TEMPEST stands for Transient Electromagnetic Pulse Surveillance
Technology.

Computers and other electronic equipment release interference to their
surrounding environment. You may observe this by placing two video
monitors close together. The pictures will behave erratically until you
space them apart.

What is important for an observer is the emission of digital pulses (zeroes and ones)
as these are used in computers and data communications. The channel for this radiation
is in two arrangements, radiated emissions and conducted emissions.
Radiated emissions are assembled when components in electrical devices
form to act as antennas. Conducted emissions are formed when radiation
is conducted along cables and wires.

Although most of the time these RF emissions are simply annoyances, they
can sometimes be very helpful. Suppose we wanted to see what project a
target is working on. We could sit in a van outside her office and use
sensitive electronic equipment to attempt to pick up and decipher the
radiated emissions from her video monitor. These emissions normally
exist at around 55-245 MHz and can be picked up as far as one kilometer
away.

A monitoring device can distinguish between different sources emitting
RF radiation because the sources emanating the radiation are made up of
dissimilar elements and this coupled with other factors varies the
emitted frequency. For example different electronic components in VDUs,
different manufacturing processes involved in reproducing the VDUs,
different line syncs, etc… By synchronizing our raster with the
targets raster we can passively draw the observed screen in real-time.
This technology can be acquired by anyone, not just government agencies.

The target could shield the emissions from her equipment or use
equipment that does not generate strong emissions. However, TEMPEST
equipment is not legal for civilian use in the United States.

TEMPEST is the US Government program for evaluation and endorsement of
electronic equipment that is safe from eavesdropping. TEMPEST
certification refers to the equipment having passed a testing phase and
agreeing to emanations rules specified in the government document NACSIM
5100A (Classified). This document sets forth the emanation levels that
the US Government believes equipment can give off without compromising
the information it is processing.

For more information concerning TEMPEST, visit The Complete, Unofficial TEMPEST Information Page.