GPS Procedure at Competitions

By Adrian Thomas

The procedure I use is:

1. Download the official waypoints from the competition organisers.

2. Check the set-up of the gps: correct time zone, correct time offset, correct map datum and coordinate format. For the BPC you will need to check the time that is used (GMT or BST or some other offset), the map datum most commonly used is WGS84, but Ordnance Survey is used in the UK for the XC league, and has been used for the nationals in the past - check current rules (e.g. at www.pgcomps.org - where there is also a gps usage guide).

3. MLR setup. the MLR is definitely the best machine to use. When you have downloaded the waypoints make sure that they have altitudes on them so (format e.g. way001 as per manual) so that you have glide angle to the next waypoint constantly displayed. I use this for final glide setting off when it reads a suitable number (on the Boomerang you are safe using 9 in still air, 6 in a headwind or if you expect to glide at maximum speed, 12 in a tailwind or if you expect sink).

Normally turnpoints will have a declared radius (400m in the nationals), check this on the comp rules. You can set the MLR to switch to the next turnpoint when it gets within a pre-selected radius of a waypoint. So make sure you get a track-point inside the cylinder you need to set the MLR so that it switches to the next turnpoint only once you have been in sector long enough to be certain of having recorded a track point. If you fly at trim speed you do 10m/s (roughly) so if you fly straight in to 350m of the turnpoint and back out it takes 5 seconds in, say 5 seconds to turn and then 5 seconds to fly out again. Combine this with your allowed position error and you will get the turnpoint. I use 0.36km, fly faster than 10m/s (usually) and have not yet missed a turnpoint.

This brings up the subject of track-log intervals. Typically people use 20 seconds on an MLR. with 1024 track points 20 second interval gives 5.68 hours of recording time (I have only twice flown for longer than that in tasks, you can switch off the gps between turnpoints to save memory if you know you are going to be up for a long time but need to have at least a minute before and after each turnpoint. Switching off is not a good practice in general though...). If you expect to fly for longer than that then you should use a longer interval and also adjust the time you spend in sector at turnpoints accordingly.

Fill or Wrap. Almost all GPS units give you the option of using fill (track log fills the memory and stops) or wrap (track log fills the memory and then starts overwriting the memory from the beginning). People are divided as to which to use - if you use fill you might not get the end of your flight and you might not get much at all if you forget to clear the tracklog before take-off. If you use wrap you always get the end, but might write over the beginning, but you don't need to remember to clear the tracklog. I use one gps on fill and one on wrap and have had both possible kinds of error but have only once lost a flight (on wrap, it turned on in my bag and ate the start).

Race procedure: set up your glider etc. get completely ready then set up your instruments prior to briefing. Turn on the gps and make sure it has a fix on take-off. Clear the track log. Make sure it is recording a new track log. Mark-enter to confirm take-off position. Go to the breifing. Enter the task turnpoints as a route. Check coordinates and height of each waypoint as you enter them. Write down the waypoints aswell (back-up in case of accidental route deletion). Check that the route distance is the same as the distance given at the breifing (the MLR will assume you have left take-off so the distance will probably not be correct).

5 minutes before you launch turn on your gps. Clear the track-log. Make sure the GPS has a position fix and is recording a track. Make sure that the route is functioning. I use the MLR on its arrow page - check that the arrow points to the first turnpoint (or start point if necessary). Take off, get high, get to the start line in time. Always use gps time for starts. Allow 100m maximum distance between yourself (distance to the next turnpoint is displayed on the arrow page) and the start-line at starts (this makes it much easier to stick with the lead gaggle). If you are not at the top of the climb at the start go anyway - it is easier to catch up during climbs than it is to catch up once the leaders have gone (unless your glider has a large performance advantage).

Turnpoints. Having followed the arrow to get to the next turnpoint with the MLR all you do is fly straight towards the turnpoint until the arrow changes to point to the next turnpoint. It only changes once you have got within your predetermined set distance from the turnpoint. I have been trying to find out whether it only changes once you have a tracklog point inside your selected radius but can't confirm whether this is true. If not then it is likely to be made so in the next software upgrade (free at PWCs).

Final glide - follow the arrow until you are at decision distance from goal. It is more important to make goal than to make it in the minimum possible time. In general it is sensible to try not to waste time climbing as you approach goal. If conditions are reasonable leave weak thermals early, and strong ones late (as usual), and pay attention to the glide-angle indication. I have found it to be very accurate - in the worlds I set off on a 38k final glide from the top of the Sierras to the beach with the glide calculator reading 9.2 and landed 1.4k short....