Subject: [Geopriv] getting position from WiFi

From: Andrew Daviel <[email-masked]>
Date: Fri, 26 Oct 2007 18:49:41 -0700 (PDT)

I just had this idea while listening to someone talk about siting 
wireless access points :

- would it be possible to get a location for WiFi enabled mobile devices 
by triangulating off 802.11 access points ?

Theoretically, it could work like GPS. You could derive an accurate 
location from an "ephermeris" of AP locations plus time-of-flight data. In practice,
I think a clock accurate to 10ns in each AP to get 10m resolution might 
be somewhat expensive, and NTP won't cut it, you'd need a hardware 
solution.

Just using the AP identity itself would give reasonable accuracy with 
normal (not high-power) access points. RFC 3825 alludes to this 
possibility, suggesting using DHCP to distribute a list of AP locations.

This technique would work indoors, while GPS and to some extent cellphone 
service do not. (cellphones won't work here; we have metal-clad buildings 
with wire in the window glass and the towers are too far away).

(The newer generations of mobile phone have combination WiFi/3G/cellphone 
connectivity and can in some cases allow a mobile Web client to roam 
seamlessly from WiFi to cellular. Future models may allow VoIP roaming 
too, so there may be some incentive to have positioning work in shielded 
locations)

-- 
Andrew Daviel, TRIUMF, Canada

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From: "Dawson, Martin" <[email-masked]>
Date: Fri, 26 Oct 2007 22:53:20 -0500

Yes - there are a number of WiFi positioning solutions already
implemented. Check out names like Ekahau, Skyhook, and Place Lab. In
fact, do a Google on WiFi triangulation and you'll find lots of prior
art.

From an architectural perspective, the specific method used to do the
location determination is a second order detail. A LIS in a WiFi access
network will invoke whichever flavour it is equipped with. The HELD
semantics for requesting location remain the same regardless.

Of course, if technology specific device-based measurements are part of
the plot, then the LIS would like to be able to invoke that measurement
capability as well. That is why the device-capability extensions draft
has been created.

Note that GPS is an overlay network specifically designed for
positioning; fully synchronized, with atomic clocks, and long code
sequences explicitly designed for accurate ranging. Using communications
networks for triangulation inevitably involves a compromise since the
access is designed for doing something else; not for location
determination. There are some terrestrial overlay networks designed to
provide location functionality - see S5 Networks for example.

Nevertheless, the good thing about using a communication network when
location is required in a communication context is that you can
guarantee that the network is where you need it. If there's no
communication then working out location is the least of your worries.
Depending on a separate network for location information creates a
serial reliability problem - which is exactly the issue when it comes to
relying on GPS in a communication application scenario. Sometimes it
just doesn't work when your communication network does - and when your
communication application needs location information. So - there's
definitely good utility in being able to use WiFi for positioning.

Hopefully, one day, widely adopted communication network standards will
include specific signalling facilities and procedures purely for
positioning purposes instead of having to make do with measurements of
signals that are just there for establishing communication and carrying
content.

Cheers,
Martin

-----Original Message-----
From: Andrew Daviel [mailto:[email-masked]] 
Sent: Saturday, 27 October 2007 11:50 AM
To: [email-masked]
Subject: [Geopriv] getting position from WiFi

Theoretically, it could work like GPS. You could derive an accurate 
location from an "ephermeris" of AP locations plus time-of-flight data.
In practice,
I think a clock accurate to 10ns in each AP to get 10m resolution might 
be somewhat expensive, and NTP won't cut it, you'd need a hardware 
solution.

This technique would work indoors, while GPS and to some extent
cellphone 
service do not. (cellphones won't work here; we have metal-clad
buildings 
with wire in the window glass and the towers are too far away).

(The newer generations of mobile phone have combination
WiFi/3G/cellphone 
connectivity and can in some cases allow a mobile Web client to roam 
seamlessly from WiFi to cellular. Future models may allow VoIP roaming 
too, so there may be some incentive to have positioning work in shielded

locations)

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From: [email-masked]
Date: Sun, 28 Oct 2007 20:00:27 -0400 (EDT)

Martin -

Excellent summary - thanks. Also should note that there are also a couple
of patents in this space. http://www.freepatentsonline.com/EP1542492.html
for example.

Regards

Carl