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SanderO 08-12-2019 08:58 AM

GPS determined speed
 
I happened to notice that my old Ray C80 and a newer, but old B&G Zeus T7 were not reporting identical SOG yesterday. They were never more than .1 knots different... but what do I attribute this to?


old technology in the C 80?
rounding algorithm?

I have several other GPS including smart phone which I did not check. The difference is small and of no consequence...

Just curious... any thoughts?

BarryL 08-12-2019 09:19 AM

Re: GPS determined speed
 
Hey,

GPS speed (and course) is based on a few things:
-How often the gps receiver updates (some old unit only receive 1 update per second (1 HZ), newer units can receive 10 updates / second)
-How much filtering is done (both horizontal and vertical)
-How often the display is updated.

GPS speed is your HISTORICAL speed - is how quickly you got to your CURRENT position from your PAST position. I don't believe any gps is accurate enough to provide .1 kt difference AT THIS EXACT MOMENT.

Really .1kt difference is the same speed. At any given time the speed reported by my B&G Vulcan, Garmin GPSMAP 76, and Iphone running Navionics will be slightly different. It's no big deal.

Barry

JimsCAL 08-12-2019 09:24 AM

Re: GPS determined speed
 
My guess would be sampling times and averaging used by each. GPS position measurements are constantly bouncing around and need to be averaged and smoothed.

pdqaltair 08-12-2019 09:44 AM

Re: GPS determined speed
 
Really, it is a simple matter of understanding how GPS works. It calculates where you are, and that calculation has some error, on the order of 3 meters. So obviously, there will be some variation in instantaneous speed. In fact, in order to be relatively certain of 0.1 knot accuracy (1%) you would need to average ~ 1000 feet. It does much better than that, for a number of reasons, but over just a few seconds there can easily be a few tenths disagreement.

We expect miracles. 0.1 knot based on a time delay from a satellite. Not happening. I though GPS was a miracle when it just gave lon/lat within a few hundred feet.

SanderO 08-12-2019 09:50 AM

Re: GPS determined speed
 
Thanks for the replies which pretty much confirm that SOG is not terribly accurate and completely useless for trimming sails.

On the other hand my B&G has supposed accuracy to .01 knots, and you can dampen to display. Forget the sample rate... but you CAN trim sails using the speed thru the water... even with a fouled bottom or transducer.

Next time I am aboard and in the harbor at slack (high) I will conduct some tests of my various speed reporters. I will need to get them all reporting the same speed and calibrate the speedo as necessary.

denverd0n 08-12-2019 10:28 AM

Re: GPS determined speed
 
Well, I wouldn't say that SOG from a GPS is "not terribly accurate." For what it is, and understanding how it is determined, it is quite remarkably accurate.

On the other hand, I agree that it is not a great tool for adjusting sail trim. For that a traditional speed log is much better.

SloopJonB 08-12-2019 10:52 AM

Re: GPS determined speed
 
I recently heard that automobile speedometers are changing over to GPS from physical sensors on the car.

I suspect that means GPS is pretty accurate.

pdqaltair 08-12-2019 11:45 AM

Re: GPS determined speed
 
Quote:

Originally Posted by denverd0n (Post 2051619658)
Well, I wouldn't say that SOG from a GPS is "not terribly accurate." For what it is, and understanding how it is determined, it is quite remarkably accurate.

On the other hand, I agree that it is not a great tool for adjusting sail trim. For that a traditional speed log is much better.

Maybe. But speed logs are notorious for fouling. And even if perfectly clean, the wind oscillates and fluctuates and sea state makes a difference. A trial horse, or even a similar boat on a close, parallel course is usually far more telling. And though it will look and feel like you are racing, you are really just learning your boat.

Targets are good, but only as a reasonableness check, because the computer does not know all of the variables.

SanderO 08-12-2019 12:51 PM

Re: GPS determined speed
 
Quote:

Originally Posted by SloopJonB (Post 2051619660)
I recently heard that automobile speedometers are changing over to GPS from physical sensors on the car.

I suspect that means GPS is pretty accurate.

at best a car speedometer show accuracy/precision to integers.

Most boat speedo do .1 knot. My B&G dies .01

Precision is perhaps a better word... GPS appears to be only precise to .1 knots.

SanderO 08-12-2019 12:53 PM

Re: GPS determined speed
 
Quote:

Originally Posted by pdqaltair (Post 2051619668)
Maybe. But speed logs are notorious for fouling. And even if perfectly clean, the wind oscillates and fluctuates and sea state makes a difference. A trial horse, or even a similar boat on a close, parallel course is usually far more telling. And though it will look and feel like you are racing, you are really just learning your boat.

Targets are good, but only as a reasonableness check, because the computer does not know all of the variables.

I want to use GPS on a motored run in calm conditions with no current to calibrate the speedo. May not be possible.


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