Track point GT-11's
Doppler for GT-31's

Send it off to Ben if your not sure, for verification.
Long as your OK with it, post it and change it later if it proves to be crook

I feel your pain! When I changed from a GT-11 to GT-31 the first session I had a very speedy trackpoint alpha which I ended up ditching for the significantly slower doppler one when I was educated by my teammates. Various academics of the GPS world have convinced me that like mineral says if you've got a GT-31 then you should be using doppler. Because of the variety of devices and interpretation software the playing field isn't totally flat but I've accepted that and by and large the best will still come to the front! There's always the conditions to think about and >23kn alphas in >25knots in chop are really good going.

The GT-11 and GT-31 are great little gizmos for calculating accurate speeds when they are in a steady state, they are heading in a constant direction or their acceleration is not too extreme. The doppler speeds they calculate are derived from a velocity measurement. Velocity is a vector measurement which has speed and direction as it's components. To get a doppler speed reading the GTs convert it to a scalar measurement which only has speed as it component.

Now if you are changing direction fairly rapidly, like you do in a jibe, the doppler calculations can lose their precision as the velocity is changing rapidly over a period of a second and the poor little gps doesn't have enough data to make a precise conversion to a scalar speed. It makes a dead reckoning calculation averaged over a second; if the gps was recording at 10 times a second it would have more data to make it's calculations.

With trackpoints our fearless little gps might lose some accuracy due to atmospheric interference but it doesn't have to deal with any velocity measurements with big changes in direction and therefore doesn't get as confused.

The trackpoint portion of the gps has got so much time on his hands he can kick back and have a brew while his doppler counterpart is working his butt off trying to work out which direction he's going and how fast. At the end of the day trackpoints, while not always highly accurate, have a greater likely hood of being right when you are doing your alpha calcualtions.

I agree with Roo. As great as our little devices are they in normal speed mode, they are struggling with the sudden change of direction in the Alphas. We need the next advance in GPS with higher hz and perhaps accelerometers to get more accurate results.
I suspect both Doppler and Trackpoint Alpha results are affected by dead reckoning in different ways. I have been posting GT-31 Doppler results this year (2009) but I am surprised that the GPSTC list does not show that. Maybe a glitch in the database?
If you see W shaped bumps in the bottom of the speed graph for your Alphas, they are (even more) highly suspect. The later software for both the GT-11 and GT-31 suffer much less from this.
GPS-SS has discussed the possibility of a Record Catagory for Alphas but we agree that we need better, more accurate and consistent devices before it would be meaningful enough for 'Records'.

www.gpsteamchallenge.com.au/sailor_session/show?date=2009-12-16&team=10

nebbian said...

Please, please observe what the technical committee decided, which is to use the most accurate method available:

Trackpoint for GT-11
Doppler for GT-31

Nebbian has spoken!!!!

Trackpoint for GT-11
Doppler for GT-31

Rexy there's snow at my house here in the Gorge![}:)] Here's the view from the house today, bit too cold to sail.


Chris I would go with trackpoints for Alpha, regardless of whether you use the GT11 or GT31. I did a test with some 5 hz binary data earlier this year and the result was closer to the GT trackpoint calculation. That's just my personal opinion, Nebbs and Hardie should have the final say on what they use in the GPS Challenge.

Apologies Nebbs, not trying to step on anyones toes. Email me me if you want to chat.

Hi Sailquik,

Yes, that's correct, it was implemented around that time. I'm sure you remember the big problems we had when some people were posting doppler, some were posting trackpoint, and there was no way of telling who was doing what.

Understandably some people decided to post trackpoints to give themselves a higher score, and equally understandably, the people who were posting doppler kicked up a big stink. It ended in tears, someone left the site due to the bad publicity around their name, much sleep was lost, and I really don't want to go down that road again. It's just not worth it.

25 (the captain of the Swan River Mob) urged all his team to post in doppler, which set the standard that it seems the rest of the community is now following. Which is a very good thing!


The only way that we can achieve a fair and fun competition is by everyone posting the most accurate data they have available, however they arrive at those figures. If all you've got is some string and a stopwatch, then use that. If you've got a trimble GPS with ten official timekeepers watching you, then use that data. This methodology 'future-proofs' the site, so that we can compare times now with times that we achieve later on with better equipment.

It is vitally important that when people have a couple of different data options available, that they choose their most accurate measurements. Otherwise the competition descends into a farce, and worse - it stops people being able to measure their own progress towards speed sailing nirvana. You've got to ask yourself, what's more important: getting one more point in a silly table somewhere on the internet, or being completely ecstatic when you know for sure that you just beat your own PB?

If anyone is unsure of any of their scores, I'm more than happy to check their numbers. Team captains also should offer to check any tracks that have any doubt around them.

Let's keep it fun

Hey Bertie,

The clever guy who is behind KA72.com made the site automatically choose doppler for GT31's, and trackpoint for GT11's. Don't ask me how he figured it out

So you're safe. Choose "Trackpoint" for your alphas, and "Doppler" for all other scores when you're posting on the challenge site, this makes it clear what you're doing.