fog. It was early in the season and although we could pick up the line of empty mooring buoys down the middle of the river on the radar, the dot indicating the vessel's position was in the field to one side of the river. If we had been relying on the chart plotter there is a good chance that we could have hit the opposite bank in an effort to move the vessel's position to the correct place on the electronic chart.

In this case the chart was on OS36 datum and the GPS was giving a WGS84 datum position.

Common sense errors

Bearings to waypoints do not take in to account any hazards in the way. Always check the chart thoroughly for any hazards on or near the projected route.

Waypoints on buoys

Placing a waypoint at a buoy is potentially hazardous, GPS positions can be so accurate that you may hit the buoy. This is especially true when the position of the buoy is taken from the almanac. Always go to a waypoint to one side of any navigation mark.

This is especially dangerous when the boat is left on autopilot and heading towards a waypoint where there is a navigation mark. I was once motoring in no wind towards the Fairway Buoy at the Needles channel when I noticed there were four other vessels all approaching the same mark from different directions, all of these vessels were under autopilot control!

The use of stock waypoints

You can buy lists of waypoints or find them in almanacs. The danger of this is that in a busy area there is a good chance that someone else may be using the same waypoint, especially in poor visibility or at night this could lead to a collision.

Input errors

The biggest cause of errors in the use of GPS is input errors. It is very easy to transpose two numbers entering 50° 18.5'N instead of 50° 15.8'N is very easy, the other common mistake is getting the decimal place wrong, 50° 15.08'N instead of 50° 15.8'N.

In confined waters both of these errors could cause problems.

The simple check is that every time you turn on to the course indicated by the GPS you must check the bearing and distance indicated by the receiver against that measured from the chart. If a route has been plotted, each leg of the course should be checked in this way before you set off.

Aerial position

It is possible for the aerial to be set too high up on the vessel. If the radio signal is able to bounce back up off the deck before reaching the aerial a faulty position will be indicated. The aerial should be set low down but where it will not have its view of the sky obscured by fittings on the yacht.

In the UK, rescue helicopters have the doors on the starboard side, it is recommended that you fit any aerial on the starboard side of your vessel to avoid blocking the access to a winch man to the rear deck in the case of a helicopter pick up.

Some hand held units will not work below decks or if the sky is obscured by parts of the vessel.

Bad satellite geometry

With visual fixing techniques it is important to chose objects that are at suitable positions in relation to each other. The same is true with the satellites used to obtain GPS positions, there is nothing you can do if the geometry between the satellites is not good except wait for them to move. Poor geometry is indicated by Horizontal Dilution of Precision (HDOP).

HDOP is indicated on receivers numerically, a figure of 1.4 is good and the higher the number the less accurate the reliability of the fixes obtained. Normally a high figure is indicated when the set is still starting up and has not locked on to sufficient satellites.

GPS jamming

Whilst extremely rare, this is a possibility. Occasionally announcements are made by the military that GPS positions may be unreliable due to jamming exercises. If it is an exercise and you speak the language of the announcement this should not be a major problem and I guess if the signal is being jammed and there is no announcement you do not want to be there anyway!

To receive email announcements of GPS jamming exercises from Ofcom join their mail list.

Aerial damage

If the aerial or the wire connecting it to the receiver is damaged you will not get a signal. If you rely on GPS a lot you should consider carrying at least one spare receiver or a replacement aerial, if it is battery powered make sure you have plenty of spare batteries as they can use a considerable number.

All of the above may cause problems if you are not aware of them, especially when sailing on a boat that you are not familiar with. Remember that no navigation system should be relied upon entirely and there must always be a system of checks in place from a completely independent source.

One potential danger of electronic systems is that they tend to lead people to do things they would not attempt without that system. An example is when a radar set is fitted, this may tempt people to make passages in restricted visibility that they would not dream of making without radar, if the radar fails during the voyage the crew may not be prepared to deal with the situation they find themselves in.

A similar situation can arise with GPS, there are many more people being drawn into long ocean passages today then there were in the past, some of these people are relying on GPS for their navigation and may not even be carrying a sextant. This type of thinking can be very dangerous when unexpected incidents occur.

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