why did old (like 90s/2000s) GPS take up to 15 minutes to get a lock, in giant bulky units, but my running watch gets it in 15 seconds?
In: 3751
Nobody’s actually posted the full explanation, at least not in the top ten comments I went through. This’ll be buried, but searchable.
1. The almanac (ephemerids, etc.) was (and is) broadcasted at a very low bitrate but is necessary for any GPS receiver to obtain a fix. The receiver needs to know exactly what orbit each satellite it wishes to use is following, since GPS works on light lag. Today it can be downloaded over the Internet, so the receiver has it very quickly.
2. Nearby WiFi networks are often catalogued and you can be located using them, but so are nearby cell towers. They all have their own unique ID, and this can be referenced to location using online services. If your phone can see WiFi BSSIDs x, y, z, and cell towers A, B, C, and D, an online locator service (Qualcomm operates one for its Snapdragon A-GPS systems) can get you to within around 50 metres in most cases.
3. (Which is often missed) Cell towers are sector-based. Your carrier will know which sector you’re on, which tower you’re on, and how strong the signal is, it then has a rough direction and range, which it can provide back to your device. If you’re doing multi-cell MIMO, you’ll be on more than one tower, enabling the carrier to triangulate you better.
What tends to happen is you first get a very rough location, from the cell tower data. This data is being piped to your smartphone constantly anyway (for things like hand-off to another tower, it’s basically “Tower 34 will be stronger than me soon, look for it and connect to it when you can”). It will then be sending out a request for an updated GPS almanac while it sends the tower ID and WiFi visibility to another service, and while it listens for GPS (or Glonass, or Galileo…) signals.
So you see the blue inaccuracy circle shrink on Google Maps as these requests complete and the Internet tells your device where it is with more accuracy. Finally, GPS will get a 2D lock (a WGS-84 surface position) which will shrink the circle to less than 10 metres, and then a 3D lock (includes height above or below WGS-84), which will get the circle down to less than one metre.
Maintaining a GPS lock is quite battery intensive, so the high-accuracy location isn’t maintained while the screen is off (unless you change this setting), and it’ll do periodic WiFi and cell tower scans to maintain background location.
Latest Answers