The steady increase of air traffic and the operation of aircraft in almost all environmental conditions demand for advanced sensor concepts for safe and efficient flight service. Furthermore the bird's perspective on the Earth allows manifold use of airborne imaging for civilian and military Earth observation. Passive microwave remote sensing is an attractive tool for exploring our environment, offering benefits from optical or infrared frequencies like simple interpretation of images, and those of microwaves like penetration through many obstacles for enhanced vision. Microwave radiometry is a measurement technique based on sampling the naturally generated microwave emission of matter having certain thermal energy content, i.e. a temperature being larger than the absolute zero. Hence, the operation can be executed covert due to the absence of a transmitter, and a radiometric sensor generates no additional electronic pollution, being of importance for avoiding radio frequency interference with other devices and avoiding burden on the anyway tight resources in available frequency bands. Modern microwave technology, in parallel to advanced imaging concepts, offers furthermore real-time operation and compact low-weight sensor design, both being main drivers in sensor construction for airborne vehicles. The paper addresses fundamental relations for two-dimensional imaging in various applications. Basic imaging approaches are outlined and discussed with respect to performance and expense. Various imaging examples of different applications are shown and discussed.
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