TEMPEST
TEMPEST is the code name for technology related to limiting unwanted electromagnetic emissions from data processing and related equipment. Its goal is to limit an opponent's capability to collect information about the internal data flow of computer equipment. Most information concerning TEMPEST specifications is classified by the United States Government and is not available for use by its citizen. Therefore, we are not permitted to disclose the various facilities where Fil-Coil delivered and installed TEMPEST filters.
The reason why TEMPEST technology is particularly important for computers and other data processing equipment is the kinds of signa1s components in a computer use to talk to each other ("square waves") and their clock speeds (measured in megahertz) produce a particularly rich set of intentional signals in a wide portion of the electromagnetic spectrum. Because the spurious emissions occupy so wide a portion of that spectrum, technologies used to block one portion of the spectrum. (as pulling the shades dosed on a window to stop the visible light portion) are not necessarily effective in another portion.
Unintentional emissions from a computer system can be captured and processed to reveal information about the target systems from simple levels of activity to even remotely copying keystrokes of capturing monitored information. It is speculated that poorly protected systems can be effectively monitored up to the order of one kilometer from the target equipment.
One way of looking at TEMPEST from the lay person's point-of-view is that it is virtually identical to the problem of preventing electromagnetic interference ("EMC") by your computer system to others' radios, televisions, or other consumer electronics. That is, preventing the emission of wide-band radio "hash" from your computers, cabling and peripherals both prevent interference to you and your neighbors television set and limits the useful signal available to a person surreptitiously monitoring.
Preventing unintended electromagnetic emissions is a related term. It is not feasible to reduce, to zero all unintended emissions. Fil-coil's goal, for example, might be to reduce the amount and quality of spurious emission until the monitoring van a kilometer away would have to be within 30 ft. before it could effectively eavesdrop on a computer system.