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Comment by cjg_ | Hacker News Reader

GPS provides accurate 3D positions. However if you know roughly your position the GPS fix can be acquired more quickly. So the barometer only helps you with an estimate Z pos, which helps you get the GPS fix faster.

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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 altitude can be computed.

edit: spelling

[1] http://gis.stackexchange.com/questions/12866/why-does-gps-po...

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.

Ah, yes of course :)

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.

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.

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_G...

The question to me is why would someone want a faster Z.

Normally people only view their GPS in 2D, granted elevation could be useful for GPS speedos and the like, but when inside a car the barometer will lack precision.

The Private Pilot (ie n00b of t3h pilots!111!) in me knows that its incredibly unlikely to have issues seeing many GPS sats when flying, and I wouldn't ever want a static source that wasn't outside.

So I think this will be only a speed optomisation at most, however combined with GPS posistion and tempurature, such barometric data could no doubt be of use to weather forecasters who are trying to model incredibly localised patterns maybe?

No matter what your screen is showing you, you are currently occupying three dimensions of space, and if you want an accurate representation of that without already knowing at least your rough location, you need to lock on to more satellites than you do if you already know approximately how high you are.

At least, such is my limited understanding.

Also if I'm plummeting out of plane in the middle the night with no sense of position, then I'm not saying relying on my phone is a good idea but knowing when to pull my chute so I don't spend too much time in the stratosphere would be pretty handy.

I mean I'm not saying it's likely but...luck favors the prepared right?

As I said I support faster GPS (my last line in the post). I think it might be a faster, data light alternative to the A-GPS thing, but I can't really see it narrowing it down that much?

I'd have thought dual tuning GLONAS and GPS was better? After-all the error for these sensors will be vast depending on where you are. My understanding (may well have changed, I don't know) is that currently most work on one or the other, not both.

A reason I am saying this is I remember the first time I flew a plane in Asia after learning in mild weathered UK I was getting challanging readings from my static source. As such I'm not sure how well it would work as an optomisation.

I'm not saying its bad, I'm not saying it's not fun for geeks considering the no doubt low cost of this scale.

But I still don't see the need for faster Z, as most smartphone users are on the terraferma?

"The question to me is why would someone want a faster Z."

Cyclists, Joggers - anybody who uses their iPhone for logging external activities is frustrated with how inaccurate the iPhone is for capturing Z. It's not completely useless, but it's not very precise.

I bought one of the handle bar mounts for my phone and have really rather accurate Z, this is also in London too, AFAIK the software isn't using map data to help either, but surely that would be a good way of doing it too. Archway Hill has always been accurate, even on my old Garmin thing.

Again I think it is mostly for faster aqusistion, as i doubt how this will help anyone who isn't keeping their phone in a correct posistion for accurate sampling. These barometric sensors weren't that accurate 5-7 years ago when I played with them, I can't imagine them helping when in a pocket or waterproofed running strap / cycle case.

GPS coordinates are calculated off what are known as psudoranges: http://en.wikipedia.org/wiki/Pseudorange. You can imagine these 'ranges' as a calculated distance to a given satellite. Knowing the altitude ahead of time allows you to calculate your 3D position faster because you don't need as many range samples, or satellites, to do accurate estimates of position.

The barometer gives you better resolution on the Z pos. So it's useful for measuring changes in altitude and the like, provided the weather conditions don't change (resulting in a change in atmospheric pressure). Though without some kind of calibration, you can't determine your altitude from the barometer alone. It can also be useful to calibrate when enough GPS satellites are visible, then continue providing the altitude, even when there aren't enough satellites in view.

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