SAR Altimetry in Coastal Zone: Performances, Limits

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SAR Altimetry in Coastal Zone: Performances, Limits,
Perspectives
Salvatore Dinardo
Serco/ESRIN
Bruno Lucas
Deimos/ESRIN
Jerome Benveniste
ESA/ESRIN
SUMMARY
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•
•
•
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Introduction
Land Contamination in SAR Mode
SAR Mode: Application in Med Sea
SARin Mode: Application in James Bay
Conclusion & Recommendations
SAR DATA SET
Study Area:
West Mediterranean Sea
Four Months of CryoSat
SAR L1B Data: May, June,
July, August
Corrections coming from
CryoSat Data, no sea state
bias applied
Sea Land Mask:
250m MODIS
Water/Land Mask..
To calculate the
distance
SAR Retracking
One of the side consequences of the SAR
focusing is that the characteristic quasistep shape of the pulse-limited waveform
is lost and the SAR focused waveforms
assume now a particular pulse-shape with
long slow-decaying tail
SAR Retracking on CryoSat
L1b Data on normalized
waveforms
One Shot Retracking (not
double one)
The retracker scheme used
to fit the model to Data is
the
non-linear
Least
Squares Algorithm (LSA) in
the formulation proposed
by Levenberg-Marquardt
Able to retrieve
epoch, wave-height
and Pu
GOF: sum of squared
residuals
Over open ocean
0.3-0.4
Land Contamination in SAR mode
The expected Land Contamination is believed much lower than in Pulse-Limited
Mode for two reasons:
• The Along Track Resolution is finer (300 meters for CS)
• At t=0, the Across Track Resolution is the same than Pulse-Limited Altimetry
(Pulse-Limited Diameter) but anyway the size is reducing with the time delay


R  RPL i  i  1 
RPL
i  1,2,3,4..
2 i
The scattering area is not anymore constant  Effect of off-nadir scattering
targets at across track is minimized
Effect of the Land In SAR mode
Waveforms at 20 Hz, one waveform each 300 meters – Optimal Conditions
Coastal Zone
Effect of the Land -> LIMITS
Track Orientation and bright targets can have effect on the level of the Land
Contamination.. Even in SAR mode !!!
How Close to the coast?
A quantitative measure for land contamination can be the GOF (Goodness of
Fitting) (aka QME):
GOF   ( yi2,data  yi2,mod el )
i
Mean of the GOF in our study area plotted vs. the distance to coast
WEST Med Results: SSH
Statistics in Study Area: SSH
For the all study area, the std of the SSH inside 20 Records Block have been computed
Mean std @ 20 Hz 5.5 cm
(1.1 @ 1 Hz)
Std of std 3cm
84% of std between 0 and
10 cm
Just In coastal Area
Mean std @ 20 Hz 6.13 cm
Std of std 7 cm
57% of std between 0 and
10 cm
Statistics in Study Area: SWH
For the all study area, the std of the SWH inside 20 Records Block have been computed
Mean std 33 cm @20 Hz ( 7
cm @ 1 Hz)
Std of std 11 cm
87 % of std between 0 and
0.5 meters
Just In coastal Area
Mean std 36 @ 20 Hz
Std of std 16 cm
62 % of std between 0 and
0.5 meters
Statistics in Study Area: s0
For the all study area, the std of the s0 inside 20 Records Block have been computed
Mean std 0.15 db @ 20 Hz
( 0.036 db @ 1 Hz)
91% of std between 0.1 db
and 0.2 db
Just In coastal Area
Mean std 0.173 @ 20 Hz
cm
83 % of std between 0 and
cm
SARin Mode: Origin Angle
Beyond the SAR Mode, CryoSat operates also in SARin mode
From
the
Phase
Difference between the
channels,
we
can
easily compute the
origin angle and then
the
across
track
distance of the first
return (closest range)
x0
 0 (rclosest ) 
    sin 

2b


1
You need to know the
closest range to
surface .. This means
you have to re-track the
SARin Data
x0  rclosest sin 
SARin Mode Application
No SARin Mode over
Mediterranean Sea
The James Bay in Canada
Covered in SARin
A series of small Islands
across track the satellite
pass
Conclusions & Way Forward
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In SAR mode, 1 cm of range noise seems a achievable objective
For SWH, the current precision is not yet the theoritical one (4-5
cm).. Works have to continue on this front and an explanation needs
to be found
For Wind Speed, a variability in sigma zero lesser than 0.04 db is
very good result
The land contamination in SAR Mode is greatly reduced but still
present in unfavourable cases; GOF (MQE) and Ground Track
Orientation to the coastline can be parameters useful to quantify the
contamination
When possible, also the SARin mode must be used in coastal zone: the
SARin mode allows to compute the across track distance of the
closest target; this parameter can be useful to discriminate the nadir
or off-nadir origin of the echo
The analysis needs to be repeated with a more set of correction
(included sea state bias) and contrasted against in situ data and
models... Some biases in SWH and SSH are expected
Conclusions & Way Forward
THANKS FOR YOUR
ATTENTION
[email protected]
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