Frequency-agile

Frequency agility is the ability of a radar system to quickly shift its operating frequency to account for atmospheric effects, jamming, mutual interference with friendly sources, or to make it more difficult to locate the radar broadcaster through radio direction finding. The term can also be applied to other fields, including lasers or traditional radio transceivers using frequency-division multiplexing, but it remains most closely associated with the radar field and these other roles generally use the more generic term "frequency hopping".

Radar systems generally operate by sending out short pulses of radio energy and then turning off the broadcaster and listening for the returning echoes from various objects. Because efficient signal reception requires careful tuning throughout the electronics in the transceiver, each operating frequency required a dedicated transceiver. Due to the size of the tube-based electronics used to construct the transceivers, early radar systems, like those deployed in World War II, were generally limited to operating on a single frequency. Knowing this operating frequency gives an adversary enormous power to interfere with radar operation or gather further intelligence.

The British used the frequency information about the Würzburg radar gathered in Operation Biting to produce "Window", aluminum foil strips cut to 1/2 the length of the wavelength of the Würzburg, rendering it almost useless. They also produced jammer units, "Carpet" and "Shivers", that broadcast signals on the Würzburg's frequency, producing confusing displays that were useless for aiming. Post-war calculations estimated these efforts reduced the combat effectiveness of the Würzburg by 75%. These countermeasures forced the Germans to upgrade thousands of units in the field to operate on different frequencies.

Knowing the frequency of the Würzburg also helped the British in their attempts to locate the systems using radio direction finders, allowing aircraft to be routed around the radars, or at least be kept at longer distances from them. It also helped them to find new operating frequencies as they were introduced, by selecting the location of known installations when they disappeared and singling them out for further study.

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