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SAR Altimetry numerical simulations over water surfaces
Christine Gommenginger, Paolo Cipollini (NOCS)
Cristina Martin-Puig, Jose Marquez (Starlab)
David P. Cotton (Satellite Oceanographic Consultants)
R. Keith Raney (Johns Hopkins Uni/Applied Physics Lab)
Jérôme Benveniste (ESA/ESRIN)
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Presentation Content
• What is Delay-Doppler Altimetry (DDA)?
• The ESA SAMOSA project
• Motivation and methodology
• CRYMPS DDA simulations over water
• First results
• Conclusions
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What is Delay-Doppler Altimetry (SAR) ?
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Conventional ALT footprint scan
Vs/c) ) ) ) ) )
RA pulse-limited footprint in effect is dragged along the
surface pulse by pulse as the satellite passes
overhead
RA pulse-limited footprint in effect is dragged along the
surface pulse by pulse as the satellite passes
overhead
)
Courtesy: K.Raney
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DDA: a fundamentally different method
Vs/c
DDA spotlights each along-track resolved
footprint as the satellite passes overhead
DDA spotlights each along-track resolved
footprint as the satellite passes overhead
) ) ) ) ) ) )
Courtesy: K.Raney
Improved along-track resolution, higher PRF, better S/N, less sensitivity to sea state,…
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ESA SAMOSA project
• SAMOSA - Development of SAR Altimetry Mode Studies and Applications over Ocean, Coastal Zones and Inland Water
• Project management: David Cotton, SatOC• Consortium members: NOCS, Starlab, De Montfort University,
Danish National Space Centre• Tasks:
1. Review state of the art (Starlab)2. Quantify improved range error in different sea states (NOCS)3. Assess recovery of short scale surface slope signals (DNSC)4. Develop theoretical model for DDA waveforms (Starlab)5. Assess capability in coastal zone and inland waters (DMU)6. Application to RA-2 individual echoes (NOCS)7. Validation with ASIRAS data (DNSC)
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Motivation• Task 2: to independently validate Jensen & Raney
(1998) on improved sea level retrieval with DDA against sea state
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Methodology
• CRYMPS: Cryosat Mission Performance Simulator• CRYMPS developed & run at University College
London/MSSL, in collaboration with ESA/ESTEC• Simulates the CryoSat platform orbit and
instrument operation, generates official Cryosat products for LRM, SAR and SARIn mode, for a given (explicit) surface
• Simulator and surface descriptors optimised for ice/sea ice surfaces
• Here, CRYMPS is applied to ocean surfaces
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Code Description SWH Swell Amplitude
Swell wavelength
PDF s.d.
F13 F1: CRYOVEX 2006, 02/05/2006
F3: CRYOVEX 2006, 30/04/2006
1.41m
0.71m
1.0 m
0.5 m
100 m
50 m
4 cm
4 cm
F24 F2: moderate sea state
F4: high sea state
4.23 m
14.1 m
3.0 m
10 m
150 m
200 m
10 cm
10 cm
C3 Realistic ocean wave spectrum (Elfouhaily et al., 1997)
1/2/3 m N/A N/A 10 cm
C1 Realistic ocean wave spectrum (Elfouhaily et al., 1997)
0.1/4/5 m N/A N/A 10 cm
FT1 Sea Floor Topography 1, variations in sea surface height, low swh, short wavelength
1.41 m 1.0 m 100 m 4 cm
CRYMPS runs over open ocean
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Example: C3 scenarioC3
+3m
-3m
Hs = 1m Hs = 2m Hs = 3m
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Example: C1 scenarioC1
+5m
-5m
Hs = 0.1m Hs = 4m Hs = 5m
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CRYMPS SAR 18kHz
Pseudo-LRM 20Hz
CRYMPS LRM 20Hz
NOCS ocean waveform retracker (Brown model)
Ocean surface DEM
CRYMPS
SAR->LRM Reduction (Starlab)
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64 pulses per burst
etc
etcLRM mode
SAR mode
PRF: 1970 Hz
Continuous
Burst period 11.7 ms
Generation of Pseudo-LRM Waveforms from SAR-mode: Revised (1/4)Extrapolate from the original pre-sum model
time
time
PRF: 17.8 KHz
within bursts
SAR -> pseudo-LRM reduction ?
. . .
Extract 1 in every 9 waveforms
We get 8 waveforms per burst:Wf1+wf10+wf19+wf28+wf37+wf46+wf55+wf64
9SAR
LRM
PRFn
PRF= =
Courtesy: K.Raney & C. Martin-PuigWork in Progress !
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20Hz pseudo-LRM
. . .
SAR
t
PRFSAR = 17.8KHz
Apply IFFT0Apply IFFT0
|.|^2|.|^2
20Hz rate Incoherent averaging20Hz rate Incoherent averaging
RDSAR
I,QI,Q
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First results
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C3 LRM Hs: 0.145m
• 16 seconds along-track• 260 LRM samples along-track
• No Power Scaling applied– No along-track variability in
peak amplitude– No Sigma0 info
• Amplitude scaled by 10-6
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C3 RDSAR Hs: 0.145m
• 16 seconds scenarios • 250 samples along-track
• No Power Scaling applied
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Ocean retracker results
C3 LRM
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Ocean retracker results
C3 RDSAR
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Conclusions
• SAMOSA will assess the improved performance of DDA w.r.t. pulse-limited altimetry to:– Retrieve higher-accuracy ocean range, detect short-scale surface
slope, extend altimetry to the coastal zone,…
• Methodology is based on Cryosat-type SAR and LRM data from the CRYMPS simulator applied to ocean surfaces
• Reduction of SAR -> pseudo-LRM still debated• First results show that:
– CRYMPS produces realistic LRM and DDA waveforms– the CRYMPS waveforms were successfully retracked with the
NOCS ocean retracker, both LRM and RDSAR
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SAR Altimetry numerical simulations over water surfaces
Thank You !
For further info, contact Christine Gommenginger