GPS Speedometers

Electrical and electronic topics for small boats
oysterman
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Joined: Fri Oct 23, 2015 9:37 pm

GPS Speedometers

Postby oysterman » Tue Jul 25, 2017 1:53 pm

My neighbor has two boats equipped with [GPS speedometers]. They seem to work okay.

[Give me your] review [of a GPS Speedometer] before I decide to get one or not.

jimh
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Location: Michigan, Lower Peninsula
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Re: GPS Speedometers

Postby jimh » Tue Jul 25, 2017 2:23 pm

For the last three decades all boats I have owned have had a GNSS receiver that can receive the U.S. Air Force GPS constellation. I don't notice any anomalies with GPS and its calculation of speed over ground. Almost every marine GNSS receiver can calculate and display speed-over-ground, so I don't know that one must have a dedicated speedometer gauge to display it. If you already have a chart plotter with a GNSS receiver, you already have a "GPS speedometer."

Note that most marine GNSS receivers do not measure speed, they only measure time delays and from that they deduce position. Speed is calculated from sequential position solutions using the distance between positions and time. A GNSS receiver is extraordinarily accurate with time measurement, so it can deduce speed from position solutions that are separated only by a short amount of time.

If you want a GNSS receiver that can actually measure speed, you need to look for a very specialized receiver that can use Doppler frequency shift measurement of the L1 carrier signal. Such GNSS devices are usually sold into the market for high-performance marine speed measurements, and you won't find them in a run-of-the-mill GNSS receiver.

For most purposes, the accuracy of boat speed over ground from GPS with WAAS augmentation is sufficient; the calculated speed will be accurate to about 0.4-MPH.

Dr T
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Joined: Fri Aug 26, 2016 6:38 pm

Re: GPS Speedometers

Postby Dr T » Tue Jul 25, 2017 5:48 pm

I would only add that any sonar (fishfinder) with a GPS has a GPS speedometer. (Mine is a Hummingbird 788ci HD).

PatSea
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Re: GPS Speedometers

Postby PatSea » Wed Jul 26, 2017 8:54 am

jimh, you bring up a point that I have been thinking about. The accuracy of GPS speed of 0.4mph. I like to fish for walleye by trolling. The desired speeds are in the 1 to 2.5mph range, but we like to dial it in within about 0.2mph. I have argued with some of my fishing buddies that it is impossible to do with GPS because the accuracy of GPS is only about 0.4mph. Comments.

Jefecinco
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Location: Gulf Shores, AL

Re: GPS Speedometers

Postby Jefecinco » Wed Jul 26, 2017 9:15 am

For the purposes of trolling at very low speeds I don't believe any electronic device will provide accuracy of 0.2 MPH. The desired accuracy is made even more difficult because the GPS provides "speed over the ground" as opposed to speed over the water and the two speeds can be wildly divergent depending upon tides and other factors.

Years ago I saw a handheld device which used a floating ball in a clear plastic tube with speed markings on the tube. The tube was "L" shaped and the short dimension was to be placed in the water with the opening pointed forward. The pressure of the water pushing into the tube would cause the ball to move in the tube. The position of the ball indicated speed over the water. They were very inexpensive.

I haven't seen one in decades but perhaps such a device would meet your requirements far better than an expensive electronic device.
Butch

Dr T
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Joined: Fri Aug 26, 2016 6:38 pm

Re: GPS Speedometers

Postby Dr T » Wed Jul 26, 2017 12:03 pm

The plastic device using water pressure is called a pitot tube when it is used in an airplane and uses air pressure. Don't know if the terminology changes for maritime use.

When you start looking at such small measurements, you are in Heisenbug territory: The increment that you want to measure is less than your ability to resolve the measurement. Such errors are inherent in any man-made measurement system. Don't sweat the details, relax, and catch fish.

Dr T
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Re: GPS Speedometers

Postby Dr T » Wed Jul 26, 2017 12:32 pm

Just to beat the dead horse a bit more. The 0.4 mph resolution accuracy is related to two things: The 5 second update rate of most commercial devices and the 1 meter accuracy of GPS. In 5 seconds, at 0.4 mph you will move about 35.1 inches in 5 seconds, and 35.1 inches is pretty close to 1 meter (39 inches or so).

Jefecinco
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Location: Gulf Shores, AL

Re: GPS Speedometers

Postby Jefecinco » Wed Jul 26, 2017 7:39 pm

Dr T,

Yes, the technical name for the hand held device I mentioned may be "pitot tube" but it is far from the devices we use on boats for speed over the water. The device had a tube diameter of perhaps 3/4 of an inch.

I'll repeat that a GPS, regards of update rate, is virtually useless to determine speed over the water in most conditions.

I agree that the best course of action for the OP is to forget about accurate speed readings and enjoy the fishing.
Butch