Simulated SWOT ocean observations
Need for the nadir altimeter Wet tropospheric correction
Oceanographic performance requirement
High resolution wide-swath altimetry anticipated from the SWOT mission
The Surface Water and Ocean Topography (SWOT) mission will carry a Ka-band radar interferometer to perform wide-swath altimetry over the global oceans. • Joint mission of NASA and CNES with
contribution from the Canadian Space Agency
• Launch planned for 2020
The future SWOT mission
Lee-Lueng Fu(1), Clement Ubelmann(1) and Rosemary Morrow(2)
(1) Caltech, Jet Propulsion Laboratory, Pasadena, CA (2) LEGOS/CNES, Toulouse, France
Sampling characteristics:
• SAR resolution from 10 -70 m, smoothed to 1kmx1km over the ocean to reduce noise to cm accuracy
• 2x60km wide swath.
• Non-sun-synch orbit with inclination of 78°, 21-day repeat period
A snapshot of sea surface height (SSH) from Jason-1/2
• Drop in energy of 2-D gridded SSH spectrum (blue) vs alongtrack spectrum (red) at 200 km Indicates the mapping resolution (AVISO product) from nadir altimeter constellations.
• The challenge of the SWOT
Mission is to observe SSH at wavelengths shorter than 200 km
The limit of resolution of Jason-1/2
mean
68%
95%
10!100!103!105!
104!
103!
102!
101!
100!10-3! 10-2! 10-1!
Wavenumber (cycle/km)!
Pow
er S
pect
ral D
ensi
ty (c
m2 /c
ycle
/km
)!
baseline threshold
Wavelength (km/cycle)!
Reference wet tropo correction
1-beam residual error
2-beam residual error
• Conventional nadir-looking one-beam radiometer is not sufficient for correcting the cross-track variability of the range errors caused by the tropospheric water vapor.
• The two-dimensional swath measurement of SWOT needs a two-beam radiometer for wet-tropospheric corrections.
• The long-wavelength accuracy of the SSH measurement is expected higher for the nadir altimeter than KaRIn.
• By combining the two measurements simultaneously, a consistent measurement from short (KaRIn) to long wavelengths (altimeter) can be obtained.
• Strong heritage of error analysis from nadir altimetry (T/P, Jason1, Jason2, Saral/AltiKa, …)
• Calibration and validation of SWOT in setting the standard for the next generation altimetry missions to continue the climate data record of sea level and improve its resolution and coverage.
à For oceanography, requirements are specified in the spectral domain
à In order to observe fine scales (~20km wavelength) in most of the regions, the level of noise has to be almost 2 orders of magnitude below conventional altimetry
Left: Global mean power spectrum of SSH estimated from Jason series satellites (thick black curve). For wavelength <100km, a linear fit has been applied following (Xu and Fu, 2012). Lower limits of the 68% and 95% percentiles are indicated. Red and blue spectra: baseline and threshold requirement for the SSH accuracy. The baseline (threshold) requirement leads to 15 km (20 km) resolution over 68 % of the ocean.
Spatial distribution of the resolved wavelength (baseline)
à < 35km everywhere
à The small values in the tropical band are caused by shallow spectral slopes that may not reflect ocean eddy signals (e.g., internal waves)
References • Durand et al. et al., 2010: The Surface Water and
Ocean Topography mission: Observing terrestrial surface water and oceanic submesoscale eddies. Proc. IEEE, 98 (5), 766-779.
• Fu and Ubelmann, 2014: On the Transition from Profile Altimeter to Swath Altimeter for Observing Global Ocean Surface Topography, JTECH (in press)
• Xu and Fu, 2012:The effects of altimeter instrument noise on the estimation of the wavenumber spectrum of sea surface height, JPO, 42, 2229-2233.
• Ubelmann et al., 2014: The effect of atmospheric water vapor content on the performance of future wide-swath ocean altimetry measurement, JTECH (accepted)
• With Jason-1 + Jason-2, only large scales >200km are captured. Eddies are severly attenuated.
• The finescale features of the reference truth field are captured by the simulated SWOT observations.
• Drastic Improvement in retrieving the geostrophic velocity (derivative of SSH).
~ 15 km
km
cm
km