[Paul Alvarez]

Can someone explain the GPS performance on the iPhone 3G? I've tried it on several occasions and sometimes when searching I only get the blue circle showing my location. Other times, after the blue circle pulsates a bit, it collapses to the blue dot that then looks like a radar pulse.

Why does it not do the latter all the time as shown in the TV commercials?

Just curious.

[mduell]

Because it's not very good.

[Mastrap]

A small GPS unit, as found in the iPhone, needs a clear line of sight to the satellite. It can get confused by tall buildings or in buildings themselves. The signal from a satellite can bounce from walls and surfaces, making it difficult to obtain 100% correct location services. This is pretty much the norm for most hand held GPS devices.

Your iPhone will report your accurate location in most outside places. It should work in a car without problems, but once you're in a building all bets are off. It simply loses touch with the satellite.

The GPS build into my car is a far more powerful system. It's also quite a bit larger and has access to more power and a far larger antenna. As a result it manages to read a signal even within concrete parking garages.

[Simon]

Because it's not very good.

Nonsense. No other $199 cell phone offers better GPS. There are definitely better GPS systems for dedicated use, but not in cell phones. And even very expensive dedicated GPS systems aren't perfect. No GPS device will give you maximum accuracy at all times.

Whenever you have obstacles (a tree can be enough) or reflections (lots of buildings or metallic structures around you) signal quality will suffer. And with that your positioning accuracy suffers. The iPhone uses A-GPS which already helps a lot compared to the "old-fashioned" regular GPS, but it sill doesn't offer exact positioning at all times. High rises, trains, tunnels, subways, etc. are bad environments. Cars are usually quite OK. Open countryside is ideal.

Whenever you get the blue circle the iPhone is showing you a measure of the uncertainty with which your position is being determined at the moment. When you get just the dot, you are getting high accuracy. This will not be better than a few m laterally at any time though.

[ghporter]

To make it very clear what's going on with any GPS, here's a little lesson.

To establish its location, a GPS unit has to get signals from at least 4 satellites that are in different parts of the sky, preferably all in different places near the horizon. With only 3 satellites, the unit can determine Lat/Long, but not altitude; with only 2, it can't do much at all. Better receiver units have as many as 12 discrete receivers, each of which can lock onto a different satellite's signal. The downlink data in those signals includes a lot of information, but especially it includes a time hack; by comparing the transmission time differences between multiple satellites, the GPS unit's logic establishes latitude, longitude, altitude, direction and speed through an "interesting" piece of linear algebra. It's also important to note that these satellites are in "half-synchronous" orbits, rising over any spot on the planet every 12 hours, and they orbit at about 11,000 statute miles in altitude. Their transmit power is 50 watts or less. That ain't much!

So let's look at the "city scenario" and see what it does. You're surrounded by buildings, most of which are built on either steel skeletons or steel-reinforced concrete. Interior partitions are framed with steel framing. Interior walls are sheathed with gypsum board, and the walls are filled with conduits and pipes. ALL of that blocks radio signals very thoroughly. So the outside of a building blocks signals from satellites "behind" it, but it also reflects signals from satellites "in front of" it. That confuses GPS units and reduces accuracy.

Further, it doesn't take much to block a GPS signal. The roof decking of my house is made of "Tech Shield" material, which is oriented strand board sheathed with a very thin layer of aluminum foil for thermal management. Not only does it block much of the summer heat, but it blocks A LOT of radio signals, including much broadcast TV and ALL GPS signals. Heavy forestation, especially in rainy weather, makes it difficult to get a good signal with a civil GPS unit. Military units use a more robust antenna than hand-held civilian models, and thus get better signal strength, so that's not a real problem for them, but your tiny little Garmin or Magellan gadget is going to have difficulty. Your iPhone, which has a VERY small GPS antenna, will have a bit more trouble.

[osiris]

I spent the weekend racing around in a convertible - the gps was amazing.
On the way home the top went up, and gps was useless unless it was up against the front window.

My Garmin handheld (the inexpensive yellow model) is slower compared to the iPhone. Acquisition seems to take a lot longer.