First Slot - Autumn 2003

Accurate? - Very. Reliable? - Not Entirely

The Real Truth about GPS

For many years flying club bars have been awash with pundits who extol the virtues of GPS and decry the timidity of the aviation authorities who are said to ignore its very existence, allegedly in the hope that the whole business will just go away. These pundits tend to scoff at the warnings that are routinely issued about the dangers of relying on GPS for navigation. The same savants mock the assertions often made that the substantial increase in busts of controlled airspace by light aircraft stems largely from a general tendency to abandon the more tedious forms of navigation and rely entirely on the new wonder gadget.

    In my last conversation with the late Ann Welch (at a GASCo meeting, as it happens) she observed that probably the greatest cause of GPS errors by far was the mistaken entry of co-ordinates by the pilot. If the pilot then depends solely on GPS for navigation he will have no way of recognising his error. Ann was a forthright lady who knew her own mind and sometimes I found myself in friendly disagreement with her, but on this occasion there was no disagreement as I was sure that she was right - she usually was.

    However, the reverse side of the argument that most GPS error are the result of pilot error when inputting is that if these errors can be eliminated - and with rigorous self discipline they largely can - then you can depend on your GPS set without further question. You can feel comfortable that you have done your homework properly and are thus entitled to rely unquestioningly upon all that your set has to say. Wrong!

    Thanks to the recently published GA Safety Sense leaflet No.25, Use of GPS I now know better. It seems that there are no less than nine different types of failure that the system is prey to. When the system works it works magnificently. Its accuracy seems astonishing and the ease with which it solves those difficult time / distance / speed calculations or assists accurate maintenance of track, positively invites total reliance on its enticing wizardry. There seems no longer to be any need for bending the overtaxed mind on old fashioned navigational problems and you can just get on and enjoy the flight. What can happen next is a sudden announcement of insufficient satellite signals and interruption of the service. If you were previously relying wholly on GPS you will now be lost. Worse still, there may be no announcement at all but the system can still let you down with false but apparently dependable information. Even if your GPS has RAIM, for which you will have paid thousands of pounds extra, the set will not be proof against all types of misinformation.

    While the authorities may have been slow to adopt a proactive approach to GPS the new leaflet now addresses the issues squarely. It is comprehensive, authorative and convincing. It ought to be handed out as a matter of duty to all their customers by suppliers of GPS sets and needs rapidly to become part of the PPL and NPPL syllabi. At present it is available only on the CAA website but if you use GPS you need to make every effort to read it. There is a lot of good stuff there, by far the most important of which is the warning that accuracy, which GPS usually has in spades, is by no means the same thing as reliability. GPS does very well at accuracy but a good deal less well at reliability and you should never confuse the one with the other.

    This recommendation applies most particularly if you are one of those who have taken to using GPS to scud run or to fly 'unofficial' IFR or marginal VFR approaches. Don't ever allow GPS's beguiling boon of accuracy tempt you into thinking that you can always rely on it. Read the leaflet and learn about the serious risks that your are running.

Nigel Everett