View Full Version : Wonky GPS

Tim Diebert
06-14-2006, 08:58 AM
First sail of the season last Sunday.

Dragged out my little Garmin Etrex that I use for a knotmeter. Although
I had done a bunch of rig mods over the off season I am quite sure that would not be reason enough for my wee little gaffer to sail 8 1/2 knots to windward.
The speed is way out and am wondering why?
Can the device be calibrated somehow (I am pretty sure not....)?

Time to throw it away and buy a new one?

I am wonder how this could happen and (other than internal electronic malfunction) what might have caused it.

Maybe it has more to do with my ship board faeries (normally involved in knot making), the thunder storms and the wicked ass squall that tried to tear my skin off.

Advice would be most welcome.

P.I. Stazzer-Newt
06-14-2006, 09:07 AM
Which way was the tide going? - I've seen 13.5 kts on a GPS with only 5.4 on the log. The GPS gives the vector sum of all the components.

John Bell
06-14-2006, 09:07 AM
Are you sure it hasn't had its units of measure reset? Kilometers/hr vs. knots would explain it. If it's giving you a good fix with a low estimated positional error (EPE), then there's no reason to doubt that the speed would be off.

Is there any current where you sail? A GPS gives only speed over ground which may be a lot different than speed through the water.

How does the speed function match with your car speedo?

Tim Diebert
06-14-2006, 09:22 AM
I am a lake sailor lads. Sorry I didn't mention that. Not likely to be much of a tide or current.

First thing I checked was if it was reading knots or not......~:0)

No, I havn't checked it with the car yet. I was going to try that later this morning. The error factor was pretty well what it usually is. I have been using this unit as a knotmeter for 3 seasons now.

One thing I didn't mention is in between last using the etrex and this first time, I moved to a new home and new lake. About 200 miles or so.
I know the manual said something about re-initializing or whatever.

I will play with it a bit more and check it to the speed of the car on the way to work and see what happens.

Previous highest speed on my boat was 6.6 knots. Carrying too much sail on the boat, carrying too much beer on the skipper and surfing down a wave wing n wing. A blistering pace.

John Bell
06-14-2006, 09:33 AM
Is that a sustained 8 knots or did it record a maximum of 8 knots?

I've found that GPS sometimes gives a wonky max speed reading.The reason for the strange readings has to do with the way the GPS measures speed. It is really measuring the distance between two fixes and the amount of time that it took to get between the two. If one fix is in error, or the software messed up the time component, then it is possible to get an anomalous value for speed. I once saw a max speed of 1,100 mph on car trip! Therefore, I take that reading with a grain of salt.

Ken Hutchins
06-14-2006, 10:02 AM
"New home and new lake" Elevation might be a factor. Does your GPS calculates the change in elevation? You might have to re set it.

Bruce Hooke
06-14-2006, 11:18 AM
I once saw a max speed of 1,100 mph on car trip!

And I once got told that I had managed to hit about 60 mph while walking in the woods. Single speed readings and max speed readings from the GPS are not trustworthy in my experience.

I believe the deal with re-initilization is simply that it can take a little longer for the GPS to aquire satellites if your location has changed significantly from the last time you used it.

Nicholas Carey
06-14-2006, 11:27 AM
One thing about getting a speed reading is that if you just turn the GPS one, look at the speed and then turn it off, you're likely to get bogus readings. As was pointed out, the "speed" obtained is time between two points. Most GPS recievers do this periodically (once a second?, 2x? The manufacturers are mum on how often). These "instantaneous" speed readings are then buffered and an average computed -- when the machine is on, the older readings are flushed from the buffer and replaced by newer readings so the reported speed is an average over the last X period of time.

Anyway, if all you do is turn on the GPS, look at the speed and turn it off, you're likely to get spurious readings, especially if you're moving slowly (as in, for instance, a small gaffer). Remember that your solution under SPS (Standard Positioning Service), with SA (Selective Availability) turned off as it has been now for some years, is accurate, at a 95% confidence level, to within +/- 20m horizontally. Which is a fancy way of saying that, 95 percent of the time, you can expect your fix to be within 20m of where it says it is.

Let's assume that your GPS computes speed once per second: How far does your little gaffer travel in one second? It's entirely possible that in that one second, it hasn't haven't travelled far enough to accurately determine a speed (6 knots is about 3m/second, so given the +/- 20m accuracy at 95% confidence level...you might have "moved" from the standpoint of the two fixes anywhere's from no distance at all to 23m. 0m/second is of course, standing still and 23m/second is 44 knots.

Hence the need to just turn on the GPS and just leave it run -- once the speed buffers are full, the speed reported will be an accurate average over the time period represented.

"New home and new lake" Elevation might be a factor. Does your GPS calculates the change in elevation? You might have to re set it.A GPS solution is always point in 3D space. So, yes, the solution includes the current altitude.

06-14-2006, 06:35 PM
I was involved in GPS in-car navigation a few years ago, and part of my job was driving around in my car with a handheld GPS unit plugged into a laptop. I logged a few hundred hours of data. When then used this data to generate maps - we were trying to make a self-generating map/route-optimization application - and I spent *days* writing a algorithim to correct for the GPS 'jumping around'.

Happens more than you think, people who navigate close inshore by GPS only are fools.

06-14-2006, 06:40 PM
:DDon't forget the Coreolis affect.

Tim Diebert
06-14-2006, 07:13 PM
.....uh.........great replies....but I think I will be a little embarrassed now.

kh and kn look a lot the same on a tiny little screen in bright sunlight.
Looks like sometime between last year and this I changed the units to metric from nautical. DOAH!

But....the good thing is I learned a bunch of new stuff about the little yellow box.

I do leave it one the entire time whilst sailing. I like to have max speed and total time moving etc at the end of the day.

So it looks as though it works as well as it ever did. It was deadly close to the speedometer in the car this morning.

Thanks and sorry for the false alarm.

Kim Whitmyre
06-14-2006, 07:25 PM
The dang things eat batteries for breakfast, lunch, and dinner. Mine says it is good for up to 12 hours (it uses 4 AA batteries), but I don't think I have ever seen such an epoch! I gotta get some good rechargeables, because buying new batteries every time I go out is a drag. Any recommendations for rechargeables?

06-14-2006, 07:42 PM
Sounds like a classic case of gronicle interference.

Tim, did you move or remount the wee beasties? This is a much greater problem with the flood of knock-off Chinese gronicles. Classic American Acme gronicles or even the french ones do not cause as much interference.

GPS' dirty little secret.

Doug Wilde

06-15-2006, 12:42 AM
Get some of those Nimh (nickel metal hydride) batteries - they have fantastic capacity and dont self discharge like nicads do. Our digital camera will eat a set of "good" alkaline AA's in about half an hour, but the nimh rechargeables last for hours. Also, you can pick up the camera after it has been sitting for weeks, and the batteries are still good to go.


The dang things eat batteries for breakfast, lunch, and dinner. Mine says it is good for up to 12 hours (it uses 4 AA batteries), but I don't think I have ever seen such an epoch! I gotta get some good rechargeables, because buying new batteries every time I go out is a drag. Any recommendations for rechargeables?

George Roberts
06-15-2006, 01:14 AM
I find that GPSs are worthless for most of the things I want to do.

I bought one that said it would compute the slope of a road. Useful I thought as I was out bicycling.

Installed it on my bicycle. Up the hill I got a consistant 15%. Tough hill. Down the hill 8-9%. Not so tough. Road that hill for a good long time. Always 15% up and 8-9% down. Same speed up and down. Alwyas 15% up and 8-9% down.

Stop at the top and get the elevation. Do the same at the bottom and a bit of math should produce a reasonable result.

50' variation just sitting at the top.


Several years ago I was out kayaking with a GPS. Recorded the location of the putin/takeout. Arrived back 2 days later. Same GPS reading. 2 hours to find the takeout.


I expect more people get wrong answers than get right answers.

Tim Diebert
06-15-2006, 09:30 AM
I have been using those new big capacity AAs as well. I use a pretty nifty charger called a LaCrosse. It conditions batteries, discharges them and charges at different rates.
I use all this stuff in my digi camera as well, not the gps. But I have another set of batts on my wish list.

I used to race state of the art battery powered r/c cars. Spent a small fortune on all that tech. The LaCross is as good a charger for AA AAA and C sized batts....for a reasonable amount of money.... as I have ever seen. Very pleased with it.

One thing, when you first use a new set of these batts the run time seems unimpressive. It takes a coupla cycles before they begin to impress. My camera will eat a good set of alkalines in no time. The NimH seem to be in the camera forever. They also, like good matched r/c batts, once at the end of the charge will dump fast. So when you are low....do something about it, don't wait.