GW52

OK, Sean asked for it, so here we go.

For what's gone on before see "the GT31 replacement" thread

Basically Sailquick and I have a couple of test units to try out.

Here's the Pros.
5hz means better accuracy.

When I first looked at the manual, I was seriously considering writing the basic instruction on my board with a texta!
But once I've become familiar with it there's no problem, the buttons are quite logical really, and easy to use inside the pacqua, although they do loose a bit of sensitivity through the plastic. Much better than using the GT31 toggle inside a pouch.
So it's possible to go to the 5 best of page and check your 5x10's on the water, no more wondering if you need one more 10s run to get the average you're after.

It's obviously smaller than the GT and almost half the weight, which means it doesn't slide around on your arm as much. Just feels more comfortable there.
It's simpler to use and setup than the GT, apart from the timing function it's all windsurfing orientated, there is a small display showing your bearings but that's all the navigation there is. So there's no great list of settings to scroll through, there's just 8 settings on one page.


Here's the Cons so far.
I'm not sure if we're just trialling an intermediate firmware, but there's a few things missing at the moment, most important in my mind is a distance read out, then a large font scroll of max speed for the session. There is a best of 5 readout but that's in small font, and if you need glasses, you won't be able to read it. To access the file you need to use a windows machine with their "utility" installed. The one they pointed us to had a "smart logger" option but if you select that, the unit doesn't log anything.

Smart logger sounds like a good idea, it logs at 1hz until the speed genie is triggered then it switches to 5hz for better accuracy, this saves memory, which at 5hz continuous last for just under 8hrs.

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OK, back to the on going saga.


Had another glitch today, processed my files on the windows laptop, so used realspeed. As seems to be the case the GW gave slightly better figures than the GT except for alphas.
I had two 21s a 20 and two 19s for my top five with the GT31, but only a 15, 13 and 12 for the GW, just those three no more.
So now I'm on the PC and using GPSarPRO, I get very similar results for both units in all divisions.

GT31 2s 31.10, 5x10 30.19, NM 22.46, hr 13.68, alpha 21.32
GW52 2s 31.16, 5x10 30.23, NM 22.49, hr 13.69 alpha 21.36.

The Gt track is much cleaner than the GW



Red is the GW52 yellow the GT31. Interesting there's a small time divergence in the two tracks going into the gybe, but there's a big spatial divergence of 12.59m. Even stranger is what happens coming out of the gybe.



The gap between the two tracks has decreased to 4.63m.
Today I had the GW on my upper right arm and the GT on my right shoulder, maybe the GT had a better satellite view so tomorrow I'll reverse them.

Can someone send me a download link for the software for transferring data off the GW-52? I have a test unit, but no software.

Dylan.

Yoyo, here's what GPSvisulizer makes of them, it's a bit easier to see the pairs of tracks when they are speed coded by colour.

Hers's some more test tracks part bike part car, it's looking more and more like the faster you go the more accurate the posistion is.

And where it's red and stationary, I'm apparently climbing around on the rooves of several neighbouring houses.

Good testing Mike.

Perhaps you are looking at Doppler Positioning rather than Trackpoint positioning? Can you switch off Doppler and map it?

RealSpeed. Try checking which is being used to draw the tracks. Right click the window and see if there is a way to select the other. (There may not be, I am just brainstorming)

Sorry Andrew, tried everything but I can't see any reference to how alpha positioning is drawn.

Overnight battery test gave me 10hrs 24min of data at 1hz continuous logging, the only interesting/worrying thing, it did 4km while sitting still in the window!

Today I took the GT31, GW52 and Fangy's Canmore for a drive. All in the same pouch on my roof rack so they all had the same good sky view.

This has improved the look of the GW tracks, they're much smoother, so does this mean it's less sensetive to satelite signals than the GT31?





This turn doesn't sink below the speed genie setting of 10 knots so there's a nice smooth 5hz track.
The next pic shows what happens when you slow down below the speed genie setting


The blue highlighted line in the tracks table is where it's changed to 1hz, speed is down to 6knots, and it changes back to 5hz at 9.76kts.
You can see the kink in the speed graph at the change point.
And the sudden change to a faceted track.
So if you're using "smart logger" and doing alphas, be sure that speed genie isn't set too high.

And here's another worrying pic
Canmore Blue
GT31 Green
GW52 Red


So how important is this positional inaccuracy?
Any idea anybody?
Guess if you're looking for hidden treasure you'd go for the Canmore of GT31

Decrepit asks a very good question: How important is the actual positional accuracy in our speed measurement?

Answer: (Bearing in mind that I am not a GPS technical expert but this is just my understanding from my reading and from conversations with those who know a lot more about it than me).

Not very important, and very important. In fact, it is almost irrelevant in one sense, but positional accuracy in another sense is critical. The Reason this sounds contradictory is simple. It is the relative positional accuracy that counts, not the actual geo location.

If the positional trackpoints are skewed north, south, east or west in a consistent manner, then it is not at all important. What is important is that when you return on your Alpha, the relative positional accuracy has not changed and we can be sure that we are within the 50m circle, or outside it as the case may be.

Critical observers of GPS tracks on maps (like Google Earth) will have noticed long ago that the tracks don't always match the roads. More often than not, they are offset slightly in one direction fairly consistently. If the offset is North for example, you find the track north of where the road goes if you are travelling east-west. You don't really notice any offset if you are travelling north-south.

The most obvious cause of this is the limited accuracy of the GPS system due mainly to atmospheric conditions affecting the GPS signals. Luckily for us, this affect tends to be consistent over the short term and may be likened to a relatively slow 'drift' in position. So in the time frame we do an Alpha, the 'drift' usually does not change enough to be a serious issue for us. (perhaps a few centimetres, but mostly less than a meter).

Another cause of this 'drift' may be the accuracy of the actual satellite position data. Satellites do 'drift' from their predicted orbits over time. This is constantly monitored by the system and updated in the system, but there may be a lag in these corrections that can affect the positional calculations.

Probably the biggest cause of inaccuracy is low numbers of satellites tracked and used in the calculation. The more satellites you track and use, the better the accuracy will be. (The position of the satellites counts also, but that too becomes less critical when you have more tracked). That is only up to the point where you have lots (maybe 8 or more?) of satellites tracked and then the previously mentioned factors become more of a factor. But this is very important for us and explains why we go to such lengths to urge people to mount their GPS in a position and orientation that give it the best view of the most sky, like on the top of your helmet. It also explains why we have had some rubbish accuracy results from GPS's worn in bad orientations like in the back pocket of a harness, or in arm bags that have slipped around to face down and so get reflected signals. It is important to mention here that water is a very good blocker of radio signals, so anything covering the GPS antenna that holds a significant amount of water (like damp Neoprene wetsuit lining material) will significantly degrade the radio signal, and therefore accuracy.

Tom Chalko emphasised to me, that from his testing of the accuracy of the GW-52, giving the antenna a clear view of the sky was critical to maximise it's accuracy potential. ie. Mount it on your helmet!, or at the very least make sure it is in a stable position with the antenna facing the sky and no radio signal blockers over it! (of course, this also applies to all types of GPS).

Back to accuracy. The reason we went to using the Doppler based speed calculations is because they are inherently far more accurate than speed calculations derived from positional data.
The Alpha category is the only one that must use some positional data, and that is only for the calculation of the proximity circle. The speeds are done with Doppler.

It is theoretically possible to use the Doppler data to calculate position and Manfred Fuchs tried this in his GPS-Results program for the Alpha a few years ago. After testing, we had to revert to using the positional data for the reason that any error in the Doppler positional data (calculated from direction of travel) is retained and accumulated. To compound matters, we found the most likely place Doppler directional errors occurred was during the gybe! One small error here can have the effect of the plotted track diverging enough to miss the proximity circle by a significant amount. So we had to revert to using the positional data for the proximity circle, which may not always be as exact as we would like, but at least it does not accumulate errors.

Another possible cause of the difference between the plotted tracks from different GPS's is that they may be using a different datum set on the shape of the earths surface to calculate their results. To keep it simple, (the only way I understand it), the earth is not a perfect sphere. In fact it is far from it. there are various models (Map Datum) that are used to correct/calculate the positional data to compensate for the irregular shape of the Earth. The recommended one for us is called WGS84. If you look in your SETTINGS menu on your GT-31, that is what you should find it set to.

I don't know what Map Datum the Canmore GPS or the GW-52 uses. They may be different, and that could also explain the offset we are seeing between positional tracks. In any case, it should not matter much as the relative positional accuracy is what we need.

There you go. More than you ever thought you wanted to know. If there are any real GPS technical experts out there who know I have got something wrong here, please feel free to correct me.

I like yoyo's house !!!

Good one Andrew, I've been meaning to do that for the last few days, haven't got around to signing up to a video site.
You see what I mean about the scrolling time.
I've emailed Kate to see if they can increase that to 3s to 5s per view.