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In recent years the security of people and critical infrastructures is of increasing interest. Passive microwave sensors in
the range of 1 - 100 GHz are suitable for the detection of concealed objects and wide-area surveillance through poor
weather and at day and night time. The enhanced extraction of significant information about an observed object is
enabled by the use of a spectral sensitive system. For such a spectral radiometer in the microwave range also some depth
information can be extracted. The usable frequency range is thereby dependent on the application. For through-wall
imaging or detection of covert objects such as for example landmines, the lower microwave range is best suited. On the
other hand a high spatial resolution requires higher frequencies or instruments with larger physical dimensions. The
drawback of a large system is the required movement of a mirror or a deflecting plate in the case of a mechanical scanner
system, or a huge amount of receivers in a fully-electronic instrument like a focal plane array. An innovative technique
to overcome these problems is the application of aperture synthesis using a highly thinned array. The combination of
spectral radiometric measurements within a wide frequency band, at a high resolution, and requiring a minimum of
receivers and only minor moving parts led to the development of the ANSAS instrument (Abbildendes Niederfrequenz-Spektrometer mit Apertursynthese). ANSAS is a very flexible aperture synthesis technology demonstrator for the
analysis of main features and interactions concerning high spatial resolution and spectral sensing within a wide
frequency range. It consists of a rotated linear thinned array and thus the spatial frequency spectrum is measured on
concentric circles. Hence the number of receivers and correlators is reduced considerably compared to a fully two-dimensional
array, and measurements still can be done in a reasonable time. In this paper the basic idea of ANSAS and
its setup are briefly introduced. Some first imaging results showing the basic capabilities are illustrated. Possible error
sources and their impacts are discussed by simulation and compared to the measured data.
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