recalculate rate on GPS and Depth gauge

[Ezgoing]

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

[willk]

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!

[RichardB]

Originally Posted by Ezgoing

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.

[willk]

Originally Posted by RichardB

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???

[jyasaki]

The only time my boat sees 50 kts is on the trailer.


jky

[RichardB]

Originally Posted by willk

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!)

[Cookee]

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!

[PeterM]

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 ! )

[PeterM]

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. )

[Poly]

Originally Posted by Blackroady

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.

[PeterM]

OK - can see your logic & how mine falls apart. I was just theorising. I guess there are pages & pages of GPs theory out there . All I know for sure is it makes life a lot easier when it works & a bit harder when it doesn't .

[jyasaki]

Originally Posted by Polwart

My GPS updates itself at the same rate regardless of speed (as far as I can tell).

Sounds right. It is, I believe, more a matter of processing speed of the GPS unit than anything.

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.

The GPS unit on the boat is a receiver. The satellites just blindly output their signals. No interfacing between the two other than the unit seeing what's been transmitted.

If you are stationary the GPS software knows this and uses repreated positions to average that out.

No it doesn't. It doesn't care. It simply reads the current position over and over. If it happens to be the same (or close to the same), fine. If not, fine, as well. It may average over a fair period, but that would likely be applied the same whether you're stationary or not.

If you set a waypoint at your current position (as a reference), then track your position without moving, but zoom way in on the screen, you'll see your position bouncing all over the place. As far as the unit knows, you *are* bouncing all over. All the GPS knows is what it comes up with from the satellite signals. So, any smoothing has to be done on dozens, or hundreds of readings, not just a few. Otherwise, it would interpret any movement as error and average it back to nothing.



Off-topic:

For your coin tossing hypothesis - the probabiliy of getting two heads, or two tails is only 50%.

I think that's wrong. If my math is right (and no guarantees here) for 2 coins you have 4 possible outcomes: Heads-Heads, Heads-Tails, Tails-Heads, and Tails-Tails. That makes your odds for Heads-Heads or Tails-Tails for a 2 coin toss 25%, not 50%.



jky

[Poly]

Originally Posted by jyasaki

Off-topic:

I think that's wrong. If my math is right (and no guarantees here) for 2 coins you have 4 possible outcomes: Heads-Heads, Heads-Tails, Tails-Heads, and Tails-Tails. That makes your odds for Heads-Heads or Tails-Tails for a 2 coin toss 25%, not 50%.

jky - yes a 25% chance of getter HH, and a 25% chance of getting TT; but a 50% chance of getting either HH or TT.

[Poly]

Originally Posted by jyasaki

No it doesn't. It doesn't care. It simply reads the current position over and over. If it happens to be the same (or close to the same), fine. If not, fine, as well. It may average over a fair period, but that would likely be applied the same whether you're stationary or not.

If you set a waypoint at your current position (as a reference), then track your position without moving, but zoom way in on the screen, you'll see your position bouncing all over the place. As far as the unit knows, you *are* bouncing all over. All the GPS knows is what it comes up with from the satellite signals. So, any smoothing has to be done on dozens, or hundreds of readings, not just a few. Otherwise, it would interpret any movement as error and average it back to nothing.

The fundamental GPS reciever part of the device is doing exactly as you say - but the front end on mine certainly processes a difference between stationary and moving (and the error drops the longer you sit in one place). If it was as you suggest the error would only improve for the first few seconds. As I understand it (I am sure there is a setting for this - but the manual is so crap I cant be 100% sure that this is what it does) there is a setting where if two consecutive measurements are withing "tolerance" of each other then it is assumed to be stationary and it continues to average - but the actual location still moves as the averaged position shifts. Different companies may take a different approach to how they handle the data.

[Texel Tom]

Nearly all the current popular GPS modules utilise chipsets rated at 1Hz including the very latest SirF111 unit.

Be wary of GPS specs which claim position update rates of 10 times per second when they are using 1Hz chips as you will actually get the same position reported 10 times! These refresh rates relate to NMEA2000 bus data and will be limited by the update speed of the source data.

TT

[RichardB]

Originally Posted by Polwart

The fundamental GPS reciever part of the device is doing exactly as you say - but the front end on mine certainly processes a difference between stationary and moving (and the error drops the longer you sit in one place). If it was as you suggest the error would only improve for the first few seconds. As I understand it (I am sure there is a setting for this - but the manual is so crap I cant be 100% sure that this is what it does) there is a setting where if two consecutive measurements are withing "tolerance" of each other then it is assumed to be stationary and it continues to average - but the actual location still moves as the averaged position shifts. Different companies may take a different approach to how they handle the data.

Most of these GPS units are also going to do quite a lot of further processing with the basic positional data. For example secondary velocity data (heading and speed) are obviously derived from the sequence of positional fixes. Tertiary like VMG will then be calculated from these compared to required course data etc. It may be the case that some sort of moving averages are used to calculate this data rather than just two consecutive positions but it's impossible to tell from the manual and as far as I can tell it works fine.

You may also find that the unit stops calculating heading info below a certain speed as the accuracy drops off at slow speeds. Mine will switch to the flux gate compass data below a configurable speed.

The Garmin units (handheld at least) still have the capability of averaging a waypoint's position over a short period of time to give a better fix. Used to be interesting in the days of SA but less useful now and pointless when on the water.

All in all, they are far from simple bits of kit!

[Ezgoing]

higuys

Many thanks for your contributions.

Has anyone any ideas on the refresh rates of the Navman depth gauges?

Thks

Peter

[Nos4r2]

Originally Posted by Ezgoing

higuys

Many thanks for your contributions.

Has anyone any ideas on the refresh rates of the Navman depth gauges?

Thks

Peter

There's an easy answer to that-stick your hand on the transducer and if you can feel them,count the speed of the clicks.

[9D280]

Originally Posted by RichardB

The Garmin units (handheld at least) still have the capability of averaging a waypoint's position over a short period of time to give a better fix.

Surely they would have to - if you are walking the hills 2.5MPH is considered quite fast!