Department of Electrical Engineering

The Pennsylvania State University

Calibration of a "Stoke's" radiometer is similar to the standard approach described above for the first two Stoke's parameters. One approach to TA calibration of the third and fourth Stoke's parameters, U and V, has been described in [7]. A pair of calibration loads, together with a polarizing grid, is assembled in a geometry which varies the U and V components of TA in a controlled way, thus allowing the necessary hardware calibration coefficients to be estimated. TB calibration of U and V can in principle be approached in either of the two ways discussed above. However, determination of the appropriate antenna patterns will require measurement of the cross-correlation between orthogonal linearly polarized components of the received fields. This is a somewhat non-standard antenna range measurement. The alternative approach to TB calibration requires that an estimate be made of the U and V components of TB during measurement of TA. This could, for example, be accomplished for a spacecraft radiometer using aircraft underflights by a "Stoke's" radiometer, similar to the approach used in [4] for a conventional radiometer.

Calibration of a SAIR requires that one step in the standard approach be modified. The role of TA is replaced by the visibility function, TVIS, which is closely related to the Fourier transform of the angular distribution of TB. The hardware calibration aspects of TA, in the standard approach, are replaced by TVIS calibration. TB calibration in the SAIR context implies a conversion from TVIS to TB, where TB is a function of look angle. This is commonly referred to as image reconstruction. This approach to SAIR calibration is described in [8]. Just as in the standard approach to TB calibration, image reconstruction can be accomplished a priori, using antenna range measurements, or inverted from modeled angular distributions of TB together with coincident measurements of TVIS.

[2] C.S. Ruf, M.A. Janssen, and S.J. Keihm, "TOPEX/POSEIDON Microwave Radiometer (TMR): I. Instrument description and antenna temperature calibration," IEEE Trans. Geosci. Remote Sens., 33(1), 125-137, 1995

[3] M.A. Janssen, C.S. Ruf, and S.J. Keihm, "TOPEX/POSEIDON Microwave Radiometer (TMR): II. Antenna Pattern Correction and Brightness Temperature Algorithm," IEEE Trans. Geosci. Remote Sens., 33(1), 138-146, 1995.

[4] J.P. Hollinger, J.L. Pierce, and G.A. Poe, "SSM/I Instrument Evaluation," IEEE Trans. Geosci. Remote Sens., 28(5), 781-790, 1990.

[5] C.S. Ruf, S.J. Keihm, B. Subramanya, and M.A. Janssen, "TOPEX/POSEIDON Microwave Radiometer Performance and In-flight Calibration," J. Geophys. Res., 99(C12), 24915-24926, 1994.

[6] L.A. Klein and C.T. Swift "An improved model for the dielectric constant of sea water at microwave frequencies," IEEE J. Ocean. Eng., 2, 104-111, 1977.

[7] A.J. Gasiewski and D.B. Kunkee, "Calibration and Applications of Polarization-Correlating Radiometers," IEEE Trans. Geosci. Remote Sens., 41(5), 767-773, 1993.

[8] A.B. Tanner and C.T. Swift, "Calibration of a synthetic aperture radiometer," IEEE Trans. Geosci. Remote Sens., 31(1), 257-267, 1993.

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