Surface%20Water%20and%20Ocean%20Topography%20Mission%20(SWOT)

1
Surface Water and Ocean Topography Mission
(SWOT) Lee-Lueng Fu (Jet Propulsion Laboratory)
2
A brief History

• 1998- Wide Swath Ocean Altimeter (WSOA) was
  selected for NASAs Instrument Incubator Program
  (IIP).
• 2001 Workshop on high-resolution ocean
  topography (chaired by Dudley Chelton).
• 2002 Development of WSOA as a payload for
  OSTM/Jason-2 began.
• 2004 WSOA was canceled due to budget problems
  (after spending 20 M on the program).
• 2005 - Joint US/European hydrology team formed
  and WatER proposal submitted to ESA (not selected
  due to programmatic issues)
• 2005 Hydrosphere Mapper and WATER were
  submitted to the NRC Decadal Survey.
• 2007- Decadal Survey recommended SWOT as a
  combined mission for land hydrology and
  oceanography applications.
• 2007 A joint informal NASA/CNES Science Working
  Group for SWOT was formed.
• 2008 Ka-band SAR Interferometry Studies for
  the SWOT Mission was selected for NASAs IIP.
• 2008 NASA has authorized funding for
  risk-reduction studies of SWOT.
• 2008 CNES plans to start a phase-A development
  of SWOT.

3
Space time sampling of radar altimetry missions
10 100 days
1000 km
100 km
WSOA
10 km
SWOT
4
SWOT Measurement Concept
A SAR interferometry radar altimeter
h H r1 cos(? )

• Precision timing of the sampling system allows
  precise ranging of the returned signals.
• Interferometry allows the determination of the
  direction of the signals from phase difference.
• Use SAR technique to achieve resolution on the
  order of 50 m.
• Spatial smoothing achieves measurement noise of
  1 cm at 1 km resolution.

5
Radar Interferometry was successfully
demonstrated by JPLs Shuttle Radar Topography
Mission (SRTM)
60m
6
A Jason pass for the analysis of SSH signals and
measurement errors
Jason Pass 132
7
SSH wavenumber spectrum
1000 100
10 km
Jason pass 132 (147 cycle average)
k-3
Instrument noise
Power density (cm2/cycle/km)
k-2
SWOT noise at 1 km resolution
3 cm noise
30 km
1cm noise
10 km
Wavenumber (cycle/km)
8
Coastal currents have scales 10 km. Velocity
accuracy from SWOT is 3 cm/s at 10 km scale or 1
cm/s at 25 km scale.
42.5º N
? h 5 cm ? v 50 cm/sec
lt 10 km
lt 10 km
41.9º N
lt 10 km
Observations made by ADCP offshore from the US
West Coast
T. Strub
9
SWOT Temporal Sampling
Temporal sampling is irregular and location
dependent.
1st 2nd 3rd 4th
10
Wavenumber spectra of SSH and measurement errors
1000 100 km
SSH
iono
wet tropo
SSB
Media errors and sea-state errors have scales
larger than 100 km and not affecting submesocale
SSH measurement.
11
Errors in coastal tide models up to 20 cm are
revealed from the Jason-T/P Tandem Mission.
What is the state of the art in coastal tide
modeling?
Andersen and Egbert (2005)
12
Besides the intrinsic science of internal tides,
they introduce 2-5 cm/sec error in ocean current
velocity. Is predicting internal tides from
models feasible ?
R. Ray/GSFC
13
Systematic errors in the SWOT measurement

• Look-angle errors due to uncertainly in the roll
  of the mast
• Range errors due to media effects
• Phase errors due to electronics
• Correction through cross-over calibration using
  both the nadir altimeter and swath measurements
• The spatial scales of the residual errors are in
  the range of mesoscale to basin scale, depending
  on the scales of the systematic errors.

14
Conclusions

• SWOT potentially can measure SSH at 1 km
  resolution with 1 cm precision, revealing ocean
  signals down to 10 km wavelengths.
• Accuracy for surface geostrophic currents is
  about 3 cm/sec at 10 km wavelength and 1 cm/sec
  at 25 km wavelength.
• Temporal sampling is a challenge irregularly
  spaced varying from 2 samples/20 days to more
  than 5 at mid and high latitudes.
• Coastal and internal tides have scales in the
  range of mesoscale and submesoscale, causing
  potential interference to ocean circulation
  studies.
• Systematic errors in interferometry pose
  challenges at mesoscale to basin scale yet to be
  quantified.

15
k-11/3
SWOT noise level
20 km
Along-track altimeter wavenumber spectrum for the
Kuroshio area for TOPEX/Poseidon, Jason-1,
ENVISAT and GEOSAT Follow On. Spectral slopes
were estimated in the 100 km to 300 km spectral
band. A k-11/3 slope is also shown (dashed
line). (Le Traon et al, 2008)
16
KE Spectrum from an OGCM simulation
Velocity error at 45º latitude with SSH noise of
1cm at 1 km resolution.
30 km wavelength
17
Finite-Time Lyapunov Exponent (T100 days)
AVISO data 1 October 2007