A couple of our local club members were planning a flight from Port Elizabeth to Cape Town in a Cessna Cutlass, a retractable 172. These chaps were taking no chances and had even purchased a brand new hand-held GPS to aid them on this daring escapade. They spent a number of days planning for every eventuality and had programmed a comprehensive route into their newly acquired GPS.
The great day dawned and our two fearless explorers took wing into a bright blue sky. About an hour later my mobile phone rang. I could tell by the background noise and the panic in their voices that it was our bold voyagers and that they had run into a spot of trouble. Their newly acquired GPS had unexpectedly gone on the blink and they had also neglected to include all their careful planning into their flight bag. Here we had a ship without a rudder. They wanted to know what to do next. I advised them the best idea would be to land at George, purchase some maps, re-plan their flight, and then continue. Even though it was simply a case of keeping the sea on their left, for two new pilots, new to the route, it would not have been a good idea to try and continue into Cape Town International. Even though they later stated that they had merely forgotten their flight planning behind I believe they had merely put all their trust in their GPS, with disastrous consequences.
How many of us are guilty of similar offences? Most of us, I believe. I once took off in a Seneca for a "quick" flight to Aliwal North and back. For some unexplained reason, I punched FAAD into the GPS instead of FAAN. I actually checked the designation a couple of times and was happy that I had the correct entry, especially since the magnetic track was exactly what I was expecting to see. The difference of course is that FAAD (Adelaide) is about one third of the distance to FAAN. On the climb-out, I remember being somewhat perplexed that the distance to go just didn't make any sense at all. Even though I knew that I had a flight of 1 hour 20 minutes ahead of me the distance to go told me that it was time to begin the descent, even though I was still climbing to altitude.
The more I stared at the wrong entry on the GPS screen, the more I kept seeing what I wanted to see. I quickly snapped out of it and saw the blatant error almost leaping out of the screen at me. There was no danger involved as this was a fair weather flight and I knew the route well but it brought home to me just how dead easy it is to punch in the wrong designator and fly blindly and trustingly to the wrong waypoint.
Just recently I flew some passengers to Harmony in the Free State. I once again punched in the co-ordinates from my brand new Aerad only to find that the GPS then tried to lead us to a point some 17nm west of Harmony. It makes a strong case for keeping sectional maps within easy reach, no matter that you may have the finest, state-of-the-art GPS on board.
Any obstructions in the area of the GPS antenna can cause a very significant reduction in accuracy. Examples of interfering obstructions include: buildings, trees, fences, cables etc. Obstructions may have the following effects, thereby reducing accuracy: reduced number of satellites seen by the receiver, reduced strength of satellite geometry (Dilution of Precision (DOP) values), satellite signal multipath, and corruption of GPS measurements. Multipath is caused by GPS signals being reflected from surfaces near the GPS antenna that can either interfere with or be mistaken for the signal that follows the straight-line path from the satellite. Other sources of error in GPS are signal delay caused by the Ionosphere, signal delay caused by the Troposphere, orbit Errors (GPS satellite position inaccuracy), and receiver noise. Remember too that any significant cloud cover can completely block out the satellite signals thereby rendering your GPS receiver completely useless.
I am aware that there are many pilots who regularly fly their own "home-made" GPS letdowns into unmanned airfields and I hope they all read this. You are begging to fly into the side of a mountain.
Recently, one of our local pilots had a portable GPS stolen out of a cupboard in his locked hangar. As the days went by it became glaringly apparent that it would never be seen again. The thief had obviously sold it for a bargain price and the purchaser may not have seen the error of his ways by buying stolen property. Imagine the owners surprise when someone came walking up his garden path, holding a GPS in front of him, tracking the signal to a waypoint called "home". Now there's one of those stories that leaves a pleasant glow!
