Re: Cell phone tracking

[Phil Karn <karn@qualcomm.com>]

> 	And in the future Phil Karn's company Qualcomm's CDMA digital
> cellphones will provide few feet accuracy position as a byproduct of the
> spread spectrum receive correlator operation on every transmitting phone
> within range of more than one cell receiving site unless they actually
> aviod trying to make the measurement.  Most of the time more than one
> cell site tracks a given phone so they can vote on which one has the
> stronger signal - given that each of these sites has a precise estimate
> of the time of arrival of transissions from each phone it takes little
> more than netting of the time base (with GPS ?) between the cell sites to
> detemine cellphone positions since the positions of the cell site
> antennas are well known.  I suspect that if the hardware and software to
> do this (mostly software) is not part of the current base station that
> certain TLAs will pay to have it developed and implemented. 

Yes, as I described in earlier mail this is certainly doable with our
CDMA system although position location was not one of the original
design goals for the system nor is it a requirement for the ones we're
currently building.

During drive tests in the demo/test system it was easy to tell by
watching a display in the cell when the test van had returned to the
starting point. We had a continuous display of round trip time which
would increase and then return to its original value. But we don't do
anything useful with this information at present. For data logging
purposes on field tests, we now use commercial Trimble GPS receivers
that have piezo gyros for dead reckoning through the GPS coverage
shadows. They work much better than the compass/wheel rotation counter
scheme used in the ETAK units we had before.

We already "net our time base with GPS" - each cell site has a GPS
timing receiver that controls frequencies and spreading codes within a
microsecond (limited mainly by SA dithering). This is needed to make
our soft-handoff work, but it does have the side effect of making
a potential location-determination system more practical.

Phil