OT Sat Nav

[ARWadsworth]

Now that is interesting. And the Steady state speed between the two?

[Alan]

In message , "dennis@home" wrote

Get real, I use Satnav for areas not local to me and therefore don't necessarily know exactly where I passed by. On a long journey I'm hardly likely to stop every 15 minutes to correct a map.

[ARWadsworth]

Who would want a map that dennise has uploaded? He cannot tell his arse from his elbow.

Thats what the SatNav is for. I actually use mine for journeys that I do know as the TraficMaster kicks in faster than radio reports.

[Andrew Gabriel]

I have a 5 year old Garmin nuvi 360. Although there are too many things about it which I don't like, it's battery life seems to be many hours - I'd guess about 4, but I've rarely run it completely flat.

If I had liked it, I was going to fit a permament USB power connector for it in the car, rather than faffing around with the cig lighter one. However, I'm really waiting for it to die or its maps to get so out of date that I can justify replacing it with one I like instead.

[dennis]

You don't have to, you press the button and it remembers where you are so you can correct it later. Of course if you never intend to go there again you don't need to, and you wouldn't want to download any corrections made by others would you.

[Andy Champ]

That's very surprising. I shall have to try some time.

Andy

[Andy Burns]

I thought that rather than calculating velocity using time difference and position difference, (some?) GPS receivers calculated velocity directly using doppler shift of the satellite signals and the known satellite positions/velocities?

[Mike Lane]

Andy Burns wrote on Aug 29, 2011:

It's a little more complicated than just doppler shift, I think, although that certainly comes into it.

I don't pretend to understand the maths behind gps calculations, but AFAIK the gps is continually solving a set of simultaneous equations for each of four satellite signals received, with the distance of the receiver from each satellite, the velocity of the receiver, and the time, as the unknowns. It does this by an iterative process, and what comes out at the end of the calculation is the precise time, position, and velocity of the receiver.

[Mike Barnes]

I don't know - I wasn't driving at a steady speed. So I expected a little variation, but the consistently higher reading of one was a surprise. FYI the car's built-in Trafficmaster Smartnav consistently showed 1-2 mph faster than the TomTom on the top of the dashboard.

[Ron Lowe]

So did you push her off, because she was inconveniencing you? Were you in China a couple of years ago?

[Ron Lowe]

Update the maps.

gpsunderground is your friend.

[Nick Leverton]

xxxxxxx approximate

HTH

Nick

[John Rumm]

It does not seem to have made a difference in this case. I download the full update set every so often - so, new maps and software, and it still does not hack this junction.

[John Rumm]

Indeed - I was not defending the accuracy of the speedos they choose to fit...

It might even be that the makers have decided that the combination of speedo error, and the prosecution tollerances used for most speed traps (i.e. 10% + 2mph) means most folks can drive about, speed at a "reasonable" amount and never get done!

[damduck-egg]

bet you had one set with a male voice and the other with a female.

G.Harman

[John Rumm]

Might it be the built in one is deliberately over reading to make it more consistent with the speedo? Not something the maker of a third party device needs consider, but possibly something a car maker might want to take into account when providing two instruments to do the same job in the car.

[polygonum]

Maybe because it is built-in it has to conform to the same regulations as the speedo. Whereas, as you say, the TomTom needn't bother.

[Mike Barnes]

Nice try but Trafficmaster Smartnav *is* a third-party device, though unlike a TomTom etc it's fitted to the car by the owner and is not portable. It's still model-independent, though, and I think it's unlikely that there's a link to the vehicle's normal speed-measurement system.

[Mike Lane]

Nick Leverton wrote on Aug 29, 2011:

OK, I should have said 'accurate'.

[Ret.]

Hmmmm. The distance around the wheel and tyre will still be the same no matter how compressed the tyre tread is at the point where it touches the road surface...