triggerfish

[Phil Karn]

>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