How big and sensitive would an antenna need to be in orbit to accomplish this? It would have to be sensitive to 1 watt transmitters. They send up many polar orbit satellites which are not too far away, so that could be a big help.

Sensitivity is not the issue. Two 1-watt walkie-talkies, one in (low) orbit and one on the ground, can (and do) communicate with each other as long as the earth isn't standing in the way. It's done on the ham radio bands on just about every space shuttle mission (like the one currently underway). Higher orbits require better antennas, but they're no big deal. The real problem with a space-based cellular telephone surveillance system is interference - the best spot beam antenna you can make would still take in *many* ground transmitters on the same channel in a place like New York City. From orbit, you see everything, whether you want to or not. This is borne out again and again with tapes from the shuttle. Often you hear nothing at all because there are so many ground stations all transmitting at the same time that none of them are recoverable. Some hams run ungodly amounts of power to get through, not because it's required for the distance to be traveled, but to stand far enough above everybody else that they can capture the shuttle's receiver. Both these systems and cellular telephones use the same modulation method - FM. Phil