Positioning systems – GPS v. three point fix

Simon Jinks investigates who might win in game of position fixing top trumps!

Simon Jinks investigates who might win in game of position fixing top trumps!

Our dependency on GPS and other Global Navigation Satellite Systems is pretty much complete. We depend on it for navigating the car, finding the nearest pub on our phone, tracking trains, planes and automobiles. The worlds’ banks and electricity companies would grind to a halt if GPS and its hyper accurate timing signals were to shut down and we obviously use it for navigating our boats.

Let’s be honest, GPS is great. At the press of a button it solves the problem of the ‘where are we’, puzzle navigators have been trying to solve since we first ventured afloat. But is it the best solution, would it win the Position Fixing version of ‘Top Trumps’?

The GPS doom mongers would be unlikely to agree. Whilst they agree the ‘End of GPS’ may not be nigh, there’s the constant worry of both known and unknown errors, lurking out of sight like the bogeyman, that could jump out and get us if we give over to GPS, over reliance and relax into complacency.

The doom mongers would quite rightly say;

‘… the Earth is undergoing high levels of solar activity over the next couple of years and this is leading to less accurate GPS positions and occasional drop of signal.’


‘GPS jammers can be bought for £50 that can block a GPS signal, this over reliance on GPS and jamming issues are why the UK and some European governments are funding National land based navigation systems which are harder to jam’


‘…what if little Johnny sits on the antenna, making it useless, or uses all the battery power to watch the new Star Trek DVD on the laptop leaving the GPS hungry for power or signal’

Personally, I welcome all advances in navigation. I happily scroll an outline plan of my day’s route on the charts on my mobile phone, before having a squint at the paper chart to confirm the facts.

When I started teaching, radio direction finding and radio lighthouses were still being taught, then Decca navigation reduced in price and took over, and for the last twenty years GPS has been the trendy kid on the street corner. Over that time I’ve found that all position-fixing systems have strong and weak points - they are all fallible and they are all great. There’s no difference between traditional and electronic positioning – it is all just positioning. They all have pro’s and con’s and you use a combination of methods to keep you safe.

GPS is susceptible to solar activity, jammers, horizontal datum errors, and vector chart error. GPS requires power, a good signal and a user who knows how to operate it – but when it is good it is very good. Often electronic charts are their downfall, as you’ll see the boat driving over land on the chart plotter in close pilotage situations.

A three-point fix is great as it is visual and lets’ face it, the way most of the world was charted. But it’s only good if you have visibility, can identify the points on the chart, convert the bearings and take them in the right order.

The seabed is often the closest you are to land so depth gives a great indication of position. Except, that it only gives one rough position line and requires you to know from where the depth is reading. It sort of tells you where you are not.

Mark one eyeball is a great way to ensure your GPS is not telling fibs by comparing the GPS position to the actual position of the dock you are sitting on. The eyeball lets you down a little on a featureless coast or at night where you cannot find reference and are unsure of gauging the distance.

Radar portrays what is there and can see through the murk, but only if you know how to use it and know whether to use bearings or ranges for the best accuracy.

So what do we do?

  • Keep a log of your position by either a regular plot on the chart or an entry in the logbook.
  • The faster you travel or the more essential your exact position, the more it should be logged.
  • Have the ability and equipment to work out a position and shape your next course.
  • Back up your position by another means (log, depth, compass, radar and mark-one eyeball)

GPS would probably win a game of Top Trumps between position systems. Whilst it may not be the final piece of the puzzle it is possibly a large piece of the jigsaw. But treat all pieces of kit on-board with the same bit of healthy mistrust and scepticism. It keeps you safe.

Simon Jinks