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Old Dec 12, 2012, 09:15 AM   #1
PracticalMac
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Question Why is GPS and Cellular combined and not seperate?

A number of Android tablets have GPS without Cellular.
So why is Apple forcing you to get Cellular if you just want GPS?

Some guesses I have is:
Shared antennas.
Cellular (Qualcomm?) chip includes both functions.
A-GPS (using cellular to quickly download updated satellite data)

or all of the above?


Also a ? about A-GPS,
does one get the satellite data even if you do not subscribe to cellular service???


EDIT:Wiki says "For billing purposes, network providers often count this as a data access", so this suggest it does require subscription, but not conclusive.
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Last edited by PracticalMac; Dec 12, 2012 at 09:29 AM.
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Old Dec 12, 2012, 09:27 AM   #2
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Quote:
Originally Posted by PracticalMac View Post
A number of Android tablets have GPS without Cellular.
So why is Apple forcing you to get Cellular if you just want GPS?

Some guesses I have is:
Shared antennas.
Cellular (Qualcomm?) chip includes both functions.
A-GPS (using cellular to quickly download updated satellite data)

or all of the above?


Also a ? about A-GPS,
does one get the satellite data even if you do not subscribe to cellular service???
Or it's just marketing. You want GPS pay the extra $100
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Old Dec 12, 2012, 11:29 AM   #3
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It's combined because its the same SOC; System-On-A-Chip is a sigle component.

A-GPS is just like regular GPS but it is assisted by cell towers and wifi networks to find your location quicker. The A doesn't mean they are downloading data, it just means the cell-towers triangulate your position quicker.
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Old Dec 12, 2012, 12:06 PM   #4
PracticalMac
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Originally Posted by darngooddesign View Post
It's combined because its the same SOC; System-On-A-Chip is a sigle component.

A-GPS is just like regular GPS but it is assisted by cell towers and wifi networks to find your location quicker. The A doesn't mean they are downloading data, it just means the cell-towers triangulate your position quicker.
That is incorrect.

AGPS does not use Cell Towers for location.

AGPS might use the signal quality measurements off *a* single tower, but it does not triangulate.
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Last edited by PracticalMac; Dec 12, 2012 at 12:28 PM. Reason: a
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Old Dec 12, 2012, 01:27 PM   #5
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Quote:
Originally Posted by PracticalMac View Post
Also a ? about A-GPS,
does one get the satellite data even if you do not subscribe to cellular service???
You're not really getting "satellite data". The signal from the GPS satellites is just used to determine location. If it's not using the GPS satellites to determine your location then it's not GPS whether it's standalone or aGPS. Most aGPS receivers can fall back on standalone mode if the Ephemeris data isn't available from the assistance servers for whatever reason.

Quote:
Originally Posted by PracticalMac View Post
AGPS might use the signal quality measurements off *a* single tower, but it does not triangulate.
That's not entirely correct. It doesn't use cell tower triangulation but GPS is basically triangulation.
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Old Dec 12, 2012, 02:09 PM   #6
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Originally Posted by takeshi74 View Post
You're not really getting "satellite data". The signal from the GPS satellites is just used to determine location. If it's not using the GPS satellites to determine your location then it's not GPS whether it's standalone or aGPS. Most aGPS receivers can fall back on standalone mode if the Ephemeris data isn't available from the assistance servers for whatever reason.


That's not entirely correct. It doesn't use cell tower triangulation but GPS is basically triangulation.
According to Wiki on ephemeris, it does actually download data from satellites.
Quote:
once the receiver picks up each satellite's signal in turn, it then downloads the ephemeris data directly from that satellite.
And yes to be accurate, triangulation is done by satellites, not cell towers.
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Old Dec 12, 2012, 03:01 PM   #7
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I've read some data from the cell towers is downloaded to more quickly establish location of satellites, so it can use some data. I need a reference though.
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Old Dec 12, 2012, 03:15 PM   #8
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I need a reference though.
http://en.wikipedia.org/wiki/Assisted_GPS
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Old Dec 12, 2012, 03:47 PM   #9
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Found some answers:

iPad 3 (and iPhone 5) use Qualcomm's RTR8600 multi-band/mode RF transceiver for 3G and 4GLTE bands which includes GPS services.

I think it is because of the SoC in the RTR8600 one gets "aGPS" (AGPS)

I do think the antenna is separate, however.

Likely cheaper to used a combined system then separate chips.

Any other opinions?
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Old Dec 12, 2012, 03:50 PM   #10
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That's certainly the reason it's true in the iPad - no dedicated GPS chip. Any other answers are Apple business decision speculation.
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Old Dec 12, 2012, 04:35 PM   #11
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Quote:
Originally Posted by darngooddesign View Post
It's combined because its the same SOC; System-On-A-Chip is a sigle component.
The wireless module is neither part of the SoC nor is it what you would generally classify as a system on a chip. The module does not function on its own. You might get away with 'ASIC' if you need a label.
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According to Wiki on ephemeris, it does actually download data from satellites.
It listens for, receives, and inteprets data from satellites, but to call that a "download" in the sense it is usually understood isn't really common usage, just like you don't generally say that your TV "downloads" broadcast transmissions.
Quote:
And yes to be accurate, triangulation is done by satellites, not cell towers.
The triangulation is actually done in the receiver, to be accurate. Neither the cell tower nor the satellite actually interacts with the phone for location purposes.

Quote:
Originally Posted by PracticalMac View Post
I think it is because of the SoC in the RTR8600 one gets "aGPS" (AGPS)
It's not the SoC; it's the fact that the package is part of the integrated logic. Almost every device with GPS and cellular data connectivity uses aGPS in some capacity--it would be foolish not to take advantage of that.
Quote:
cheaper to used a combined system then separate chips.
Cheaper, less redundant, more space-efficient, more power-efficient, and better integrated into a mobile device.

The standalone GPS chips that you might find in a TomTom or Magellan GPS are not really suitable for a mobile device.
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Old Dec 12, 2012, 04:36 PM   #12
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That's certainly the reason it's true in the iPad - no dedicated GPS chip. Any other answers are Apple business decision speculation.
Simple, concise, and accurate.

Clearly you do not belong on the Internet.
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Old Dec 12, 2012, 05:15 PM   #13
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Originally Posted by lianlua View Post
It listens for, receives, and inteprets data from satellites, but to call that a "download" in the sense it is usually understood isn't really common usage, just like you don't generally say that your TV "downloads" broadcast transmissions.
Excellent details, thanks!

one items puzzles me.
It sounds like the GPS will "build" a library of data listening to the satellites.

But for Assisted, does it instead receive a file of like data?

I also assume if in with Wi-Fi connection to internet it will also download the data (as in Cell is turned off).
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Old Dec 12, 2012, 06:50 PM   #14
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Originally Posted by PracticalMac View Post
Excellent details, thanks!
No problem!
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It sounds like the GPS will "build" a library of data listening to the satellites.
Not a persistent library. GPS satellites constantly broadcast a bunch of information, but it's only one way. The satellites constantly tell you where they are and what time it is, with an extreme amount of precision. It doesn't do any good for the GPS receiver to log data into a historical library--all of the satellite data is basically useless once it's transmitted, except for certain details that only change every few hours or days.

The receiver listens to that data and compares the timestamp of the satellite to the time when the message is received. Knowing the location of the satellite and how long it took to receive the signal tells you where you are on the planet.

It takes a comparatively long time to listen for long enough to get all the location information from the satellite, because it is a fairly low bandwidth transmission. That's where assisted GPS comes in.
Quote:
But for Assisted, does it instead receive a file of like data?
Radio transmissions from the satellites are slow. Terrestrial radio transmissions, even on relatively bandwidth-constrained 2G/3G networks, is much faster. In a nutshell, assisted GPS gets data on the location of GPS satellites from cell towers or other sources, rather than waiting for the complete set of data on the slow satellite signal.

That's what makes it faster to find your location with aGPS.
Quote:
I also assume if in with Wi-Fi connection to internet it will also download the data (as in Cell is turned off).
That depends on the specific GPS hardware and software in your device. Some get their data over the Internet and some get it only from the cell towers.
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Old Dec 12, 2012, 11:48 PM   #15
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Originally Posted by ActionableMango View Post
Simple, concise, and accurate.

Clearly you do not belong on the Internet.
That's the first time that has been said about me.

Uh......

Thanks?

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