1. 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)

2. Presentation Content What is Delay-Doppler Altimetry (DDA)?
The ESA SAMOSA project
Motivation and methodology
CRYMPS DDA simulations over water
First results
Conclusions

3. What is Delay-Doppler Altimetry (SAR) ?

4. 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
The footprint of a conventional pulse-limited altimeter moves along the surface at (essentially) the same rate as the progress of the satellite along its orbit. Typically, the altimeter transmits a pulse 1000 to 2000 times per second (the so-called pulse repetition frequency). Since the speed of the satellite (projected onto the surface) is about 6.7 km/s, the effective along-track length of the footprint is increased in proportion to the number of pulses so averaged.
The half-power width of the antenna beamwidth projected onto the surface is (typically) about 15 km, which is several times larger than the pulse-limited footprint.
Courtesy: K.Raney

5. DDA: a fundamentally different method) ) ) ) ) ) )
Vs/c
DDA spotlights each along-track resolved footprint as the satellite passes overhead
Improved along-track resolution, higher PRF, better S/N, less sensitivity to sea state,…
A delay-Doppler radar altimeter uses signal processing techniques to resolve a small along-track footprint. The altimeter in effect “stares” at each resolved surface location for as long as that particular cell is illuminated by the antenna as the spacecraft passes over head. The minimum size of such a delay-Doppler-resolved cell is about 250 meters, which is a constant (determined by system parameters and viewing geometry). Note that each cell is viewed over a larger fraction of the antenna beam than the pulse-limited; thus more data is gathered, which leads to substantial benefits (reviewed in subsequent materials).
If only one such cell were illuminated, the measurements could not keep up with the forward velocity of the antenna footprint. The system gets around this objection by operating many of these spotlight beams simultaneously. A typical design would generate 64 parallel Doppler beams, but not all of these fall within the main lobe of the antenna. Thus, in practice there will be date from ~40 beams combined in the processor to generate the final (averaged) waveform from each resolved cell.
Courtesy: K.Raney

6. 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:
Review state of the art (Starlab)
Quantify improved range error in different sea states (NOCS)
Assess recovery of short scale surface slope signals (DNSC)
Develop theoretical model for DDA waveforms (Starlab)
Assess capability in coastal zone and inland waters (DMU)
Application to RA-2 individual echoes (NOCS)
Validation with ASIRAS data (DNSC)

7. Motivation
Task 2: to independently validate Jensen & Raney (1998) on improved sea level retrieval with DDA against sea state

8. 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

9. CRYMPS runs over open ocean
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
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 C3
Realistic ocean wave spectrum (Elfouhaily et al., 1997)
1/2/3 m
N/A C1
0.1/4/5 m
FT1
Sea Floor Topography 1, variations in sea surface height, low swh, short wavelength
1.41 m

10. Hs = 1m
Hs = 2m
Hs = 3m
Example: C3 scenario C3
+3m
-3m

11. Hs = 0.1m
Hs = 4m
Hs = 5m
Example: C1 scenario C1
+5m
-5m

12. CRYMPS SAR 18kHz Pseudo-LRM 20Hz
Ocean surface DEM
CRYMPS LRM 20Hz
CRYMPS
CRYMPS SAR 18kHz Pseudo-LRM 20Hz
SAR->LRM Reduction (Starlab)
NOCS ocean waveform retracker (Brown model)

13. SAR -> pseudo-LRM reduction ?
Generation of Pseudo-LRM Waveforms from SAR-mode: Revised (1/4)
Extrapolate from the original pre-sum model
LRM mode
etc
PRF: 1970 Hz
Continuous
time
64 pulses per burst
Burst period 11.7 ms
SAR mode
PRF: 17.8 KHz
within bursts
etc
. . .
time
Extract 1 in every 9 waveforms
We get 8 waveforms per burst:
Wf1+wf10+wf19+wf28+wf37+wf46+wf55+wf64
Work in Progress !
Courtesy: K.Raney & C. Martin-Puig

14. 20Hz rate Incoherent averaging
20Hz pseudo-LRM
SAR
PRFSAR = 17.8KHz
. . . t
I,Q
RDSAR
Apply IFFT0
|.|^2
20Hz rate Incoherent averaging

15. First results

16. C3 LRM Hs: 0.145m No Power Scaling applied Amplitude scaled by 10-6
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

17. C3 RDSAR Hs: 0.145m 16 seconds scenarios 250 samples along-track
No Power Scaling applied

18. Ocean retracker results
C3 LRM

19. Ocean retracker results
C3 RDSAR

20. 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

21. SAR Altimetry numerical simulations over water surfaces
Thank You !
For further info, contact Christine Gommenginger