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)

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

What is Delay-Doppler Altimetry (SAR) ?

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

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

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)

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

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

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

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

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

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)

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

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

First results

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

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

Ocean retracker results C3 LRM

Ocean retracker results C3 RDSAR

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

SAR Altimetry numerical simulations over water surfaces Thank You ! For further info, contact Christine Gommenginger cg1@noc.soton.ac.uk