The cell tower signal is just an aid in getting the first fix.  After that the 
GPS part of the chip takes over.  I was likewise getting position / speed data 
on inavx on my iPhone while "off watch" on the Annapolis to Newport race this 
year.

Tim

> On Oct 2, 2015, at 6:04 PM, Jerome Tauber via CnC-List 
> <cnc-list@cnc-list.com> 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, 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.
>>> 
>>> 
>>> _______________________________________________
>>> 
>>> Email address:
>>> CnC-List@cnc-list.com
>>> To change your list preferences, including unsubscribing -- go to the 
>>> bottom of page at:
>>> http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com
>>> 
>> 
>> _______________________________________________
>> 
>> Email address:
>> CnC-List@cnc-list.com
>> To change your list preferences, including unsubscribing -- go to the bottom 
>> of page at:
>> http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com
> _______________________________________________
> 
> Email address:
> CnC-List@cnc-list.com
> To change your list preferences, including unsubscribing -- go to the bottom 
> of page at:
> http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com
> 
_______________________________________________

Email address:
CnC-List@cnc-list.com
To change your list preferences, including unsubscribing -- go to the bottom of 
page at:
http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com

Reply via email to