I would like to locate people in a building to +/- a meter or so.  I
suppose differential GPS will not work because of multi-path errors. 
Inertial systems might- but could be costly.  Is there some technology
or product that uses differential/Doppler techniques with 'wall
penetrating' VHF or UHF signals?

Hello!

Since you are looking for an application that works inside a building,
any outside signal source like GPS is probably going to be a problem. 
A solution with a internal signal source is probably the answer.

While no off-the-shelf solution exists just yet, a few applications
are on the right path:

"Wi-Fi Software Tracks You Down"
http://zdnet.com.com/2100-1105-962587.html

Ekahau Corporation
http://www.ekahau.com

"The Ekahau Positioning Engine (EPE) 2.0 can locate 802.11-connected
wireless PDAs, laptops and other terminals--including voice over IP
telephones--to, the company claims, within about a meter."

You can see a recorded demo of the system at
http://www.ekahau.com/products/positioningengine/demo.html.  From the
website it seems that average accuracy is about 0.9 meters (about 35.5
inches).  If your location is fixed, you can adjust the error
correction vectors to make the measurements more accurate.  Anyway, it
looks like it meets your requirements.

The Wi-Fi (also known as 802.11b) wireless technology has become very
commonplace and low-cost over the last few years, and if previous
technology trends are any indication it will be virtually "free" to
install in hardware before the decade is out.

I could see the Ekahau system used in conjunction with a setup where
all your users carry PocketPCs or some other transmitter that uses
802.11b.  I've also read articles about a company named Vocera that is
developing a "Star-Trek like" communicator that uses 802.11b.  It
seems a little more convenient to carry than a PDA.  Use this with the
Ekahau engine and you're only a server away from "Bridge to Picard"!

Vocera Communcations Badge:
http://www.vocera.com/products/products.shtm

Good luck with your project, it sounds really cool!

Search terms:
http://www.slashdot.org -> "802.11 location"
google: "star trek communicator 802.11b"

Fine.  I got the answer- may not like it, but that is the breaks!!  Pay Koz-ga.

Virtual Reality techniques often require tracking of human locations.
Here is a link to a survey of human movement tracking technology

http://www.cs.sfu.ca/~amulder/personal/vmi/HMTT.pub.html

GPS to that level of precision is not practical at this moment, maybe
never.  You are correct about the differential and multipath errors. 
You also get massive signal attenuation in the core of most solid
buildings.

Questions:
1) What type of building is it?  
2) How many floors, walls, etc?
3) How much are you willing to spend?
4) Can the person be connected to a wire or is this wireless only?
5) How obtrusive can the mobile part be?
6) How many people do you want to track at once?

Have you thought about RF Id tags?  Each tracked item would get one. 
They are small.  The building can be wired with zone sensors that
track each tag as it passes.  If you need more resolution in an area
(bathroom vs waterfountain) then you install more zone sensors with
smaller zones.  If the building is large and you need lots of
resolution, this can be expensive.  The good part is the tags are
cheap.  Once installed, cheap tags and a high tag recycling rate make
it easy to run.  The upfront costs are not small.

  -dave

Would that give you a position within a meter of accuracy?

RFid tags operate differently than GPS.  You can set up zones that
will give you precision to 1 meter (or whatever you want) but you have
to have a receiver in the middle of each such zone.  This means you
need to space receivers about 2 meters apart, which can get expensive.

I need to know more about the application to give you a really good
answer.  I know I am not supposed to post private info like e-mail
addresses here to avoid free contact outside this web site, but I
don't know how to give you a better answer for your specific need if I
don't find out more.  Without more info, here is my best guess at what
will work for you.

I assume you want to track people moving inside a limited area such as
a building or a campus of buildings.  Usually, people move through
defined travel points such as entrances, elevators, escalators, etc. 
These are ideal for locating sensors.  With a bit of cleverness, you
can actually determine which direction the person is moving in by how
the sensors report.  Establish these choke points around the building
and you will have your people located.  You can space recievers around
choke points on a floor of the building to determine how traffic
flows.  You can move receivers into less populated areas as needed to
get more resolution and accuracy.  The part that people carry is cheap
and may not even need a battery.  If it does, the battery operated
ones are very small and use a lithium or hearing aid battery to power
it.  I have never actually used these systems, but I get asked to
research and report on them every few years as an alternative.

I have also heard there are some clever barcode scanners that are good
for several meters.  That might get to be a bit wierd, though, since
it requires everyone to wear a visible barcode.  Very Big Brother.

It all boils down to what you need.  Without knowing more about the
specific circumstances, I can only guess.

  -dave