[StuJones]

Its all about the mechanics (or science) of GPS.

GPS navigation is based on the triangulation of signals from a minimum of three satelittes 20,000km away. There are 24 satellites in three orbit planes that cycle twice a day. So introduce buildings and signal loss and things go a bit awry! Also depending on the time of day, satellite location, etc you can be working with signal interferance from horizon, heat haze, etc.

Then there is the accuracy thing. Many vendors will quote +/-15m, 10m, etc but if you read in to the details they all declare an additional +/-100m for the United States Department of Defense-imposed Selective Availability Program. This is the overlying error code the USD impose so that only they have the accurate stuff. So yes its true that in a sequence of measurements they are +/-15m accurate but in relation to map position and repeatable could be up to +/-100m. During military opperations this Selective Availability can be up to km's for 30sec's or more!!!

So how does in car GPS seem to be more accurate that +/-100m, well the smarts in the systems make up for errors in signal in open areas by using dead reckoning algorithms. So if you are heading North at 50km per hour and loose signal the system 'estimates' position for a while until it re-engages. In the city you turn corners, etc and its lost signal or has poor signal strength then the dead reckoning system gets 'lost'. The really accurate systems have digital gyro's to account for this, dont think BMW/Mini have this?

So thats the technical on GPS. Great tool, but not always on the mark

Stu

[Todd!]

Thanks for that interesting insight Stu...

I take it you work in some form of satellite tracking or mapping company?? Or just an interest for you?

Its interesting how only the USD systems are accurate!!

Then again the car isnt used to guide missiles is it??

Todd

[StuJones]

The US Defence designed, built and paid for the system. We just get to use it for free as the signal is there anyway. So their view is that they can do what they want with it. The funny thing is that now in the US there are so many companies using it for their business (trucking, couriers, taxi, etc) that they are now lobbying the govt to release the error codes.

US military know their 'enemies' have GPS on their weapons systems (heck they probably sold them the stuff!!) so that when they want to use it they change the error codes to what they want.

[warped]

Stu gives a very good description on Sat Nav however most cars have advanced onboard mechanisms to compensate for the error code ... most vehicle Sat Nav systems have onboard electronic gyroscopes that are fairly accurate in maintaining your location even when you change direction repeatedly ... I have seen one manufacturer even claim that they only need the satellites to establish an initial location, onboard systems are accurate enough to determine location without satellite assistance

The MINI system has a gyro and is also aware of which gear you have selected to assist in dead reckoning ... from my observation the MINI system also stores what satellites it should 'expect' to see based on its current global position ... turning the system on from a known location satellites are locked in less than 10 secs ... turning the system on in an unknown location (like being on a boat to Tasmania for 20 hours ) means the system takes at least 2-3 minutes to establish its location

And to answer your original question Tim ... my system has only ever done this once ... other than that one time where the system thought I was about 100m into the harbour instead of on the bridge I have never had a problem in the CBD ... based on my experience your system does not appear to getting a decent lock ... how many satellites do you typically pickup? Typically I see 6-7 but never less than 5

[enzothemaddog]

I know its not a Mini Nav system, but I use a Navman Bluetooth GPS receiver linked with a HP 5500 Pocket PC. Although I have a freestanding GPS I can always seem to pick up more than 3 satellites (5-7) yet inner CBD is always terrible, mind you its understandable with the multiple 50 storey buildings in the city.
If you have a really bad spot, I could do a satellite response test with the GPS to test it (signal strength).
Everyone just remeber that the GPS network is a US military owned/controlled network, so if it goes out for more than five minutes it probably means theres a security scare somewhere around the world. (GPS was shut off for 12 hours after 9/11 - worldwide)

Quick fact***: I couldnt buy Nordost cabling for my stereo for about a month after 9/11 because it has a defence contract with NASA (they control everything)

[robbo mcs]

Hi,

I have had only very minor dropouts in the city in my other cars. However, ocaasionally the system goes haywire in random locations. I was once on the cumberland highway, and it told me my altitude was -200m

I have the BMW CD (not DVD systems) and find them frustratingly slow at calculating routes, especially recalculations if you miss a turn etc

Cheers

Robbo

[StuJones]

elevation errors in GPS are the significantly more of the positionings. some of it is due to the difficulty in calculating height from long distance radio signals.

Also the other issue is that GPS works on the earth being a perfect sphere (which it definately is not) which then leads to a thing called geoid-spheriod seperation. There is about 40m difference across the country. Add to this gravity anomolies caused by high magnetic ground deposits.

Key thing is more satelites at near vertical to your position the better, as they drift to the horizon the worse it gets. Two can give you a quick fix, three provide redundancy and the more you get after this just adds to the accuracy.

Here endeth the lesson

[StuJones]

as a post script to all the technical crap!! I think that despite the occaisional mis-direction I was very impressed with both Warped's and Ozmini's GPS on the Tassie trip. Definately saved the day a few times!!

[IML337]

Thanks for all the info guys, after making my post the other day I thought about it and realised that expecting 100% accuracy out of a GPS system in the CBD is a bit of an ask... I guess that coupled with all the other crap I've had to deal with from my nav system (two screens replaced, three computers replaced, loss of sat connectivity for days on end and other issues, even after all the replacements) I was a bit angry at the time.

Quote: Originally Posted by robbo mcs

I have the BMW CD (not DVD systems) and find them frustratingly slow at calculating routes, especially recalculations if you miss a turn etc

frustrating isn't the bloody word for it (I have the old CD system too)

[AndrewH]

Bring on the EUs Galileo system – greater accuracy, significantly lower cannoning effect in CBD areas due to much wider spectrum spread and the fact that the signal will penetrate buildings and guaranteed availability cause it is not at its heart a military system. Only problem is that it won't be compatible with existing GPS systems. Oh, and it's due to come online in 2008.

[enzothemaddog]

Quote: Originally Posted by AndrewH

Bring on the EUs Galileo system – greater accuracy, significantly lower cannoning effect in CBD areas due to much wider spectrum spread and the fact that the signal will penetrate buildings and guaranteed availability cause it is not at its heart a military system. Only problem is that it won't be compatible with existing GPS systems. Oh, and it's due to come online in 2008.

Add to that date a reported 12 year waiting before total system is in place. Yay Galileo!

[hambone]

Hi, just wondering what version of software that people have on their Sat Nav on Australian delivered cars. I have a 2004 built S and have SW 4-1/20. Has anyone got any later versions and what are the updates ? I have read of overseas updates and hacks to update the nav voice etc, just wondering what people are getting with their delivered cars ? Ta....

[Mike Clarke]

Welcome to MINI2 hambone


I've merged all the threads I could find on Sat Nav into this one so hopefully all future Australian Sat Nav discussion will take place here.

[IML337]

Quote: Originally Posted by hambone

Hi, just wondering what version of software that people have on their Sat Nav on Australian delivered cars. I have a 2004 built S and have SW 4-1/20. Has anyone got any later versions and what are the updates ? I have read of overseas updates and hacks to update the nav voice etc, just wondering what people are getting with their delivered cars ? Ta....

Check the interior & ICE forum here on MINI2

When updates come out people post links to the CD images of the updates. You just download them (usually 100MB+) and burn them to CD. Then you just chuck the CD in the drive and turn the accessories on.

I documented it onc, take a look here:
http://iml337.com/iml337/mcs/nav_upgrade/
(I've upgraded mine stacks of times over the years)

I've never seen (or wanted) any voice changes