(T) sat-nav no reception

having just praised my abarth for having no probs so far, got in it this morning and the sat-nav isnt getting a signal. is this normal due to weather conditions etc, has anyone else had this, will if fix itself, answers on a postcard please.

Well the system will likely use satellites to get its bearings, it will need at least 2 to triangulate. Could be quite possible that there are currently none in range.

Hang on, now don't get me wrong, I am not a know it all on GPS, however, I do understand triangulation.....it requires 3 signals I do believe to work, right?

If you think of it graphically in 2 dimensions, sure, 2 points work, however, as the globe isn't flat, you need a third signal for the third dimension. Right?

OT anyway.

DOG; is it permament or just in a certain place? Have you checked your receiver (I assume aerial on roof) isn't damaged etc.?

hmm yeah you could be right! I remember reading that with mobile phones you need 2, but signal strength comes into play then.

i was back out this afternoon and it was fine. must have been weather or satellite or something. hopefully just a glitch.

Probably, wouldn't worry unless it happens a few more times.

You only need 2 sats to get a rough position, you need 3 to get the height data, and obviously the more sats the better. even with the 3 or more sats the height data will still be messed up, (IIRC due to the US military not wanting 100% accuracy on civilian equipment) I used to work with fairly highend Differentially corrected GPS stuff (accurate to 1cm) and even it could still have dodgy height.
Obviously you need line of sight to the sats though, even tree's/leaves can get in the way.

MiniG

Hang on, visualise it, 1 signal - you get a surface, 2 signals you get a line, only with three signals can you actually get a point and make it work properly (if in doubt, try it, two fixed points with two fixed lengths of say paper) do it 3-dimensionally. You get a curve which could me you're either at point x either side, higher up, but who's to say that the minimum point is the position?

If you have worked in the field, please explain why my mode of thought is wrong.

I've noticed my SatNav does this occaisonally - I only noticed when the Nav software started telling me to turn right on the motorway because it thought I was actually on the road 200m parallel to the motorway!

I found that power cycling the satnav clears the problem - looks like a software bug to me. It appears that when it loses the sattelite it uses other car telemetry (and a built in compass) to guesstimate where you are - after a while the inaccuracies build up and it thinks your in a different location to where you really are. Useful feature in the Dartford tunnel though - map shows you dissapearing under water in the tunnel :-)

Just for the record - I've never seen mine show less than 5 satellites on the status screen, normally its 6,7 or 8!

BTW I am running 9.8 software

Paul

what do you mean by power cycling pjp

By 'power cycling' I mean simply press the on-off button to turn it off - then turn it back on so it reboots the software and hey presto it works again.

Paul

thanks m8 but touch wood it hasnt happened again.

Since sat nav is basically a GPS system tied to a map it actually takes 4 satellites.

http://www.nasm.si.edu/exhibitions/gps/work.html