A couple of sessions lately have recorded 0 for alpha.
Any reason why this is happening?
Your gybe most likely has a big fat arse.
If you're checking in KA72 it won't tell you, if you use realspeed and change the proximity then you can suss it.
Glad you're back out on the water Anita
It's not the ark that matters, you can do a 70m wide gybe, but you have to return upwind to get within 50m at the bigging and end of the U.
But yes I agree, if there's nothing in your tracks that conforms to the formula, no more than 500m and no more than 50m apart at start and finish, you'll get a 0
It sounds like more of a system issue. Unless Windxtcasy is sailing out one direction and then turning the GPS off.
Has the new GPS been set correctly?
My new gps is not recording alphas either, but there's definitely tracks that should qualify. Checking my device I think maybe I have the logging speed threshold set too high at 8 knots. I was probably dropping below that on most gybes today. Hence getting more than 1 sec recording intervals during the gybes and KA72 rejects them for alpha calculations. I've reset it to 4 knots, will see how it goes.
+1 Ian
I was getting the same problem and changed to 1 sec intervals and 4 knot minnimum and its working again.
Thanks Sailquick, that all makes sense, I'm sometimes still in the old computing days when we were always running out of memory. You've moved with the times. I was wondering what the difference between "on" and "on fix" was. Not "glued on" but "on" when there's a satellite "fix"
"On" records data all the time. "On-Fix" only records data when the GPS engine can resolve a locational fix.
"On" is very useful because we still have satellite data recorded. It has a continuous record of what happened.
This is the same GPS I have been using for ages and I have not knowingly changed any of the settings. I don't have realspeed so I can't chack proximities but most of my turns on that session were tacks so proximity should not have been an issue there. Tacks used to qualify as "alphas" on KA72. Obelix also got a zero for alphas. I have never had a zero in a session before except for 1 hr. Has something changed on KA72 so it now differentiates tacks from gybes?
As mentioned elsewhere (eg www.seabreeze.com.au/forums/Windsurfing/Gps/ka72com-updates-announcement/ ) there has been some work done recently on KA72.com around a number of things, one of which was addressing spurious Alphas that seem to have been a bugbear of a number of WA users.
After those changes, some people are recording 0kt Alphas on some sessions (and a very small number of people have been recording them on EVERY session.)
So far, every time I've run the same file through GPSResults, it has also given a 0 for Alpha.
Without getting too deep into it, each doppler point (recorded on a GT-31 or GT-11) also records (in addition to your lat and long coordinates) a speed and a direction of travel. If the speed and direction of travel don't make any sense (they say you are going NW at 20kts, but your next point is actually to the NE) then the point is considered "bad" and eliminated from the Alpha calculation, thereby killing the whole gybe.
If you record spurious data like this on every single gybe in your session, then you wind up with a 0kt Alpha.
Things you can do that will lower the chances of this kind of thing happening:
1) Check your settings (the link to the mtBest settings guide has been posted many times.) The worst offenders so far have had their recording interval set to >1 sec, which is not good.
2) Check your settings again. Also, consider setting a 0kt min speed as suggested earlier in the thread. This setting can contribute to a 0kt Alpha (but not in the way suggested above.)
3) Try sailing with a second GPS. Post both files and compare. You might just have a dodgy GPS.
4) Check you are actually doing an Alpha! Most people have difficulty understanding exactly how it works. Make sure you understand what you are supposed to do.
5) Take heart, the faster you do your Alpha, the more accurate the measure becomes (and the more consistent across the different software.) Maybe you just need to get better at it!
Cheers,
Dylan@ka72
I had a detailed look at your track from 10 Jan, and it looks to me that the problem is with you having a minimum speed of (I'm guessing) around 4 knots. With this threshold, there is a significant pause on each gybe as you slow right down.
Your best Alpha on the day was around 12 knots, but because it had an 11 second pause in it right when you were changing direction, both GPSResults and ka72 couldn't reconcile the doppler info with the physical coordinates.
Where ka72.com expected you to be at the end of the 11 seconds turned out to be about 16m away from where you actually were. It might not seem like much, but it is above the threshold for error in ka72, so the gybe was dropped from consideration. All the others had similar issues.
I think you'll find your results much more palatable now.
Cheers,
Dylan.
