speeds , fact and fiction ?

It used to be a standing joke in the late 70's and 80s that Ford speedos always overread by pretty much 10%, I know a few people who were pulled over by Plod for doing silly speeds and thought they were "lucky" that Plod said they were going much slower than the speedo said.
 
To get back to the subject of exaggerating speeds: I saw 13.9 knots on the gps last time I was out and as far as I am concerned that's nearly 15! :D
 
benjenbav said:
To get back to the subject of exaggerating speeds: I saw 13.9 knots on the gps last time I was out and as far as I am concerned that's nearly 15! :D
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Thank you Martin, with most of your posts you manage to make me laugh out loud

I guess that was wind WITH tide, blowin' a eight? and downhill.

ps. where do you get the smilies on this new format?
 
Thank you Martin, with most of your posts you manage to make me laugh out loud

I guess that was wind WITH tide, blowin' a eight? and downhill.

ps. where do you get the smilies on this new format?
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Too kind!

For me the smilies are to the right of the box where I am typing this. I did change the settings in "User cp" a few weeks ago but I can't remember what I changed.
 
Kipper said:
Because its a Golf, and it has been clocked by road side traps and its cock on every time. Last week it recorded 40mph on the sppedo and the roadside trap but tomy said much less, trust me she's not mad.
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Since it's measuring the time an object takes to get from one position to another, surely a GPS will only measure speed accurately when you're travelling in a straight line?

(Disclaimer: I nearly failed "O" Level Maths. And Physics.)
 
The gps is responding to the time it takes for the signal to get to you (boat)from a particular satellite, as it is receiving signals from several satellites at different locations in the sky suspect it somehow compares the different time signals to achieve a fix by timing the doppler effect and coming up with your speed over the ground irrespective of your heading or turning rate.As you get closer to one satellite you will be getting further away from a second ?
Presume the GPS must have its own internal clock to compare the delay in the received signals and that the existing satellites are geo stationary.
The new EU system,if memory serves is not geostationary and can give a more accurate fix in the polar regions.
 
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it's not irrespective of speed or turning rate. A gps system has algorithms to average out readings over a short period (in a car, the gps has to cope with you going under bridges, through areas with high buildings etc etc) so turning suddenly at speed, with no other input, can throw off the speed and direction readings. However, in a straight or straightish line, with good satellite visibility, which you will usually get at sea, compared to in a city, and at a reasonably constant speed, then the speed reading is very accurate.
 
BrendanS said:
in a car, the gps has to cope with you going under bridges, through areas with high buildings etc etc
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Yes and no, Brendan.
Car systems (I mean, not TomTom and the likes, but the more sophisticated navigation systems offered as OEM options) do not rely just on GPS input.
There are also an electronic compass, the car speedo and the cartography constraints: all of them feed these navigation systems.
That's why they can immediately understand if your car is turning left or right when starting from a stop, and why they still keep track of your position even in tunnels.
No gps alone could handle that, regardless of how sophisticated its algorithms can be.
For boats and planes, obviously the gps alone is more than good enough.
 
I was talking about standalone systems like Tomtoms, not those that have input from the car, and also talking very generally about the constraints. It's always possible to go further and more detailed in an explanation, but then it becomes very long, and not so understandable to people who may not know how the things operate in detail. It's a fine balancing act, as whippersnapper and I were discussing recently about DNA databasing, and where to draw the line in making the information easily understood by all, or delving into huge technical detail, so as not to be challenged by anyone that does understand the subject in detail
 
A GPS reading can be anything you want, especially with older sets. It's ok after an hour or so, but on instant read out it's rubbish. It's forever catching up, or holding back. I've had 90 knots out of it on occasions.
 
MapisM said:
Yes and no, Brendan.
Car systems (I mean, not TomTom and the likes, but the more sophisticated navigation systems offered as OEM options) do not rely just on GPS input.
There are also an electronic compass, the car speedo and the cartography constraints: all of them feed these navigation systems.
I did wonder how Jessica does it: going round a roundabout, she will sometimes say "Take the exit NOW", which on a small roundabout can't possibly be based on position alone.
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that's down to cartography on the whole, if a standalone unit, with no car input. Can be thrown completely out of kilter if the map is out of date, or no good satellite signal
 
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