Millimeter-wave (MMW) and infrared (IR) sensors are frequently used in autonomous multiple sensor systems because their operating frequencies allow relatively compact designs to be realized. Objects that may be difficult to differentiate when only a single sensor is used are often distinguished with a multiple sensor system that exploits several signature-generation phenomena to gather data about the objects and scene of interest. Relatively high probabilities of object detection and classification, at acceptable false alarm levels, can potentially be achieved in inclement weather, high-clutter, and countermeasure environments by using operating frequencies that cover a wide portion of the electromagnetic spectrum. Signatures generated by multiple phenomena also expand the amount of information that can be gathered about the location of vulnerable areas on targets. This is important in smart munition applications where autonomous sensors guide weapons to their targets without operator intervention. Multiple sensor systems are used in civilian applications as well, such as space-based sensors for weather forecasting and Earth resource surveys. Here narrow-band wavelength spectra and multiple receiver configurations, such as active radar transmitters, passive radar receivers, and infrared and visible sensors, provide data about temperature, humidity, rain rates, wind speed, storm tracks, snow and cloud cover, and crop type and maturity. Acoustic, ultrasound, magnetic, and seismic signature-generation phenomena are also used in military and civilian applications, but these are not addressed in detail in this book. However, their data can be fused with those of other sensors using the algorithms and architectures described in later chapters.
A sensor consists of front-end hardware, called a transducer, and a data processor. The transducer converts the energy entering the aperture into lower frequencies from which target and background discrimination information is extracted in the data processor. A seeker consists of a sensor to which scanning capability is added to increase the field of regard. Seekers may be realized by sensors placed on single- or multiple-axis gimbals, IR detector arrays illuminated by scanning mirrors that reflect energy from a large field of regard, frequency sensitive antenna arrays whose pointing direction changes as the transmitted frequency is swept over some interval, or phased array antennas.
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