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.
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.
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 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.
Passive electronically scanned array (PESA) radars, introduced in the 1960s, used a single microwave source and a series of delays to drive a large number of antenna elements (the array) and electronically steer the radar beam by changing the delay times slightly.
The radar unit knows which frequencies were broadcast, and amplifies and combines only those return signals, thereby reconstructing a single powerful echo on reception.
[3] An adversary, unaware of which frequencies are active, has no signal to see, making detection on radar warning receivers extremely difficult.
When the user wishes to place a call, the cell phone uses a negotiation process to find unused frequencies among the many that are available within its operational area.