How inaccurate???

Hi Harrow. There has been a lot of side by side testing done now over a couple of years and the results are pretty clear. The differences are in the variation between units. The variation run to run in the Garmin using trackpoints can be up to over half a knot. The differences in the latest GT-31 can be as little as 0.02 of a knot. When rankings or event results are decided by less than a tenth of a knot, this is very significant. Using Garmins it would be possible to have ten competitors within half a knot and the actual finishing order could be entirely random or the reverse given the margin of error.

Here is an excellent reference if you want to delve a bit more into statistics: http://www.gps-speed.com/tests.html

Thanks sailquik/mathew,

That is exactly the sort of info I was after. I really want a unit with external batteries for extended cycling/camping/hiking holiday trips, so for a mug like me who is just curious if he is managing 24 or 28 kts, the eTrex 'H' sounds good value at $127. Also, it now has a new high sensitivity antenna, so perhaps this will add to it's accuracy.

Regards,
Harrow.

The Navi also allows car charger and laptop/computer charging so if you are on a cycling trip you could charge at an internet cafe.

Don't forget the Navi with it's SD card can record months of data at 1 second intervals... a Garmin just hours unless you increase the interval time significantly.

Ah, I get it all now. USB gives +5 volts supply, which is used to charge all sorts of things.

I already have a cigarette socket in-car USB charger for my HP Pocket PC that gives a +5 volts USB supply, so no doubt this can be used to charge the GT-31 on a trip.

Only one question now....with the rechargeable battery being built-in...how long until it dies iPod style? Anyone successfully changed over the internal battery?

The battery probably can be changed at a hobbie shop but it involves splitting your Navi apart which probably wont do the waterproofing much good. The original is stuck to the inside back of the case with double sided tape. Wouldn't want to split the alu battery covering trying to get it off. Li-ion batteries burn explosively if punctured or shorted.

I find I can get several sessions on a charge. There are probably a 1000 charges in the battery. That's a lot of use.

How much is a GT-11 gps complete with waterproof bag?

AL green at Caloundra city sailboards still has some GT11 for $150.

Ive had my Navi almost 2 years now and no Probs touch wood

Yesterday my Garmin Venture gave a result .74 kn faster than my GT-31.
Don't know how to prove it but I believe the GT-31 is the most accurate because it uses Dopler.
Herby always posts his Dopler results, but because Lyn uses a Garmin, well her results are Trackpoint
The GT-31 is much lighter (weightwise) than the Garmin.

Herby

Anyone know the going price for a GT31? - and how do they differ to the GT11?

Herby. The display speeds of all the various gps units are actually derived from Doppler calculations and tend to be very accurate. Unfortunately, most GPS units are not set up to record this data for later use as we can with the GT-11/31. I used to use my Foretrex the same way but now I have a couple of GT-11/31's I use one of them on my forearm for continuous feedback instead.

No knowledge about prices but the GT-31 has a different model GPS chip. It has a Sirf3 chip v's the GT-11 which had a Sirf2 chip. The new chip is much more sensitive and is able to pick up and use very weak signals. This was brought home to me recently at the Al Green event when I had a GT-11 and GT-31 in the same Aquapac. Late in the afternoon there were only 6 satellites in the sky and two of these were very low in the horizon. The GT-11 could not pick them up and had only 3-4 signals which makes the tracks invalid. The GT-31 was able to pick up all 6 satellites and produce data that was valid and useful. I was glad I had it because a couple of my best runs were done during this period and they would not have counted in my average if I was relying only on the GT-11. I must say that this was the first time I had this problem with the GT-11 for such an extended period but it only takes loss of sats for a few seconds to invalidate a run that could have been a new PB.

Correct! (subject to possible slight differences in filtering). And this has been stated in just about every discussion of GPS's since the Doppler capability was discovered a couple of years ago.

If you don't want to download, then the GT-31 is still far better because it will save the best top speeds in the genie for recall at the end of the session. Not only do you have your best top speed, you have all the rest of the top dozen or so listed as well!

I cannot agree that if you don,t want to download then your displayed performance will be correct. Neither Lyns Etrex or my Venture displays are ever the same as we get out of Realspeed, or another program GPS Trackmaker. Sometimes the error is above and sometimes below.

In my case the Garmin is always different to the GT and when I put them thru RealSpeed the GT is always closer.

True. The display speeds are often slightly different between GT-11/31 and Foretrex. Usually not much different and usually in the same small order of difference that you get between two side by side GT-11's. Remember that these are unsynchronised GPS and you are talking about a peak 1 second speed. It will seldom ever be the same part of that best 1 second (unsynchronised).
The Garmins also round the best speed to 1 decimal place. The GT's display to 2 decimal places. If you take this into account they are usually pretty close.
The speeds you get from Realspeed for the Garmins will be based on Trackpoints. The speed on the display will be calculated from Doppler info. I would expect them to be different.
The Doppler speeds calculated by RealSpeed from the GT-11/31 bin file are almost always extremely close to the max display speed.
If you look at the 10 sec max speeds saved in the Speed Genie from a GT-11/31 they will also usually be extremely close to the RealSpeed output (but rounded to 2 decimal places) but remember that display speeds can't always be relied on as there is none of the error checking/filtering that you get in the analysis programs.