Jamming communications and radar has a long history, going back to at least to World War II. Some early versions of stealth technology involved aircraft generating a signal at the same frequency as the radar. More sophisticated versions of the technique were used during the Vietnam War.
Variations in that method are used today, said Stan VanDerWerf, president of Advanced Capitol, a consulting firm and a former chief of electronic warfare and avionics at the Warner Robins Air Logistics Center at Robins Air Force Base. "Enemy radar is looking for ours," he said. "But we have a jammer, which receives energy from the radar, emulates the signal and sends it back out."
It won't make the target plane invisible, but it will make the radar operator's screen show misleading information that's harder to decipher. A similar technique is used to fool radar-guided missiles into under- or over-estimating the speeds of their targets.
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With digital communications, the problem of interference has become more complicated. Digital signals generally operate at lower power than their old-line analog cousins, so they are more vulnerable in some respects.