Calibration of scanning low frequency microwave radiometer

摘要

The scanning low frequency radiometer (SLFMR) is a narrow-band (24 MHz) radiometer operating at 1.413 GHz. It uses a Dicke-switched reference load and a null sensor to match a noise temperature to the brightness temperature of a target. A Butler matrix is used to steer an 8 × 8 phased array antenna into eight beam directions. Calibration is required to obtain sea surface salinity estimates from the instrument as it is flown over an area to be mapped. Sea surface temperature, sea state, beam incidence angle and down-welling brightness temperature of the air affect the instrument readings. Consideration must also be given to the effect of solar and galactic radiation reflected off the sea surface into the instrument. This paper focuses on instrument calibrations which are needed to account for the effects of various temperatures measured by sensors at key locations within the instrument. Calibration of the SLFMR was performed by Prosensing before delivery, but it became clear that recalibration was necessary for each flying campaign. Long-term stability of the instrument and appropriate calibration parameters in light of those suggested by the manufacturer are discussed. These are second-order corrections which arise from small variations in temperature - the whole cabinet has a feedback control loop to hold the cabinet temperature at a nominal 40°C. A multi-variate linear approach is used to calibrate the instrument for a set of coefficients associated with the temperature measurements. Coefficients were evaluated for all beam positions and checks were made with the SLFMR pointing upwards to scan across known sources of the sun, moon and the centre of the galaxy. Further calculations were made in a room where the target temperature was kept constant. These calibration procedures removed imbalances between the responses at different beam positions. Success of the procedure is demonstrated with some early salinity maps made in the Great Barrier Reef Lagoon in north-east Australia.