Cell phone tracking
Mike McNally writes:
Jef Poskanzer writes:
It looks like at least some switches in Amerika are already equipped to read out locations for individual phones.
This is not actually that surprising. All they need is to know which phones are using a band on a cell site, and they narrow the search down to a relatively small area. I seriously doubt that they can do triangulation (I mean, they *could*, but there's not much likelihood that the cellular operators would incorporate something complicated
A company I am familiar with which does specialized classified interception systems for the NSA and other TLA's has built just such a system for the TLAs. It can locate a cellphone to within a few feet just as soon as it starts transmitting - it uses time of arrival techniques to triangulate the cell phone's position. How many of these are installed and where I do not know, but the technology has been developed and is in use. The system is multi-channel and can keep track of many cellphones at once - but as a practical matter it isn't hard to monitor the control channels and paging channels to locate the phone of interest and identify which of the 866 channels it is transmitting on so even simple doppler DF technology might work. Considering that the LA area is the biggest cellular market in the country it wouldn't surprise me that some of the these systems are installed there. 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. Dave Emery die@pig.jjm.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
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Phil Karn