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GPS sensitivity: hot start vs. A-GPS (PND vs. mobile)

 
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Leofly
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PostPosted: Sat Feb 21, 2009 5:10 pm    Post subject: GPS sensitivity: hot start vs. A-GPS (PND vs. mobile) Reply with quote

I’m trying to collect some data about A-GPS benefits in real life and my main question now is whether assisted GPS (supported by mobile phones) is really more advanced than a standalone PND with similar chipset but assuming I have made fix in good conditions for a short time before?
To my understanding, the assisted data is actually the same as the data my PND can collect after first fix under open sky, so that it should behave like A-GPS for a while. Am I correct here?
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mikealder
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PostPosted: Sat Feb 21, 2009 6:08 pm    Post subject: Re: GPS sensitivity: hot start vs. A-GPS (PND vs. mobile) Reply with quote

Leofly wrote:
To my understanding, the assisted data is actually the same as the data my PND can collect after first fix under open sky, so that it should behave like A-GPS for a while. Am I correct here?

You are quite correct - if you use the the GPS device on a regular basis there is no advantage in messing about downloading the A-GPS files.

We have even seen examples where the A-GPS download is incorrect and can mess up a device - I stopped using the download quite some time ago as it serves no purpose for a device used on a regular basis (every few days) - Mike
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Leofly
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PostPosted: Sat Feb 21, 2009 7:20 pm    Post subject: Reply with quote

Mike, I believe you are talking about so-called long term ephemeris. They can speed up the first fix, but are useless for satellite search.
On the contrary, cellular assistance can help GPS both in satellite search and in the following navigation. But this is just a theory, because once the GPS device knows where it was last time (+/-10km does not matter, I think) and keeps integrated clock working, it has actually everything, the mobile operator can provide.
At the end I read about some difference between European and US services (such as fine time aiding in America and coarse time aiding in Europe) which should make sense, but unfortunately there is no fully deployed A-GPS service in my area, so I can't even try. And I don't have a mobile with GPS.
Did you try to compare the mobile phone with activated A-GPS function and a PND when it can make hot start?
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robertn
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PostPosted: Mon Feb 23, 2009 1:52 am    Post subject: Reply with quote

Another case where AGPS makes a difference is if you recently used you GPS, but it was long distance away from where you are now. The typical case is when you jump on a plane and fly across the country.
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Leofly
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PostPosted: Tue Feb 24, 2009 9:45 am    Post subject: Reply with quote

Basically, what I’m asking is whether mobile based A-GPS indeed outperforms internet based A-GPS?
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Leofly
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PostPosted: Thu Mar 05, 2009 10:49 am    Post subject: Reply with quote

http://www.abiresearch.com/Blog/Telematics_Blog/581

Unfortunately, the relentless activity in the GPS-handset, GPS-chipset and navigation software space is not matched by a similar trend in the roll out of A-GPS network infrastructure by European carriers, most of which remain passive

any comments?
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philpugh
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PostPosted: Thu Mar 12, 2009 3:03 pm    Post subject: Reply with quote

robertn wrote:
Another case where AGPS makes a difference is if you recently used you GPS, but it was long distance away from where you are now. The typical case is when you jump on a plane and fly across the country.


In theory this is correct - but in practice it's usually not worth the hassle.

Last year I flew from Manchester to Nice and my nuvi 660 (which was on to get us to the airport, but off for the 4 hours of waiting, flying etc) took about 1 minute to lock when turned back on. That's less than the time it took to set the car up for driving - checking the mirrors, seats ...

A-GPS seems to be driven by certain authorities (notably in the US) requiring to be able to locate users of mobile phones, rather than as any practical service to users. In it's full implementation the network suppliers will have very precise information about where you are - it only takes a little legislation for agencies to get at this information.
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robertn
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PostPosted: Thu Mar 12, 2009 8:38 pm    Post subject: Reply with quote

I agree wioth what you say.
A-GPS solves a problem that no longer exists. When it was developed, in the early 2000's, GPS chipsets were significantly less sensitive, and prone to problems getting the first fix. The idea in those days was to go outside, into a big open feild or carpark, and get a fix before doing anything (Searhc the archives for examples of advice given.....). A-GPS would have been useful then. Now, the recievers pick up any wif of a signal, and therefore get the first fix more reliably.

(the authorities track your evey move now, they don't need A-GPS to that)
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