iPhone users ’ experience with GPS is so quick , so inst - on , that Apple ’s Wednesday response about positioning cross on iOS might almost seem bilk :
Calculating a phone ’s positioning using just GPS artificial satellite data can take up to several minutes . iPhone can abridge this metre to just a few seconds by using Wi - Fi hotspot and cell column data to rapidly find GPS satellites .
Several minutes ? Does n’t my iPhone take just seconds to figure out where I am ?
You can see your iOS device get a location fix when you open Maps.
Well , yes … but only when it prosecute in a set of trick to avoid a protracted process that was de rigueur when GPS receiver first appeared . In simplifying matters , Apple ’s not being entirely precise about how this all work out and what it ’s doing . So let me explain where Wi - Fi and cellphone phone tugboat meet into the equating .
12.5 minutes to locate
Early GPS receiver involve 12.5 minutes from a frigid start to prevail a lock ; later locks in the same realm could still take minutes . If you move around a GPS receiver off for a few weeks or moved it more than a few hundred miles , a cold scratch line might be required again .
GPS relies on two factors to create a set of accurate coordinates for where you ’re resist : meter and space . GPS satellite pass around precise time signals using a built - in atomic clock along with their current emplacement . They also broadcast the location of all other satellite in the sky , called the farmer’s calendar .
Every 30 seconds , a GPS satellite broadcasts a time cast , its current locating and some less exact position information for other GPS satellites . It takes 25 of these program ( thus , 12.5 second ) to obtain the full list of satellite locations . This info has to be decoded for a receiver to then by rights translate signals from the satellites that are within range .
If you have sex the place of four satellites and the time at which each sent their position information , you — or , rather , your Global Positioning System pass receiver — can calculate to within 10 time the parallel , longitude , and elevation of your current location along with the precise current time . With three satellites , you lose elevation , but a gimmick can still track movement fair accurately . Standalone Global Positioning System receivers can lock in at the same time on multiple satellite , and get across more than four . Other technique can improve accuracy , too .
But , heck , I do n’t have 12.5 minute . I ’m a meddling man ! Give me that locating quicker !
Giving GPS an assist
So GPS cow dung and gear makers come up with a legion of shipway to cut the wait , called Assisted GPS ( AGPS ) . alternatively of rely on live downloads of position data from satellites , next location can be estimated accurately enough to figure out scratchy satellite posture , and get a fix at which point even more up - to - date information is retrieve . These estimates can be downloaded via a web connection in seconds or even calculated right on a machine .
The current time can also be used as a clue . With a accurate current time , fragmental satellite data point can be decoded to gain a faster lock or project out the appropriate info to use . In CDMA internet , such as that used by Verizon , GPS - synchronized atomic time is call for for the internet ’s canonic operations , take a crap it a simple topic to have such info uncommitted . ( In fact , CDMA cell towers have GPS units built in to maintain respectable atomic time synchronization . )
you could see your iOS gadget get a position fix when you open Maps . These extra are what makes GPS into AGPS . Though a set of people misunderstand AGPS and remember it ’s some faux Global Positioning System system , that ’s not the case : AGPS call for a GPS pass catcher to operate . Apple ’s iPhone and 3 thou iPad models include AGPS , as do nearly all compete devices with GPS chip , notably Android phones . ( AGPS allow the use of much inexpensive and simpler GPS circuits in phones , reducing toll and assault and battery drainpipe . )
This is where Apple ’s financial statement on Wednesday deviate from full accuracy . Apple uses AGPS for native GPS - lock improvements , and Wi - Fi meshwork and cell tower locations are additional factor in providing a fast initial connection along with meliorate GPS accuracy .
Cellular carriers have super accurate GPS measurements of the locations of all their pillar . With a database of such towers , you could take measurements of the signaling strength of those within range — which may be dozens — and trilaterate to discover an area that overlap among them . ( Trilaterationinvolves overlapping regions to find an intersecting area ; triangulation use the measurement of angles to find a eye point . )
But cell towers are too far aside from one another to allow for GPS - similar preciseness , and they do n’t work well in less - populated areas , even suburb , where less insurance coverage is necessary than in an urban surroundings .
Looking for Wi-Fi
Apple , Google , and others rick to Wi - Fi positioning for that . Wi - Fi positioning , arise bySkyhook Wireless , originally required specially equipped trucks with Wi - Fi antenna and highly raw GPS receiver that drove around metropolis to get net identifier ( the unequaled ironware address beam by Wi - Fi infrastructure station ) and relative sign strengths at billions of tip . As with cadre towers , if you have enough networks and enough signal strength information , you could come close a office .
It ’s niggling for a networked equipment like a smartphone to take a snapshot of nearby Wi - Fi and cellular source and pass it to a host that respond with an approximate circle of coordinates . According to Apple ’s Q&A , the fellowship takes it a step further by caching subset of data about nearby web and towers to reduce web activity and rush up such search . This also shifts some computation to the headphone or tablet and out from location services . And it means if there ’s no electric cell or Wi - Fi connection usable , such location information can still be utilitarian . ( This explains a closed book I ’ve pick up in which a Wi - Fi - only iPad or an iPod touch can ostensibly calculate a hole when it ’s off the meshing . )
Apple first unveiled Wi - Fi and cell - tug emplacement with the original iPhone in an update to iPhone OS in other 2008 . This was Apple ’s sop to the fact that the original iPhone lack a GPS pass receiver , which did n’t first appearance until the iPhone 3G.
you may see either or both of these in action whenever you bring up the Maps app . Typically , you see a expectant blue circle appear about like a shot , a resolution of what must be a reference of the local database . The Mexican valium becomes small-scale as more information is used , still from cellular and Wi - Fi root , to create a better trilateration . This information is also used to provide more hint into decoding the best Global Positioning System satellite information , allowing the enjoyment of quite humble fragments of information or even in the altogether signal to get a good lock . at last , the traffic circle becomes a exclusive dot when Io is confident it has a solid GPS lock .
Out of the trucks, into the cloud
Apple switched from usingSkyhook Wirelessas a data supplier starting in iOS 4.0 for the iPhone and iPod soupcon , and with the iPad - only 3.2 freeing . There ’s a reasonableness for that . When your iOS machine use Wi - Fi locating , it institutionalize a shot of the current signal landscape to Apple , which says in the Q&A that this information is used anonymously to crowdsource better datum . But this is incredibly useful market intelligence activity , too , about how often user are relying on locating updates , and where those user are bunch . It can be used for advertising targeting and other purposes beyond those in apps that look for placement information .
Of naturally , the Apple Q&A nominate it decipherable that not every Wi - Fi snap is institutionalize to Apple ; clear some algorithm drive which are send to meliorate the database . By crowdsourcing , Apple avoids give birth to send trucks around . Google has apparently mesh the same approaching , using its Street View trucks to affix Android - based crowdsourcing . ( It ’s gotten in trouble two ways about this . First , for patently accidentallyrecording unencrypted Wi - Fi net information , which ensue in mulct , colonization , and halt such scanning in many res publica . Second , for allegedlystrongarming two major handset makersinto changing their deals to rely on Google ’s data . )
GPS is , in fact , rocket skill at many levels . But the path in which Apple combines and supplements information from multiple sources to create a fast and accurate lock explains why to us as substance abuser it ’s all “ GPS ” and just works .
[ Glenn Fleishman is a senior contributor to Macworld , and writes on a regular basis about wireless data point . His late ledger is Take Control of iPhone and iPod sense of touch Networking & Security for iOS 4 ( Take Control Koran ) . ]
