[2031] in cryptography@c2.net mail archive
Re: Mobile phones used as trackers
daemon@ATHENA.MIT.EDU (Phil Karn)
Fri Jan 2 19:00:43 1998
Date: Fri, 2 Jan 1998 15:50:09 -0800 (PST)
From: Phil Karn <karn@qualcomm.com>
To: andreas@artcom.de
CC: cryptography@c2.net, karn@qualcomm.com
In-reply-to: <y8aoh1vvnyu.fsf@horten.artcom.de> (message from Andreas Bogk on
01 Jan 1998 23:17:13 +0100)
>Well, in the case of GSM, the propagation delay between the terminal
>and the cell is measured and compensated in order to have tighter
>timeslot boundaries. The resolution of this measurement is about 500
>meters.
IS-95 CDMA uses a chipping rate of 1.2288 Mc/s, which is roughly
comparable to the GPS C/A code (1.023 Mc/s). So one would think it
could do as well as GPS in locating users, but there are problems.
As a passive (receive-only) system, GPS requires that you see 4
satellites to determine the four unknown values (latitude, longitude,
altitude and clock offset -- the latter being required even if you're
only interested in the first three).
Active closed-loop ranging systems (e.g., Qualcomm Omnitracs and some
proposed cellular ranging schemes) can use fewer measurements because
any clock offset cancels back at the sending station. But you are
often plagued by multipath propagation problems in urban environments.
Spread spectrum is good at dealing with the adverse effects of
multipath when you're communicating. But when you're also ranging, you
need that direct (unreflected) component so you can get an accurate
time delay measurement. And most of the time in urban canyons you
simply don't have a direct component to work with; it's all multipath.
Phil
