Data Processing Description

SMMR Processing History

Conversion of the raw instrument voltage counts to the microwave brightness temperatures for each of the ten SMMR channels includes application of the automatic gain control (production of "normalized antenna counts"), radiometric calibration, separation of the cross-polarized signals, and correction for space spillovereffects. Once gridded, the brightness temperatures are also corrected (with the exception of the 10 GHz channels) for long-term instrument drifts and errors depending on ecliptic angle. For a detailed description of these procedures, refer to Gloersen (1983, 1987), Francis (1987), and Gloersen et al. (1992).

Though NSIDC no longer distributes the Nimbus-7 SMMR Polar Radiances and Arctic and Antarctic Sea Ice Concentrations data on CD-ROM, this documentation is retained for users who have previously acquired the data via CD-ROM.

The following two paragraphs are taken from Gloersen et al. (1992), sec. 2.2.2, and are repeated here to clarify the difference between the data distributed on CD-ROM (Gloersen et al. 1992, derived from TCTs) and the data distributed on tape (Comiso and Zwally 1989, the CELL tapes) and on NSIDC's 1-Volume SMMR CD-ROM (1989):

The procedure for converting raw SMMR antenna signals (counts) to microwave radiances entails taking the instrument output data stored as digital counts on tapes called TATs (a misnomer originally meaning Antenna Temperature Tapes) and applying a series of calibrations and corrections. First, a prelaunch calibration was carried out based on tests done in a thermal vacuum chamber at the Jet Propulsion Laboratory. These tests measured the response of each of the ten SMMR channels to a blackbody target operated at a variety of temperatures ranging from 100 K to 350 K. The tests are described in Gloersen and Barath (1977) for the Nimbus 7 SMMR and in greater detail in Njoku et al. (1980) for the Seasat SMMR. Second, a semiempirical correction was made for procedures, are stored on tapes of calibrated brightness temperatures (TCTs). The data are then further adjusted for two additional corrections, one for long-term instrument drift and one for instrument errors dependent on ecliptic angle. These final two corrections are described in the subsections following the discussion of the postlaunch calibration.

Before the production of the TCTs, SMMR radiances were averaged into cells with larger sampling intervals, approximately equal to the integrated fields of view, and recorded on CELL tapes. The CELL data format was designed primarily to produce sea-surface temperatures from SMMR radiances, and for this purpose, the additional averaging was deemed advantageous. However, the CELL data have also been used by a number of investigators in the polar regions. These data, not corrected for all aspects of cross- polarization leakage (described in [Gloersen et al., 1992, sec. 2.2.3]), instrument drift, and ecliptic-angle dependent instrument errors, were previously mapped onto polar stereographic grids and are archived at the National Snow and Ice Data Center (Comiso and Zwally 1989). We have chosen to use the TCTs in preference to the CELL tapes, since the TCTs store SMMR data for each sampling interval, which is at most half the diameter of the integrated field of view for a given channel, thereby preserving the inherent spatial resolution of the SMMR data.

With the exception of the 10.69 GHz channels, empty grid elements within the swaths resulting from the "drop in bucket" remapping (See sec. B.2) have been filled by interpolation with an average of the brightness temperatures in the surrounding elements. The brightness temperatures of the 6.00, 18.00, and 37.00 GHz channels have been corrected for instrument drift and sensitivity variations with ecliptic angle. They have also been adjusted to correspond more closely to modeled oceanic brightness temperatures and consequently those from the DMSP SSM/I. See Gloersen et al., (1992) sec. 2.2.2 for details. Data from the 21 GHz channels have been omitted, because the drift in the 21 GHz H-pol channel is so great as to render the implemented correction for polarization mixing useless after the first 2 years of the SMMR time series.

Warning to Users of 10 GHz Brightness Temperatures

The 10.69 GHz brightness temperatures included on this CD-ROM have not been corrected for instrument drift or errors dependent on ecliptic angle, nor have empty grid elements within the swaths been filled by interpolation. The 10.69 GHz brightness temperatures were included by popular request of those responding to the questionnaire accompanying the SMMR Polar Data Sampler CD-ROM. The drift patterns of the 10.7 GHz channels differ from the other SMMR channels in that the warm reference drifts more than the averaged oceanic brightness temperatures. A scheme for correcting this behavior has not yet been developed.