GPSfix - If you don't do it regularly, still do it?

[PaulQ]

I use a laptop and don't leave the dock connected all the time. These days, I hook it up every few weeks. It usually does a little "GPSfix" update with the sat. locations.

My question... is it worth doing this update every few weeks or is it only worthwhile if I get the GPSfix every week?

If getting the updates every few weeks (as opposed to weekly) is pointless, I will probably only hook up the dock every couple months.

I think I'm wondering if the GPSfix sat locations "expire" after a week or so and it goes back to the same geenral searching method.

I hope this makes sense.

Thanks!

 

[dhn]

I use a laptop and don't leave the dock connected all the time. These days, I hook it up every few weeks. It usually does a little "GPSfix" update with the sat. locations.

My question... is it worth doing this update every few weeks or is it only worthwhile if I get the GPSfix every week?

If getting the updates every few weeks (as opposed to weekly) is pointless, I will probably only hook up the dock every couple months.

I think I'm wondering if the GPSfix sat locations "expire" after a week or so and it goes back to the same geenral searching method.

I hope this makes sense.

Thanks!

I would think you would only need to get a quickfix prior to actually wanting to put the unit to use.

My understanding is that each quickfix file represents data good for the next 5 (or is it 7?) days.

So, if you don't plan to use the unit in February, for example, you needn't bother getting regular--or even weekly fixes-- that will be of no value to you.

 

[PaulQ]

Ah - I should have said that I *do* use the unit every week. I just wonder if I need to put it in the dock a lot. I usually charge it through the cig lighter.

 

[SShadow]

LOL I hook mine up several times a day... I guess I'm a little anal. It's more of a curiousity of seeing if I get any more updates...

 

[Robm]

You best bet is to update as often as you can...if you can update more often..great, if not, so be it

 

[kichu1979]

To understand what GPSfix is all about you would need to understand how GPS calculate your postition. For the initial position calculation your GPS chipset need
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 a data from the satellite 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 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 15dbHz to 28dbHz.

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.

 

[ptras09]

So one more question on top of this. When you download the QuickFix does it replace the old one with it, or does it just download more data to the TomTom filling up the hard drive?

 

[flyer1024]

It's just a couple of files in a folder which are overwritten with the new every time you download. very small files.

 

[kichu1979]

So one more question on top of this. When you download the QuickFix does it replace the old one with it, or does it just download more data to the TomTom filling up the hard drive?

Only the latest files are loaded on device and constantly overwritten.
The folder is called "ephem" and there are two or three files inside.

packedephemeris.ee - ~ 50kB - the essential part

and some txt files with expiration information that TT HOME uses.

On you PC however TT HOME does not delete old files and keeps them
in My Documents\Tom Tom\ Downloads\ehepemeris folder.

 

[The Spook]

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...

Except when you have a TomTom with Global Locate's Hammerhead GPS chipset, in which case tomtom sends you a file with GL's version of ephemeris data called Long Term Orbit (LTO) data.