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TTFF in a different country using Leadtek 9543LP engine

 
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Tim_D
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Joined: Nov 24, 2004
Posts: 4

PostPosted: Wed Nov 24, 2004 2:02 pm    Post subject: TTFF in a different country using Leadtek 9543LP engine Reply with quote

Hello,

I am a design engineer involved in a new gps application based around the Leadtek 9543LP module.

The application requires a GPS fix in under 3 minutes. After this time, the GPS engine is shut down to conserve power and will re-acquire its position in 20 minutes (the user is not expected to move much). If the engine has not achieved a 'lock' then it will hunt quite agressively for 3 minute periods.

There is a default almanac loaded - so the engine is getting it's TTFF from a COLD start each time.

I have sent the application over to the USA for evaluation and have had some feedback from the engineers testing the unit that it is not locking (in full view of the sky) They have implied that all European GPS experience such problems and have made a suggestion that we keep the GPS powered for 30 minutes initially.

I have approached the supplier of the gps who have said it should be approximately 45 seconds cold start TTFF worldwide....

Can I believe them? Rolling Eyes Has anyone experience of getting a fast lock after stepping off the plane in a distant country?

There is a chance something has broken in transit but it is unlikely.

Many thanks for your comments in advance.

Tim
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DavidW
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Joined: 17/05/2003 02:26:21
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Location: Bedfordshire, UK

PostPosted: Wed Nov 24, 2004 7:30 pm    Post subject: Reply with quote

If the module is SiRF based, as many Leadtek GPSes are, is there any possibility of using the variant of trickle mode that returns a one-off fix then returns the GPS chipset to a low power state - rather than powering down the chipset completely? I think it may be called something like "push to fix", but I haven't got time right now to go reading through the SiRF documentation.

If you're really totally powering down the chipset, you could be losing the ephemeris each time.


45 seconds feels rather optimistic if all the GPS chipset has to go on is an almanac. I just tried issuing a cold start command to a SiRF based GPS which had a good but not perfect view of the sky and it took 106 seconds to fix.

If the GPS has a last position (even if it's a few tens of miles out) and its real time clock is running, it probably can manage a 45-50 second TTFF. Can you see what's happening with the chipset when it's trying to fix from your 'cold' state - is it finding the right satellites almost immediately and just having to get an ephemeris, or is it more work than that? If the latter, I doubt you'll get a first fix in less than 50 seconds unless you're lucky.



David
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Tim_D
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Joined: Nov 24, 2004
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PostPosted: Thu Nov 25, 2004 9:44 am    Post subject: Reply with quote

Thanks for the reply.

Yes, it's a SiRF chipset and no, unfortunately we can't use PTF. The application requires a complete power down. When there is a requirement to locate the user, the chipset is powered up with only a default Almanac, the ephemeris have to be downloaded each time.

Tests in the lab have given typically 55 seconds, but up to 1min 30 seconds is not uncommon. This isn't a problem - however if it's going to take longer than 3 minutes anywhere in the world, then I could look pretty silly as the unit powers back down after 3 minutes to save power.

As far as I understand, the default Almanac tells the module where the satellites are. But... what position would it think it's in initially with no time or position data - could it look for a scattering of satellies around the world so that it gets at least 3, then it calculates its position and therefore loads up the correct satellites for that location and time?

In practice, when I power up, I see various satellite numbers - it gets a lock with 3, then most of the other sattelite numbers change and it starts downloading from them too.

I have got a demo of a gps simulator in a weeks time - so can 'move' the position of the unit around the world and all will... hopefully... become clear.


Regards,

Tim (Luton, Beds)
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DavidW
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PostPosted: Thu Nov 25, 2004 3:47 pm    Post subject: Reply with quote

What you could do is initialise the GPS with wildly incorrect position data - look at the SiRF documentation for the syntax of the command - and see what happens.

If the GPS chipset has a stored position and time it uses those with an almanac to work out which satellites to listen for. With 12 channels and, from memory, 32 PRNs to check, it doesn't take too long to locate something and start to get an ephemeris.

The slowest fixes come when the stored position and/or time are bad; the GPS chipset will spend some time trying to locate the satellites it things are overhead before giving up and searching - at least that's my experience of SiRF chipsets, though it may be firmware version dependent.


You can learn a lot about what's going on by watching with software like Leadtek Navigator or SiRFdemo. You get better information on what's happening if you switch the GPS and software to SiRF binary mode and enable development messages.

I'm about ten miles north of Luton, by the way - do consider setting the location in your profile, as it sometimes helps to know where in the world posters are.



David
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Tim_D
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Joined: Nov 24, 2004
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PostPosted: Wed Dec 01, 2004 5:42 pm    Post subject: Reply with quote

Just to let you know the outcome...

After meeting with the applications engineer, there is a firmware upgrade required to fix some 'bugs'. In addition to this the DGPS needs turning off as this can apparently cause problems (sounds like another bug).

Thanks for your suggestions.

Tim
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DavidW
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PostPosted: Wed Dec 01, 2004 8:33 pm    Post subject: Reply with quote

I'm curious as to which firmware version(s) are involved - it's always helpful to have a feel of which SiRF firmware versions may be buggy.



David
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Tim_D
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PostPosted: Wed Dec 15, 2004 12:50 pm    Post subject: Reply with quote

Just got back from the trials.

Yes I was recommended the next version up which is 2.3.2 which (apparently!) has a whole load of fixes in it. Not that they will tell you officially though.

We're being tempted to look at the SiRF 3 now instead, looks the Bees Knees....
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DavidW
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PostPosted: Wed Dec 15, 2004 11:29 pm    Post subject: Reply with quote

As you can see from my signature, my Fortuna Clip On has 2.32 GSW2 (standard SiRF) firmware. I've noticed a couple of bug fixes over 2.31.

I've not seen any detailed information on SiRFstar III yet - but it looks impressive.



David
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