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What has allowed us to go from a unit (eTrex for example in year 2000) that would take many minutes to acquire GPS lock and be unable to do so indoors, to a modern android phone that will lock in a matter of seconds even inside on the ground floor of a multi-story building?

Is it simply receiver sensitivity?

Keith
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As with most things, there are several things that have made acquisition time faster:

  1. The very first GPS's could only receive a signal from one satellite at a time. It would have to switch quickly between satellites to make it seem like it was doing more. Modern GPS's can receive 12 to 24 satellites at a time.

  2. Decoding a GPS signal is computationally complex. Modern GPS receivers have more "decode units" (sorry, I forgot the official name for them), allowing them to lock onto the signal quicker.

  3. Modern GPS receivers are more sensitive and can receive weaker signals.

  4. Some GPS receivers have a barometric altimeter, which normally helps the GPS get a more accurate position fix when not many satellites are visible-- but also helps get the initial fix quicker.

That being said, there are some things that smartphones do that normal GPS receivers cannot. I suspect that this is the true reason for the new Android phones quick fix, and for getting a fix indoors.

Smartphones use three different types of positioning methods: GPS, Cell Tower triangulation, and WiFi. With cell tower triangulation it basically detects the signal strength to/from several cell towers and triangulates from that. WiFi positioning is basically detecting a WiFi hot spot that has a registered latitude/longitude.

Cell Tower triangulation and WiFi is fairly fast, but not very accurate. When you bring up a map on your phone it will first use cell tower triangulation for its position. When the GPS receiver gets a fix it will switch to using the GPS. If you are indoors where GPS signals can't reach then it might never switch to using GPS. That's why your position on the map may change significantly (and several times) during the first minute or so of using your phones map.

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    Knowing approximately where it is from the other methods, and which satellites are above the (natural) horizon and so potentially receivable could be a help in speeding things up, too. – Chris Stratton Nov 20 '11 at 01:39
  • To expand on the WiFi point, it is worth noting that the phone software is distributed with databases containing IP addresses and coordinates, and the devices can (and do) send location and access point information back upstream to improve on the data. more info – deizel. Nov 20 '11 at 06:55
  • The question was orientated towards the answers in points 1,2 and 3 from David's answer. It makes sense. Chris, the eTrex knows what satellites are above the horizon, as long as it is where it was when powered down and has correct date/time, but it still won't even lock on even one of them when inside. The android phone will lock on 7 satellites within 60 seconds, inside the house with all radio turned off thereby making the other methods of Assisting GPS unavailable. Deizel's point is also interesting here - If the GPS is so good now, is there a need for all this IP-location collection? – Keith Nov 20 '11 at 12:51
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    Don't forget the ephemera database. A "pure" GPS unit trying to get a completely "cold" fix might need up to 10 minutes to download a complete error-free copy from a single satellite's telemetry bitstream. An Android phone can just fetch a copy of the same data from a server via http over the internet in < 200ms. If satellites stagger the ephemera data, a unit capable of receiving multiple streams at once could probably scrape a complete aggregate copy from multiple satellites in under a minute. – Bitbang3r Dec 03 '12 at 22:34