A fast imaging MMW radiometer system for security and safety applications

Kurzfassung

The SUMIRAD system (SUM Imaging Radiometer) is a fast fully-mechanical scanning radiometer system which can provide close to real-time quasi-optical millimeter-wave (MMW) images. Its original purpose is reconnaissance in a multi-sensor vehicle based forward looking surveillance system. Hence SUMIRAD observes the road area in front of the vehicle in order to enhance the knowledge of the actual threat situation and detect automatically suspicious objects. As a MMW system it offers the advantage to detect and localize objects and persons under all atmospheric obstacles and also extents the surveillance capabilities behind non-metallic materials like clothing or thin walls, covers and thin vegetation. SUMIRAD was developed as a cost-effective fully-mechanical scanner using only two MMW radiometer receivers. The innovative and light-weight design satisfies the demand for a very large field-of-view (FOV: 30° in elevation x 80° in azimuth) in combination with a high frame rate (up to 1 image per second) and a spatial resolution well below 1 degree. The broadband receivers operate in W band (82 -102 GHz) providing sufficient radiometric sensitivity of better than 2 K at the minimum image acquisition time of 1 second. Longer times, up to 90 seconds per image, could be selected for higher radiometric sensitivity (<< 2 K). The compact integration of the complete receiver electronics allow to change easily the polarization of each receiver between horizontal (H) and vertical (V). Both radiometer receivers cover nearly the identical FOV. Hence two different imaging modes are possible depending on the selected polarization mode. In the first mode (identical polarization) the superposition of both nearly identical images improves the radiometric sensitivity by about a factor of 1.4. This mode is useful for short image acquisition times. In the second mode (different polarization), the two independent images provide different polarimetric information simultaneously from the same scene. The previously mentioned imaging capabilities allow operating SUMIRAD for a multitude of applications. Beside the originally intended vehicle based operation under driving conditions, the static operation of the system from a fixed position can monitor fast or slow changes of a scene. For example, such applications are the volume estimation of actual truckload through non-metallic containers, through-wall imaging for the detection of objects or persons and the long term observation of weather phenomena in a maritime environment. In general the system can be used for many ground-based observations where close to real-time radiometric imaging at large FOV in combination with a long observation period is useful. The paper addresses the operational principle of SUMIRAD and discusses the requirements of key imaging parameters. Imaging results from several measurement campaigns are presented.