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iPhone GPS

Discussion in 'APPLE iPhone, iPad Tablets and all iOS Devices' started by spleck, Jul 25, 2008.

  1. spleck

    spleck Tool
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    What has been your experience with the iPhone GPS?

    At times it's been able to pinpoint the room of my house that I'm in, while at other times it tracks off by 30-50 feet (still in a straight line though) with lags and drop outs (while I was walking outside with no tree cover/partly cloudy).

    Is it possible the GPS is getting faulty data from the network? Are there different power modes for the GPS that might allow for more accurate tracking than what Maps is enabling?

    I never used the location feature on the old iPhone, and it only sporadically works on my wife's T-mobile phone, but how much of a benefit is this over the cell tower triangulation? It seems to me with the specs of the GPS chip in the iPhone that we could have gotten an extremely accurate chip worthy of attaching to a decently engineered antenna. Is that not the case?
     
  2. bobolito

    bobolito Diamond Senior Member
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    If you are indoors, then there is no satellite communication. Therefore, iPhone will rely solely on WiFi and cell towers to guess your location which is not accurate. GPS systems like the one on the iPhone are not accurate when standing or walking. They're more appropriate when you are moving at car speeds.
     
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  3. spleck

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    Maps gives a separate indicator when you have a GPS lock vs triangulation. I do get a satellite lock indoors. The Hammerhead chip is similar to the SiRF chip I have that will work in a concrete basement. The only differences could be software (which could control power level) and antenna.

    What do you base that on? The GPS chip in the iPhone is the same type of chip used in handheld GPS.

    So really, my question is does the chip have a more accurate mode/power setting, or are we limited by the antenna design?
     
  4. RadioRaiders

    RadioRaiders RF Black-Belt
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    When you have a GPS satellite signal indoors, that's thanks to differential or Assisted GPS (most GPS enabled phones use AGPS), which is not a direct SAT link, but rather repeated signal from ground stations (same how you can get satellite radio indoors). If you are in an area not covered by AGPS (like in a cave in Africa, or maybe Middle Georgia :p) you will not get any GPS signal indoors. If you get AGPS coverage in a concrete basement, then you must have very good AGPS coverage and/or a very sensitive chip in your AGPS device.

    ...so I guess AGPS and "direct" GPS can give differing accuracies. Anyway, GPS should be a little off I beleive, as the US govt purposely makes it be off by a few meters for public use (only govt/military devices can give 100% accurate positioning). ...or since you are asking specifically about the iPhone, maybe it uses some Apple proprietary "iGPS" system the rest of us are not aware of? :p

    More info on AGPS:
    Assisted GPS - Wikipedia, the free encyclopedia

    More info on differential GPS:
    Differential GPS - Wikipedia, the free encyclopedia
     
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  5. spleck

    spleck Tool
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    AGPS is just downloading the ephemeris (and possibly almanac data) from an assistance server since each data block takes 30 seconds to transmit. Further, without that data, the GPS unit has to search for any satellite, rather than the ones it knows are available at its position. It simply decreases the time to first fix--ie getting a lock from initial power-on. There is no such thing as an area "covered by AGPS". It's just whether or not you get the data from an assistance server that has already acquired the data (which will expire) or you get it from a satellite.

    If you're signal is too poor to get a lock, you can also go outside, retrieve the ephemeris data, get a lock, then go inside and maintain that lock. My Garmin is 17 dB more sensitive while tracking than while searching. I don't have data on the iPhone's Hammerhead search capabilities, but its MORE sensitive than my Garmin while tracking (just 1dB though).

    There are no ground based differential GPS signals received by retail GPS units or cell phones. Most will support tethering for a basic DGPS system, but nothing is received wirelessly. Ground based systems are used primarily in aviation and have limited ranges.

    Consumer units use WAAS, which is space based. It consists ground based reference stations that transmit differential corrections to the satellites, which are then received by your unit from these additional satellites to increase accuracy. There IS a WAAS coverage area, which is basically the entire US. There's also EGNOS and MSAS that work similiarly in Europe and Japan.

    Yes, there is an extra military/government signal (the Precise code) that will significantly improve accuracy, but its encrypted and not available for public use. There's also selective availability which will make your location wander all about the map, but hasn't been used in a few year.

    I feel that I am quite familiar with how GPS works, so I apologize if I gave the impression that my questions were about GPS in general. I was more interested in the capabilities of the hammerhead chip in the iPhone, or how GPS chips have been implemented in other phones. I still don't know if it has different power modes, if there is a limit to the current implementation of the iPhone's location based services, or if they just put in a junk antenna.

    The 30-50 foot accuracy I was seeing could be due to a non-WAAS lock, which my Garmin will do to save power. The drop outs and lags though are inexcusable with a clear view of the sky. As I've said, I have had a satellite lock indoors with the iPhone. My Garmin can get a satellite lock in some crazy places, and it details which satellite its locked onto and signal strength for each.

    TomTom switched to the Hammerhead, and some users suggest it was providing better reception than the SiRF it replaced. This chip SHOULD be as good as the best handheld GPS units are. I'm already starting to see a big difference noted online from Garmin moving their antenna around and decreasing signal strength... :(

    Some links I've run into:
    Hardware:AGPS - Openmoko
    Confirmed: Garmin nuvi 700 units less sensitive than older 600 nuvi models (GPS Magazine)
    GPS Receiver Chip Performance Survey
     
  6. RadioRaiders

    RadioRaiders RF Black-Belt
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    ...so how then do you manage to have satellite positioning indoors? :confused: Every time I go indoors with a GPS I lose the SAT signal. I thought AGPS helped to compensate for the loss of LOS to the SAT, but I guess I misunderstood. I thought it helped with real-time positioning, but now I realize it just helped to start it up quicker.
     
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  7. Jay2TheRescue

    Jay2TheRescue Resident Spamslayer
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    A-GPS does help it lock quicker, and depending on the unit it will extrapolate your position if you loose signal. With TomTom the display switchees to B&W when the signal is lost. I've driven through tunnels where as soon as I entered the screen went B&W. It kept tracking me (I'm assuming by extrapolating my speed and calculating the distance of the tunnel) and I exited the tunnel and my display turned to color right on cue.

    -Jay
     
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  8. spleck

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    Also called dead reckoning, or inertial navigation. Usually a gyro or motion sensor.
     
  9. Jay2TheRescue

    Jay2TheRescue Resident Spamslayer
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    Would a phone have one of those? Both my Tilt and my old iPAQ 6515 did this running the TomTom Navigator software. I just figured it knew I went in the tunnel, and it knew how fast I was going and did the math...

    -Jay
     
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  10. spleck

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    I used to have an old Garmin eTrex that was just about useless. No WAAS so it wasn't great in the first place, but I couldn't use it on trails while mountain biking because the tree cover made it drop out too much. I grabbed a model bases on the SiRFstarIII and it handled tree cover, indoors, etc.

    L1 C/A signal used for civilian position is 1.023 MHz, but spread spectrum puts it on 20 MHz of bandwidth. I don't think many building structures block 1.5 GHz too much, so -160 dBm sensitivity at such a slow bitrate should make indoor reception possible. I actually think the -160 dBm is not so much a limit of the GPS chips, but more about information theory--I think its a hard limit, but I REALLY don't want to pull out my school notes on that.

    Truthfully, no GPS is going to work in real concrete basement, but tracking 12+ satellites can make up for multi-path and reflections with a reduced accuracy, so I guess that's how the good ones do it.
     
  11. spleck

    spleck Tool
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    I've seen some of the integrated GPS chips advertise dead reckoning with a motion sensor, but I don't remember which ones. It's probably just the TomTom software doing the math like you said. If you're going in a straight line at the same speed, it would be right on. Once you got signal back, I think its like less than a second to get an exact fix again. Some of the older in-car nav systems that were fairly large had gyro (probably electronic gyro, not a real spinning mass) dead reckoning that could handle turns and changes in speed. I think that was probably before this current generation of super sensitive chips that can track 20 satellites simultaneously.

    Does the TomTom software have any power management settings? What about for the GPS settings on the Tilt/iPAQ?
     
  12. Jay2TheRescue

    Jay2TheRescue Resident Spamslayer
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    There's no power management settings for GPS that I know of for either phone. GPS does suck the life out of the battery. One thing I do notice though is that TomTom cannot track if you trun off the screen, so you can't turn off the screen and just listen to voice prompts when hiking or biking. I make the best of a bad situation by dimming the screen all the way when operating like this. In extreme situations I'll put my SIM card in another phone, and put the phone into airplane mode. Airplane mode and dimming the screen I can usually squeak out up to 4 hours of GPS usage on the Tilt. The iPAQ could only operate standalone on battery power for about an hour.

    -Jay
     
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  13. RadioRaiders

    RadioRaiders RF Black-Belt
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    Actually the L1 signal is 1575Mhz......1.023 is the chip rate...but anyway not too far off ;) But anyway, regardless of the frequency, that signal is coming from very far away and is very weak. Last I checked, you need LOS to get GPS signal, and indoor GPS is not possible, except via ground base station triangulation. I don't think reflections are strong enough to provide an accurate GPS positioning indoors. But maybe I'm not up-to-date with the newest chips. Maybe it's possible. I'll have to read up on it in some free time... (are you really sure you get GPS indoors? :confused: Maybe it's some "dead-reckoning, like Jay talked aout?)

    -160dBm is extremely low sensitivity.
    -174dBm is floor for thermal noise, so -160dBm is posssible I guess, but really pushing the limit.
     
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  14. spleck

    spleck Tool
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    At this point, I'm just going to hope the issue is not the most likely issue: poor GPS antenna design.

    Based on what I've read, there are a number of other issues that can lead to poor GPS performance, most of them relate to reduced power requirements:
    tracking the minimum 4 satellite rather than 16 possible, also meaning no WAAS
    using expired ephemeris data from the location server
    performing server based location determination (gps data sent to server for processing--maybe lower resolution)
    reduced or no gain to the amp to save power

    So I still have hope for the iPhone GPS. It's hard to test when I only have one program that uses it (Google Maps), and it interfaces through Apple's LBS in a x.00 version of a product that already has x.01 and x.1 revision in the pipeline.

    By the way, here's quote from Infineon: "The Hammerhead chip provides the industry’s highest sensitivity for deep indoor signal tracking, full support for assisted and autonomous modes, and highly accurate navigation in the toughest signal environments."
     
  15. spleck

    spleck Tool
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    That extreme power usage is what makes me hope the current iPhone software is enabling some low power mode, and ONLY the low power mode since there is no turn-by-turn software available yet. I'll have to get the GPS running on the iPhone and see how long it will run. I'm not sure it will be any longer than your tilt though... :(

    Yeah, I just was pointing out its using a LOT of bandwidth (jamming resistance) for relatively little data. Around 0.05 bits per cycle. L1 carrier is actually 1575.42 MHz :biggrin: I can't find anything that specifies the exact bandwidth though. I saw mention of it being spread to 20 MHz and that the FCC block is 100 MHz, but couldn't find any secondary sources for that.

    I'm positive. :D I remember back when SiRF announced their chip, the -160 dBm sensitivity was ridiculously low and worthy of making a press release. Now all the chips are within a couple dBm. There are some AF documents (public of course) that specify -158.5 dBm as the lower limit for L1 reception, which makes me think it had to do with information theory.

    14 dBm over the thermal noise floor at 1.023 million chips per second using BPSK spread spectrum over a 20 MHz bandwidth makes for some gibberish equation that goes beyond my interest level at the moment! Thanks for reminding me about the thermal noise floor though! That was the missing variable that made me think limit was acceptable enough to not bother questioning. ;)
     
  16. RadioRaiders

    RadioRaiders RF Black-Belt
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    Thanks for the link. I'm always wary of Infenion and Qualcom "magic-chip" sales pitches, but there has to be some truth to it. And it does seem GPS receiver chip sensitivity has improved alot the last few years:
    High Sensitivity GPS - Wikipedia, the free encyclopedia

    ... I've got an 8 year old Garmin with a B/W screen. I keep seeing these crazy new Tom-Tom stuff, but haven't taken the "plunge into the future" and got one yet (not sure if I'm afraid of the new ones, or just like to be old-skool :p)

    ...anyway, interesting thread. I've now increased my GPS knowledge from "Misinformed / minimal knowledge" up to the "Huh?" level :p
     
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  17. spleck

    spleck Tool
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    Wow! Thank you for that link: "Given that High Sensitivity GPS receivers may be up to 30 dB more sensitive this is sufficient to track through 3 layers of dry bricks or up to 20 cm of steel reinforced concrete for example."
     
  18. RadioRaiders

    RadioRaiders RF Black-Belt
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    I raised an eyebrow when I read that too, but if you consider at ~1.5Ghz an 18" concrete wall is ~15dB loss, and a brick wall is ~5dB loss, then if the new chips offer 30dBm improvement, it could be true, in a theoretical (..or sales pitch) way ;).

    ...anything written by "lab people" (ie: "in theory") or "sales people" (ie: "buy our new wonder-chip") has to be taken with a grain of salt. Until I try it myself, or hear "real-world" experiences from reliable people, I'll remain skeptical...
     
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  19. spleck

    spleck Tool
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    How often is an 18" concrete wall oriented perpendicularly to a GPS satellite signal?

    What are you using to calculate attenuation through objects at various frequencies? Anything convenient and easy to use that you can share? Or just some graphs?
     
  20. RadioRaiders

    RadioRaiders RF Black-Belt
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    I guess more realistically, trees, house roofs, etc. would be in the way. The attenuation would vary alot depending on all the factors, but I could imagine if you are in an average house basement, the attenuation between you and the sky above could be around 30dB. GPS reflections would be weak, since the signal itsself is very weak by the time it reaches the ground.

    There's algorithms out there. But the more reliable stuff is based on years of experience and then summed up to industry averages. For example, I got these wifi attenuation averages from either Cisco or IEEE (can't remember which, but anyway either are trustworthy sources):
    RadioRaiders WLAN Range

    For cellular signals it's roughly the same. I think for (1900/2100Mhz) cell-planners usually plan around 20dB attenuation for dense urban areas, rural areas maybe 10-15dB. 850/900Mhz networks would be maybe 2 or 3 dB less.

    Companies who make RF simulation tools (like Ericsson, Nokia, Aircom, etc.) do extensive research in radio loss on different materials. Like cars, everyones milage may vary, but usually everyone is in the same general area in terms of loss, within a couple of dB's.
     
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  21. viewfly

    viewfly Mobile RF Advisor
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    Hi spleck.
    I first read your post today and wondered why one would be concerned with an accuracy of 30 or 50 feet. That sounds pretty reasonable. Rereading it, do you mean it tracks a course 30 or 50 feet offset from a road?

    In any case, I read an apple press release or interview somewhere that remarks that because of the small antenna size in the iPhone, it would be hard to use for turn by turn routing, esp in cities. I have a Garmin Nuvi 350 GPS. I picked the 350 because unlike the 200 or even one above it, it has the high sen GPS chip in it. I live in a wooded area...we call them tree tunnels in CT. But even out in the open, with 8 satellites, I will get only 15 feet after a couple of minutes. Before that it will start at 100feet accuracy, then go down to 30 feet, and after longer time, to 15 to17 feet.

    But in fact, for walking or in the car, it follows the road quite well.

    I got the feeling that finding yourself in your room, was a coincidence perhaps? GPS's aren't that good (well consumer verisons). BTW, assisted GPS is not the same as differential GPS, D-GPS is a much more higher class of GPS,. Perhaps you were locking onto less satellites another time, and hence the less accuracy. Besides getting a quicker fix, the A-GPS system makes use of the immense processing power of the cell towers to quickly do the math, that the iPhone wouls struggle with. Perhaps the towers were not fully supporting your phone at that time?

    I am quite satisfied with my iPhone GPS, considering that I paid only 199 for the phone compared with 300 for the Garmin. I do notice that, when stationary, the 'blue dot' will move or jump around, sometimes more accurately, sometimes less. The blue dot appears with a blue halo, which probably represents the accuracy. Regardless it gives me a good fix on where I am. This week in Puerto Rico, it told me well enough where in San Jaun I was, since I was quite lost and the street signs not always obvious.

    How would you attach a better antenna to the iPHone?
     
  22. spleck

    spleck Tool
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    Both really. (Mainly I'm picky and want to try to get the most out of everything! :D) In one case it tracked a course 30-50 offset from the road (it's worked in the same spot other times). The main problem I had was that it was having dropouts where the tracking would stop and then speed to catch up. THAT is unacceptable on a treeless road, with no large structures and only a partly cloudy day. The chip in the iPhone is a high sensitivity chip capable of better than 15 ft accuracy just like a high end Garmin Nuvi. The conditions I was in should have allowed that.

    That was by some magazine writer that many had disregarded, but he may have been right.

    When you get a good lock with 9ft accuracy, they ARE that good.

    I think the iPhone is fine for figuring out where in town you are and getting directions somewhere. In fact, its better than fine for being integrated into phone as small and powerful as the iPhone.

    Right, now the iPhone is centering the dot in the approximate area of my house, probably within 15-20 feet. However the pulsing ring covers about 80 feet diameter and the fixed blue shaded area is about 160 feet in diameter. This is with no network connection and wi-fi off (I had wi fi on and let it sit for a few minutes to apparently get ephemeris data since it took about a minute and also to download the map). It was able to get a GPS lock after losing it, without network support.

    I believe that is marketing propaganda. While GPS calculations are not easy, the GPS chip is capable of calculating the location many times per second without support of the network. But yes, there are 4 modes:
    MS-based (calculation of position in mobile handset)
    MS-assisted (calculation of position in base station)
    Autonomous (no assistance by network)
    Enhanced autonomous (using four day assistance data)

    My theory is that MS-assisted is the lowest power option, while enhanced autonomous is probably the most power hungry using GPS and network.

    Me too! It will be interesting to see if TomTom's software will perform as well as a standalone. If they implement some kind of dead reckoning like Jay has seen, it will probably be fine even without any kind of higher power/more accurate mode.

    I assume you can't unless they put a connection on the dock connector. I raise the antenna issue, because its pretty obvious the cell antenna is still in the bottom of the phone, since you can affect signal strength by moving your hand around.
     
  23. RadioRaiders

    RadioRaiders RF Black-Belt
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    Actually, after reading some more, I think my original understanding of AGPS wasn't too far off, in that it relays info (yes, ephemeris data, but other positioning info as well) to AGPS enabled devices when regular GPS coverage is not good (ie: indoors). And since this AGPS is sent to you via the cellular networks, then you could in fact say that there are areas of AGPS coverage or not, depending if you are in range of a cellular network that provides AGPS or not.

    I would say the best, and probably only way to get GPS positioning when indoors would be to have a high-sensitive GPS receiver (-160dB) working together with an AGPS server...which isn't too far off from what I originally said (...before second guessing myself ;))
    Here's some interesting info from Andrew antennas:
    [​IMG]
     
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  24. spleck

    spleck Tool
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    You're right. The initial acquisition requires a higher signal strength than tracking. AGPS data basically seeds the information necessary for tracking and probably a rough location.

    When playing with it yesterday, I don't recall if I was able to get AGPS data through wi-fi--I'll have to check again. I was happy though that autonomous mode works. Now I just need a map program that doesn't require a network connection.

    It was interesting that my home wi-fi is now GPS cataloged. The iPhone was able to get a rough location as soon as I connected to my router.
     
  25. Jay2TheRescue

    Jay2TheRescue Resident Spamslayer
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    Navizon catalogs WiFi access points... Someone running the Navizon software must have driven by your house.
     
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  26. RadioRaiders

    RadioRaiders RF Black-Belt
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  27. Jay2TheRescue

    Jay2TheRescue Resident Spamslayer
    Super Moderator Senior Member

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  28. RadioFoneGuy

    RadioFoneGuy Powered by HTC FUZE
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    IMO multi-tasking devices are not going to perform as well as a device that only has one purpose. A Stand alone GPS device depending on the quality of course is going to blow away a smart phone.

    I have an external GPS antenna from Garmin that just plugs into the Com1 port on my computer and its pretty damn accurate until you get into some thick trees then you lose some over head cover and rely more on satellites that are at horizon level, which will know you down to only 2D perception.


    You have to also think that a GPS satellite is some 50-75 (cant remember for sure) miles away from earth surface at a Freq of 1.5 mhz which you can use PCS (1900) as a vague reference for propogation characteristics. Even if you had a satellite 50 miles away blasting 1.500 mhz at 1000 watts by the time it gets to the surface its still a weakened signal where LOS is everything.

    It is possible to be on GPS in a basement due to deflection/reflection but the location would be off due to delay.

    A good test is test the internal GPS vs a Bluetooth. I have a blue tooth GPS as well but mine is not as accurate due to the fact it plugs in a cigarette lighter and doesnt have 360 degree plus verticle LOS.
     
  29. RadioRaiders

    RadioRaiders RF Black-Belt
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    Actually 12,600 miles above us :eek:

    [​IMG]

    ...so yea, it's very weak by the time it hits the ground. Also very vounerable to atmospheric conditions, and things like solar flares.

    GPS really should have LOS to the SATs, but with things like super-sensitive receiver chips and help from AGPS, I guess some indoor coverage is now possible.
     
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  30. spleck

    spleck Tool
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    The iPhone uses Skyhook, but the same principle. It doesn't appear that the client catalogs data, but it was awfully coincidental. I might change my MAC and test it out.

    That's what I was hinting at, although I would prefer a Garmin. It's been said that NavTeq (Nokia) has better US data, while Tele Atlas (TomTom) has better European data.

    Definitely. Tracks on my Garmin overlap to within a few feet when backtracking. To me it set the bar for sensitivity. In contrast though, the iPhone in my pocket is a better GPS than the Garmin in my other car, out of reach. Multi-purpose devices are getting better, with a slightly larger device they could have put in an antenna as good as a stand alone--which is really the only difference I can see other than maybe a higher noise floor with all those other electronics in there.
     

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