On 12/16/2012 05:40 PM, Graham / KE9H wrote:
In addition to knowing where the GSM cell site is, you time stamp the time of arrival of a specific feature in the cellphone signalling system. If the cellphone is heard by three (or more) cell sites, then you can calculate the location of the cellphone within the cell site using the time-of-arrival and speed-of-light calculations.
Since GSM is a TDM system, the arrival time of the TDM slot is the typical feature to measure. The active base station already does this, and send trimming values to steer the hand-set to stay within it's time-slot. This way you have a range measure, but it isn't enough for triangulation, so you would need to have another station measuring it too, but GSM will not usually do that, so you need to have a secondary receiver, tuned to the neighbour frequency in that direction.
The CDMA systems inherently depend on knowing time to sub microseconds in order to function. You can extract similar information from the signalling systems in CDMA.
You inherently have the same knowledge in GSM, it's just that you don't care about absolute time, but relative timing between the hand-set and the base station is being measured and steered.
GSM stations being phase-aligned have better hand-over properties, and thus releases the channel in the cell the phone is leaving quicker, and thus increases the capacity... and allows new customers in and thus more money. So, well-timed base-stations is good for GSM too.
Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
