4 ms·

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/jVJhPyouWDP

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/jVJhPyouWDP https://plus.google.com/112413860260589530492/posts/jVJhPyou... EDIT: relevant thread

mikeash 14y ago
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.
- archivator 14y ago
  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.
  - mikeash 14y ago
    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.
- calciphus 14y ago
  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.
  - anonymfus 14y ago
    >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.
    - mikeash 14y ago
      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.
      - kaiuhl 14y ago
        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.
        mikeash 14y ago
        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 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.
- juiceandjuice 14y ago
  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.
- ConstantineXVI 14y ago
  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.
  - deleted 14y ago
    [deleted]
  - mikeash 14y ago
    As I pointed out, my completely isolated GPS still gets a lock pretty much immediately, once it has the almanac. That is the limiting factor. It's hard to see how a barometer would help much, especially since it could be off by 1000ft or more when not calibrated (i.e. always.)

mikeash 14y ago

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.

archivator 14y ago
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.
- mikeash 14y ago
  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.
calciphus 14y ago
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.
- anonymfus 14y ago
  >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.
  - mikeash 14y ago
    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.
    - kaiuhl 14y ago
      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.
      - mikeash 14y ago
        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 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.
juiceandjuice 14y ago
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.
ConstantineXVI 14y ago
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.
- deleted 14y ago
  [deleted]
- mikeash 14y ago
  As I pointed out, my completely isolated GPS still gets a lock pretty much immediately, once it has the almanac. That is the limiting factor. It's hard to see how a barometer would help much, especially since it could be off by 1000ft or more when not calibrated (i.e. always.)