CAMEX-3 Polarimetric Scanning Radiometer (PSR)
Table of Contents
Introduction
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 homepage contains a vast amount of information about the instrument itself as well as data from other field campaigns, contact information, and an impressive set of links to other similar sites.
Instrument Description
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 (seen in the image to the left) 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 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.
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.
Naming Conventions and Formats
CAMEX-3 data for PSR is tarred into one large file for ease in distribution. This file is named:
dc8psr_total_data.tar
which when 'untarred' reveals the following files and directories:
- camex3_data_flight_catalog.doc & camex3_data_flight_catalog.pdf--provides a description of the various CAMEX3 maneuvers listed by PSR serial numbers.
- flight_description.doc & flight_description.pdf- in tabular form gives date, PSR flight code, UTC time, location/event/objective and comments.
- level23a_description.doc & level23a_description.pdf- describes the meaning of the various data planes within any level 2.3a data file
- software_description.doc & software_description.pdf- Self explanatory.
- 17 sub directories of the form 1998_mmdd/ where mm is the month (08 or 09) and dd is the day of the month.
- display_23a.zip- contains text file list of flight dates corresponding to the 17 sub-directories above, and Matlab files for use in processing the data.
In each of the daily directories (1998_mmdd) is a subdirectory named level2.3a that contains yet a further subdirectory named SL referring to straight and level flight data. Therein are the Matlab version 5.3 m-files necessary to render calibrated brightness temperature maps from the PSR CAMEX3 level 2.3a data. The files are of the form:
- 1998_mmdd_fffp_look.jpg are image files where fff is the frequency of the observation, p is polarization and look is either 'back' or 'front'. Example: 1998_0915_215v_back.jpg.
- L23annnn.mat and L23annnn.nc are data files used in Matlab.
References
Piepmeier, J.R., A.J. Gasiewski, M. Klein, V. Bohm, and R.C. Lum, "Ocean Surface Wind Direction Measurement by Scanning Microwave Polarimetric Radiometry", Proceedings of the 1998 International Geoscience and Remote Sensing Symposium, presented in Seattle, WA, July 6-10, 1998.
Gasiewski, A.J., Piepmeier, J.R., McIntosh, R.E., Swift, C.T., Carswell, J.R.,Donnelly, W.J., Knapp, E., Westwater, E.R., Irisov, V.I., Fedor, L.S., and Vandemark, D.C., "Combined High-Resolution Active and Passive Imaging of Ocean Surface Winds from Aircraft," Proceedings of the 1997 International Geoscience and Remote Sensing Symposium, pp. 1001-1005, presented in Singapore, August 3-8, 1997.
Piepmeier, J.R., and Gasiewski, A.J., "High-resolution Multiband Passive Polarimetric Observations of the Ocean Surface," Proceedings of the 1997 International Geoscience and Remote Sensing Symposium, pp. 1006-1008, presented in Singapore, August 3-8, 1997.
Piepmeier, J.R., and Gasiewski, A.J., "Three-level, 1 GS/s Digital Correlator for Wideband Polarimetric Radiometry," Proceedings of the 1996 IEEEMTT-S International Microwave Symposium, pp. 1339-1341, presented in San Francisco CA, June 17-21, 1996b.
Piepmeier, J.R., and Gasiewski, A.J., "Polarimetric Scanning Radiometer for Airborne Microwave Imaging Studies," Proceedings of the 1996 International Geoscience and Remote Sensing Symposium, pp. 1688-1691, presented in Lincoln, NE, May 27-31, 1996a.
"Remote Sensing of Ocean Surface Wind Vectors by Passive Microwave Polarimetry - PSR Airborne Validation Study," A.J. Gasiewski, J.R. Piepmeier, M. Klein, and V. Boehm, Report to the NPOESS Integrated Program Office, Silver Spring MD, December 20, 1999.
Piepmeier, J.R., and A.J. Gasiewski, "High-Resolution Passive Microwave Polarimetric Mapping of Ocean Surface Wind Vector Fields," accepted for publication in IEEE Trans. Geosci. Remote Sensing, September, 2000.
Contact Information
To order these data or for further information, please contact:
Global Hydrology Resource Center
User Services
320 Sparkman Drive
Huntsville, AL 35805
Phone: 256-961-7932
E-mail: support-ghrc@earthdata.nasa.gov
Web: http://ghrc.nsstc.nasa.gov/
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