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Microwave Earth Remote Sensing

Jason-1 Project

Jason is a NASA oceanography mission to monitor global ocean circulation, discover the tie between the oceans and atmosphere, improve global climate predictions, and monitor events such as El Nino conditions and ocean eddies. The Jason-1 satellite carries a radar altimeter and it is a follow-on mission to the highly successful TOPEX/Poseidon mission. It is joint mission between France and USA. The satellite was launched in May 2000. Click here if you would like more background information on NASA's Jason-1 mission.

The Jason-1 project at the Brigham Young University MERS lab delt with finding the causes of the Electromagnetic (EM) bias in data received from the TOPEX/Poseidon and Jason-1 satellites and correcting the bias. The project was funded by NASA and headed by Dr. David Arnold, who, as a Ph.D. student at MIT, investigated the EM bias for his thesis.

The EM bias problem is associated with measurements taken of the mean sea level. High frequency waves reflect from the troughs more than from the crests of ocean waves. Thus, the estimated mean sea level is usually lower than its actual level. This difference is called the EM bias. Correction algorithms exist that remove this bias, and are accurate to within 4 cm. The Jason-1 research project aims to provide an algorithm which will lead to even more accurate measurements -- to within 1 cm.

There is evidence that the EM bias:

  • has a much better correlation with short wave modulation than with wind speed.
  • can be more accurately modeled with wave slope.

  • is affected by long wave tilting.
    Theoretical modeling was used in parallel with experimental work to explore the underlying causes of the bias as well as the height and frequency dependence of the bias.

    Modulation of the short wave is caused through direct hydrodynamic modulation or via modulation of the wind field. In either case the modulation is a function of wave slope. Since the EM bias is strongly dependent on short wave modulation, we can predict a dependence of the bias on wave slope. This prediction is supported by results from the Bass Strait experiment with indicated that the EM bias has a better correlation with wave slope than wind. By using wave slope in the correction algorithm it is hoped that the variability in the corrected EM bias can be significantly reduced.