Surely they just need to program a mobile phone to set off the bomb after having no signal for 30 seconds or so - then when the jamming signal starts (or comes in range) the countdown starts also...

But I wouldn't fancy being the one to turn it on unless I was really sure the signal was good!

In case anyone has forgotten, in the good old days the "bad guys" used a TIMER to set their bombs off...

(And yes, there's one in allmost every modern mobile phone... but a simple kitchen timer would be a lot easier...)

Seeing as the terrorist plan is presumably to hurt as many people as possible, a timer would be a bit hit and miss. Remote detonation means it can go off whenever Mr. Insurgent wants.