Accuracy of GPS Speed Below 3-MPH

Articles about GPS, GLONASS, GALILEO, WAAS and other satellite navigation systems
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Accuracy of GPS Speed Below 3-MPH

Postby PatSea » Thu Aug 24, 2017 8:55 am

What is the accuracy of the GPS speed at very low speeds like 1 to 3 mph? I enjoy walleye fishing and most of the time we troll with a desired speed range (through the water) from about 1.0 to 3.0 mph depending on many factors such as lure being used and water temperature. We use our gps units to determine our speed and to control our speeds to within +- 0.2 mph. But I have always maintained that the gps speed is only accurate to approximately 0.4 mph based on reading I have done. Anyone care to expound? Thank you.

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Re: SOG v. STW

Postby Jefecinco » Thu Aug 24, 2017 11:27 am


I seem to remember an earlier discussion on this subject.

Since GPS can only deduce speed over the ground (SOG) as opposed to speed through the water (STW) any reading of speed at idle would seem to require some averaging over a few minutes. Even then, unless there is absolutely no wind or current, the GPS readings would be essentially meaningless.

Dropping a leaf in the water and seeing how long it takes to float away may provide better certainty of speed through the water. Experience will soon tell you if your speed is optimum for catching Walleye. Try idle speed, no luck, try a little more, etc.

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Re: Accuracy of GPS Speed Below 3-MPH

Postby jimh » Fri Aug 25, 2017 9:20 am

Most GPS receivers do not measure speed. They report a calculated speed based on the distance and time in traveling between two positions. Since a GPS receiver has very precise time data, most of the error in speed calculation comes from the error in position solution.

The error in position solution is affected by many elements. Use of precision augmentation like the FAA WAAS will improve accuracy of position. Accuracy of position will also improve with favorable geometry of the satellites with respect to your location; this parameter is provided as the horizontal dilution of precision (HDOP). Instances with a low metric for HDOP ( HDOP near 1.0) will improve accuracy.

I don't know of a study that specifically looked at speeds below 3-MPH. Based on the method of GPS position finding, it seem reasonable to infer that as actual speed approaches zero, the percentage error in GPS speed must increase.

For someone with interest in the topic, I propose an experiment: record the speed calculation reported by a GPS that is in a fixed position; record about 100 speed measurement; add all the reported speeds together and divide the sum by 100; this may be an average error for very low speed of movement.

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Re: Accuracy of GPS Speed Below 3-MPH

Postby jimp » Fri Aug 25, 2017 11:17 am

North Pass, Juneau, Alaska
image3.jpeg (84 KiB) Viewed 7252 times
Interesting question.

I think it is fairly accurate, but as Butch mentions it is speed over ground, not through the water. I adjust engine speed a tiny bit and it shows on the GPS. It shows increases of 0,1 knots at times. Accurate? Don’t know, but seems right.

I trolled my old 1990 Revenge 22-W.T with a 9.9 kicker just above idle. I needed speeds between 1.7 knots (2.0 mph) and 2.0 knots (2.3 mph) while trolling for different types of salmon. Kings (chinook) needed a slower speed, silvers (coho) faster. But sometimes while trolling along the shore at slow speed, the shore was ripping by quickly, speeds showing was 2.5 to 3 with the current. Going against it the speed sometimes dropped to 0.

The final determination to how fast I was going was determined by my trolled whole herring baits. Kings like a slow rolling bait, silvers a faster rolling bait. If I was trolling for kings and my bait was spinning too fast, then I had to slow down more. If trolling for silvers and the bait had a very slow roll, speed up.

Sometimes the fish are there. Sometimes they’re not. Sometimes there’s a feeding frenzy by the boat and they hit your bait as soon as you drop it in the water and you don’t even have a chance to put the rod in the holder. Other times you go home with a clean boat as you got skunked.

Don’t over think the problem.