Development and characterization of fully polarimetric noise injection radiometer for MIRAS

Abstract

An L-band noise injection radiometer (NIR) has been designed and implemented by Helsinki University ofTech- nology Laboratory ofSpace Technology f or the SMOS (Soil Moisture and Ocean Salinity) mission ofESA (1). The work is performed as a part of ESA's MIRAS Demonstrator Pilot Project-2 (MDPP-2) under a subcontract for EADS-CASA. Other partners in the MDPP-2 NIR project are Toikka Engineering Ltd. and Ylinen Electronics Ltd. NIR will work as a part ofthe MIRAS (Microwave Imaging Radiometer Using Aperture Synthesis) instrument. Its main purpose is (1) to provide precise measurement ofthe average brightness temperature scene for absolute calibration of the MIRAS image map and (2) to measure the noise temperature level ofthe internal active calibration source f or individual receiver calibration. The performance of NIR is a decisive factor ofthe MIRAS perf ormance. The challenge in the implemented, so-called blind correlation, method is the fact that there is additional noise in the correlated signal due to using the noise injection method. The main objective ofthis paper is to demonstrate the f ofthis technique. I. INTRODUCTION The precision of a noise injection radiometer is based on comparing the measured signal to two reference sources, the noise temperatures of which are known. This will remove the effect of the receiver gain and offset variations. The length of the noise pulse is then proportional to the antenna temperature (2). NIR will also be used in the MIRAS array as a regular receiver unit for interferometric image creation. In addition to measuring the horizontally and vertically polarized antenna noise temperature and the calibration net- work noise temperature, the MDPP-2 NIR was designed to provide fully polarimetric measurement capability. The Stokes parameters are retrieved using the same correlator, which the MIRAS uses for solving the correlation for the interferometric image creation. The so-called modified Stokes parameters are defined under the Rayleigh-Jeans approximation as (3)