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 >
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