Fully-polarimetric passive scanning imager at millimeter wavelengths

In this paper we present SPIRA (Scanning Polarimetric Imaging Radiometer), a fully polarimetric and transportable imager at 91 GHz that is being developed at IAP. It will be used for measuring signatures of the Earth surface in all four Stokes parameters and can be transported and put into an imaging-ready status by two persons within hours. The mechanical imager scans in azimuth and elevation whereby an image of 120×120 pixels (field of view 30°× 30°) is acquired within less than 600 s. The instrument is controlled by a PC under Linux OS, achieving a nearly real-time performance by using a specially designed kernel module. The brightness temperature distribution of a scene is sampled with a 90° offset antenna at an angular resolution of 0.5°. The polarized signal is separated by an orthomode transducer into two orthogonal components and then down converted to an intermediate frequency band between 2 and 4 GHz by two sub-harmonic mixers that are pumped by a common local oscillator. The four Stokes parameters are extracted with an analog broadband correlator, consisting of a hybrid network and quadratic detectors. Raw images in the four Stokes parameters are immediately displayed and stored for postprocessing. Calibration is performed by adding noise from a switchable noise diode in each channel and by an external load at ambient temperature. Polarimetric calibration of the whole system is done by using coherent and thermal polarization generators.