Does anyone know how many gps satellites the iphone 3G gps chipset can read signals from? Am curious about that and no one in the Apple stores can answer it. You really need at least 3 to triangulate well. But more is better.
My Garmin gps can read from 12 if they are accessible. Usually get at least 8 plus strong signals. Am sure the iphone doesn't read that many and doesn't need to for its purposes. But how many signals can it read?
The product brief for the PMB2525 used in iPhone 3G doesn't specifically state how many satellites it can track simultaneously, but the product brief for Broadcom's BCM4750, which is based on the same IP from Global Locate, can track 24 satellites at once.
I am a little confused. My Verizon phone was able to roam on GSM because they used TDMA. Tell it was shutdown. The phone recognizes it as Analog. If PCS has TDMA, It could be technically be used on GSM.
Originally Posted by Tabla
Y'know, I'm used to hysterical 14-year-old ******** on the internet, but this is exceptional. Never before in human history have so many nerds hyperventilated so publicly over so little.
The iPhone's GPS is quite a bit less accurate than dedicated devices. Dunno if this is hardware or software, or both, but it is very noticable. Good enough for casual use, like looking for the nearest Starbuck's, but not so sure it'll be good enough for turn-by-turn.
You actually need four to get a good solution, because the clock in the receiver is nowhere near accurate enough (i.e. atomic clock quality), and with the GPS satellites zinging past at thousands of miles an hour, you need to know the exact time to determine their location (and hence, your own).
What a GPS unit calculates, is the ratios of distance from the satellites. Having a fourth satellite lets the unit slide the ratios back and forth until they meet at a point. This also gives the unit's clock offset.
A-GPS sometimes has timing and other aids to use fewer satellites at the unit.
For those who are curious, one reason you want more channels is so the unit can track more satellites at a time. Four or five give accurate results. More channels allow the unit to track spare satellites in case one or more go out of sight for any reason (such as turning a corner).
the iPhone should be able to lock to as many as is visible to the phone. All the satalites aren't avaiable at once so whichever of the 32 (i think it's 32 of them) that are there it will use. On my Blackjack II using Garmin Mobile XT i see about 6-9 most times but it only locks 4-5 of them, the ones with the strongest signals.
Thanks for the replies. I agree that tracking 4 or more satellites is best. That's why I said more than 3 is better. You can triangulate well enough on 3 but it isn't the best for placing you where you really are.
I thought GPS satellites were in geosynchronous orbit or just plain geostationary. Is that not correct?
Would like to know for sure how many the iphone 3g can lock on though. Wonder if that info is out there somewhere? I can't seem to come up with it.
24 minimum - with on orbit spares, it's 32 right now.
OK I got pulled away from the comp with the reply window open so I missed this reply before I sent mine. Are you saying the iphone will track that many or that is how many are normally in the sky at a given location to use for positioning?
Thanks for the replies. I agree that tracking 4 or more satellites is best. That's why I said more than 3 is better. You can triangulate well enough on 3 but it isn't the best for placing you where you really are.
Right, because with only three satellites your GPS has to assume you're more or less at sea level. For every meter it's wrong about the altitude, the position will be approximately a meter off horizontally.
I thought GPS satellites were in geosynchronous orbit or just plain geostationary. Is that not correct?
A very common misconception. They're in medium orbits that circle the globe twice a day.
Another misconception is that their orbits enclose the earth, including the poles. They don't. Their inclined orbits carry them directly over 55 degrees north latitude down to 55 south.
24 minimum - with on orbit spares, it's 32 right now.
And I believe the spares are live, in order to give receivers more chance at a good geometry.
That is, they're all live except for number 32, since many receivers assumed there would be only satellites numbers 1-31... and they choke on an id of 32.
Many people are mentioning that you need 4 sats to get a full GPS lock, but I've had multiple times on my old Garmin E-Map where I've gotten a full lock with position, speed and Altitude (airborne doing 500mph, over Texas at 36,000ft) with only three weak GPS signals while on an airliner. It probably wasn't pinpoint accurate (i think I remember seeing accuracy around 50-75 feet) but it worked without a 4th signal and that unit wasn't WAAS compatible......
I tend to get about a 3 yard or so accuracy when I locate myself. I can't really vouch for being in motion though.
Same here. Standing still, the GPS can usually locate me within a few feet (using satellite view to verify my actual position on the map). I have no problem with GPS accuracy at all. In fact, it's been working better than many standalone units I've used previously. At high speeds there's definitely a lag between the actual position and the position indicated on the map. I believe this to be software issue rather than a hardware issue, so hopefully it could be fixed at some point.
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Originally Posted by gpatrick900
- yeah, don't listen to that guy!
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