1. SAR Altimetry in Coastal Zone: Performances, Limits, Perspectives Salvatore Dinardo Serco/ESRIN Bruno Lucas Deimos/ESRIN Jerome Benveniste ESA/ESRIN

2. Introduction Land Contamination in SAR Mode SAR Mode: Application in Med Sea SARin Mode: Application in James Bay Conclusion & Recommendations SUMMARY

3. SAR Altimetry Benefits With respect the Pulse Limited Altimetry, the SAR altimetry ensures: Along Track Resolution reduced by a factor of 10 (@SWH=2 m) and constant with the Sea State, thanks to the Coherent Doppler focusing The SNR ratio improved by more than 10 dB, thanks to the Coherent Doppler focusing A theorized improvement of factor 2 in measurement precision (SSH, SWH, WS) thanks to higher number of incoherently summed looks Ref. “Resolution and Precision of a Delay-Doppler Radar Altimeter”, K. Raney 300 meters

4. 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

5. SAR Retracking Able to retrieve epoch, wave-height and Pu SAR Retracking on CryoSat L1b Data on normalized waveforms One Shot Retracking (not double one) GOF: sum of squared residuals Over open ocean 0.3-0.4 One of the side consequences of the SAR focusing is that the characteristic quasi- step shape of the pulse-limited waveform is lost and the SAR focused waveforms assume now a particular pulse-shape with long slow-decaying tail 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

6. SAR Measured Precision Geophysical Parameter 1 Hz Precision Noise in Range 1.08 cm Noise in Wave-Height 7.0 cm Noise in Pu<0.04 db To assess the retrieval performance of the SAR retracker, we considered a 10 seconds long segment from a summer pass over the North of Caspian Sea (very low Sea Dynamics) and we calculated the mean of the standard deviation of the range, wave-height and Pu inside 20-records block

7. 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 The scattering area is not anymore constant  Effect of off-nadir scattering targets at across track is minimized

8. Effect of the Land In SAR mode Waveforms at 20 Hz, one waveform each 300 meters – Optimal Conditions Coastal Zone

9. Effect of the Land -> LIMITS Track Orientation and bright targets can have effect on the level of the Land Contamination.. Even in SAR mode !!!

10. How Close to the coast? A quantitative measure for land contamination can be the GOF (Goodness of Fitting) (aka QME): Mean of the GOF in our study area plotted vs. the distance to coast

11. WEST Med Results: SSH

12. Statistics in Study Area: SSH Mean std @ 20 Hz 5.5 cm (1.1 @ 1 Hz) Std of std 3cm 84% of std between 0 and 10 cm Mean std @ 20 Hz 6.13 cm Std of std 7 cm 57% of std between 0 and 10 cm Just In coastal Area For the all study area, the std of the SSH inside 20 Records Block have been computed

13. Statistics in Study Area: SWH Mean std 33 cm @20 Hz ( 7 cm @ 1 Hz) Std of std 11 cm 87 % of std between 0 and 0.5 meters Mean std 36 @ 20 Hz Std of std 16 cm 62 % of std between 0 and 0.5 meters Just In coastal Area For the all study area, the std of the SWH inside 20 Records Block have been computed

14. Statistics in Study Area:  0 Mean std 0.15 db @ 20 Hz ( 0.036 db @ 1 Hz) 91% of std between 0.1 db and 0.2 db Mean std 0.173 @ 20 Hz cm 83 % of std between 0 and cm Just In coastal Area For the all study area, the std of the  0 inside 20 Records Block have been computed

15. SARin Mode: Origin Angle Beyond the SAR Mode, CryoSat operates also in SARin mode x0x0 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) You need to know the closest range to surface.. This means you have to re-track the SARin Data

16. 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

18. 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

19. THANKS FOR YOUR ATTENTION salvatore.dinardo@esa.int Conclusions & Way Forward