GPS Information

Gliding. The ultimate high!

GPS - the ultimate navigation aid?

The purpose of this web site is to gather together useful information for glider pilots. This section of my web site is concerned with GPS (global positioning system), but somehow I suspect you've already guessed that. GPS is also known as GNSS - global positioning satellite system.

Garmin 12 XL, a popular GPS receiver with a basic moving map, at an affordable price. Like all models it displays distance to a turning point, ground speed, track bearing and ETA among other things. It can hold a list of common turning points and their latitude and longitude details, so that the user doesn't need to enter this information by hand when using it. It can also have a list of turning points updated by connecting ot to you PC. This model has been superceeded by several others which look much the same, and the by the sexier looking GPS II and GPS III range. While these newer models have more features, and more detailed maps, models like the 12XL will still navigate you just as accurately to a turning point.

When GPS first became available to the general public, many pilots viewed using them as cheating. The skills of navigating that they had acquired over many years, the knowledge of the appearance of the countryside from the air that enabled them to know where they were at a glance (some experienced pilots never bothering to look at the map most of the time), all these advantages were being wiped out by a richer pilot simply investing £1000 or so in a black-box - well a grey one to be more accurate - and looking at a small screen. I thought the same for a while.

Until that is I was flying with a group of friends one day in 1991 or '92 that turned out to be as poor as we had initially thought, before foolishly taking to the skies. We found ourselves spread out over an area of green fields, trying to guess just how close we were to the various landmarks a few miles away from us. If we were within three miles of a particular town, we could just and no more make it home, maybe. If we were three and a half miles, we had a problem. More than that and we were doomed. A voice came over the radio, calling the only glider in the spread out group of about eight of us who had a GPS fitted (a military glider). "Barry, how far are we from home?" "16.7 miles" was the instant reply. You could almost hear the inaudible groans from the rest of the group. Barry's Discus, with it's familiar wing shape, was visible just that little bit closer to home than the rest of the gaggle. And even his 16.7 miles was out of range for most of us, struggling at about 2500'.

I was converted immediately. I wanted a GPS Forget navigation skills being made redundant, this was what we needed to know to prevent final glides becoming just a bit too final.

The EW flight recorder. It stores the height, latitude. longitude, and time, at regular intervals. Some models can also be linked to an engine noise detector for use in motor gliders.

And simultaneously along came the original Skyforce logger. And then the very popular EW barograph, shown above, gained logging facilities. Things have never looked back since then, even though sometimes in 1997 to 1998, reading postings in r.a.s. from American pilots showed that they were busy reinventing the wheel, trying to use handheld computers and laptops (!!) to act as loggers and wondering if anyone had software to display the traces. Even though four or five years earlier the UK had accepted logger traces as competition evidence, with flights being analysed by some excellent British software, which is still the standard that others have to try to match. The silliest posts I ever read in r.a.s were from some US pilots about 1998/99, complaining that since GPS was now accepted in US comps, they would give up gliding rather than buy a flight recorder. Silly because these guys were all sole owners of high performance Standard class, 15 m and 18 m gliders, and claimed they couldn't afford the vast expense of a flight recorder. But the requirement was only that the winner of their Nationals needed flight recorder evidence if s/he wanted to enter their World Team! Too silly for words. GPS is the way to go.

For badge claims and competition use, there is an official group of FAI approved flight recorders, although some countries and some competitions, at the director's discretion, can accept other flight recorders as well. It's best not to assume things but to check with your national gliding organisation or the competion organisation. For example one of my Garmins has the firmware version one point before the FAI approved version. It's okay for competitions in the UK, but not for badge claims.

The original GPS satellites were launched from 1978 onwards and finally achieved their target of at least 24 satellites (21 active and 3 spares) in 1994. There is a budget available to continue launching more as the original ones wear out. For some time the US government imposed a built in error in the signals used to make civilian receivers less accurate than the military ones. This was known as Selective Availability (SA for short). During the Gulf War the military couldn't get their hands on enough military spec receivers in time and many servicemen were using civilian ones (a friend received a rush order for a dozen receivers from the Navy at one point!). To keep these receivers' output accurate SA was switched off - and for a time they forgot to switch it back on again. But as use of GPS increased, it was eventually decided to switch off SA permanently, from 12:04AM EDT May 2, 2000, except in times of war possibly.

To see the result of switching off SA, visit these links with a before and after view of data collect on accurary of GPS. Test report 1 , Test report 2 For all practical purposes, GPS now places you within a few metres of your exact positon. Some people argue at gliding comps that if the system is out by 5 or 6 m, they shouldn't get a penalty when they miss a start line or sector by this sort of distance. But everyone else got it right and was scored by the same system, and who knows, perhaps they were further away than they claim. To miss by that sort of distance, you have to have started your turn about 50 m earlier - I know, I've done some calibration runs at a ground target, turning on the instant it says I'm 0.00 km away and examining the recorded data on the ground. So they were out by a bit more than they'd care admit.

If you want to learn more about GPS, try these other pages on this site.

GPS - How it works is a very simple explanation for those who don't like mathematical or scientific jargon. I've added a some links to the manufacturer's more detailed explanations as well.

GPS Links Page is for those who'd like to read more about GPS on other sites.