GPS errors

[OldSchool]

I remember earlier this year Frank (Panther/Geronimo36) posted a picture of his gps with some outrageous speed on it (like 3600mph or something). I saw a few speeds this year on mine that I'm 100% positive were exaggerated.

Does anyone know why? sun spots?? ....or is it global warming? Did anyone else see this on their boat?

The picture below is after the Cambridge OPA race weekend. I saw some boats go 112mph.....but it wasn't mine!:rofl::rofl:

[yesrej]

i had it happen on my handheld gps. i watched it happen to. right when i looked down it went from 68 to 69 then to 96 then back to 69. i thought i was seeing things but after we stopped i checked top speed and she read 96.3:biggrinjester::biggrinjester: boat was faster then i thought....

[Trim'd Up]

I don't know what causes it but it happened on my handheld too.
My boat only made it up to 111 before it slowed back down. That's only about double of my previous best. ;) Must have been the cool fall air!

[Salesmanship]

I remember earlier this year Frank (Panther/Geronimo36) posted a picture of his gps with some outrageous speed on it (like 3600mph or something). I saw a few speeds this year on mine that I'm 100% positive were exaggerated.

Does anyone know why? sun spots?? ....or is it global warming? Did anyone else see this on their boat?

The picture below is after the Cambridge OPA race weekend. I saw some boats go 112mph.....but it wasn't mine!:rofl::rofl:


Just curious. Did you go under any bridges?

[OldSchool]

Just curious. Did you go under any bridges?

Yes

[glassdave]

i have seen it many times we spent a weekend prop testing a little 16 foot boat that i had and were just trying to hit 50 with the thing. On one of my runs i recorded a speed of 163mph. Its just a little Sidewinder runabout with a Merc 140 in it. I attested the speed to the fact that i am a professional throttle man, my friends however, did not buy it . . . .


I did achieve the best speed of the weekend (50.00000000109) out of all my friends . . . this is test pilot Chuckie aka "Boneless Chuck" he only saw 47.63424 . . .note helmet and chase copter . . . at these speeds you must be prepared :D

http://www.youtube.com/watch?v=7zfDuiyYAZI

[cigdaze]

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.

[yesrej]

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.



ok i did start to read your explanation but i cant stop looking at your avatar.:biggrinjester::biggrinjester:

[glassdave]

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.



thank you Mr Wizard :D


OPA allows us up to 3 spikes in gps readings while racing to accommodate these error readings

[VetteLT193]

I never understood how GPS became THE speed indicator. It's good as an average but for instant speed indication it's the wrong technology.

[cigdaze]

I never understood how GPS became THE speed indicator. It's good as an average but for instant speed indication it's the wrong technology.

I agree. It's a reasonably good indicator, but my no means is it perfectly accurate.

[fund razor]

I never understood how GPS became THE speed indicator. It's good as an average but for instant speed indication it's the wrong technology.

No doubt. Back in the old days, we calculated speed by throwing a friend off of the boat and, judging by his injuries... we calculated speed. Kind of like the Fujitsu damage scales for tornado wind speed measurement.

[OldSchool]

ok i did start to read your explanation but i cant stop looking at your avatar.:biggrinjester::biggrinjester:


That's Nick's wife. He post it for his pal catmando.:rofl::sifone:

I hope that thisd thread doesn't turn into a "Plane on a treadmill" thread!!!! :D

[cigdaze]

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.

[cigdaze]

That's Nick's wife. He post it for his pal catmando.:rofl::sifone:

I hope that thisd thread doesn't turn into a "Plane on a treadmill" thread!!!! :D

I wish.

And why? That was one of the best threads ever! :biggrinjester:

[OldSchool]

The opposite holds true when rotating backwards.

.

Is that directed towards GlassDave??? :D:sifone:

[macjazzy]

There is an issue with some GPS units that can be affected in various ways by a new sat launched last Fed. Its called PRN32, the issue becomes when the GPS sees this Sat. it was never programmed for and it gets confused giving strange readings or just refusing to lock on at all.

Most GPS manufacturers have a software patch to cure this problem. Do a search on PRN32 problems or just check the website for your GPS to see if they have this problem and what the fix is.

[glassdave]

i guess its better than rotating upside down which has somewhat of an adverse affect on overall speed readings . . .

[Bobthebuilder]

Hmmm. So I can't believe my maximum speed of 370 MPH ? LOL ( top right of GPS pic ) It was like that out of the box. Last boat. I had wondered if that was air speed if shipped by air but unit had no antanae or power. Mystery.

Bob

[Tommy Gun]

i guess its better than rotating upside down which has somewhat of an adverse affect on overall speed readings . . .

Or in Stecz' case...rotating from behind. :03:

[Salesmanship]

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.

Like from going under a bridge?

[Sea-Dated]

Mine always screws up, only shows 69 when I KNOW we are at least doing 90. :rofl::rofl:

[cigdaze]

Like from going under a bridge?
You bet, that'll do it sometimes.

[Perlmudder]

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.

I was sitting in class today reading this thread, and since im a GIS major, most my classes are geography. I asked the prof and he gave pretty much the exact same answer! He also said it might have to do a bit with where it is mounted in the boat.

[Beak Boater]

Craig....was that the day you had on that tin foil hat?

[OldSchool]

That's everyday Randy!!! I've got it on right now!:biggrinjester::biggrinjester:

[Trim'd Up]

Do tinfoil hats increase top speed?

[LaughingCat]

thank you Mr Wizard :D


OPA allows us up to 3 spikes in gps readings while racing to accommodate these error readings


Holy crap. i started reading the first 8 words and immediately thought "thank you Mr. Wizard." you beat me to it.

[LaughingCat]

I was sitting in class today reading this thread, and since im a GIS major, most my classes are geography. I asked the prof and he gave pretty much the exact same answer! He also said it might have to do a bit with where it is mounted in the boat.

FYI, if you peg the throttles, then throw your handheld GPS from the rear bench through that cabin, you'll add 30 MPH to your top speed.

Haven't tried it, but drinking my third Screwdriver and it sounds logical.

[ImaPoser]

Crap. Why did I sell the cruiser? It was one fast SOB.

http://holton.smugmug.com/photos/383883152_E3Wer-M.jpg

[Pure Energy]

My real time is acurate buy my max speed has given 2 false readings this season, 102 and 141.

[Ratickle]

No doubt. Back in the old days, we calculated speed by throwing a friend off of the boat and, judging by his injuries... we calculated speed. Kind of like the Fujitsu damage scales for tornado wind speed measurement.

:rofl::rofl: I guess that's how Stecz ended up looking like he does?

[glh]

It's Bush's fault....


Obama will have that fixed minutes into is reign after walking across the Potomac...

[Perlmudder]

FYI, if you peg the throttles, then throw your handheld GPS from the rear bench through that cabin, you'll add 30 MPH to your top speed.

Haven't tried it, but drinking my third Screwdriver and it sounds logical.

20 foot larson that goes 85?! i like! :sifone: