at 1031 miles, three hours or so.
If I understand correctly, though, if the transponder is turned off, the aircraft would still have a radar signature, but lack all the information that the transponder transmits such as altitude, heading, horizontal and vertical speed etc. If it's off the radar completely, it's likely that the plane is down or flying very low, below the radar's reach. The latter is unlikely, since there would be eyewitnesses
Yes, but we all know that eyewitnesses are wrong in "almost every crash"... ;0)
Most transponders don't transmitt anything but a code IDing the aircraft and alitude. the range is determined from the delay between the ground (or airborne AWACS can do it, as can most fighters) "radar" transmitting the "interogation" and the reception of the beacon/transponders reply. That gives you position, (lat/lon) and alittude. The rest is calculated by a tracking filter in the software of the "radar" system. (i.e. if you know where is is at some time, and when it is again at a know time later (scans are usually 10-15 seconds) you can inferr it's speed and heading and rate of climb/dive. )