Wierd GPS speedo readings

[Magicfloat]

Been running my new pontoon boat( I know,not a real performance boat,don't shoot me) It has a Humminbird 363 combination depth finder/GPS. Inexpensive unit but has worked well for the money.Boat consistantly runs 39 mph GPS. Took it out today,it was very windy,water on the lake close to whitecapping.Cruising along WOT at 39 mph,suddenly it jumps to 44,then 53,then 57,impossible speed for this boat. Throttle back to what tach should indicate 15,GPS is at 25. Pull back to idle which normally is .4 mph,GPS says 4.1. Shut all systems down,power back up,idle in gear.4 mph as normal. Nail throttle,39 mph for about 1/2 mile,then same thing,45,49 55. WHat could cause this, wind,rough water,or just a cheap GPS? Never seen such before.

[FastDonzi]

Are you sure the speed reading is GPS and not from a paddle wheel on the transom? I have never heard of a gps reading anything other than perfect.

[MacGyver]

Maybe the receiver is cheap and can't track the satellites while in choppy waters. Hard to believe. But I bet it came from China :rolleyes:

[cigdaze]

From a previous thread:
http://www.seriousoffshore.com/forums/showthread.php?t=1411


A GPS does nothing more than provide a location fix; an X-Y-Z (latitude, longitude, elevation) position. This is the basic measurement, known as a fundamental unit. Speed, heading, course, eta, etc. are all derived, or computed values using your delta position over a given amount of time, usually fractions of a second. Most GPSs provide speed data not as instantaneous readings for every delta, rather they use moving averages over say 5 or 10 discretized movements to compensate for GPSs inherent errors. Remember, most of the GPSs we use are in the range of +/-2 meters with WAAS enabled, but more often +/-5 meters accuracy. This is where we see occasional spikes of troughs in velocity indications. If, for instance, one positional fix is -15 meters, and the next of +15 meters, that's a total error of about 90 feet. This error over say even 250 milliseconds amounts to about 30mph error. All it takes is the gain or loss of a few milliseconds during clock re-sync (because the internal GPS clocks are synced with satellite time at set intervals), or the loss/acquisition of satellites giving slight positional errors, and you'll see some weird indicated speeds.



One more thing: For those with your antennas mounted high, you'll see the effects of rotationally-induced delta velocities in addition (or subtraction) to your translational velocity. When re-entering the water from a launch for example and the boat quickly goes from bow-high to level again, the positive angular velocity will add to your longitudinal velocity. The opposite holds true when rotating backwards.

This can be easily witnessed when sitting idle and bobbing around in some waves...I'll bet your gps is alternately reading between 0 and say 3 mph, just from the swaying.

Then there's this:
http://www.guardian.co.uk/technology/2009/may/19/gps-close-to-breakdown

[cigdaze]

P.S.

I have a max speed on my GPS of 104mph....no way in hell that's true.

:cool:

[fund razor]

P.S.

I have a max speed on my GPS of 104mph....no way in hell that's true.

:cool:

My 21 foot Nova once acheived 87 gps with a single merc 260. :D

When I read the title of this thread, I remembered your post that you quoted. One of the best explanations of gps errors out there.

When I first learned about this, I played with forcing velocity averaging that were crazy. :D

[Magicfloat]

Thanks,Cig. Apparently just a fluke, occasionally showing up due to GPS capabilities? Been working great for a couple weeks,only today did it get these weird readings. Guess it is nothing to worry about. Thanks for sharing your knowledge on this.

[fund razor]

Thanks,Cig. Apparently just a fluke, occasionally showing up due to GPS capabilities? Been working great for a couple weeks,only today did it get these weird readings. Guess it is nothing to worry about. Thanks for sharing your knowledge on this.

That's what you get from an actual rocket scientist. :D
Cigdaze is da man.

But on topic, watch it over a period of time with this information in mind, in different circumstances. It's one of those things that you may not notice, but once you know.... you will notice. Like Nick described with the rocking.

I threw a handheld one a pretty good distance once to see if I could force a speed error, and I did.

[cigdaze]

You got it, Magic! :)

[cigdaze]

My 21 foot Nova once acheived 87 gps with a single merc 260. :D

When I read the title of this thread, I remembered your post that you quoted. One of the best explanations of gps errors out there.

When I first learned about this, I played with forcing velocity averaging that were crazy. :D

Funny thing is, I started typing it all out again, and then I'm like...wait, I remember doing this once already...off to the search I went.
;)