I just received a GT-31 and used it just 1 time. I noticed that the unit reads speeds when not moving. If I turn it on and leave it still on a table, it continuously reads random speeds up to even a few knots. Don't know if this is normal, or my unit is troubled, and whether this will affect sailing datas or not.

Any help?

Hi Giorgio,

If you do a search of this sites forums you will probably find previous discussion around this topic.

The short answer is that it is normal and nothing to worry about. Some of our tech experts have done quite a bit of geostationary testing and have found that in good satellite reception, the variation you will see is generally less than the error margin they have calculated for the instrument. Remember that the speeds that you see are in random 'directions' and effectively cancel themselves out in real sailing.

On a table inside a house you will see more fluctuation due to reception interferance.

Hi Andrew,

thanks for the answer, I'll do the search, maybe you can point me to the right thread?

Anyhow, the "idle" speed readings I see go from a fraction of a knot to almost 2 knots: not bad, if that is comparable with "the error margin they have calculated for the instrument" and generally less than that.

Dont worry about it Giorgio. GPS cannot pinpoint the location exaclty until it is moving. That is why it needs several sattelites to track. When it is stationary the unit will appear to move within about a 5m radius from its actual location - which is quite a big area and the reason why it can give up to 2knt readings. As soon as you move it then this radius reduces. (I am no expert, this is just my understanding of it)


I used the GT-31 2 times now. In both cases, results look unreliable. The first time I used the unit set at 1 sec interval, the second time I tried it at 2 sec. In both cases I see many 'impossible' results (the second track contains a lot of trackpoints at speeds close to 50 knots) and I see logged speeds are 'sawtooth' like, alternating 1 sec at very high readings and the following at about 1/2 such and so on.  All those weird speeds do not look like spikes on the track diagram, but are 'blended' into what looks like a 'regular' track.

I am starting to think that the system used by the Foretrex201 (using the doppler datas for calculating future points and narrowing error margin, logging only position datas) is way more reliable, plus makes it easier to detect spikes.

Possibly something wrong with your unit then. I hope (and am sure) someone who is a bit more knowledgable on here can have a look at your track for you to see. These GT31's are very accurate, much more so than the others, hence why the others are not allowed for record verification. I am on my 3rd unit now, keep two running and my oldest one died for some reason (all bits work when plugged in but not when unplugged........been in contact with locosys and recieved possibly the best customer care I have experienced from a company).

Have you closely followed the settings from the mtbest website? It sounds like either a setting is off or something could be wrong with the unit.

Must be my usual luck then.

I'll check again the settings, but am sure it's all OK. Is there anyone willing to have a look to the tracks?

Another hint is: when I open the data with GPSAR, I see from the SpeedSatistics that  best "50 m run" speeds and even best "100 m run" speeds are higher than "best speeds". I did a test logging a short track with my car. Already checked the settings are same as suggested on the mt best page.

Part of the same track, over which I was driving the car at about constant speed, when analyzed with the track points data table shows trackpoints alternating speeds of about 30, 15 and 0 knots.


I would find another "GT31 user", to check the results of same track, with the two GPS. Be sure of the same settings on 1 sec and, before the run, reset the max past speed. 



Hi Giorgio,

Like Andrew says many discussions have been held in the past. Don't test in a car but on the water. Standing still does show speeds (sometimes up to 2 knots). The reasons have been described above. The error margin while sailing will be extremely small. I am sure with the right settings and using two GT-31 devices you will find the speeds shown will be nearly completely the same. If you can find another GT-31 user at your local spot you can check for yourself.



Hi guys, thanks to all for your help!

OK I understand that the unit will detect random speed values when standing still.

The problem now is, do I use it in a car or when sailing (in an aquapac on my left arm), still my GT-31 logs totally sick speeds. Very often I find "sawtooth" speeds (i. e.: 0 knots - 15 knots - 30 knots - 0 knots - 14.5 knots - 31 knots - 0 knots - ... and so on) and irregular log rate (1 sec - 2 sec).

I set the unit using the setup file from mt. best site. I am logging in the internal memory (no SD card, and not willing to use any) and reading with GPSAR. Has anyone else experienced such behaviour? Could it be some setup problem, or is my unit defective?

This behaviour you describe sounds very unusual. If you have double checked the set up and you are logging at one second maybe there is a problem with the reception in your GPS, but I have not seen anything like this before.

Why are you not using an SD card? The use of the card is one of the great advantages of this GPS. It enables the logging of the full data set in the .SBN format and recording time that is virtually unlimited. (The internal logger records an abbreviated data set in the .SBP format and is limited to a few hours before it overwrites) The card also enables the fast and easy transfer to computers via a card reader.

Are you using the older GPSAR? If so that might be your problem as I think it only displays and calculates from positional trackpoints and it is possible that that could be the casue of the sawtooth effect you see. Send me your trackfile and I will run it through the latest software.

sailquik (at) hotmail dot com

Hi Andrew,

I sent you an email with the files attached.

I checked my GT-31 many times, I also found a weird thing, seems the flash memory capacity is 16256 instead of the supposed 20480 (DATA LOGGER / SAVE TO CARD, in the small lines under the menu it displays: "xx/16256-0-0-0" and (below) "xxSBP0-0-0:0:0:0").

I am not using the SD card as the instrument is OK for me the way it is (I mean, supposing it was working) and don't want to mess up with even 1 more electronic device such as SD card or card reader. The older GPSAR is OK for my use, I just want to see on screen what my sailing was like and the speed statistics. I ran the evaluation version of GPSresults for a while, it is even nicer than GPSAR but the upgrade is not worth the fee for me.

I bought another GT-31 with mine, for a friend who is using it with no SD card and reads results with GPSAR; everything works OK with that one. Anyhow, the instrument is supposed to work perfectly well with no SD card and provide .gpx files that can be read with GPSAR or Garmin Connect with no limitation to specific software (or memory device).

Giorgio, you can contact Locosys directly and ask their help, I did when I had a problem (still unsolved, think the unit broke after 3 years of use in a leaky aquapak) and they were extremely helpful, they have very good customer service. Roger was the person who helped me -, alternatively go on their website and use a contact form.

I checked again with my friend and his (properly working) GT-31 is just like mine, memory capacity 16256. Maybe there was a change in production, not a big issue anyhow. Andrew Daff checked my files and found the .sbn ones are OK. This points to the file convertion, as the instrument logs in .sbn and conversion in done by the Navilink software, so hopefully it's not the GT-31. I'll contact Roger.

Thank to all you guys for the help!

Can it be a problem with the GPSAR-software? I remenber when I used the free version of GPSAR, I got some wrong speed-results. Like 32  knots on 100 meters and less over 2 seconds, and I certainlu had not sailed 32 kots. I have since used GPSResults, and don't see these wierd results.


Best regards