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If you have line-of-sight to only three GPS satellites then you don't have enough information to compute your altitude [1]. If you can see four satelites, the

If you have line-of-sight to only three GPS satellites then you don't have enough information to compute your altitude [1]. If you can see four satelites, the altitude can be computed. edit: spelling [1] http://gis.stackexchange.com/questions/12866/why-does-gps-positioning-require-four-satellites http://gis.stackexchange.com/questions/12866/why-does-gps-po...

archivator 14y ago
Just tried it out and my phone doesn't report any position (however inaccurate) if it's only seeing 3 satellites. Also, although it's possible in principle, I doubt the timing information on a consumer phone is accurate enough to get a possible fix from 3 data points.
cjg_ 14y ago
Ah, yes of course :)
StavrosK 14y ago
If you have line-of-sight to three GPS satellites, you can compute all three of your coordinates (one will be impossible and can be ruled out), assuming you have an atomic clock locally. If you're like most of us and don't, then you need a fourth (and fifth, and sixth, and seventh) satellite for timing adjustments. Until then, you'll get no fix.
- Retric 14y ago
  You don't need a local atomic clock for fairly accurate positioning using 3 satellites it just takes longer. Basically, what you need to know is distance from each satellite and an atomic clock let's you find that directly. However, GPS satellites are moving so much faster than you that at over time you narrow down the possible distances involved fairly quickly. Though this does take a lot of processing and is not on any consumer device I know of. Also of note, phones get a reasonable fix using base towers which can speed up GPS calculations though I don't know of any phone that does this.
  - correctifier 14y ago
    Almost all phones do use the base tower to speed up getting a fix with Assisted GPS: http://en.wikipedia.org/wiki/List_of_devices_with_Assisted_GPS http://en.wikipedia.org/wiki/List_of_devices_with_Assisted_G...

archivator 14y ago

Just tried it out and my phone doesn't report any position (however inaccurate) if it's only seeing 3 satellites. Also, although it's possible in principle, I doubt the timing information on a consumer phone is accurate enough to get a possible fix from 3 data points.

cjg_ 14y ago

Ah, yes of course :)

StavrosK 14y ago

If you have line-of-sight to three GPS satellites, you can compute all three of your coordinates (one will be impossible and can be ruled out), assuming you have an atomic clock locally. If you're like most of us and don't, then you need a fourth (and fifth, and sixth, and seventh) satellite for timing adjustments. Until then, you'll get no fix.

Retric 14y ago
You don't need a local atomic clock for fairly accurate positioning using 3 satellites it just takes longer. Basically, what you need to know is distance from each satellite and an atomic clock let's you find that directly. However, GPS satellites are moving so much faster than you that at over time you narrow down the possible distances involved fairly quickly. Though this does take a lot of processing and is not on any consumer device I know of. Also of note, phones get a reasonable fix using base towers which can speed up GPS calculations though I don't know of any phone that does this.
- correctifier 14y ago
  Almost all phones do use the base tower to speed up getting a fix with Assisted GPS: http://en.wikipedia.org/wiki/List_of_devices_with_Assisted_GPS http://en.wikipedia.org/wiki/List_of_devices_with_Assisted_G...