The Jason-1 Project (1996)
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
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.