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Thread: Navi GT-31

  1. #1
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    Navi GT-31

    Having been bought a GT-31 for Xmas and spent quite some time finding out how to set it up, load all the necessary software and select a good download/analysis software package, I thought I would start a thread here to help bring out all the best practise and advice.

    I have yet to use mine on the water so my experience is somewhat limited but this is what I have learned so far.

    1. The pack no longer comes with a DVD ( despite reference to it on the pack!) so all the software and info you need is now only accessible from the Locosys web site.
    2. The info there is pretty good in terms of helping to understand basic operation of the menus, how to charge it, and how to enable the device to be used with a SD card rather than relying just on the flash memory within the device. There is also reference to specific set it up for speed sailing using " Speed Genie"
    3. The device ( if set up with Speed Genie) is tailor made to enable max speeds, max average speed over 10 secs, and best average of 5x10 sec runs.........which is what is required to enter the league table on the GPS Speedsurfing website. None of that prevents the device being used as well for cycling, rambling, orienteering etc . GPS Speedsurfing/windsurfing speed events in general, use Knots as the units however. Knots are imperial, so it does mean once set that way, all other measures in the device are also imperial. It is fairly easy to change in "settings" though, if you wanted to use the GT-31 for other activities...and then change back to knots before a windsurf session.
    4. Firmware has to be updated via the Locosys site............easy enough.
    5. GT-31 has its own USB driver and download software called Navilink.....you need to install and use that to update the firmware but thereafter, downloading can much more easily be done by removing the SD card and using a card reader.
    6. What is missing from all the info available ( as far as I have find anyway) is instruction and advice on actually using the device once set up! Fortunately, from the point of view of using the Speed Genie function, it is pretty straightforward. In effect it consists of little more than powering up and strapping it to your arm in an aquapac. All the clever stuff is done by software packs into which you load the files after a session.
    7. I tried a couple of different download software packages but settled on GPS Results V6. The GT-31 ( if configured to write to the SD card) creates "SBN" files. You merely copy these to your PC from the SD card using a card reader, then load them into GPSResults. GPS results then shows both your Tracks and all the trackpoints you have recorded both in a table and graphically. GT-31 records a trackpoint every second. GPS Results enables the removal of spikes and automatically calculates your max speed and 5x10 secs max average...........which can then be loaded into your account on GPS Speedsurfing with a single button press. You can also see your track on a Google Earth overlay.
    8. GT-31 has been supplemented with firmware written specifically to enable it to act as an approved GPS device for speed sailing. This is called the speed genie and various settings you apply enable it to work almost totally automatically. Once GT-31 is switched on and locked...and strapped to your arm.......there is no need to press any buttons etc, until you finish your session. It switches to whatever display you have chosen once you go over the minimum speed you have set for it. After each run it shows the max and 10s average max. Once you have finished you can then unlock it and check back on a history of 14 Max recordings and 10 Max 10 sec recordings. Of course once you download your SBN files you can analyse the complete days sailing. The data is automatically broken into separate runs based on either when you drop below the minimum speed you set ( e.g. you stop!) or if a change of heading greater than 90 degrees is recorded ( i.e. you gybed/tacked).

    I did a dry run, taking the device with me for a quick drive around in my van just to create some SBN files and use the GPS Results software. So far I have been impressed with how easy and reliable all of that was.

  2. #2
    Senior Member Redsurfbus's Avatar
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    9. They no longer produce the GT-31, once stocks are sold we need to find another bit of kit that is as accurate. None of the other GPS tracking systems use doppler so are not as accurate. There are others out there that have a 10x higher frequency, recording position in tenths of a second but this is deemed to be less accurate than every second with doppler.
    You can also use ka72.com to analyse your data. I use GPSresults which is easy to use. Others use GPS Action Replay which gives better (or rather easier) analysis of alphas.


    .....A word of advice - use it in the aquapak but double bag it, get a sandwich bag or something like that. Aquapaks are leaky when hitting the water at 30 odd knots unless you get all the air out of it before sealing it. They will not break due to a bit of water BUT the screws go rusty. They occasionally suffer from a freeze (happened to me 3 times with 3 different units over about 6 years), easy to fix with a screwdriver but not with rusty screws......either that or you leave it until the battery dies and recharge it. Trouble is the battery in it lasts a very very long time.
    Last edited by Redsurfbus; 4th January 2014 at 11:17 PM.

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    Senior Member Capie's Avatar
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    Theoretically, measuring speed using Doppler (the change in frequency as an object passes a point - think police car passing with siren) would not be more accurate. It is in fact demonstrably less accurate at low speeds (including the kinds of speeds most of us windsurf at). The traditional way of measuring speed is calculating distance traveled in a specific time. As I understand it, Doppler calculations can only compete with the traditional method given the inaccuracy built into the civilian gps system. Basically, if you have a small sample time, then you have probably only traveled a few metres and that is difficult to detect using civilian gps. That said, the traditional method is very, very accurate and good enough for aviation, most other forms of boating, cycling etc. Also, the change in frequency is equally difficult to detect in a short sample.

    Many other devices are capable of doing Doppler calculations but this function is not enabled given that is considered not to be useful. Suunto have talked about enabling it in their Ambit range but it seems only speed sailors are interested in it.

    Tests done with various devices show that the GT-31 is no more accurate than an iPhone or most of the sports watches out there. The difference between these devices is negligible. The advantage of the GT-31 is simply that it is a standard device, not that the method it uses to calculate speed is more accurate.
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    Senior Member Redsurfbus's Avatar
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    The use of doppler alongside the traditional positional calculations allows for much better accuracy in regards to acceleration as it is the calculation of acceleration between 2 points. The GT31 is considerably more accurate than an iPhone, especially when alphas are being calculated. With a standard GPS the calculation is done in a polygon as each position is 1 second, and travelling at 30knots plus this is a considerable distance. With doppler the true curved path taken is much more accurate.
    This is a very simplified explanation of what is available on the GPSSS and elsewhere to justify their choice in using doppler.

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    ...............or if you like the techie stuff, this article demonstrates the accuracy of Doppler as implemented by the GT 31 using SDOP and a SiRF3 chipset

    http://nujournal.net/SDOP.pdf

    ...........not quite as simple as the police car siren methinks!!

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    Quote Originally Posted by Capie
    Tests done with various devices show that the GT-31 is no more accurate than an iPhone or most of the sports watches out there. The difference between these devices is negligible.
    Care to provide references for this? I think this statement is only true for positional data. Doppler data from the GT-31 are more accurate than positional data from other GPS units.
    Quote Originally Posted by Redsurfbus
    The use of doppler alongside the traditional positional calculations allows for much better accuracy in regards to acceleration as it is the calculation of acceleration between 2 points. The GT31 is considerably more accurate than an iPhone, especially when alphas are being calculated. With a standard GPS the calculation is done in a polygon as each position is 1 second, and travelling at 30knots plus this is a considerable distance. With doppler the true curved path taken is much more accurate.
    Since the measurements are still only taken every second, even with Doppler, there is no such thing as a "true curved path taken". In fact, the alpha measurements are where the GT-31 data are least accurate. For examples, check the graphs at http://boardsurfr.blogspot.com/2013/...etter-gps.html
    The examples given there show that much better accuracy can be reached with more modern GPS units. Development of a better GPS unit specifically designed for windsurfing is going on, and I hope that we'll be able to buy units sometime later this year.

  7. #7
    Senior Member Capie's Avatar
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    Quote Originally Posted by boardsurfr View Post
    Care to provide references for this? I think this statement is only true for positional data. Doppler data from the GT-31 are more accurate than positional data from other GPS units.
    I wasn't able to find it easily so will have to dig. I think it was an article in the German Jahrbuch last year but have no idea where I put that. I did find some forum posts though. This one posts some results from a GT-31 and a Garmin Foretrex with the Foretrex showing 82,796km/h and the GT-31 82,595km, i.e. a difference of 0.1 knots - http://www.surf-forum.com/archive/index.php/t-1959. This post suggests the same difference with a Forerunner - http://www.gps-speedsurfing.com/defa...val=26328.html.

    However, the article I'm thinking about compared a Garmin Forerunner, iPhone (a 4S if I remember correctly) and a GT-31. For speed measurements, the difference between the devices was less than 1%. The GT-31 was the least accurate of the three for positional data because it uses an outdated chipset.

    Is there any data to prove that the GT-31 is more accurate than the popular watches or the latest smart phones?

    At the end of the day, 1% is a big enough difference for the guys at the top of the speed ladder. There's probably a big difference in the rankings between people doing 40 knots and 40.4 knots. Which is motivation in and of itself for standardising onto one device. My point though is that the argument that the GT-31 is significantly more accurate is overstated. It's more about all using the same ruler than one ruler being significantly more accurate. The fact is that in other discplines where speed measurements are significantly more important (like aviation, sailing and rallying) than bragging rights on a website, the GT-31 is nowhere to be seen.
    My Boards: 2016 Fanatic Falcon TE 129, 2014 Patrik Slalom 115 vII, 2014 Patrik Slalom 92l, 1992 Windsurfer One Design, 2012 Fanatic Freewave 85l
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