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Old 28 August 2008, 16:19   #1
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recalculate rate on GPS and Depth gauge

Hi guys

I have a Raymarine C Series and navman depth gauge.

Does anyone know how often the Raymarine recalculates your position? Same question for the navman depth gauge.


Next question. Does anyone know the distance travelled per second for the different boat speeds?


thks
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Old 28 August 2008, 16:39   #2
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Haven't a clue, but facinated as to why you need to know?? Perhaps it's something I didn't even know I needed to worry about!
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Old 28 August 2008, 17:39   #3
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Quote:
Originally Posted by Ezgoing View Post
Hi guys

I have a Raymarine C Series and navman depth gauge.

Does anyone know how often the Raymarine recalculates your position? Same question for the navman depth gauge.


Next question. Does anyone know the distance travelled per second for the different boat speeds?


thks
Not sure about Raymarine but Garmin units usually state a refresh of 1 second for most of their units. There are complicating factors if you are using a NMEA bus from GPS unit to Plotter as this may also slow transmission but if it's an integrated unit, this shouldnt factor in. Generally assuming 1 - 2 max sec is probably fine.

On boat speed the conversion equation is Speed(in kts) * 1852 / 3600 to give metres per second. So 1 kt = 0.514 m/s So obvious approximation is to divide boat speed by 2 to give speed in m/s (i.e. 10 kts = approx 5 m/s etc).

So at 50kts () your position may easily be 25 - 50 m out from the reported position on the GPS as a result of this issue. (I've noticed at least this 'lag' on some in-car GPS units.) But if you are that worried about 25m positional error on a boat due to speed factors, chances you are probably going rather too fast

No idea about the depth guage, I'd guess its of the same order.
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Old 28 August 2008, 17:44   #4
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So at 50kts () your position may easily be 25 - 50 m out from the reported position on the GPS as a result
at 50kts you're looking at the GPS???
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Old 28 August 2008, 19:18   #5
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The only time my boat sees 50 kts is on the trailer.


jky
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Old 29 August 2008, 04:47   #6
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at 50kts you're looking at the GPS???
Nah - fraid not, I can just squeeze 45 with and empty boat and fumes in the tank. 40-42 with a normal full family load!

(But 50 makes the maths easier!)
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Old 29 August 2008, 08:51   #7
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Mine does 91.57 mph, and I look at the GPS speed to alter the trim and lifter to get the maximum speed from the boat- it is on a repeater from the main GPS using NMEA.

I find the refresh rate perfectly acceptable!
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Old 29 August 2008, 09:12   #8
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Surely the slower you go the slower the GPS updates ? - the change in position is slower at slower speed therefore the rate of change of position (which is basically how GPS Updates) means is does not give such an accurate reading ( %) wise .

IN reality - the change from 1-2 knots is a 100% increase in rate of chage of position. At 50 knts to 60 knts its 20% increase - when you go quicker the 'differance' becomes a lower % and therefore more accurate .

Not sure I have explainedthat too well , but the time differance between reciept of GPS signals form the satellites is greater meaning you have a bigger & therefore more accurate number to average ?

Think of tossing a coin twice - in theory 50/50 - in practice very likely to 100/0.

If you do it 100 times far far less likley to be 100/0 & far more likely to be 50/50 .

I woudl bet it is all irrelavant as the screen you look at may not be able to update as quick as the actual GPS ' position' - especially the chart plotting ones. OH and yes I can see the GPS at 50knts ( when its calm enough for me to get there ! )
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Old 29 August 2008, 09:15   #9
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I think you can change the Raymarine sounder refresh rate - if thats what you mean ? (1-15, 1 being slow & 15 being fast - not sure what this equates to in 'real' per second readings though. )
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Old 29 August 2008, 09:44   #10
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Surely the slower you go the slower the GPS updates ? - the change in position is slower at slower speed therefore the rate of change of position (which is basically how GPS Updates) means is does not give such an accurate reading ( %) wise .

IN reality - the change from 1-2 knots is a 100% increase in rate of chage of position. At 50 knts to 60 knts its 20% increase - when you go quicker the 'differance' becomes a lower % and therefore more accurate .

Not sure I have explainedthat too well , but the time differance between reciept of GPS signals form the satellites is greater meaning you have a bigger & therefore more accurate number to average ?

Think of tossing a coin twice - in theory 50/50 - in practice very likely to 100/0.

If you do it 100 times far far less likley to be 100/0 & far more likely to be 50/50 .

I woudl bet it is all irrelavant as the screen you look at may not be able to update as quick as the actual GPS ' position' - especially the chart plotting ones. OH and yes I can see the GPS at 50knts ( when its calm enough for me to get there ! )
I think you are wrong!

My GPS updates itself at the same rate regardless of speed (as far as I can tell). The rate at which the boat recieves information from the satellite has nothing to do with its speed. The satelite doesn't even know the boat exists.

Your %age error in speed doesn't make sense. The GPS doesn't measure speed, it measures position. It calculates speed based on two positions at a known time interval. You are correct though at low speed the GPS will have bigger %age errors on the speed but not for the reason you said.

If you are stationary the GPS software knows this and uses repreated positions to average that out. On my Lowrance handheld unit you can clearly see that the EPE (estimated position error) gets lower (better) when it sits in one spot for a little while. [Now given that no two readings are actually identical - how does it know if you are stationary or just moving really slowly? there is a setting somewhere in the software which tells it to treat two measurements within a certain range as being stationary]

For your coin tossing hypothesis - the probabiliy of getting two heads, or two tails is only 50%. It depends if you call that "very likely" or not. Its exactly the same chance as getting one head and one tail.

Finally A good plotter will have a screen refresh rate of 15 fps or better (15 fps is almost a smooth cursor movement). Which would be perfectly adequate for keeping up with a 50 knot speed on a sensible scale.
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