Editing Signals intelligence (section)

===Signal detection===
Even if a signal is human communications (e.g., a radio), the intelligence collection specialists have to know it exists. If the targeting function described above learns that a country has a radar that operates in a certain frequency range, the first step is to use a sensitive receiver, with one or more antennas that listen in every direction, to find an area where such a radar is operating. Once the radar is known to be in the area, the next step is to find its location.
[[File:SpectrumAnalyzer-Superhet.png|thumb|Simplified [[spectrum analyzer]] display of [[superheterodyne receiver|superheterodyned]], [[amplitude modulated]] signals.]]
If operators know the probable frequencies of transmissions of interest, they may use a set of receivers, preset to the frequencies of interest. These are the frequency (horizontal axis) versus power (vertical axis) produced at the transmitter, before any filtering of signals that do not add to the information being transmitted. Received energy on a particular frequency may start a recorder, and alert a human to listen to the signals if they are intelligible (i.e., COMINT). If the frequency is not known, the operators may look for power on primary or [[sideband]] frequencies using a [[spectrum analyzer]]. Information from the spectrum analyzer is then used to tune receivers to signals of interest. For example, in this simplified spectrum, the actual information is at 800&nbsp;kHz and 1.2&nbsp;MHz.
[[File:DirectionalSpectra.png|thumb|left|Hypothetical displays from four spectrum analyzers connected to directional antennas. The transmitter is at bearing 090 degrees.]]
Real-world transmitters and receivers usually are directional. In the figure to the left, assume that each display is connected to a spectrum analyzer connected to a directional antenna aimed in the indicated direction.

====Countermeasures to interception====
Spread-spectrum communications is an [[electronic counter-countermeasure]]s (ECCM) technique to defeat looking for particular frequencies. Spectrum analysis can be used in a different ECCM way to identify frequencies not being jammed or not in use.