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From: JF Mezei on 28 Mar 2010 00:20
nospam wrote (in the USB thread)

> that's incorrect.



http://www.apple.com/ipad/specs/
Location
* Wi-Fi
* Digital compass
* Assisted GPS (Wi-Fi + 3G model)
* Cellular (Wi-Fi + 3G model)

http://www.apple.com/iphone/specs.html
Location
* Assisted GPS
* Digital compass2
* Wi-Fi
* Cellular

http://www.apple.com/iphone/specs-3g.html
Location
* Assisted GPS
* Wi-Fi
* Cellular


If the 3G didn't have the GPS chip in it, but the 3GS does, then the
term "Assisted GPS" on the spec sheets does not mean that it has the
actual GPS chip in it, just that it can triangualte your position based
on Wi-fi and/or 3G signals.

It would be much clearer if Apple had the word "GPS" (alone), along with
"Wi-Fi" and Cellular to properly describe the actual capabilities to get
your location. Having GPS with Cellular and Wi-Fi implies "assisted
GPS". BUIt having Cellular and Wi-Fi only would confirm there is no GPS
chip and it is only through the 3G and Wi-Fi that it obtains your location.



On the other hand, the ipod-touch doesn't claim "Assisted GPS". The
specs mention nothing about "Location". But the "features" on Maps mention:

"iPod touch finds your location using known Wi-Fi hotspots"


So I find it interesting that the 1-800-MYAPPLE folks, after checking
with a technician, would have told me explicitely that only the 3GS has
the actual GPS receiver in it.

And BTW, any decent GPS remembers its last position, so when you turn it
on, it doesn't take half an hour to obtain a fix. Even after a very
long flight, a GPS doesn't take that long to get its new position, as
long as its almanach si current (almanach is the orbital parameters of
the 32 or so satellites).


So, my conclusion is that I do not trust the specs nor the 1-800-MYAPPLE
, and I would have to really test a unit outside of a city to really
know if it has a real GPS in it or not.

(or go to one of those sites where they take it apart and inventory the
various chips found in it and would point to the GPS receiver chip to
confirm it has a real GPS in it).

From: JF Mezei on 28 Mar 2010 00:34
Alan Baker wrote: (in the USB thread)

> I'm sorry, but the idea that gps has to "look for satellites" is utter
> nonsense.


> Knowing roughly it is doesn't give a GPS receiver any information which
> will help it "find" those satellites.

Actually, it does. The Doppler effect is significant (and is usd by GPS
to get your position, speed etc). While satellites may be broadcasting
on fixed frequencies, the signal doesn't arrive at the GPS at those
frequencies due to the shifting caused by Doppler.

So whether a satellite is approaching or getting farther away from you
is significant.

So, if you have an up to date almanach, and you know your approximate
location (such as "north east USA"), then the GPS receiver knows which
satellites it can expect to be overhead at this time, and which ones are
approaching and which ones are getting further away and can thus
greatlty reduce the frequency scanning it must do to test what
satellites it is getting a signal from (and at what frequency).

If you are moving at a significant speed, it takes longer for the GPS to
lock in because it has to test more frequencies since the doppler effect
from your movement shifts the frequencies at which satellites are being
received.

If you last had your GPS in north-east of north america, and the next
time you power it on, your are in western australia, the GPS will take
quite a while to get its position unless you tell it where you are. This
is because it will assume you are still in north america and look for
satellites it would expect to be overhead. When that process totally
fails, it will then scan through all known satellites with different
frequency shifts for each satellite to see which ones it can see. Once
it has acquired enough satellites to gets its position, the rest of the
process is fast.

If you give the GPS a hint of where you are ("Western Australia", this
allows the GPS to start scanning with the satellites that would be
expected overhead, and this will get the GPS to find the prerequisite 3
satellites much faster since it will skip the statellites that would not
be expected to be overhead at this time over western australia.

Having said this, if you just move across town and turn your GPS back
on, the unit will get a fix very quickly because that doesn't change
much what is expected to be overhead in terms of satellites.

Where wi-fi and
cellular become usefull are in urban canyions (manhattan for instance)
where the view of the sky is very narrow and you can't get a very good
GPS fix.

GPS requires a lot of CPU computational power in real time. It will
drain your batteries much faster on an apple device. This is why
assisted GPS is added, to save on batteries whenever it is possible.
From: nospam on 28 Mar 2010 00:38
In article <4baed922$0$11010$c3e8da3(a)news.astraweb.com>, JF Mezei
<jfmezei.spamnot(a)vaxination.ca> wrote:

> http://www.apple.com/ipad/specs/
> Location
> * Wi-Fi
> * Digital compass
> * Assisted GPS (Wi-Fi + 3G model)
> * Cellular (Wi-Fi + 3G model)
>
> http://www.apple.com/iphone/specs.html
> Location
> * Assisted GPS
> * Digital compass2
> * Wi-Fi
> * Cellular
>
> http://www.apple.com/iphone/specs-3g.html
> Location
> * Assisted GPS
> * Wi-Fi
> * Cellular

which clearly states that they all have an assisted gps.

> If the 3G didn't have the GPS chip in it, but the 3GS does, then the
> term "Assisted GPS" on the spec sheets does not mean that it has the
> actual GPS chip in it, just that it can triangualte your position based
> on Wi-fi and/or 3G signals.
>
> It would be much clearer if Apple had the word "GPS" (alone), along with
> "Wi-Fi" and Cellular to properly describe the actual capabilities to get
> your location. Having GPS with Cellular and Wi-Fi implies "assisted
> GPS". BUIt having Cellular and Wi-Fi only would confirm there is no GPS
> chip and it is only through the 3G and Wi-Fi that it obtains your location.

the assistance is via the cell/wifi network.

> On the other hand, the ipod-touch doesn't claim "Assisted GPS". The
> specs mention nothing about "Location". But the "features" on Maps mention:

the ipod touch doesn't have *any* gps.

> "iPod touch finds your location using known Wi-Fi hotspots"

right, because there is no gps whatsoever (which i find annoying,
hopefully the next model will have one).

> So I find it interesting that the 1-800-MYAPPLE folks, after checking
> with a technician, would have told me explicitely that only the 3GS has
> the actual GPS receiver in it.

call back, you'll get a different answer. :)

> And BTW, any decent GPS remembers its last position, so when you turn it
> on, it doesn't take half an hour to obtain a fix. Even after a very
> long flight, a GPS doesn't take that long to get its new position, as
> long as its almanach si current (almanach is the orbital parameters of
> the 32 or so satellites).

true, but that only lasts so long. after a few minutes, a decent gps
will get a fix within seconds. however, after a long flight, it will be
far enough away from its last known position that it will do a cold
start, which will take a while.

> (or go to one of those sites where they take it apart and inventory the
> various chips found in it and would point to the GPS receiver chip to
> confirm it has a real GPS in it).

that will confirm it one way or the other. i assume by april 3rd
evening it will have been torn apart and posted. :) maybe even by the
afternoon.
From: JF Mezei on 28 Mar 2010 00:41
nospam wrote: (in the USB thread)

> ephemeris is sent every 30 seconds, and if the gps doesn't get an
> accurate download, it will have to wait for another iteration.

Ephemeris for one satellite is sent every 30 seconds. And that is
assuming the GPS has found the signal from at least one satellite.

Until it has VALID ephemeris for the satellites it is receiving signals
from, it can't know exactly where they are and thus can't calculate your
current position. But the more satellites it is receiving a signal from,
the faster the almanach is downloaded to the GPS.

Having said this, an almanach is valid for 1 week. And even after that
week, the old almanach is accurate enough to let the GPS get a fix
before having updated the almanach completely.

So turning your unit on after a few days off isn't a big deal in terms
of getting the GPS to lock in. Having to wait for a new almanach happens
less than a handful of times in the life of a GPS if you used it regularly.
From: Alan Baker on 28 Mar 2010 01:07
In article <4baedc61$0$13643$c3e8da3(a)news.astraweb.com>,
JF Mezei <jfmezei.spamnot(a)vaxination.ca> wrote:

> Alan Baker wrote: (in the USB thread)
>
> > I'm sorry, but the idea that gps has to "look for satellites" is utter
> > nonsense.
>
>
> > Knowing roughly it is doesn't give a GPS receiver any information which
> > will help it "find" those satellites.
>
> Actually, it does. The Doppler effect is significant (and is usd by GPS
> to get your position, speed etc). While satellites may be broadcasting
> on fixed frequencies, the signal doesn't arrive at the GPS at those
> frequencies due to the shifting caused by Doppler.
>
> So whether a satellite is approaching or getting farther away from you
> is significant.
>
> So, if you have an up to date almanach, and you know your approximate
> location (such as "north east USA"), then the GPS receiver knows which
> satellites it can expect to be overhead at this time, and which ones are
> approaching and which ones are getting further away and can thus
> greatlty reduce the frequency scanning it must do to test what
> satellites it is getting a signal from (and at what frequency).

Incorrect.

All the satellites transmit on the same frequency.

>
> If you are moving at a significant speed, it takes longer for the GPS to
> lock in because it has to test more frequencies since the doppler effect
> from your movement shifts the frequencies at which satellites are being
> received.

Complete and utter nonsense. Compared to the speed of light, *nothing*
in the system is moving "at a significant speed".

>
> If you last had your GPS in north-east of north america, and the next
> time you power it on, your are in western australia, the GPS will take
> quite a while to get its position unless you tell it where you are. This
> is because it will assume you are still in north america and look for
> satellites it would expect to be overhead. When that process totally
> fails, it will then scan through all known satellites with different
> frequency shifts for each satellite to see which ones it can see. Once
> it has acquired enough satellites to gets its position, the rest of the
> process is fast.
>
> If you give the GPS a hint of where you are ("Western Australia", this
> allows the GPS to start scanning with the satellites that would be
> expected overhead, and this will get the GPS to find the prerequisite 3
> satellites much faster since it will skip the statellites that would not
> be expected to be overhead at this time over western australia.

The GPS system uses code division multiple access to receive information
from all satellites in the system on two frequencies. The doppler shift
plays no role in picking up a particular satellites transmissions.

--
"The iPhone doesn't have a speaker phone" -- "I checked very carefully" --
"I checked Apple's web pages" -- Edwin on the iPhone
"It is Mac OS X, not BSD.' -- 'From Mac OS to BSD Unix." -- "It's BSD Unix with Apple's APIs and GUI on top of it' -- 'nothing but BSD Unix' (Edwin on Mac OS X)
'[The IBM PC] could boot multiple OS, such as DOS, C/PM, GEM, etc.' --
'I claimed nothing about GEM other than it was available software for the
IBM PC. (Edwin on GEM)
'Solaris is just a marketing rename of Sun OS.' -- 'Sun OS is not included
on the timeline of Solaris because it's a different OS.' (Edwin on Sun)
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