TT BT - Cold/Warm Starts

tezzer · Occasional Visitor Joined: Jan 10, 2004 Posts: 16

Hi,

Need a little advice.

In the reviews of TT Bluetooth, there are warm and cold start comparisons with other GPS kit. Could I ask the definitions of cold and warm starts?

I'm finding that following a recharge (which I guess constitutes a cold start), it can take between 30 and 40 mins to pick up a satellite signal which is not that great really - is this the same for others?

Thanks
Terry

Darren · Posted: Sat Jan 31, 2004 12:45 pm Post subject:

A cold start is typically one where more than 12hrs has elapsed since the last fix.

When you aquire a fix, ephemeris data is downloaded to your receiver's almanac which tells the GPS receiver which satellites it can expect to see in the sky at your location. This data is valid for a period of time and aids the receiver in achieving a fast lock at power on.

A 'Warm' start is one where the GPS has been switched off for only a short period and the ephemeris data is still valid. It knows what satellites it should expect to see and so can aquire a speedy fix

After a period of time with the receiver 'OFF' this data becomes stale. Whilst GPS satellites are in geo-stationary orbit the earth is constantly rotating beneath them and so the ephemeris data needs to be re-downloaded after a long period. This is the 'Cold' start and downloading this data explains the increased time necessary to aquire a lock. This is also necessary where the unit has been moved a large distance since the last fix i.e. a Transatlantic flight. It's ephemeris data may still be time valid but would in this situation be stale as it related to a a view of the sky in a different part of the world!

You don't say what GPS Receiver you have but 30+ minutes is way too long given a good sky view. I would expect it to take no longer than 10minutes worst case and typically 2-3mins for a cold start.
_________________
Darren Griffin - Editor

stevex · Occasional Visitor Joined: Jan 21, 2004 Posts: 15

Hi

I was having simioar problems and it was taking ages to get a fix. I called TT in Holland to establish what the problem was.
The guy i spoke to was really helpful and whilst on the phone, it went like this.

Q' "Have you got anything in your screen, like heat reflective material or similar"
A' "No" or so i believed.

Hold the receiver outside the car pointing at the sky.

I did this and it got a fix within 30seconds....from cold.

He then suggested moving it to different places on the dashboard. I did this and after a while it would lock on, but would quite easily lose the fix.

I don't know if you have seen the windscreens where there are a load of black dots behind the rear view mirror, i have them on my Peugeot 307 and they are on my partners BMW.

This was the last place i thought of putting the receiver. Eventually out of frustration, itried it.

Everytime since i get a fix within 15 seconds, from cold and in many different locations. Even between walls and buildings.

I assume from this that there is possibly something in my screen that i didn't know about causing problems...

Hope this helps you...

Stevex

tezzer · Occasional Visitor Joined: Jan 10, 2004 Posts: 16

Hi Stevex/Darren

Thanks for the replies - really interesting - I have a Tomtom Bluetooth GPS, fyi.

I guess the issue is going to be the heat reflective windscreen I have on my peugeot 307 - in fact, I was surprised i could get a decent signal on the dashboard - I can usually access 5-7 satellites, but it can fluctuate to 2-3 at times.

I guess the trick with this is to get a signal from cold start outside the car before setting off....

Stevex - how have you mounted your gps on the rear view mirror?

Thanks
Terry

stevex · Occasional Visitor Joined: Jan 21, 2004 Posts: 15

Hi Terry

What a coincidence, you have a 307 too.

If you have a look, you will see that the rear view mirror is quite a deep one. If you have the magnetic plate, with a sticky pad on, (they come in the TT car kit) use that and stick it on top of the mirror.

I took the receiver off the plate for hard wiring and just use it on the magnetic plate.

Any more info needed, just shout...

Stevex

DavidW · Posted: Sun Feb 08, 2004 7:35 am Post subject:

Just a little correction to Darren's excellent explanation of ephemeris data and the like.

GPS satellites are not in geostationary orbit. They're in a much lower mid earth orbit at around 20000km - which is why their position moves as observed from the surface of the earth. If they were geostationary, their position with respect to the earth would appear fixed.

The satellites used for SBAS (WAAS, EGNOS and similar) are geostationary.

David

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