And here I thought you were watching Bonanza down there.
On 10/2/2015 8:40 PM, Graham Collins via CnC-List wrote:
It doesn't have a cell receiver, and it works very well offshore.
During a recent offshore race I was watching the course while I was
supposed to be sleeping.
Graham Collins
Secret Plans
C&C 35-III #11
On 2015-10-02 7:04 PM, Jerome Tauber wrote:
It works well below deck when you are in cell range. How does it
work below deck with just a GPS signal offshore? I don't want to
belabor the point. Jerry
Sent from my iPhone
On Oct 2, 2015, at 4:36 PM, Graham Collins via CnC-List
<cnc-list@cnc-list.com> wrote:
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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--
Boat_Sig Cheers,
Jeff Nelson
Muir Caileag
C&C 30
Armdale Y.C.
Halifax
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