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RE: In-building location determination using wireless?

The wireless is an interesting idea, getting the signal strength is not =
hard (done at the driver level).  The variation that Paul points out we =
can call noise.  This noise is high because 2.4Ghz (the frequency that =
802.11b runs at) is terrible for multipath and attenuation by common =
building materials (which is why it was chosen).  A common technique to =
mitigate noise is just add more signal (SNR ratio).  If three access =
points are not enough (this is what you would need to define a plane) =
add a couple more.  In your software make a decision for each access =
point:  does the signal strength make sense?  If not, don't use it.  =
Another step that could be made is directional antennas, most access =
points allow you to add them.  Now you can put bounds on your algorithm =
by knowing you are in a given cone.  (This will be very "fuzzy" because =
of the multipath effects.)   Overall the 802.11b would work well if you =
were in a warehouse (CMU Robotics Institute for example) or a cubical =
farm or a real farm.  The reasons it would not work well is not the =
networking protocol but the frequency that it runs at.  This is why you =
see these types of "relative" positioning systems done at other =
frequencies.

- Jason W Fox
- Electronics Engineer
- (202) 404-3736
- SPG, ENEWS, Code 5707.56
- Tactical Electronic Warfare Division, Naval Research Lab
- Washington, DC

-----Original Message-----
From: Steve Barr [mailto:]
Sent: Friday 30 May 2003 22:14
To: 
Subject: Re: In-building location determination using wireless?

McCarty, Paul wrote:
> I would say this is next to impossible.

Oh, if you search the web it's clearly been done; the
accuracy may not be thrilling, but it could be good
enough for some uses (where are the nearest exits, am
I near my desk and moving toward it or away from it,
etc).

> I know there are some hooks to measure signal strength, but all it =
takes
> is one wall thicker then another and you lose signal strength. Or a
> floor say? You'd be better off writing coordinates on the wall! :-)

This afternoon I did joke with a coworker about OCR'ing the
office numbers to get location.

> Another alternative is they sell GPS repeaters.

Thanks; I didn't know that.

> I would suggest looking into an intertial/accelerometer sensor.

Thanks; that's an interesting idea.  I guess you could choose
a spot at which to activate it at each location (home, school,
work, etc.)

I am interested in workstation-class systems (whatever
that means) ;-) as wearables, so it would be reasonable
to assume the processing power would be there to compute
the location from either wacky wireless signals or
other sensor data.

Mainly I was hoping to leverage existing work so I
could concentrate on the aspects of wearables I
want to pursue without losing time recreating
existing work.  I guess I can always fake it and
enter the locations manually.

Steve
--=20
http://www.stevebarr.com
100% my opinions.

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