gpsd and AGPS

Alexey Feldgendler alexey at
Mon Oct 1 09:40:00 CEST 2007

On Fri, 28 Sep 2007 21:21:54 +0200, Ken Yale <kyale at> wrote:

> You have 4 great ideas to discard or supplement the location cache:
> 1)  CellID database inside the phone.

...or on a server, updated with contributions from many users.

Note that such database can even be used by non-GPS-capable devices (e.g.  
running OpenMoko) to find their approximate location -- you can't get any  
precision, but at least it gives you the city and district, enough for  
tasks like finding a café nearby.

> 2)  Power-on after a long timeout with a prompt to discard location.
> 3)  Flight mode with no destination typing.

This one should probably also have a prompt. Software shouldn't try to be  
smarter than the user.

> 4)  Change cell network.

With (1) working, this will rarely cause loss of location; the current  
location aid will rather be replaced with location data associated with  
the CellID. If the new location aid doesn't contradict the previous one,  
we're having a case of overlapping networks, and the previous data (more  
precise) shouldn't be discarded.

> Each of these sparks additional ideas and opportunities for improving
> autonomous GPS:
> - invalidate location cache from sort of "travel manager" application
> that knows you're flying because of schedule, change of timezone,
> invoking "flight mode", etc.  Could tie into the airport destination
> planning to get a rental car, directions, local information, etc.

Could be useful, but only if it's transparent to the user and behaves  

Another issue is driving through tunnels. Garmin receivers used to have  
this problem: when you drive into a tunnel, the signal is lost, and upon  
exit, the last valid location (entry point) is used as aid. Norway (where  
I happen to live, so I'm affected by the issue) has a lot of tunnels,  
including the longest one in the world which is 24.5 km long. After  
driving though such a tunnel, the cached location is a hindrance rather  
than an aid. As the result, immediately after exit the device could show  
your location in some unrelated point. They fixed this in newer versions  
of the firmware by using the map data: when you enter a tunnel, the device  
looks up on the map where its exit(s) are (sometimes tunnels have branches  
inside), and uses those locations as aids when the satellite reception  

Garmin devices also show your inferred location while in the tunnel, based  
on the tunnel shape on the map and your average speed during some time  
before entry. Neo has a technical advantage here because its  
accelerometers allow to perform some dead reckoning (combined with map  
data, if available).

Alexey Feldgendler <alexey at>
[ICQ: 115226275]

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