• Callahan, P. S., C. S. Morris, and S. V. Hsiao, 1994: Comparison of TOPEX/POSEIDON σ0 and significant wave height distributions to Geosat. J. Geophys. Res.,99, 25015–25024.

  • Chen, G., B. Chapron, J. Tournadre, K. Katsaros, and D. Vandemark, 1997: Global oceanic precipitation: A joint view by TOPEX and the TOPEX microwave radiometer. J. Geophys. Res.,102, 10457–10471.

    • Crossref
    • Export Citation
  • Cudlip, W., J. K. Ridley, F. Strawbridge, A. Harris, and C. G. Rapley, 1994: Detecting surface roughness and moisture variations in deserts. Proc. Second ERS-1 Symposium—Space at the Service of Our Environment, Hamburg, Germany, European Space Agency, 849–853.

  • Elfouhaily, T., D. Vandemark, J. Gourrion, and B. Chapron, 1998: Estimation of wind stress using dual-frequency TOPEX data. J. Geophys. Res.,103, 25101–25108.

    • Crossref
    • Export Citation
  • Fu, L.-L., E. J. Christensen, C. A. Yamarone, M. Lefebvre, Y. Menard, M. Dorrer, and P. Escudier, 1994: TOPEX/POSEIDON mission overview. J. Geophys. Res.,99, 24369–24381.

    • Crossref
    • Export Citation
  • Guzkowska, M. A. J., C. G. Rapley, J. K. Ridley, W. Cudlip, C. M. Birkett, and R. F. Scott, 1990: Developments in inland water and land altimetry. ESA Contract Rep. 7839/88/F/FL, 418 pp. [Available from ESTEC, 2200 AG Noordwijk, Netherlands.].

  • Hayne, G. S., D. W. Hancock, and C. L. Purdy, 1994a: TOPEX altimeter range stability estimates from calibration mode data. TOPEX/POSEIDON Res. News,3, 18–22.

  • ——, ——, ——, and P. S. Callahan, 1994b: The corrections for significant wave height and attitude effects in the TOPEX radar altimeter. J. Geophys. Res.,99, 24941–24955.

    • Crossref
    • Export Citation
  • Kennett, R. G., and F. K. Li, 1989: Seasat over-land scatterometer data, part II: Selection of extended area land-target sites for the calibration of spaceborne scatterometers. IEEE Trans. Geosci. Remote Sens.,27, 779–788.

    • Crossref
    • Export Citation
  • Quartly, G. D., 2000: The gate dependence of geophysical retrievals from the TOPEX altimeter. J. Atmos. Oceanic Technol.,17, 1247–1251.

    • Crossref
    • Export Citation
  • ——, T. H. Guymer, and M. A. Srokosz, 1996: The effects of rain on Topex radar altimeter data. J. Atmos. Oceanic Technol.,13, 1209–1229.

    • Crossref
    • Export Citation
  • ——, M. A. Srokosz, and T. H. Guymer, 1999: Global precipitation statistics from dual-frequency TOPEX altimetry. J. Geophys. Res.,104, 31489–31516.

    • Crossref
    • Export Citation
  • Ulaby, F. T., R. K. Moore, and A. K. Fung, 1986: Microwave Remote Sensing: Active and Passive. Vol. III, From Theory to Applications, Artech House, 1120 pp.

  • Witter, D. L., and D. B. Chelton, 1991: A Geosat altimeter wind speed algorithm and a method for altimeter wind speed algorithm development. J. Geophys. Res.,96, 8853–8860.

    • Crossref
    • Export Citation
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Monitoring and Cross-Calibration of Altimeter σ0 through Dual-Frequency Backscatter Measurements

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  • 1 James Rennell Division for Ocean Circulation, Southampton Oceanography Centre, Southampton, United Kingdom
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Abstract

The normalized backscatter, σ0, observed by an altimeter at its Ku band is used to infer wind speed above the ocean surface, and for the dual-frequency TOPEX altimeter, algorithms exist to calculate wind stress and rainfall from simultaneous observations at its two frequencies (Ku band and C band). However, the creation and application of such algorithms rely on the long-term stability of an altimeter, or even coreferencing the values obtained by one altimeter to those of another. This paper proposes a method of monitoring an altimeter’s backscatter values using the constancy of the correlation between the values at its two different frequencies. Although the amplitude of the scattering by the ocean surface may vary by more than an order of magnitude, the coherency of its behavior at different spatial scales enables it to be used as a constant reference surface for dual-frequency altimetry. Using the observed close correlation between σ0C and σ0Ku for each TOPEX cycle, the drift in σ0 calibration may be assessed through the shifts in the mean relationship. Application to the first 150 cycles of data from TOPEX side A shows good general agreement with the already applied corrections (derived from cycle averages of σ0); however, these corrections have overcompensated slightly leaving a remnant drift of 0.03 dB yr−1. In a particular instance, the response of the altimeter can be seen to take a day to recover from an instrument shutdown. Initial investigations for TOPEX side B (which apart from the antenna is an identically constructed but separate instrument) confirm that dual-frequency methods can be used to cross-calibrate nonsimultaneous sensors.

Corresponding author address: Dr. Graham D. Quartly, James Rennell Division for Ocean Circulation, Southampton Oceanography Centre, Empress Dock, Southampton S014 3ZH, United Kingdom.

Email: graham.d.quartly@soc.soton.ac.uk

Abstract

The normalized backscatter, σ0, observed by an altimeter at its Ku band is used to infer wind speed above the ocean surface, and for the dual-frequency TOPEX altimeter, algorithms exist to calculate wind stress and rainfall from simultaneous observations at its two frequencies (Ku band and C band). However, the creation and application of such algorithms rely on the long-term stability of an altimeter, or even coreferencing the values obtained by one altimeter to those of another. This paper proposes a method of monitoring an altimeter’s backscatter values using the constancy of the correlation between the values at its two different frequencies. Although the amplitude of the scattering by the ocean surface may vary by more than an order of magnitude, the coherency of its behavior at different spatial scales enables it to be used as a constant reference surface for dual-frequency altimetry. Using the observed close correlation between σ0C and σ0Ku for each TOPEX cycle, the drift in σ0 calibration may be assessed through the shifts in the mean relationship. Application to the first 150 cycles of data from TOPEX side A shows good general agreement with the already applied corrections (derived from cycle averages of σ0); however, these corrections have overcompensated slightly leaving a remnant drift of 0.03 dB yr−1. In a particular instance, the response of the altimeter can be seen to take a day to recover from an instrument shutdown. Initial investigations for TOPEX side B (which apart from the antenna is an identically constructed but separate instrument) confirm that dual-frequency methods can be used to cross-calibrate nonsimultaneous sensors.

Corresponding author address: Dr. Graham D. Quartly, James Rennell Division for Ocean Circulation, Southampton Oceanography Centre, Empress Dock, Southampton S014 3ZH, United Kingdom.

Email: graham.d.quartly@soc.soton.ac.uk

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