GPS units acceptable for Team Challenge

Ok, i am going ask the simple one.
I have a flooded GT31 and need a new device.
Have not sailed for what seems forever (injury, work ....life) and its time to sail again.
What do I buy in Australia today?
(and will it work on KA72)

thanks
Climber

I think you did a great job to bring the GT-11 to our attention Roo. How much 'initial development work' you did on the design has never been shown.

You also saw the potential of the Doppler speed measurement at the same time as Tom Chalko, but it was Tom who did the science to back up his theories and contacted Locosys and convinced them to make the GT-31 in a form which we could use better by implementing the 'SDOP' into a binary file format, and it was Tom who conceived and designed the 'Genie' function that is so useful to us and which other developments have extended.

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sailquik said..
I think you did a great job to bring the GT-11 to our attention Roo. How much 'initial development work' you did on the design has never been shown.

You also saw the potential of the Doppler speed measurement at the same time as Tom Chalko, but it was Tom who did the science to back up his theories and contacted Locosys and convinced them to make the GT-31 in a form which we could use better by implementing the 'SDOP' into a binary file format, and it was Tom who conceived and designed the 'Genie' function that is so useful to us and which other developments have extended.

Seven months of development work before the Navi GPS was usable for speedsailing mate! Further time was spent developing the unit to export in binary format that could be used for post processing to increase accuracy, which included the SDOP data long before the GT31. I did the science to back that up and had the binary data files reparsed to contain the necessary information.

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sailquik said..

climber said..
Ok, i am going ask the simple one.
I have a flooded GT31 and need a new device.
Have not sailed for what seems forever (injury, work ....life) and its time to sail again.
What do I buy in Australia today?
(and will it work on KA72)

thanks
Climber

Simple question. No simple answer. It depends on what you want from it.

If you want to sail just for fun and fellowship, the Canmore Gporter 102 or cheap Android phone with GPS-Logit will probably suit you.

If you aspire to getting into he top rankings or competition with your mates, and contribute 'jelly beans' to your team score, then you need to get a GW-52.

Oh, and get a really good waterproof arm bag for your device. For a phone, that is the H2OAudio bag, and for the others it is a Paqua arm bag. You have to keep them seriously dry or they will die like your GT-31.

The eagle has landed! These 5 lines from Andrew summarise 35 pages of forum postings across 5 or 6 topics since July 2014.

Thanks Captain Kangaroo for the gt success you sound very clever anybody that can do that stuff always amazes me and its nice to know a bit of its background and this thread has been super!

Personally I think this is all beside the point, it's what happens on some phones when the display goes off that worries me, then you can get errors in the order of 100%. ie 40kts instead of 20. but that's not all, instead of recording at steady 1hz, there can be gaps in the data that vary between a few seconds to almost a minute.
We don't know which phones besides the Sony xperia E1 and the Alcatel pixie 3(4) are affected.
I know the Samsung Ace3 isn't affected, and it would be good to add to this list.
Then there is the issue of satellite sensitivity, not as bad as the display off syndrome, but can also cause big errors if the device isn't worn optimally, and with the Sony xperia E1, it can still report using 10 or more sats, when it's obviously struggling to get a good signal, there's no clear indication that anything's wrong apart from it having a big spike that the other gpss don't.

So although under good conditions the phone's are producing very acceptable results, they are much more likely to produce bad results than the GTs GW, or canmore.
Unless it's a phone that isn't affected by the display going off and has high sat sensitivity with a good signal to noise ratio.
As we don't have a definitive list of such phones, it's very hard to say that results from any phone will be acceptable all the time.

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boardsurfr said..

sailquik said..

Roo said..
Here's some comparison measurements from today.

Just got back from a mission to the coast. Ran with GPSLogit on Alcatel One Touch Model 4037T Android 4.4.2 phone on left upper arm and 2 GT31 on right upper arm. I have included both doppler and trackpoint measurements as it gives you a good idea how clean the data stream is.

Max and 2 sec were all at the same time stamp of 8:43:50

Max: Phone 42.590 GT31-1 42.181 +/-0.272 GT31-2 42.259 +/-0.253 Trackpoint: Phone: 42.951 GT31-1 42.533 GT31-2 42.566
2 sec: Phone 42.492 GT31-1 42.143 +/-0.272 GT31-2 42.172 +/-0.253 Trackpoint: Phone: 42.685 GT31-1 42.400 GT31-2 42.358
5 x 10 sec: Phone 38.781 GT31-1 38.558 GT31-2 38.563 Trackpoint: Phone: 38.834 GT31-1 38.830 GT31-2 38.875

10 sec run Phone GT31-1 GT31-2
1 9:16:18 203.4m 39.540 202.5m 39.357 +/- 0.255 202.5m 39.356 +/- 0.253
2 9:43:54 201.7m 39.206 199.4m 38.765 +/- 0.289 199.4m 38.757 +/- 0.258
3 9:56:03 200.9m 39.054 199.5m 38.773 +/- 0.286 199.5m 38.782 +/- 0.284
4 9:21.07 196.8m 38.259 195.7m 38.035 +/- 0.291 195.7m 38.004 +/- 0.277
5 9:32:56 194.7m 37.849 194.8m 37.859 +/- 0.265 194.9m 37.877 +/- 0.254

All in all the phone acquits itself pretty well.

'Pretty well'???

Well, I guess so if you are measuring with a stretchy string!

I have to say that an objective view of this 10 sec. run data is that the phone is over reading constantly by up to half a knot. Especially since the data from the two GT-31's is consistently extremely close and within 0.01 and 0.03 Kts of each other on all the 10 second runs. (this is why they are so good!)

It can be observed that the Maximin 99% error range of the GT-31's is between 0.253 and 0.291 Kts. This says that we can be 99% sure that the speed is at least what the reading says.

There are different ways of looking at Roo's data. Four of the five 10-second numbers of the phone are within the range for the GT-31 speeds; only run #2 is outside of the GT-31 range, by 0.16 knots. To correctly compare the two numbers, however, we would need accuracy estimates for the phone numbers, too (which are not available). But it seems reasonable to assume similar numbers as for the GT-31 (+/- 0.25-0.3 knots); then, the speed ranges overlap for all 5 runs. In other words, the numbers are not statistically different.

The next question is what the +/- numbers actually mean. I'd love to see an actual description. The "99% sure" refers to 3 standard deviations and normal-distributed random errors. Typically, however, +/- numbers give a single standard deviation. Even if the numbers would indicate 3 standard deviations, a non-normal distribution of the errors would reduce the probability to a number below 99%.

The fact that the two GT-31s that were both worn on the same arm give very similar numbers does not prove that the numbers are accurate. Error sources in the GPS data, for example from atmospheric distortions, will be virtually identical for two devices right next to each other. An ideal test would involve one phone and one GT-31 on each arm.

That said, there does indeed appear to be a bias in the reported speeds between the phone and the GT-31s. But we do not know for certain whether the phone overstates the speed, or the GT-31s understate it. Assigning the error to the phone is jumping to conclusions. It is at just as likely that the GT-31 underreports the doppler speeds (similar to the way speedometers in cars are allowed to over-report speed, but not under-report it). I have seen slower reported Doppler speeds in GT-31 data when compared to a 5 Hz ublox GPS that appeared to be more accurate (but this is just anecdotal). The 5x10 positional speed data, which are very similar for all three GPS units and higher than the doppler speeds, also point towards the GT-31s under-reporting speeds. Maybe Roo knows something about bias in the GT-31 firmware when reporting doppler speed.

This is just one example of a data set. But it is a significant difference in that all the 10 second runs are significantly faster. Of course you would have to study a lot more comparison runs from this particular combination to see if this is truly a consistent pattern, but if this was just random range within the GT-31 range, we would expect to see just as many variations below the GT-31 readings in a large sample, and since there are none even close to that in this small sample, it is a big worry. As you said, that is a strong indicator of a bias.

On the other hand, most of the testing here has not shown that significance of bias, but some has. There are so many combinations of phones and GPs chips and and antenna and they way they are implemented that we are struggling to find and list those we can recommend. More Doppler data comparisons like that which Roo submitted would help a lot.

We don't compare GPS worn on different arms when doing testing for accuracy as we cannot guarantee that they are seeing the same number of satellites with the same reception. The body and head can easily block signals and the orientation of the units can be quite different and change the geometry of satellite reception. In fact, most of my and Tom Chalko's testing is done with the GPS worn on the top of the head. A lot of Toms testing of many scores of GPS, GT-11, GT-31 and more recently GW-52 was done with arrays of 10 or more mounted on the roof rack of his car. He actually claims, supported by a whole lot of Math that is beyond my grasp, that the GT-31 is far more accurate that the SDOP error values suggest, (by a factor 10) and that the GW-52 beats it by a significant margin again. (subject to ideal reception). We choose to take a very conservative view and are therefore very confident of it.

The trackpoint data is almost totally irrelevant. We saw a long time ago that it is subject to far greater error variation and almost always reports higher speed due to a number of known factors, including the inherent error of positioning and often lack of resolution in the positional data (grid Effect). To put it simply, the positional points not only vary in the line of travel, (which tend to cancel each other out) but to the side of the direction of travel. This sideways variation leads to a slight Zig Zagging of the plotted track which effectively causes over reporting of the speed. Some data from higher the Hz units is really bad. To use positional data for speed calculation we would need far higher positional accuracy than the consumer GPS chips can produce. Experiments with RTK and base station correction are promising to bring that level of accuracy in the not too distant future. Roos laudable attempts to post process GT-11 data apparently failed to get the desired results, I am guessing, due to the poor quality of the data the Chips were capable of providing. Since he did not share his methods and results with us, we don't really know.

As to the accuracy of the GT-31 and GW-52 using Doppler derived data. There are numerous scientific papers and studies going back 10 years or more that report the method and reasons why the Doppler derived data can be used for high accuracy speed calculation if implemented correctly. A massive amount of testing was done by Tom Chalko and others including Manfred Fuchs to confirm the speed accuracy of the GT-31 in particular. I personally took part in one of Manfred's tests at the Luderitz speed canal in 2013 comparing both GT-31 and Ublox10hz data against the official timing gates. In those tests the GPS's fared extremely well, (and the UBlox based test units even better) and the few very small variations from the official speed timing gates were pretty evenly spread slightly higher and lower, and, as I remember, well within the reported error values of the GPS's tested.

But as Decepit has pointed out. There are still issues with data from some Phones that is not related to their inherent GPS accuracy. Spotting these errors when comparing them directly with a GT-31/GW-52 is not difficult, but when a sailor is using them as their only source of data, and they are inexperienced in recognising errors, we could end up with quite a significant amount of junk data, and that can easily upset a keen, close fought competition, even on GPS-TC.

We have no issues with new sailors who are finding their interest and just want to get involved at a fun level using phones with GPS-Logit to see their results and post. How we implant their use in the GPS-TC challenge competition is the question we are working on.

That surprises me! Google worked for me.

I think there were some listed on Toms paper you found. I suggest you contact him.

Manfred also wrote up some of his research but not sure if it was published or just circulated amongst the tech group at the time. I think he published a lot on his website as well but not sure if it is still there. I suggest you contact Manfred as well

I think I might have saved copies and links to some stuff. I will see if I can find something for you and email it if you like.

edit: Try searching under these names. They did quite a bit as I remember.

RMIT University, Melbourne I think.

JASON ZHANG,
KEFEI ZHANG,
RON GRENFELL,
ROD DEAKIN

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jamesf said..
>>>>
Hi Decrepit, my Experia E1 display stays on but there are few settings you need to do to make it work.

In GPSLogit, there is a checkbox saying "screen on" in the app. It seems like you have to have this box ticked AND "start recording" activated.

Also in the phone settings you can set the display off time.
- Go to settings --> Display --> Screen Timeout

I also disabled all of the Sony "power saving" features.

One of my teammates has a Pixie and his last track had stacks of spikes. I'd say the screen must have gone off - thanks for the tip.

Yes, it's essential you have "screen on" ticked, and of course "start recording".
But unfortunately, there's a few things that over ride the "screen on" button, like accidental operation of any of the phone buttons, even a crash at speed will do it, if the screen isn't protected. My E1 turned the display off because of a faulty earphone connection.

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John340 said..

Mike,

My Xperia E1 still operates (including speed chat) when the screen display accidently turns off

Hmmm, well Mine does as well, but it certainly gives some very funny results while doing so, like 40kts instead of 20kts.

here's a look at the Samsung Ace 3 and the Sony xperia E1 going for a ride on the car roof.
On the way out, both displays were off, on the way back both displays on.
The Ace 3, which is the bottom display, has both to and fro fully recorded, but the xperia E1 only has the return journey normal, the trip out has most of the data missing.





I wasn't listening to hear what happened to speed chat, but this sort of data is a long way from ideal.

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boardsurfr said..

There are different ways of looking at Roo's data. Four of the five 10-second numbers of the phone are within the range for the GT-31 speeds; only run #2 is outside of the GT-31 range, by 0.16 knots. To correctly compare the two numbers, however, we would need accuracy estimates for the phone numbers, too (which are not available). But it seems reasonable to assume similar numbers as for the GT-31 (+/- 0.25-0.3 knots); then, the speed ranges overlap for all 5 runs. In other words, the numbers are not statistically different.

The next question is what the +/- numbers actually mean. I'd love to see an actual description. The "99% sure" refers to 3 standard deviations and normal-distributed random errors. Typically, however, +/- numbers give a single standard deviation. Even if the numbers would indicate 3 standard deviations, a non-normal distribution of the errors would reduce the probability to a number below 99%.

The fact that the two GT-31s that were both worn on the same arm give very similar numbers does not prove that the numbers are accurate. Error sources in the GPS data, for example from atmospheric distortions, will be virtually identical for two devices right next to each other. An ideal test would involve one phone and one GT-31 on each arm.

That said, there does indeed appear to be a bias in the reported speeds between the phone and the GT-31s. But we do not know for certain whether the phone overstates the speed, or the GT-31s understate it. Assigning the error to the phone is jumping to conclusions. It is at just as likely that the GT-31 underreports the doppler speeds (similar to the way speedometers in cars are allowed to over-report speed, but not under-report it). I have seen slower reported Doppler speeds in GT-31 data when compared to a 5 Hz ublox GPS that appeared to be more accurate (but this is just anecdotal). The 5x10 positional speed data, which are very similar for all three GPS units and higher than the doppler speeds, also point towards the GT-31s under-reporting speeds. Maybe Roo knows something about bias in the GT-31 firmware when reporting doppler speed.

The +/- numbers are easy to explain. Tom Chalko and Manfred Fuchs tested many GPS units side-by-side, over long periods -> collecting lots of data. The tests including using both stationary and moving scenario - resulting in lots of highly correlated data. They then looked for worst-case values - what +/- is the "within 99" range.


Your interpretation of the error-range as "not statistically significant", needs clarification. Phones use a "chip antenna" that is much smaller than the patch antenna used within the GT31 - this greatly affects the reception capability of the device. With this knowledge, there is no reason at all to think a phone would have anywhere near the same accuracy. Can you clarify how you came to this conclusion ?


The GPS mathematical model specifically has to take into account moving satellites and a moving frame of reference; adding any bias makes the GPS calculations harder to solve as you have to add the bias in 3 dimensions. What evidence suggests that the GT31 under-reports the doppler speed ? [ Worth noting that the GT31 has been compared to video timing systems, which have a known error rate (as that is bounded by the frame rate). ]

So assigning a large error to the phone is the correct thing to - you must assume that your data is wrong, then try to prove that it is correct. One way to do that is empirically - you create a large dataset, then use the worst-case sample as the worst-case-error.

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Te Hau said..
>>>>>>>>>>>>>>>>

''All in all the phone acquits itself pretty well. ".........???

As long as it has a very good sky view, and the display is on, then the data from any of the phones I've tested are within 0.5 of a knot.
It's what happens with some phones, that don't have an excellent sky view, and whose display has accidentally turned off, that's the problem. Then the error can be up to 100% out

Pixie 3(4) results from today, were acceptable until I crashed near the end of the session and the display turned off.

Here's my results using GPSResults with filters on
GW52 2s 29.558, 5x10 28.367, 1hr 16.115, alpha 22.110, NM 28.231, dist 93.308
Pixie 3(4) 2s 48.450, 5x10 28.481, 1hr 16.280, alpha 22.055, NM 28.385, dist 89.763

And the 48.450 wasn't even on the water, it was at the rigging spot just before I turned the unit off, there's 2 more results above the GW52 2s, one on the water one shown in the houses on the other side of the road.
If I turn the filters off I get another 48 on the water, and a 46 and 37.

Phone was in a paqua midi with foam surrounding it and a hard plastic cover over the face, this seems like it should be enough protection of the controls to stop accidental activation, but apparently it isn't.
The good thing about these errors, is they're so far out something is obviously wrong and a look at the file will show it, there's big holes in the data, so at least if anybody is suspicious of a post like this, it can be easily verified.

yes I guess I should try KA72 to see how well that filters out the junk.
But I suspect it won't do a better job than GPSresults.
Watch this space.

OK here's what KA72 has made of the mess, and yes it is slightly better than GPSresults, only 43 instead of 48
Results
2 Second Peak (kts): ?43.037
5x10 Average (kts): ?28.482
Top 5 5x10 speeds: (1) ?29.527
Top 5 5x10 speeds: (2) ?28.963
Top 5 5x10 speeds: (3) ?28.044
Top 5 5x10 speeds: (4) ?28.009
Top 5 5x10 speeds: (5) ?27.869
1 Hr (kts): ?16.348
Alpha 500 (kts): ?22.055
Nautical Mile (kts): ?28.382
100m peak (kts): ?29.932
Total Distance (km): ?95.118

So if I hit the "post to GPSTC" button, I'd have a 43knot PB.
And unless somebody else queried the error that's where it would stay.

New world records coming up