Basic principles of GPS

[HandyMac]

I'm asking this question out of simple curiousity.

I know that TomTom and other GPS systems need to acquire signal from 3+ satellites in order to work, but I'm curious as to what methodology is used to calculate the position to within a few metres? It all seems rather accurate to me!

The satellites are obviously hundreds of miles up in the sky, and potentially thousands of miles away from the GPS systems which are using their signal.

I know this isn't two-way communication between the GPS unit and the satellites whizzing around above, so clearly the satellites must be sending out standard signals similar to the way that a radio station does. But I wonder what the satellites are sending? Is it their own position in the sky, a standard time signal from which delays can be measured? Or something else?

Just curious.

Andrew

[Beefy]

Try this guide from Garmin for a full explanation of how GPS works in laymans terms:

http://www.garmin.com/manuals/GPSGuideforBeginners_Manual.pdf


_________________
Beefy
High Quality Pubs - http://www.pocketgpspoi.com/reviews/tomtompoi/High_Quality_Pubs.zip
TomTom ONE

[HandyMac]

Thanks Beefy - that was exactly the sort of explanation I was looking for.

Andrew

[Hunnymonster]

For those not willing or able to wade through the whole technical guff, it boils down to solving equations for 4 variables.....

In this case the variables being time, latitude, longitude and altitude - recalling my O-level algebra, 4 variables means you need (at least) 4 equations to solve (hence 4 satellites) - though with additional knowledge (altitude should be >0 and <10000m for instance) it's usually possible to discount one of the 2 solutions offered with 3 equations(satellites) to get a close approximation to a solution(position).

Crudely it's a triangulation excercise from your position on the ground to the satellites in the sky - hence why sometimes with a limited view of the sky the position can be a bit flaky (the base of the triangle is small) and sometimes with a 'big sky' view it's excellent (the 4 sats are well spaced across the sky)

[Dave]

We also have several thorough articles on how GPS works and accuracy:-

http://www.pocketgpsworld.com/howgpsworks.php
http://www.pocketgpsworld.com/egnos.php

We see you’re using an ad-blocker. We’re fine with that and won’t stop you visiting the site.
Have you considered making a donation towards website running costs?.
Or you could disable your ad-blocker for this site. We think you’ll find our adverts are not overbearing!