Joe - that is a common misunderstanding. While the Iphone does not need cell
service for positioning it actually does use cell tower triangulation for
position and is not very accurate or fast without it. Moreover, if you are
below deck you will not get an adequate GPS signal. This is from the internet.
MotionX-GPS
Does MotionX-GPS require a cellular network?
The iPhone 5, 4S, 4, 3GS and 3G use an A-GPS (Assisted-GPS) chipset which uses
cell tower triangulation to speed up GPS signal acquisition. Cellular coverage
is not needed to acquire a signal, however the signal acquisition will be much
quicker if you have data coverage.
Without data services, it can take 15 minutes or longer to acquire a signal.
This is simply because it takes longer to determine which satellites to use out
of the 31 available around the world. With data services, it typically takes
under a minute, but it can take up to 5 minutes.
How the iPhone knows where you are
By Glenn Fleishman, Macworld
iPhone users' experience with GPS is so quick, so instant-on, that Apple's
Wednesday response about location tracking on iOS might almost seem baffling:
Calculating a phone’s location using just GPS satellite data can take up to
several minutes. iPhone can reduce this time to just a few seconds by using
Wi-Fi hotspot and cell tower data to quickly find GPS satellites.
Several minutes? Doesn't my iPhone take just seconds to figure out where I am?
Well, yes… but only when it engages in a set of tricks to avoid a lengthy
process that was de rigueur when GPS receivers first appeared. In simplifying
matters, Apple’s not being entirely accurate about how this all works and what
it's doing. So let me explain where Wi-Fi and cell phone towers fit into the
equation.
12.5 minutes to locate
Early GPS receivers took 12.5 minutes from a cold start to obtain a lock; later
locks in the same region could still take minutes. If you turned a GPS receiver
off for a few weeks or moved it more than a few hundred miles, a cold start
might be required again.
GPS relies on two factors to create a set of accurate coordinates for where
you’re standing: time and space. GPS satellites broadcast precise time signals
using a built-in atomic clock along with their current location. They also
broadcast the location of all other satellites in the sky, called the almanac.
Every 30 seconds, a GPS satellite broadcasts a time stamp, its current location
and some less precise location information for other GPS satellites. It takes
25 of these broadcasts (thus, 12.5 minutes) to obtain the full list of
satellite locations. This information has to be decoded for a receiver to then
properly interpret signals from the satellites that are within range.
If you know the position of four satellites and the time at which each sent
their position information, you—or, rather, your GPS receiver—can calculate to
within 10 meters the latitude, longitude, and elevation of your current
location along with the exact current time. With three satellites, you lose
elevation, but a device can still track movement fairly accurately. Standalone
GPS receivers can lock in simultaneously on multiple satellites, and track more
than four. Other techniques can improve accuracy, too.
But, heck, I don’t have 12.5 minutes. I’m a busy man! Give me that location
faster!
Giving GPS an assist
So GPS chip and gear makers came up with a host of ways to shorten the wait,
called Assisted GPS (AGPS). Instead of relying on live downloads of position
data from satellites, future locations can be estimated accurately enough to
figure out rough satellite positions, and get a fix at which point even more
up-to-date information is retrieved. These estimates can be downloaded via a
network connection in seconds or even calculated right on a device.
The current time can also be used as a clue. With a precise current time,
fragmentary satellite data can be decoded to gain a faster lock or figure out
the appropriate information to use. In CDMA networks, such as that used by
Verizon, GPS-synchronized atomic time is required for the network’s basic
operations, making it a simple matter to have such information available. (In
fact, CDMA cell towers have GPS units built in to maintain better atomic time
synchronization.)These extras are what makes GPS into AGPS. Though a lot of
people misunderstand AGPS and think it’s some faux GPS system, that’s not the
case: AGPS requires a GPS receiver to work. Apple’s iPhone and 3G iPad models
include AGPS, as do nearly all competing devices with GPS chips, notably
Android phones. (AGPS allows the use of much cheaper and simpler GPS circuits
in phones, reducing cost and battery drain.)
This is where Apple’s statement on Wednesday deviates from full accuracy. Apple
uses AGPS for native GPS-lock improvements, and Wi-Fi network and cell tower
locations are additional factors in providing a fast initial connection along
with improving GPS accuracy.
Cellular carriers have extremely precise GPS measurements of the locations of
all their towers. With a database of such towers, you can take measurements of
the signal strength of those within range—which may be dozens—and trilaterate
to find an area that overlaps among them. (Trilateration involves overlapping
regions to find an intersecting area; triangulation uses the measurement of
angles to find a center point.)
But cell towers are too far away from one another to provide GPS-like
precision, and they don’t work well in less-populated areas, even suburbs,
where less coverage is necessary than in an urban environment.
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