What is correct, Speedo or sat nav?

jase1

Strider590 wrote: »

Well the faster they travel compared to earth, the lower the orbit, but it's only our perception of speed relative to us. A satellite in higher orbit doesn't over take a satellite in low orbit, it's position in the sky is the same but it simply travels faster to keep up (relative to earth).

A satellite in higher orbit will be "overtaken" by one in lower orbit. Geostationary satellites keep the same position relative to the Earth's surface, the moon takes longer as it is further away. A GPS satellite is about twice as close as a geostationary satellite, takes half the time to orbit (exactly 12 hours) and is travelling somewhat faster than the geostationary satellite.

GPS satellites aren't actually geostationary at all.

Lum

Also worth noting that the yanks can turn off the GPS system globally, or in select areas, at their discretion. Another reason why GPS isn't allowed as your only speedo.


It's still more accurate when used correctly though.

Strider590

Oh and GPS is unaffected by cloud, rain, snow or fog..... It can be affected however by water more than a mm (or so) deep surrounding it's antenna... Which is why portables need to be kept dry.

GPS frequency was specifically selected for this reason.

Higher wavelengths in the EM spectrum, travel through such "obstacles" fairly unhindered....... Coming right down into the visible spectrum, this is why yellow lights are better in foggy conditions (Used in France) and why your 21 watt tail lights are as bright in severe fog as your 55 watt headlights.

Sat-nav also does not recieve "data" from GPS satellites ("hello, you are here at XYZ"), it's calculations are based on it's distance between as many satellites as it can "see", which as I mentioned are set distance apart and traveling at an exact pre-determined speed of 3.87km/s.

epninety

Lum wrote: »

As it happened, it was over 10 years old so didn't need to go through IVA, and thanks to a !!!! up by the MOT tester (reported the mileage as 28,000 miles not 28,000 km) the DVLA never asked for proof of conversion and I was able to do it after registration using this thing which is a legal way of displaying your speed.

And I wouldn't ever fit a borrowed set of different sized wheels and tyres to a Land Rover to get the speedo to pass SVA/IVA... oh no, not me :rotfl:

(Actually, my current speedo can be calibrated from the dashboard, either over a measured mile, or by direct setting, so it isn't a problem for me.)

adouglasmhor

birkee wrote: »

Linked to the vehicle means just that. Whatever the link method. Mechanical or electronic detector. The vehicle speed is what determines what the speed readout is on the indicator. Whether it is mechanical or digital.

Turn off all the satellites, and see what speed readings you get.
Satellites are detached from the vehicle, therefore they cannot indicate your speed, particularly (as pointed out earlier) in tunnels and other situations.
Or perhaps, you think you can drive through the Mersey Tunnel at 100mph, because your sat nav isn't reading your speed?

An electronic or the earlier eddy current speedometer is not "linked" to the vehicle though is it? You just made that up.

And I have allready mentioned tunnels twice now.

jase1

> Sat-nav also does not recieve "data" from GPS satellites

Well it does, just not information that can be used by itself to directly infer anything.

epninety

Strider590 wrote: »

Oh and GPS is unaffected by cloud, rain, snow or fog..... It can be affected however by water more than a mm (or so) deep surrounding it's antenna... Which is why portables need to be kept dry.

Not quite true - signal levels drop significantly in foggy weather. The signal is also distorted by long paths through the atmosphere and ionosphere, and by ground reflections at the horizon - which is why your GPS will try to ignore satellites close to the horizon, as their timing accuracy is degraded.

Strider590 wrote: »

Sat-nav also does not recieve "data" from GPS satellites ("hello, you are here at XYZ"), it's calculations are based on it's distance between as many satellites as it can "see", which as I mentioned are set distance apart and traveling at an exact pre-determined speed of 3.87km/s.

It does not receive 'personalised' data, however it does receive identity and time information from each satellite, updated 'almanac' data (describing the latest orbit corrections for each available satellite), and health information for each satellite. Since it knows something about the positon of each satellite, it solves the system of equations describing the signal delay from each satellite to arrive at a solution for both position and current time.

Lum

epninety wrote: »

And I wouldn't ever fit a borrowed set of different sized wheels and tyres to a Land Rover to get the speedo to pass SVA/IVA... oh no, not me :rotfl:

Ok now that is an awesome way of doing it! Might require some wheelarch modifications to get it to work on my car though!

Strider590

epninety wrote: »

Not quite true - signal levels drop significantly in foggy weather. The signal is also distorted by long paths through the atmosphere and ionosphere, and by ground reflections at the horizon - which is why your GPS will try to ignore satellites close to the horizon, as their timing accuracy is degraded.

http://gpsinformation.net/gpsclouds.htm

cyclonebri1

birkee wrote: »

The point you missed, was that I KNOW speeding is speeding.
You also missed the point that a speed indicator must meet certain criteria as required by law. Whether they are checked or not, doesn't negate that law. They clearly should be tested, to pass the MOT. Also, it must be a speed INDICATOR, not an overall speed calculator as in a satnav.

However the point you missed is that you did not acknoledge the point I made. Speeding is speeding no argument, measure it everywhich way you like it's either speeding or it isn't??

But they aren't, which to me indicates they are not essential.

There are purpose built satelite derived speedos available.
My knowledge ends when sva became repugnant so .......... it does not matter how you measure it, so long as its accurate.

theory or practice?? yours and my choice;)