SMS coverage is much deeper than voice coverage...it appears that one of the passengers' mobile picked up an SMS sent to that mobile, about half to one hour after the accident...

Call data records (CDR) contain information of the service provider administered location number connected to calls made/received or SMS made/received...location number normally pertains to a collection of cells...in the case of the service provider that delivered SMS for the passengers' mobile, the service provider had done a poor job of design and mapped it to a specific cell...but that turned out to be very good for the investigators to zero in on the area of crash very quickly...

I think the cell phone towers pick up cell phones in its region of coverage, even if that cell is not receiving or making calls. Or may be, that cell phone received calls/messages. Three towers in that region picked up the presence of that cell phone in their area of coverage. AP police intelligence gave this info and by deducting the common area (like A intersection B intersection C in Set Theory), the area of search was brought down to 5 - 10 sq km and in the second pass, one AF copter identified the crash site.

Coming to more serious stuff, remember, accident investigation is an extremely demanding task and is quite like murder investigation. Very careful collection and preserving of evidences, detailed forensic, metallurgical and other examination of collected evidences and then
arriving at the probable causes, elimination of causes etc are to be done.


Reason that it is inefficient to have one cell mapped to a location number is that when a call is made, all cells connected to the last location number known of the receiver is paged...that's a lot of computing resources and relates to a lot of wasted investments for service provider if design of network is not optimal...

You have copter/plane crashes and then you have telecom network crashes...sometimes I am sure you all have experienced unavailability of telecom services...most (good) service providers take network outages as seriously as those connected with airplanes etc. consider crashes...very rigorous crash commissions are done and learnings from the same are applied to make sure that the specific problem is not repeated again...