The airborne, imaging, polarimetric EMIRAD system employs Ku (16 GHz) and Ka (34 GHz) band polarimetric radiometers of the correlation type. Each correlation radiometer uses two receivers that are connected to the vertical and the horizontal outputs of a dual polarized feed horn, analog detectors to find the first two Stokes parameters, and a fast complex digital correlator to find the 3'rd and 4'th Stokes parameters. The digital correlator features 3-level A to D conversion and a 1540 MHz sampling rate. The radiometric sensitivity (8 msec integration time) for the vertical (or horizontal) channel is 0.35 K. Thus the sensitivity for the first and second Stokes parameters is 0.5 K. The sensitivity for the correlation measurements, i. e. the third and fourth Stokes parameters, is 0.57 K (8 msec integration).
The imaging antenna is based on an offset 1m aperture parabolic reflector scanning around a vertical axis, and illuminated by microwave horns pointing upwards along this axis. The beamwidth of the antenna is 2° at Ku band and 1° at Ka band. The cross polarization due to the offset parabolic reflector is below 25 dB. The maximum scan angle is +/- 25°, and the incidence angle on the ground is constantly 55° at Ku band and 50° at Ka band for this conically scanned system. Due to the fixed feed horns and the scanning reflector, significant polarization mixing takes place when scanning away from straight aft. This is corrected in the data analysis, based on the measured scan angle and the full set of Stokes parameters.
The antenna and the receivers are mounted on a cargo pallet, which is posi-tioned on the loading ramp of a C-130 aircraft. The ramp is closed during take-off, landing, and transit, but lowered to its horizontal position when measure-ments are to be carried out. The antenna thus has an unobstructed view of the scene below and aft of the aircraft. The ground speed of the aircraft is 70 m/sec during operation with the ramp open. The flight altitude is 2000 m, and the +/-25° scan then results in a swath of 2000 m. For the 2000 m altitude the footprint on ground is roughly 100 m at Ku band, and 50 m at Ka band. 8 msec sampling and integration time in the radiometers ensure contiguous imaging at the highest frequency.
The polarimetric signatures of the wind driven sea only show small azimuthal variations in the range of a few Kelvin. At the same time they are quite dependent on incidence angle and pointing geometry which may result in polarization rotation. Thus aircraft attitude must be carefully monitored, and the radiometer system includes an inertial navigation unit mounted directly on the antenna frame. The antenna attitude is thus measured to within 1/10 of a degree. Using this information data is corrected for unwanted attitude variations.
A 5 GHz single polarization radiometer (H or V may be selected) can be mounted and operated simultaneously with the polarimetric channels. The footprint corresponding to 2000 m flight altitude is 400 m.