QuickGPSfix

[dcgh2007]

What happens when you go on holiday and can't download quickGPSfix? I know that this is to tell your Tom Tom where the satellites are and this speeds up the start process. However, when you're away from home is it sufficient to switch your Tom Tom on daily to reposition itself or will the start up just take longer and longer until you get home and hook up to your computer again?

[Darren]

QuickFix is an aid, it is not essential. All it does is peed up fix in 'some' situations and your device will continue to function perfectly even if you never again use it.
_________________
Darren Griffin

[Calomax]

[Trevor1234]

Hi dcgh2007
Lots of users (including myself) have never used the quickGPSfix. I have travelled in France, Germany and Belgium without any troubles at all. The Go devices are pretty good at getting a fix, unlike the old GPS receivers that first came on the market some years ago.

Calomax, stop taking the pees out of the GPS Staff.
_________________
Tomtom Go 720.
Navcore 9.510
Central and Western Europe v855.2884
GPS World Traffic cameras
Home 2.7
iPhone 3G

[Darren]

I'm a very fast two finger typist if I leave out the odd letter here and there
_________________
Darren Griffin

[Calomax]

[Stewart166]

I was recently in this situation, holidaying in Europe. I found that I didn't really notice a significant delay provided that when I turned the device on the car was in the same place that it had been when turned off. What REALLY confused it was when coming out of an underground car park where the signal had been lost. In those circumstances it could take up to 2-3 minutes to work out what was going on. Of course by then I was heading the wrong way in some terrifying Italian one-way system

[Trevor1234]

Hi Stewart166
Your experience as you came out of the carpark can happen anywhere anytime but usually the first fix happens quite quickly. For instance, the overnight ferry trip from Portsmouth to St Malo in France is quite a long journey yet my Tomtom has had my position fixed before I have even left the Ferry ramp at St Malo. However, like you I have also suffered delays in the past and have landed up leaving my departure point and going in the wrong direction but I’m afraid that that’s just one of those things.

However, if you have used your ‘Go’ recently it will get a fix quickly because it has the data stored from its last fix. But if you have not used the device for 3 months then the quickGPSfix should speed things up significantly.
_________________
Tomtom Go 720.
Navcore 9.510
Central and Western Europe v855.2884
GPS World Traffic cameras
Home 2.7
iPhone 3G

[Darren]

Here's a good article to read:

http://www.pocketgpsworld.com/howgpsworks.php
_________________
Darren Griffin

[Trevor1234]

Hi Darren
That’s the most interesting article I have read on the subject. Thanks. When I think of the work involved to get the info to my device and the calculations that then have to be performed by my device I feel quite humbled.
I used to sit in the cinema watching James Bond use his Gizmos but never dreamed that I would one day treat such devices so casually and carelessly.
I promise to never swear again when my 'Go' takes its time to get a fix.
_________________
Tomtom Go 720.
Navcore 9.510
Central and Western Europe v855.2884
GPS World Traffic cameras
Home 2.7
iPhone 3G

[IanS100]

I'm stunned just how quickly the TomTom does get a fix. I owned a Road Angel Navigator before this & it wasn't unusual for it to take 5 mintes plus for a fix, & if the weather was overcast often as not it wouldn't get one at all. In contrast the my 520 gets a fix sat in my lounge when I'm updating it on the computer & even in my car when shielded by the metal roof.

[kichu1979]

To understand what GPSfix is all about you would need to understand how GPS calculates your postition. For the initial position calculation your GPS chipset needs to find at least 4 satellites with strong enough signal (28 dbHZ or more) and keep those satellites and the signal strength for about a minute to download data from the satellites that is essential for calculating your position (called ephemeris).

If at any time it looses the signal of any satellite or the signal will go below 28dbHZ it has to start again at track that satellite for another minute. Real life scenario you are driving between high buildings (urban canyon) and the received gps signal keeps changing all the time.

Now the GPSfix file you donwload from the internet is a part of solution from your GPS chip maker company called SiRF and their solution is called Instant Fix (I Edition) or A-GPS (assisted GPS). THe file contains specially prepared ephemeris data that is valid for 7 days that your GPS chip uses instead of the one received from satellites for calculating your first fix.

This allows the chip to skip that so called "download ephemeris from satellite part" and start calculating your position imeediately after powering on. And this takes around 5-15 secons on average.

Also one important point is that the singal strenght required for downloading epehemris data from satellites is 28dbHZ, signal strength required for calculating your position once your GPS have the ephemeris is only 15dbHZ.

So valid GPSfix file on your TomTom allows your device to calculate you position quickly 5-15 sec instead of around a minute (when stationary),
and lowers the minimal signal strength required for calculating your position from 28dbHz to 15dbHz.

If at any time your GPS chipset finds the downloaded ephemeris is not valid or highly out of date it calculates your position traditional way... 4 satellites, 28dbHz signal continously for around a minute.

And by the way... real epehemris downloaded from satellites is valid only for around 4 hours.

You can also spot terms called hot start / cold start when reading about GPS performace, ie. time to first fix.

The hot start time means starting the chipset with valid epehemris data and takes around 5-15 seconds for most chipsets depending on conditions.

Cold one is the opposite, no ephemeris data, takes usually around a minute, considering the requirements I worte before are met.

[Trevor1234]

Hi IanS100
Yes, I too get a fix sitting in my living Room. In fact it gets a fix even when I do not want it to.
Example… If I sit here and plan an itinerary that I want to use in France later this month and then run the route it will insist on starting the journey from my present location because it has a fix. This can be irritating but of course it would not happen if I was sitting at the bottom of a coal mine or in a room surrounded by Lead.
Prior to owning a Tomtom Go I had a PDA and Tomtom 5 and that gave you the option of having the GPS on or Off. It worked a treat and I wish the ‘Go’ gave the same choice. However, most users never seem to use the Itinerary mode and are more than happy to get a fix so easily and I suppose I should too.
_________________
Tomtom Go 720.
Navcore 9.510
Central and Western Europe v855.2884
GPS World Traffic cameras
Home 2.7
iPhone 3G

[dhn]

To Darren.......

I think Kichu's explanation is worthy of a 'sticky'. It really explains things well....even for an idiot like myself

Well done. Kichu!

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