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Comment by cryptoz | Hacker News Reader

I was really hoping for a barometer so I could recruit iOS users to pressureNET. No such luck. Maybe next time.

Yes I think you're the cat I was looking for!

I recently got a Galaxy Nexus and I remember reading here a while ago about your app that crowd sources barometer readings from phones that have it but I couldn't remember what it was called when I got the GNex. I'll go sign up. Thanks.

Cool project dude!

But why on Earth does any phone ship with a barometer? Weather is the only application I can think of, and surely it's easier just to pull weather conditions from the network.

It's mainly[0] to give the phone/tablet a rough altitude reading to make GPS locks faster.

[0] https://plus.google.com/112413860260589530492/posts/jVJhPyou...

EDIT: relevant thread

That makes no sense. My iPhone takes two seconds to get a solid GPS lock and it doesn't have a barometer. My stand-alone TomTom car GPS takes about 30 seconds to get a lock from a cold start, which is entirely attributable to the time required to download the satellite positions over the extremely slow 70s-era channel that they're broadcast over. Actually solving the equations takes no appreciable time on anything resembling modern hardware.

As far as I know, aGPS as found on modern devices is all about getting that almanac faster (which is why my iPhone only requires two seconds), not about using a rough fix to then get a better fix.

I realize that isn't your post, but it seemed appropriate to reply here since you posted the link.

Many android devices support actual GPS, not just aGPS, so you can use them in places where you have no cell signal (say, while hiking).

Your iPhone takes 2 seconds because it's never a cold start. You're always connected to at least one cell tower you know the location of, you have several nearby wifi access points, etc. It might not be a feature you need for your limited use case. Cool. Then you won't miss it.

>Many android devices support actual GPS, not just aGPS

Every phone with A-GPS support actual GPS. A-GPS is additional feature on top of regular GPS.

There are actually two different kinds of A-GPS as best I understand it.

One is the bad kind where GPS only works when you have a cell signal. It offloads a lot of the processing to the cell tower. This is used by cheap phones to comply with FCC requirements for location reporting for 911 calls, and isn't found much elsewhere.

The other is the good kind where it grabs the almanac off the network when possible for faster cold start times. This kind still works fine when you have no connection, it just takes longer to figure out your location when you turn it on, because it has to download the almanac the slow way.

They're basically unrelated except for the name, which is confusing.

You are misinformed. Assisted GPS is simply a feature that reduces the time-to-first-fix of GPS. Triangulation from cell towers exists, but nobody calls it A-GPS.

I've used the iPhone 3G, 3GS, 4, and 4S extensively in the wilderness. Every version since GPS was introduced has been "real" GPS.

I'm not talking about cell tower triangulation, I'm talking about offloading computation to more capable hardware located remotely. It exists:

http://en.wikipedia.org/wiki/Assisted_GPS#Modes_of_operation

I can't find anything about chipsets that only do that, but I'd be surprised if nobody had ever done it.

You don't need cell signal for the GPS to work - in fact, you don't even need to have a SIM inserted or the WiFi to be on... It just locates you faster (that's why it's called "Assisted")

A-GPS is about downloading the almanac over the fast ground data link. Solving the equations for partial signals is DIFFICULT and is where most innovation is these days.

Your phone takes two seconds (and so does mine) probably because you have WiFi on and you live in the States. Try living in the middle of nowhere in, say, Serbia, and see how fast you get a lock, regardless of WiFi.

Except that my years-old TomTom can figure out my location as fast as it can download the almanac. Clearly the equation-solving part is not a limiting factor there.

Actually the 4S is much faster at a fix than the 4 because it has more satellites to look at (GLONASS). More satellites means a faster, more accurate location fix.

It can't be assumed you'll always have a data connection to grab aGPS data, or that your particular nearby WAPs and cell towers are properly mapped out. Having extra data poins is rarely a bad thing.

MEMS barometers are commonly used for relative altimetry (e.g. stair climb calculations on activity trackers, etc).

How accurate are they for relative altimetry indoors? I could imagine climate control throwing in a decent margin of error if all the inputs/outputs (vents and blowers?) aren't balanced correctly.

Edit: They probably carry barometers already, but AC installer people could make use of phones with this hardware.

I don't think HVAC systems will do much damage there. I have a gadget on my airplane that gives altitude accurate to the foot, and that's while moving 60MPH or more through potentially turbulent air.

I am thinking more about actual differences in atmospheric pressure between indoors and outdoors, or even between rooms in a building. If the ventilation system is installed correctly this shouldn't happen, but if it is not then you can cause either an overpressure or underpressure in the building.

I don't think buildings will sustain much of a pressure difference. For example, a 10ft-equivalent pressure difference will exert a roughly 16 pound force on a standard residential door, which would be highly noticeable, and it's probably tough to seal a building well enough to contain that kind of pressure.

Hmm, I had no idea that pressure differences between altitudes were that great. Fair enough.

Altitude measurements. My (standalone) GPS 10 years ago had it as one of its main features. The extra hardware needed on a phone can't be that much.

GPS gives you altitude already, no need for a barometer. It's not hugely accurate, but it's more accurate than an uncalibrated altimeter.

but a properly calibrated barometer is WAY more accurate than GPS. 10 years ago I could track a two meters offset (that's the difference between your head and your feet!).

Indeed, but I can't think of any way to calibrate the thing other than looking up your location and pulling down weather data, or simply forcing the user to put in an altimeter setting.

They're very useful for fine-tuning altitude readings.

So it knows when you change floors for indoor maps?

Oh. You were serious. I feel bad for laughing now.

Yeah, what a loser for collecting weather information... ?

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