GPS errors

[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


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.

 

[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: