TEMPEST monitoring is passive if cannot be detected. The computer emits compromising radiation which can be reconstructed from a remote location. There is no need to ever come near the target. No reason ever to go back to change a lousy bug like the Watergate buglars. It can be performed from an office or a vehicle with no chance of discovery. The premise is very simple.
An electronic device emits some low-level electromagnetic radiation. Whenever an electric current changes in voltage level it generates electromagnetic pulses that radiate invisible radio waves. Similar to the ripples caused by dropping a small rock into a quite pool of water. These electromagnetic radio waves can carry a great distance.
Computer monitors like televisions contain an electron gun in the back of the picture tube which transmits a beam of electrons (electric current). When the electrons strike the screen they cause the pixels to evanesce. This beam scans across the screen from top to bottom very rapidly in a repetitive manner, line by line, flashing on and off, making the screen light and dark, creating the viewed image. These changes in the high voltage system of the monitor generate the incoherent signal that TEMPEST monitoring equipment receive, reconstruct and view.
We have found that most monitors emit signals in the 2 to 20 MHz range although harmonics are fairly strong and can be intercepted. Radiated harmonics of the video signal bear a remarkable resemblance to broadcast TV signals although various forms of sync must be restored.
Associated unshielded cabling can act as an antenna and increase interception range. Emissions can be conducted down power cables and supplies. Computers attached to unshielded telephone lines are easy prey as the telephone line acts as an excellent antenna. Printers and their cables are not immune either. The average computer setup in the home of office could be compared to a base station transmitting its signals all over the neighbourhood.
Put quite simply, it is easy for someone with basic electronics knowledge to eavesdrop on you while you are using a computer. They might not be able to steal everything from the hard disk but they can view anything you do...see anything you see...