The Scanning Polarimetric Imaging Radiometer (SPIRA) is a passive microwave imaging system operating around 91 GHz. It consists of a two orthogonally polarized receiver channels and an analog adding correlator network with 2 GHz bandwidth, which can measure all four Stokes parameters simultaneously by scanning the scene with an offset parabolic reector on an elevation over azimuth scanner. In October 2008 the SPIRA instrument has participated in the joint Swiss-German Radiometer Experiment Thun where it has been operated in parallel with two PMMW systems of Fraunhofer Institut für Hochfrequenzphysik und Radartechnik and an IR camera. During this measurement campaign different camouage kits, vehicles and persons with hidden threats have been observed together with reference objects. This paper gives an overview of the three different instruments and discusses selected images of the joint measurement campaign
Millimeter wave passive imaging systems constitute a good compromise between resolution and penetration depth for a variety of imaging applications. In an outdoor scenario, the cold sky radiation, interacting with the reflectivity characteristics of the targets, constitutes the main source of contrast in the acquired images. In indoor applications such a source is not available, and higher thermal sensitivity is required. Alternatively, one has to provide an artificial illumination to the scene in order to increase its dynamic range. The implementation of an active source for a passive radiometer can, under certain conditions, increase the contrast of the images acquired and add extra information to the measurement. With such a setup, outdoor systems can be used for indoor observations (the absence of cold sky radiation is compensated with active illumination). The subject of our study is to better understand which kind of source and which setup can provide a diffuse illumination over the targets. This topic was investigated by conducting observations of various indoor scenes with two radiometers in the W-Band, using noise and continuous wave (CW) sources as illumination. In this paper we present the results achieved and our conclusions in order to provide an efficient illumination for indoor environment.
Abstract-High reflective materials in the microwave region play a very important role in the realization of antenna reflectors for a broad range of applications, including radiometry. These reflectors have a characteristic emissivity which needs to be characterized accurately in order to perform a correct radiometric calibration of the instrument. Such a characterization can be performed by using open resonators, waveguide cavities or by radiometric measurements. The latter consists of comparative radiometric observations of absorbers, reference mirrors and the sample under test, or using the cold sky radiation as a direct reference source. While the first two mentioned techniques are suitable for the characterization of metal plates and mirrors, the latter has the advantages to be also applicable to soft materials. This paper describes how, through this radiometric techniques, it is possible to characterize the emissivity of the sample relative to a reference mirror and how to characterize the absolute emissivity of the latter by performing measurements at different incident angles. The results presented in this paper are based on our investigations on emissivity of a multilayer insulation material (MLI) for space mission, at the frequencies of 22 and 90 GHz.
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