navigation by G.P.S

GPS as an illicit substance and how to handle it without sinking into total dependency.

... if you can confine your habit to one of moderation, you may survive and even prosper ...
 reproduced from GASCO

For my part I use GPS all the time that I am flying outside my local area and regard it as something almost magical in its ability to solve navigational problems that used to take much cudgelling of an overtaxed brain. What is your estimate for DUNNO?. an earnest controller would ask. After the initial wave of panic and the shaming response. Standby, (It is written that every proficient navigator must always have to hand their estimate for the next three waypoints - so what sort of navigator does that make me'?) I would then embark upon a tortuous mental calculation that went something like:

My groundspeed is about 10' KTS and I should think that I am about here (or might it be there?)  If my first position is correct there is about 21 miles to DUNNO but if it is the second it is only /6 miles: I shall make a masterly compromise at 18. So it's 18 miles at 107 KTS and Time equals Distance over Speed (or is it the other way around?). So 18 over 107 equals ...God Knows. lf it was 107 over 18 it would work out at about five or six. But the answer must be in hours as knots are nautical miles per hour. It ' can't be five or six hours. Or can  it? Well anyway - I do know that ' if I was flying at 120 KTS it would be half of 18 in minutes. That's nine minutes from when l was at where I thought that I might be and that was at four minutes to and so my estimate should be a bit after five minutes past. Say six minutes past. Past what? lf it's now two minutes to twelve, French time, what's that in Zulu?

And so on. Such an innocent request from a controller can take all the fun out of flying. These days I simply consult my GPS and instantly respond: ', 10 minutes from now. However, everything brings ', its own price and with GPS it is the risk of total dependency. Yes, we are into illicit substances here.


A Garmin 430. Common equipment for IFR aircraft

That nice avionics salesperson tempts you with such an attractive little proposition. For about £300 you can have a proper aviation hand held set and for half that a non aviation set that will be just as accurate, but will not know where EGLL is unless you first tell it. Give it a try, you'll like it! Says the helpful salesperson, and by golly, you will like it. In no time at all you will be venturing out to distant places and scraping home in low scud confident that your little friend knows just where you are, even if you don't. Before long the craving will be luring you into moving maps, colour, terrain and all the bells and whistles.

The thing is that if you can confine your habit to one of moderation, you may survive and even prosper, but if you let it take you over, so that you become a total dependant then, sooner or later, doom awaits you. Doom in this instance will take the form of your finding yourself overhead EGLL (London, Heathrow) when your magic little friend has definitely assured you that you were nicely on track from Popham to Biggin Hill - well, 0.3 miles right of track, to be precise.

In nine cases out of ten, it is not actually your little friend's fault but yours. You entered some nonsense into the set and it just did what you said. The most common fault is to get one of the 15 digits that make up a lat/long co-ordinate wrong. Or you might have meant to enter EGKB (Biggin Hill) as your destination but absentmindedly entered EGSG (Stapleford). Mistakes of this sort are easily and often made. Much less often, it's your little friend lets you down. It runs out of batteries or satellite signals and dies on you. At least you will then be aware that all is not well but very occasionally it will mislead you in spite of your fingers having pressed all the right buttons and in the right order too.

Thus, you cannot rely totally on GPS because occasionally it will mislead you. Consequently you must always run some other navigation system as a check, which is a bit of a pain when you thought that all your navigation problems had been solved. Nonetheless, if you are to avoid all the unpleasantness that follows busting controlled airspace, you have to keep running that check.


A Bendix/King Skymap Ill. Easier to use and less error prone than earlier or simpler sets

For IFR pilots your check system will probably be VOR/DME and if you have an RNAV set it will mean merely entering your waypoints in your RNAV set as well as your GPS set and making sure that both sets are giving you the same information. Without RNAV you will need to check VOR/DME co-ordinates from time to time.

For VFR pilots, I'm afraid it's back to the old map reading, watch and compass. Here are a few tips:

1 Enter the route in your GPS set the night before. This gives you time to draw track lines on your map, construct your PLOG, check that you have entered your waypoints correctly and familiarise yourself with your GPS set once more. If your set is panel mounted, get a software simulator of your GPS for your PC and set up the route on that the night before.

2 Avoid lat/long co-ordinates and use instead bearing and distance from somewhere already in your database. Thus if, say, the south western end of the twin canals in Cambridgeshire is a waypoint, it is much easier to define and enter this as 001 deg and 22 nm from BKY than it is to deal with N522133E0000215. Furthermore, in the air, when the brain atrophies, 22 miles North of Barkway means something that you can quickly check, while those 15 co-ordinates mean nothing.

3 Choose waypoints and routes that are easily recognisable from the air, even though that means creating User Waypoints.

4 If you can use a 'second generation' GPS set, such as a Bendix/King Skymap III with a large moving map and a joystick for moving a pointer around so as quickly to define waypoints, you will be considerably less error prone.


A handheld GPS set

If you keep your visual navigation going alongside your, GPS nav. you will soon detect when things are not as they should be and in this way you will cleverly avoid the dreaded consequences of over dependence. If you also keep in contact with a controller providing a Radar Information or Flight Information service they can often save you from the error of your ways before things get serious. With GPS, plus a visual nav. check plus an information, service your navigation should be foolproof.

Nigel Everett