Point of Contact:

Albin J. Gasiewski

NOAA Environmental Technology Laboratory

R/E/ET1

325 Broadway

Boulder, CO 80303

Office Phone: (303) 497-7275 (O)

FAX: (303) 497-3577 (F)

E-mail: agasiewski@etl.noaa.gov

PSR Homepage: http://www1.etl.noaa.gov/radiom/psr.html

Instrument Description

The Polarimetric Scanning Radiometer - PSR The Polarimetric Scanning Radiometer (PSR) is a versatile airborne microwave imaging radiometer developed by the Georgia Institute of Technology and the NOAA Environmental Technology Laboratory for the purpose of obtaining polarimetric microwave emission imagery of the Earth's oceans, land, ice, clouds, and precipitation. The PSR consists of a set of five polarimetric radiometers housed within a gimbal-mounted scanhead drum. The scanhead drum is rotatable by the gimbal positioner so that the radiometers can view any angle within 70 deg. elevation of nadir at any azimuthal angle (a total of 1.32 sr solid angle), as well as external hot and ambient calibration targets. The configuration thus supports conical, cross-track, along-track, fixed-angle stare, and spotlight scan modes. The PSR supports several observational objectives: (1) to provide polarimetric imagery of the upwelling thermal emission field at several of the most important microwave remote sensing bands covering X to W band in octave intervals and including the 22.235 GHz water vapor line. The current implementation provides full Stokes vector (four parameters Tv, Th, TU, and TV) capability at 10.7 and 18.7 GHz, tripolarimetric capability (Tv, Th, and TU) at 37.0 and 89.0 GHz, and dual polarization capability (Tv and Th) at 21.5 GHz; (2) to provide the above measurements with absolute accuracy of better than 1 K for Tv and Th, and 0.1 K for TU and TV; (3) to provide radiometric imaging with both fore and aft look capability (rather than single swath observations); (4) to provide conical imagery at a variety of surface incidence angles; and (5) to provide image resolutions appropriate for spatially- resolved studies of precipitating and non-precipitating clouds, mesoscale ocean surface features, and satellite calibration/validation at Nyquist spatial sampling rates. The scanhead was designed for in- flight operation without the need for a radome (i.e., in direct contact with the aircraft slipstream), thus allowing precise calibration and imaging with no superimposed radome emission signatures. Moreover, the conical scan mode allows the elements of the modified Stokes' vector to be observed without polarization mixing. The entire PSR assembly (including both scanhead and positioner) has been designed for integration into any of three NASA aircraft: the DC-8 (nadir-7 port), Orion P-3B (aft end of the P-3 bomb bay), and ER-2 (Q-bay). The PSR scanhead contains an 80486 PC, an analog correlator bank, and five total-power radiometers installed inside a 20" (~51 cm) diameter and 20" (51 cm) long rotating drum. The radiometer antennas are orthogonal linearly-polarized corrugated feedhorns with grooved rexolite lenses. A single dual-band antenna is used for the X- and Ka-band channels. Antenna diameters were chosen to provide beamwidths of 8o (for 10.7, 18.7, and 21.5 GHz) and 2.3o (for 37.0 and 89.0 GHz). All main beam efficiencies are in the 95-97% range and all on-axis cross-polarizations exceed -27 dB. The scanhead package includes a monochrome CCD telephoto video camera boresighted along the antenna main beams. The camera incorporates an orange filter and vertical polarizer to maximize the contrast of ocean foam against the ocean background and clouds against either ocean or land backgrounds. The PSR has been integrated into the nadir-7 port of the DC-8 for CAMEX3 and will be primarily operated in a 52o conically-scanned mode. A unique new moving-map radiometric display for the PSR will be used to provide near real-time brightness temperature imagery for both scientific and flight operations purposes. The display will provide conically-scanned brightness temperature maps at the five PSR bands and for several polarizations. Several major improvements in the scanhead and data system have been implemented in preparation for CAMEX-3. First, the 18-GHz feedhorn and receiver was upgraded to cover two radiometric bands and now includes a fully-polarimetric channel at 18.6-21.7 GHz, and a dual-polarized water vapor channel at 21.3-21.6 GHz. The new K-band receiver is thus sensitive to emissions from both the surface and integrated water vapor. The new receiver is also a direct detecting type rather than superheterodyne. Second, each of the five radiometers will incorporate sub-interval calibration hardware to supplement the standard hot- and ambient-view calibration. The sub-interval calibration hardware switches noise diodes into the RF inputs of the receivers at ~100 msec intervals, pulsing each diode both on and off so as to allow estimation of the radiometer gains and offsets at time intervals comparable to the drift time of receivers. In this manner, uncalibrated gain and offset drift caused by 1/f noise is significantly reduced. Third, the digital correlators in the original scanhead were replaced by analog adding correlators incorporating quadrature hybrid arrays. The analog system will allow measurement of all four Stokes' parameters for the 10- and 18-GHz channels, and the first three Stokes' parameters for the 37- and 89-GHz channel. Finally, using the new canted pyramidal calibration load the absolute accuracy of the PSR is expected to be ~1 K, thus providing excellent capabilities for absolute intercomparisons of PSR measurements with radiative transfer model calculations and TMI measurements.