The WatER Mission in Europe: how can it help science?

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The WatER Mission in Europe how can it help
science? By Vivien ENJOLRAS(1,2), Ernesto
RODRIGUEZ(2), Paul BATES(3), Nelly MOGNARD(1,2),
Anny CAZENAVE(1,2) (1) Centre National dEtudes
Spatiales, 18, av. Edouard Belin, 31400 Toulouse,
France (2) Laboratoire dEtudes en Geophysique et
Oceanographie Spatiales, 14 av. Edouard Belin,
31400 Toulouse, France (2) Jet Propulsion
Laboratory, 4800 Oak Grove Drive, Pasadena CA
91109, United States (3) University of Bristol,
University Road, Bristol BS81SS, United
Kingdom Author to whom correspondence should be
addressed - Email vivien.enjolras_at_gmail.com
WatER Orbit over Europe
Abstract
WatER Main Parameters
The Water Elevation Recovery (WatER) mission,
which was recently submitted by an international
team to the Earth Explorer Core Mission of ESA,
is dedicated to the determination of surface
water extent, height, and slope. The WatER
instrument consists of a Ka band Radar
Interferometer (KaRIN) coupled with a nadir Ka
altimeter (AltiKa) for filling the nadir gap and
for risk reduction in the KaRIN calibration.
There is a strong need to determine the value
added science that can be attained from various
spatial and temporal samplings of surface water
storage and movement. A Virtual Mission has been
implemented for a while and helps to answer this
question. Recent large floods that occurred in
1995 on the river Meuse in Northern Western
Europe have led to heightened interest in flood
forecasting systems in this region. The VM
concept has been applied to this river. Simulated
WatER data have been generated and ground
processing techniques have been tested in some
specific cases. Flood forecasting improvement by
assimilating generated WatER data in models is
currently under study. Keywords interferometry,
hydrology, Europe

• Main instrument Ka-band radar interferometer on
  Prima platform
• Two 50 km swaths on either side of the nadir
  track
• Spatial resolution varying continuously from 70
  m x 5 m (near range) to 10 m x 5 m (far range).
• First product amplitude maps of the
  co-registered returns
• Identification of Water Bodies
• Second Product interferometric phase map
• Estimate of the topography using timing and phase
  difference measurements (relative and absolute
  elevation)
• Nadir gap filled with a Ka-band nadir altimeter
  (AltiKa), also used in the calibration of KarIN
• 6 am-6 pm Sun-Synchronous Orbit of 16 days (no
  yaw steering impact and power consumption
  optimization)

WatER KarIN Theoretical Performances
Preliminary Scene File Creation

• The main contribution to the global height error
  budget comes from the instrumental error related
  to the instrument signal to noise ratio (SNR).
  The amount of power returning to the instrument
  is mainly dependent on the roughness of the
  surface (characterized by the standard deviation
  of small gravity-capillary waves)
• The instrumental error budget is a function of
  the SNR, the signal wavelength, the
  interferometric baseline and the geometry of
  observation. The bigger the resolution pixel, the
  better the budget. However, a too big pixel will
  cause phase wrapping water and land will also
  mix more easily, impacting the height error
  budget as well. It is presented for a multi
  looking process of 4 looks

Developed Program Chart
Performances over Meuse

• The orbit is generated with a time step related
  to the azimuth resolution intended to be reached
  (basis 4 Looks)
• Instrument thermal noise (see KarIN theoretical
  performances) is brought in the process relying
  on the desired multi-looking process, directly
  impacting the ground pixel azimuth resolution
• External sources of errors wet troposphere,
  attitude (roll angle), baseline length knowledge,
  onboard phase calibration.
• A way to cope with these errors is to use as a
  reference interferometric phase map the one
  generated with the best ancillary information of
  topography (SRTM,). Looking at the difference
  between the real maps and the simulated reference
  maps (averaged over hundreds of meters pixels to
  smooth the data) enables to estimate the tilts
  caused by roll and wet troposphere
• The extraction of the river data is based on an
  amplitude threshold in the amplitude map and the
  a priori knowledge of the position of the
  observed river (see scene file creation)
• The phase unwrapping (going from interferometric
  phase to absolute height above the ellipsoid)
  begins in the Far Range, where the altitude of
  ambiguity is the greatest (an accurate altitude
  (DORIS) is used through the process)
• To improve the height error budget, two
  independent processes are computed on the raw
  elevation data
• Cross River Average considering the same
  curvilign absciss gets the same elevation
• Along River Moving Fit, characterized by its
  river length computation and the a priori
  elevation basis from the a priori scene file
  creation
• Precision depends on the water brightness and the
  length and width of the water body

Conclusion and Perspectives
References

• A WatER Virtual Mission has been implemented and
  first tested through a very critical observation
  scenario on a narrow river such as the Meuse in
  Europe
• The single-pixel performances of KarIN between
  30 centimetres and 2 meters can be really
  improved thanks to the great quantity of data,
  even on a narrow river such as the Meuse, by
  doing some ground post-processing across river
  and along river
• Having a strong reference topography such as
  SRTM (Shuttle Radar Topography Mission) helps to
  retrieve the effects of external errors,
  especially roll and wet troposphere errors,
  leading to centimetric residual errors
• One year WatER data are currently generated over
  the Meuse and are delivered to flood forecasting
  specialists to look at the improvement brought by
  the assimilation of WatER data to their models.
  The important in situ networks, especially in the
  Netherlands, is used (from Remco Dost, ITC,
  Enschede, The Netherlands).

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