Sweet session Denis, straight to the top of the rankings for the 2017 USA Speed Challenge. https://www.gps-speedsurfing.com/default.aspx?mnu=event&val=478 You're the one to chase now! Fastest windsurfer in the USA for 2017....yeehaa!

Denis, that is great, especialy for 98L., every one is jelouse of you now. молодец!

Strange data there, check this out......

https://www.seabreeze.com.au/forums/Windsurfing/Gps/Locosys-GW-60

There has been quite a discussion going on here re the GW60 accuracy.

They would probably be interested in your data.

Seems that the unit mounted on the wrist at 5 Hz has lots of movement (and spike potential) particularly during a gybe.

The Aussie blokes have found that the most accurate data comes with the unit mounted on the head, inside the helmet lining.

Ok, @500 mystery resolved after reviewing my files again today. I did 23.6 knots alpha on my first jibe. GW-60 did not record that because I had history data showing on the display and decided to clear the data after first run. All @500 results are showing within two tenths of a knot. That is great.

Short distance speeds are definitely different. I don't think it is wearing it on the wrist. At high speeds and with short gusts, one could expect different results from 1hz vs 5hz recording. I was just thinking it wouldn't be huge difference, but it is. Sailing in steady wind should produce more comparable results. Will see....

For what it is worth, below are some low speed comparison numbers between my GW-60 and GW-52 taken while sailing on Bonaire. The GW-60 on the right wrist, GW-52 on upper right arm.

Great speeds guys!

Hi Boro, I shall point out your session to Decrepit and Sailquik, who are our main GPS testers over here. I am sure they will be very interested in that discrepancy.

Maybe it is time I bought a GW60...

Nice speed, guys! Although it seems you'll need a bit more wind to bump Dean from the 1st place in the 2017 5x10 ranking and the #1 overall spot.

Boro, I'd like to look at your tracks, too. The difference between the GW-60 and GT-31 is much higher than anything I've seen. 

Thanks Anita, that is indeed interesting. The data I've seen between the gt31 and Gw60 is much closer than that, usually well within 0.5kts.

If your speed is changing very fast, then the 5hz is definitly an advantage as it can "centre" the peaks.

If you are using GPSResults, you can check the SDoP (accuracy) data on both the GT31 and the GW60.

I would like to see the files, if you could send them to "decrepit at westnet dot com dot au" (put together in the right way of course), I'll get back here with my opinion on what's going on. I'll also share them with sailquick, he's the real expert. Roo may also be interested in them.

Yes Barton that's the sort of result I get between the GW52 on my head and the GW60 on my wrist, except the GW52 is usually slightly higher. And the GT31 is the same order of difference lower again.

I sent files to Roo, Peter and Decrepit. We'll see what they find out...

I checked the data. The GW-60 data have unusually high SDOP values, from 1.2 to more than 2. I usually get values around 0.3-0.7. In side-by-side comparisons of 2 watches, I have seen that the data are quite unreliable if the SDOP is that high (in other words, this is true error).

Therefore, the data from the GT-31s should be used for this session, not the GW-60 data. 

Boro said he had the watch on the front hand, using an under grip, on his runs.  That probably killed the reception - even more so than usual because he's a tall, muscular guy with big wrists. But it's also possible that the watch is faulty.

I did a little walking test with my watch either facing up or facing down. I see about 5x differences in SDOP values, with values between 1 and 2 similar to Boro's data when facing down. So his watch seems to be fine, but the effect is larger than imagined. 

The effect is rather dramatic. With the watch facing up, you can make out each step I took. Facing down, not so.

It was as I expected, poor reception from the smaller antenna of the GW60. As soon as the number of sats drops to 8 and below the error spread increases between the GT31 and SW60. The SDOP goes up dramatically also. By setting the filters to min sats 8 and max HDOP of 1 I got very similiar results in GPSResults for the 2 second runs and brought the max readings closer together. I need to check the signal strength for the sats to confirm that it's due to poor reception due to the smaller antenna.

Conclusion, in areas with poor satellite visibility make sure the antenna is always pointing to the sky on the GW60 and if possible wear it as high on your body as possible.

In Boro's data, the top speed runs were in the first 30 minutes, when the GW-60 tracked only 7 satellites. When it picked up #8 and 9 after 35-40 minutes, the SDOP values dropped about 2-fold. This was the first time the watch was used at this location, after a previous use ~50 miles away, which could explain the slow addition of satellites #8 & 9. 

In addition, the SDOP values are ~ 2x higher when the watch was facing down, which was on the speed run tack. 

Together, that gave ~ 4x higher errors and SDOP values. With a "maximum search" algorithm, this will typically lead to speed overestimates, as seen here. 

I plan to write this up in detail when I get to it, and will post a link.

To quote: "In Boro's data, the top speed runs were in the first 30 minutes, when the GW-60 tracked only 7 satellites. When it picked up #8 and 9 after 35-40 minutes, the SDOP values dropped about 2-fold. This was the first time the watch was used at this location, after a previous use ~50 miles away, which could explain the slow addition of satellites #8 & 9. "

One big problem with the above theory, the GT31 tracked the same 7 satellites during the same time period. It then picked up #8 79 seconds later than the GW60. As I said the poor reception from the smaller antenna didn't allow for a high confidence in the dead reckoning result as the signal wasn't strong enough from the fewer sats used. The GW60 and GT31 used the same sats 02,06,12,17,19,24 and 25 for their calculations in those first 30 minutes. For the four fastest runs both the GPS units used the same 7 sats.

Thank's Borisfor send ing me your files.

Here's what I've found,

If you're only doing speed runs in one direction, wear the watch on the trailing hand so that the antenna has a better sky view.

(The antenna is under the "GPS" sign on the band).

Here's the GPSResults view of the SDoP differences.

This is the watch, you can see the +/- values are high, and the envelope on the speed graph is very wide.

And here's the GT31, the SDoP is much better, the +/- numbers are much lower and the accuracy envelope is much narrower, although the number of satellites is the same, they must be getting a better signal

So I'm totally in agreement with Roo and Peter.

Some of our guys are using a strap extender so they can wear the watch on their upper arm

There's a screen flip option so you can wear it with the watch pointing towards you and the antenna pointing at the sky.

I haven't tried this yet as I take my numbers from a GW52 on my head, but I suspect it will dramatically improve accuracy.

Thank you guys for looking into his. I just edited the results using GT-31 data.

Next time, I'll test it with antenna pointing up and see how it goes....

I have been able to reproduce a 2 knot increase in 2-second speeds simply by having the watch face down. This does not appear to be a watch-specific problem, since the GW-52 and GT-31 show a very similar increase in SDOP values. Having an armband slip to the inside of the arm could theoretically produce the same problem - but that's uncomfortable and not the same as using an undergrip. There's plenty of detail at http://boardsurfr.blogspot.com/2017/05/the-fastest-way-get-faster.html

Today, I had it on my left arm, antenna pointing up on speed runs. Nearly indentical results to GT-31. Within hundreds of a knot. 

I think that's the easiest way out, if only going down wind in one direction.

Trouble is we have spots that work both ways, then I think the extended arm ban is the answer, brings the watch closer to your eyes, and it's always facing you, much easier to read while flying. Must invest in one.

Where do you get one?

Amazon has a bunch of them, for example https://www.amazon.com/Baaletc-Extender-Fitness-Tracker-Wristbands/dp/B01N9GQEEJ/

I'll order one, I like the idea of having the watch on the upper arm. Maybe I can read it better there.