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 (1s and 0s) as these are used in computers. 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 emissions are simply annoyances, they can sometimes be very helpful. Suppose we wanted to see what project a target was 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 radiation because the sources emanating the radiation are made up of dissimilar elements and so 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.

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