Sentinel-3 calibration and Validation Planning Meeting ESA/ESRIN, Frascati, Italy, 20-22 nd March 2012 Road-mapping the Way Forward for Sentinel-3 Topography

Sentinel-3 calibration and Validation Planning MeetingESA/ESRIN, Frascati, Italy, 20-22nd March 2012

Road-mapping the Way Forward for Sentinel-3 Topography Mission SAR-Mode waveform Re-tracking

over water surfaces

J Benveniste, P Cipollini, C Gommenginger, C Martin-Puig, D Cotton, S Dinardo, NOC (UK), Starlab (Spain), SatOC (UK), ESA/ESRIN (Italy)


Introduction• This talk deals with findings and challenges

relevant to the Sentinel-3 Radar Altimeter over ocean

• We will see:– What is special about the Sentinel-3 Altimeter – What are the challenges – Validation Exercise: height inter-calibration with

Jason-2 and SWH calibration w.r.t. wave buoys– Conclusions


Vs/c) ) ) ) ) )

In LRM, the pulse-limited footprint moves across the surface pulse by pulse as the satellite

passes overhead

In LRM, the pulse-limited footprint moves across the surface pulse by pulse as the satellite

passes overhead

)

Courtesy: K. Raney, APL/JHU

PRF: ~2 kHz

Conventional Low Resolution Mode (LRM)

3-7 km depending on sea state


Vs/c

SAR altimeter uses Doppler frequency effect to spotlight a

small area along-track (250-300 m) resolved by each burst as the

satellite passes overhead

SAR altimeter uses Doppler frequency effect to spotlight a

small area along-track (250-300 m) resolved by each burst as the

satellite passes overhead) ) ) ) ) ) )PRF: ~20 kHz

SAR altimeter (Cryosat-2, Sentinel-3): fundamentally different!

250-300 meters

Courtesy: K. Raney, APL/JHU


At t=0, the SAR footprint is no longer circular but rectangular !!

At t=0, the Across Track Resolution is the same as Pulse-Limited Altimetry (Pulse-Limited Diameter) but the footprint size reduces with the time delay

The footprint area is no longer constant with time delay

..4,3,2,12

1 ii

RiiRR PL

PLt=0t<0 t>0

RED LRM GREEN SAR


Multi-looking

• Doppler-beam selection/weighting & incoherent integration of beams from multiple bursts starting at the same ground cell (to reduce the speckle noise) i.e. multilooking

• Note the peaky waveform shape and the long decaying tail due to varying footprint area in SAR Mode

128 gates

1D multi-looked 20Hz waveform


• More independent looks improved retrieval precision– Two-fold improvement according to numerical

studies by Jensen & Raney (1998)

• Finer spatial resolution along track– ~300 meters along-track

• Higher SNR – ~10 db more

• Better performance close to land– especially for track ~90° to coastline

• Less sensitivity to sea state

Predicted advantages of SAR over LRM


THE FUTURE: Sentinel-3 Surface Topography Mission

• LRM: open ocean• SAR: global coastal ocean +

sea ice• SAR open loop: ice sheet margins• SAR open/closed loop: land & inland

waters

THE PRESENT: Cryosat-2• LRM: most of open ocean

+ most of land

• SAR: some oceanic & coastal regions + sea ice

• SARIN: few pilot regions (ocean & land) + land ice


The ESA SAMOSA Project:• Study funded by ESA STSE, led by David Cotton (SatOC)

– Starlab, NOC, De Montfort University, DTU Space & expert support from Keith Raney (JHU/APL)

• Objectives & Methodology– Quantify range retrieval accuracy in pulse-limited and SAR mode as a

function of significant wave height• Develop physically-based models for SAR altimeter ocean waveforms• Apply physically-based models to SAR ocean waveforms

– Done for both simulated and real Cryosat SAR waveforms over ocean

– Investigate method to reduce SAR mode data to pseudo-LRM– Applications to airborne SAR altimeter data, SAR waveforms over

inland water (DMU), coastal regions, sea floor mapping (DTU Space),…

• Development and Validation of SAMOSA SAR re-tracker with Cryosat-2 SAR mode data over ocean


Next-generation SAMOSA models• SAMOSA1 gives acceptable fit to Cryosat-2 SAR waveforms BUT…

– Circular antenna beam, no across-track mispointing– Non-physical behaviour for large mispointing angles– Over-estimates significant wave height (Hs)

• Next generation model: SAMOSA2– entirely new physically-based formulation developed by Starlab– Accounts for asymmetric antenna beam, ellipticity of the Earth, along-

AND across-track mispointing, non-linear ocean wave effects – Physically correct response to mispointing– Improved fit to Cryosat-2 SAR waveforms– BUT… not fully-analytical & computational expensive


SAMOSA3 model• Simplification of SAMOSA2 while keeping its

advanced features• fully-analytical, robust and computationally

fast !!


• With SYMMETRIC antenna beam and no Earth ellipticity effects:

– SAM3 and SAM1 are equivalent !!

Comparing SAM1, SAM2 and SAM3

These two entirely independent theoretical models converge !

These two entirely independent theoretical models converge !


SAMOSA 3 Project:• Study funded by ESA STSE, led by David Cotton (SatOC)

– Starlab, NOC, • Objectives & Methodology– Develop and characterize SAM3 model (Trade-off TN)– Deliver to the Sentinel-3 STM PAD a Detailed Processing Model (DPM)

of a SAR ocean waveform re-tracker based on the best SAMOSA model to operationally retrack Sentinel-3 SAR-Mode L1b waveforms

– Validate Retracker using Cryosat Data in high and low sea conditions


SAMOSA 3 DDM and Re-tracker:• Physically-based model developed

by Starlab from first principles• Analytical ( by Bessel Functions)

solutions to model the Delay Doppler Maps (DDM) for the full span of Doppler Frequencies

• Model depends on epoch, significant wave height, Pu, surface rms slope, and mispointing angle(s),

• The model independent variables are the Doppler Frequency and the Time Delay

• The waveforms are retracked by Least- Square Fitting Algorithm (Levenberg-Marquard)


Model Behaviour: effect of Sea State (SWH)– SWH affects the width of lobe of the SAR waveforms

SAMOSA3 model:Dependence of waveform shape on SWH


Model Behaviour: effect of mispointing– Roll mispointing affect shape of the SAR waveforms– Model insensible to pitch mispointing– Effect significant only for mispointing greater than 0.1 deg


Effect of along-track mispointing: negligible


Effect of across-track (roll) mispointing: changes waveform tail

0.3°

0.5°

0.1°

0.0°


Challenges of SAR altimetry


Challenge 1: LRMSAR transitionsLRMSAR


LRMSAR transitions (cont.)• Methodology to reduce SAR mode data to

pseudo LRM has been developed by Starlab (RDSAR method)– Tested and validated with simulated SAR

waveforms: • Full Bit Rate (FBR) SAR can be successfully transformed

into pseudo-LRM waveforms that can be re-tracked with Brown model

• RDSAR anyway does not give the same range noise as LRM (less number of accumulated independent looks)


Challenge 2: Coastal ZonesWaveforms 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 !!!

The land contamination in SAR Mode is greatly reduced but still present in unfavourable cases; MisFit and Ground Track Orientation to the coastline can be parameters useful to quantify the contamination


Challenge 3: Inland Water


Validation Exercise over open sea


CryoSat-2 and Jason-2: Range error in SAR and LRM as a function of sea state

• Lots of CryoSat-2 data in LRM and SAR mode since July 2010 but not collocated (SAR and LRM are exclusive modes)

Comparison of CryoSat2 SAR with Jason2 LRM

Focus on small area in Norwegian Sea

July 2010 – March 2011Wide range of sea

states


SAM3 SSHKsiyinput = 0.01 deg

Variability in SSH (Norwegian Sea)

Factor of 1.43 reduction in SSH error with CryoSat-2 L1B SARSAR perf will improve with reprocessed CryoSat-2 L1B dataFactor of 1.43 reduction in SSH error with CryoSat-2 L1B SARSAR perf will improve with reprocessed CryoSat-2 L1B data

CryoSat-2 SAR (Hs < 2m)SSH std 20Hz: 4.907 cm

1Hz: 1.097 cm

Jason-2 LRM (Hs < 2m)SSH std 20Hz: 7.012 cm

1Hz: 1.568 cm


CryoSat-2 SAR (Hs < 2m)Hs std 20Hz: 0.425 m

1Hz: 0.095 m

Jason-2 LRM (Hs < 2m)Hs std 20Hz: 0.536 m

1Hz: 0.120 m

Variability in Hs (Norwegian Sea)

Factor of 1.26 reduction in SWH error with C2 L1B SARFactor of 1.26 reduction in SWH error with C2 L1B SAR

SAM3 SSHKsiyinput = 0.01 deg

Slight bias in SAR Hs ?Slight bias in SAR Hs ?


Bias in SAR Hs ? Validation against buoys

Buoy wave height data from UK Met Office

CryoSat-2 SAR data and buoys collocated within 50km and 30 minutes

Data for July 2010 - May 2011


• Linear relation between C2 SAR Hs and buoy Hs over full range of sea states

• Small bias < 0.5 m; Std ~ 0.3 m• Hs bias is directly linked to roll

mispointing used as input to the SAR retracker– use of platform mispointing data

as input to the SAR retracker should reduce this bias

– Need to account for known small systematic bias in roll mispointing for C2 (Scharroo, pers comm.)

CryoSat-2 SAR Hs against buoys

SAM3 SWHKsiyinput = 0.01 deg


Conclusions• Performance of CryoSat-2 SAR should improve with optimised SAR

retracking that uses platform roll mispointing as input– Both variability and SWH bias– Same analyses to be repeated in a less dynamic region– Analyses of CryoSat-2 SAR performance against pseudo-LRM products

• In summary:– CryoSat-2 SAR mode produces very good data over ocean– we have a fully-analytical, robust and computationally efficient model

that successfully retracks CryoSat-2 SAR waveforms– SAR shows improved performance compared to LRM for both SSH and

SWH

SAR opens up a wide range of new scientific possibilities over ocean, coastal zone and inland water

SAR opens up a wide range of new scientific possibilities over ocean, coastal zone and inland water

Ready for Sentinel-3!

Ready for Sentinel-3!



www.altimery2012.org

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Sentinel-3 calibration and Validation Planning Meeting ESA/ESRIN, Frascati, Italy, 20-22 nd March 2012 Road-mapping the Way Forward for Sentinel-3 Topography