Hi Jerome
I must disagree with that statement. My sony tablet with built in GPS
works perfectly below decks, it is what I use for anchor watch.
Graham Collins
Secret Plans
C&C 35-III #11
On 2015-10-02 12:42 PM, Jerome Tauber via CnC-List wrote:
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 <http://www.macworld.com/author/Glenn-Fleishman/>,
Macworld
iPhone users' experience with GPS is so quick, so instant-on, that
Apple's Wednesday response about location tracking on iOS
<http://www.macworld.com/article/159501/2011/04/apple_location_data_response.html>
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 <http://en.wikipedia.org/wiki/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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