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Old 29 August 2008, 10:33   #11
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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 .
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Old 29 August 2008, 12:10   #12
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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.


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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.


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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.



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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%.



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Old 29 August 2008, 15:21   #13
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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.
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Old 29 August 2008, 15:29   #14
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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.
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Old 29 August 2008, 17:01   #15
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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
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Old 29 August 2008, 18:15   #16
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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!
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Old 02 September 2008, 17:00   #17
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good dscussion

higuys

Many thanks for your contributions.

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

Thks

Peter
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Old 03 September 2008, 03:51   #18
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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.
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Old 03 September 2008, 11:15   #19
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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!
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