By Adrian Thomas
Free flying (xc) GPS uses: 1. finding out how far you have gone when you land (providing you remembered to mark take-off). Not very useful.
2. Finding out how fast you are going on glides. Perhaps surprisingly, this is very important. It turns out to be very difficult to accurately fly straight downwind when trying to maximise your glide - except by using the GPS to tell which direction gives you maximum groundspeed. It can also be very useful when attempting to cross large valleys, and when going round corners or through junctions in valley systems to figure out which way the valley-winds are blowing at your height.
3. With the GPS snail-trail on the map page, with the scale set to fine resolution, you draw circles as you thermal. The drawing can be used to find a thermal again (compensating for wind drift) if you lose it, or if you have to leave it to get a turnpoint.
Competitions: your track-log (possibly also marked waypoints) is used to determine whether you have gone round the course. The obvious things your gps needs to do therefore are:
1. record sufficient trackpoints to cover the time you spend on course (maybe 6 hours).
2. Record trackpoints sufficiently often that you can cut corners at the waypoints to save distance on track and still have a point in sector.
3. Record trackpoints even if you forget to turn it on.
4. Not wrap round and eat up the trackpoints from the start of your flight (which could lose all your points)
These are not entirely mutually compatible unless you have a gps logger, which is expensive.
On, for example, a garmin 12, recording a trackpoint every 20 seconds is a reasonable compromise between points 1 and 2 (gives you just over 6 hours, in 20 seconds you might fly 200m so getting a point in sector shouldn't be too hard).
Leaving the track log on wrap solves 3 but not 4. Remembering to clear the track log before launching solves 4 but not 3.
I use two gps units one on wrap, one on fill, and try to remember to clear them both. (you'd use two cameras if it was film, so why not have a backup gps?).
Sectors: now that gps degradation has been turned off we generally have a 400m radius beer-can sector at each turnpoint . You need to see the gps reading less than 0.39km to the turnpoint to be sure you are in the beercan. Once it reads 0.39 press mark, enter and go.
In practice I set the gps on the compass arrow for navigation (fly to where the arrow points), and then switch to the map page once I am within a couple of k from the turnpoint. I then draw the snail trail around the turnpoint, switching to the 2k resolution to make sure I am in. The rings option on the map-page allows you to put a ring round the pilot position that has the same radius as the sector. To minimize the track on course I fly so that the turnpoint is just inside the ring, check that the distance to the turnpoint reads less than 0.39, mark, enter and leave. Then goto the next turnpoint.
It is a good idea to leave the gps units on while you pack up, but make sure you switch them off before putting them away otherwise the one on wrap will eat the start.
GPS specific turnpoints: some software allows you to enter your finish point on a day as the nearest point you have attained to the next turnpoint. This means you can fly towards the turnpoint over unlandable terrain, get as close as possible, mark - enter(which may not be necessary) then fly away and land somewhere safe.
Hope these are useful. I lost all my points the first time I used a gps, by allowing the wrap function to eat the start and first turnpoint. Incomplete camera data and no backup gps...