Scanning Polarimetric Imaging RAdiometer

Institute of Applied Physics, University of Bern

SPIRA: Technical Description

 

The Scanning Polarimetric Imaging Radiometer (SPIRA) is a fully polarimetric imager in 90-92 GHz band.

 

The instrument can measure simultaneously the 4 Stokes parameters for an observed scene using two orthogonally polarized receivers with 2 GHz of bandwidth.

 

The antenna consists of a dual polarized corrugated feed horn and a 90-axis parabolic reflector with an effective focal length of 70cm and a projected aperture of 45cm.

 

The scene is scanned in two dimensions: the off-set parabolic reflector is rotated around the horizontal bore-sight axis of the feed horn (elevation), whereas the antenna and the polarimeter support are rotated around the vertical axis (azimuth).

 

The receiver consists of two heterodyne chains, which are both connected to the feed horn through an orthomode transducer (OMT).

 

In each channel an additional noise signal from a switchable noise diode can be injected through a directional coupler, which is used together with an ambient temperature blackbody target to calibrate the gain and system temperature of the receiver at the end of each antenna scan.

 

Optimum pumping of the mixers and fine adjustment of the phase shift between the channels are achieved by variable attenuators and a phase-shifter in the local oscillator path.

 

The down-converted signals are amplified by low-noise IF amplifiers and band-limited to frequencies between 2 and 4 GHz.

 

For polarimetric observations it is necessary to determine the complex cross-correlation between the two IF signals. For that reason an analog adding correlator is used, which consists of a network of 90 and 180 hybrid couplers followed by six square law detectors and video amplifiers.

 

From the 6 output voltages the four Stokes are thus reconstructed.

The Instrument

SPIRA Radiometer on IAP building

Block diagram of SPIRA receiver

Human Body scanned by SPIRA, an hidden metal plate is revealed by the instrument (center torso)

Introduction

Instrument

Our work

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Introduction

Instrument

Our work

Gallery

Contact Us