A GPS and Cold Ocean Brightness Temperature Calibration of the ERS-2 and TOPEX/Poseidon Microwave Radiometers

Sea-level change studies from altimetric satellites are reliant on range stability of the sea surface heights computed from orbital positioning and geophysically corrected data. One such correction, namely the wet tropospheric delay induced by the highly variable atmospheric water vapor content, is provided by radiometers onboard ERS-2 and TOPEX/Poseidon (T/P). In this study the long-term stability of the ERS-2 microwave radiometer (E2MR) and the T/P microwave radiometer (TMR) are investigated with the observed drift in the brightness temperatures approximated by reference to the coldest temperatures over the oceans. The E2MR stability is characterized by a gain anomaly fall in 1996 and a drift in the 23.8 GHz channel. For the TMR, investigations show that the dominant drift is about 0.2 K/year in the 18 GHz channel over the first 7–8 years but stabilizing and even decreasing slightly thereafter. In contrast, the 21 GHz and 37 GHz channels are comparatively stable. Utilizing correction formulae a modified wet tropospheric range is inferred from “small-change” analysis of the radiometric correction given on the altimetric Geophysical Data Records. The accuracy of this formulism is validated by independent comparison against GPS derived wet tropospheric delays inferred at 14 coastal IGS stations with near continuous data from September 1992 through to the present day. Comparisons between GPS results for ERS-2 and T/P show that the E2MR path delay is 14 mm short. For T/P, the spatial distribution of the wet tropospheric enhancement is further investigated to show that the nonuniformity can equate to a deviation in sea-level height change of about 0.1 mm/year compared with global average sea-level change. Finally, the altimetric range stability of T/P is revisited by comparison against time series from the global network of tide gauges. Analysis shows that the validated TMR drift correction results in a residual trend of −0.27 ± 0.11 mm/yr which is not significant at the 3σ level.

Keywords:

• radiometers
• altimetry
• calibration/validation
• sea-level
• GPS

Acknowledgments

The Authors would like to express their thanks to the NASA JPL group for the provision of precise orbit and clock products and the GIPSY/OASIS II software used in this experiment. Additionally, we acknowledge with thanks the Global Sea-level Observing System (GLOSS) fast center, University of Hawaii Sea-level Center for the provision of tide gauge data. We also express our thanks for the detailed and insightful comments provided by the reviewers of this article.

Notes

^a ERS-2 before anomaly on 26 June 1996.

^b After the anomaly: units for a _i consistent with Eq. (2).

AVISO. 1994. AVISO user handbook: Merged TOPEX/POSEIDON products. AVISO report AVI-NT-01-101-CN, Edition 2.1.

CERSAT. 2001. Altimeter & microwave radiometer ERS products—User manual (V2.3, July 2001), C2-MUT-A-01-IF, CERSAT - IFREMER.

Eymard, L., and S. A. Boukabara. 1997. Calibration-validation of the ERS-2 microwave radiometer, ESA, Noordwijk, The Netherlands, Final Report of European Space Agency. Contract 11031/94/NL/CN.

Obligis, E., N. Tran and L. Eymard. 2003. ERS-2 drift evaluation and correction, CLS/DOS/NT/03.688.

Stum, J., F. Mertz, and J. Dorandeu. 2001. Long-term monitoring of the OPR altimeter data quality, Ifremer Contract no. 00/2.210 052.