Go Back   Starboard Forums > Free Forum

Reply
 
Thread Tools Display Modes
Old 7th March 2009, 02:16 AM   #1
Per
Member
 
Join Date: Aug 2006
Posts: 237
Default GPS accuracy?

Hi all..
I've got myself a Locosys GT-31 GPS for measuring speed on my boards.
Even when I don't move the GPS it shows speeds up to 3 knots at times.
This must affect the accuracy a lot (up to 10 %). Did I really sail 31 knots? Or maybe 28?

Aren't these GPS quite unprecise, or should I change something?

Any feed back is very welcome

;-)
Per
Per is offline   Reply With Quote
Old 7th March 2009, 02:40 AM   #2
bensen
Member
 
Join Date: Aug 2006
Location: Denmark
Posts: 21
Default

Hi Per
I work as at captain on board High Speed Crafts.
Among a lot of equipments, we got 3 different GPS, used for speed measurements.
I own several GPS to be used handhold.
When I compare the equipment on board the vessel with my personal GPS, the accuracy is exactly the same.

I trust in the readings (most of the time....;.)


Bensen
www.bensens.dk
bensen is offline   Reply With Quote
Old 7th March 2009, 04:33 AM   #3
Per
Member
 
Join Date: Aug 2006
Posts: 237
Default

Hi Bensen.
I'm a sailor myself, and I trust my GPS too (most of the time - I still keep a good look out).
Anyway, for trimming my boards speed 2 knots make a lot of difference.
Let's say you have a certain trim (all other factors being excluded) giving you a speed of 30 knots. Your GPS though says only 28 knots (due to unaccuracy). You change trim, head for another run, and this time your speed is only 28 knots but your GPS claims 30. You think your trim made you faster, but the fact is you lost speed. Ithink a GPS should have an accuracy within 0.25 knots to be reliable.
Anyway I'm still working to understand my new toy..

;-)
Per
Per is offline   Reply With Quote
Old 7th March 2009, 04:44 AM   #4
bensen
Member
 
Join Date: Aug 2006
Location: Denmark
Posts: 21
Default

Have you ever tried this: http://trail.motionbased.com/trail/p...kValue=7713213

Quite interesting.

/Bensen
www.bensens.dk
bensen is offline   Reply With Quote
Old 7th March 2009, 06:19 AM   #5
Roger
Dream Team - School Guru
 
Join Date: Aug 2006
Posts: 2,112
Default

Hi Per,
Rest assured that the GPS is accurate.....very accurate.
Try these 2 tests:
1/ Turn on the GPS (on shore) and set it down on something solid.
Give it a minute to locate all the satellites it can.
Take a look at it after a couple of minutes.
Does it still show motion?
Probably not would be my guess.
The 3 knots you are seeing could be one of the following:
Current induced.
Drift due to the wind moving you and your board along.
Your hand motion (if it's attached to your arm).
2/ Place the GPS in a visible place on the dashboard of your car.
Give it a minute to find the satellites.
Drive along slowly, then increase your speed a bit.
At < 10 mph, the GPS should be very accurate to 1/10 mph-m/s- knots.
Since we are no longer "burdened" with selective availability (SA for short), the positonal
accuracy of a GPS unit is down under 1 meter if the unit is allowed clear access to the full range of satellites in it's viewing area.
Even in the days of SA, (a slow sine wave error signal that varied and caused the positonal accuracy to decrease to around 30 meters (factor of 10)) the CHANGE in position accuracy was still very accurate as the SA signal was very slow.
The intent was to cause deliberate positonal inaccuracy.
Now, with no SA, and better GPS units that can see more satellites, it's become very accurate.
Oh, if you check the accuracy when moving with the GPS in your car, and the speedometer and GPS don't agree, the inaccuracy is most likely "built in" to the car speedo system by the factory.
My Mercedes Sprinter Van speedo (analog) has a built in 3 mph (reads 3 mph high) to keep me a little scared of getting a ticket and give me a 3 mph "cushion" to prevent tickets.
I've had as many as 3 portable GPS units running in my van at the same time, and all of them indicate within 0.1 mph while the speedo reads 3 mph higher.
So, if I'm cruising at 60 mph on the speedo (steady speed on the cruise control with RPM steady with in 10 rpm) the GPS units will all read 57 mph +-0.2 mph.
I used the recorded and printed GPS readings recently to have a traffic ticket for 70 mph in a 55 mph zone reduced to 62 mph for a greatly reduced fine and no points on my license.
Hope this helps,
Roger is offline   Reply With Quote
Old 7th March 2009, 02:42 PM   #6
Per
Member
 
Join Date: Aug 2006
Posts: 237
Default

Hi Roger.
You're always good for a detailed answer. Thanks a lot.
I made the first test. I placed my GPS outside under a clear sky on top of my litter bin. It didn't move at all. Anyway it measured a max speed of 1.98 knots and an average speed over 10 seconds of up to 0.98 knots - while NOT moving at all!!!
I guess something is completely wrong with it. I have a friend who made a run with his old Garmin and this new GT-31 and they disagreed with as much as 6 knots!!!
My old Garmin E-trex never showed any sign of being more than 0.2 knots unaccurate..
Does anybody know the GT-31? Is there anything in the set up that I may have forgotten?

;-)
Per
Per is offline   Reply With Quote
Old 7th March 2009, 05:13 PM   #7
Romas
Member
 
Join Date: Dec 2006
Posts: 20
Default

http://mtbest.net/setting-up-navi-GT31.htm
http://www.gpspassion.com/forumsen/t...OPIC_ID=112357

Last edited by Romas; 7th March 2009 at 05:16 PM.
Romas is offline   Reply With Quote
Old 7th March 2009, 06:54 PM   #8
Per
Member
 
Join Date: Aug 2006
Posts: 237
Default

Hi Romas.
Thanks A LOT ;-).
Anyway I tried to bring my GT-31 for a run in my car. As soon as the speed increased to above 10 km/h it appeared to be very precise indeed. Actually I was not able to keep the car on an exact speed precisely enough. It looked like the GPS vas precise within 0.2km/h.. Strange.. I will check your links even more and take care that the GPS is updated..
My first goal is 32.5 knots on my iSonic 122 this week-end..

;-)
Per
Per is offline   Reply With Quote
Old 12th March 2009, 03:16 AM   #9
Phill104
Member
 
Join Date: Aug 2006
Posts: 297
Default

Sounds odd. You shuld ask this question on http://www.gps-speedsurfing.com/

Try testing it in a position a meter or so from where your bin was. There is a grid effect (which I find hard to explain in words) that may explain what happened on that occasion.

In my tests so far the gt31 is very accurate.
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
All the latest windsurfing news in one place.
Phill104 is offline   Reply With Quote
Old 12th March 2009, 03:37 AM   #10
Ian Fox
STARBOARD
 
Join Date: Aug 2006
Posts: 532
Default

Hi Phill,

"Grid effect" can affect accuracy of speeds derived using Lat / Long methods,
however it is eliminated by the use of Doppler based, which is what (and why) GPSSS
now choose to base all accurate and record timing on Doppler results.

The speed accuracy of the GT-31 (correctly configured and using Doppler) is very precise.
-and significantly free of errors seen from older or lat/long based units/results.

Cheers ~ Ian
Ian Fox is offline   Reply With Quote
Reply

Bookmarks

Tags
None

Thread Tools
Display Modes

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +7. The time now is 10:33 PM.