SAR Altimetry in Coastal Zone: Performances, Limits, Perspectives

Salvatore DinardoSerco/ESRINBruno LucasDeimos/ESRIN

Jerome BenvenisteESA/ESRIN

• Introduction• Land Contamination in SAR Mode• SAR Mode: Application in Med Sea• SARin Mode: Application in James

Bay• Conclusion & Recommendations

SUMMARY

SAR DATA SETStudy Area:

West Mediterranean SeaFour Months of CryoSat SAR L1B Data: May, June, July, AugustCorrections coming from CryoSat Data, no sea state bias applied

Sea Land Mask:

250m MODIS Water/Land Mask.. To calculate the distance

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

Land Contamination in SAR modeThe 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

..4,3,2,12

1 ii

RiiRR PL

PL

Effect of the Land In SAR modeWaveforms 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 !!!

How Close to the coast? A quantitative measure for land contamination can be the GOF (Goodness of Fitting) (aka QME):

i

elidatai yyGOF )( 2mod,

2,

Mean of the GOF in our study area plotted vs. the distance to coast

WEST Med Results: SSH

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

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

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

SARin Mode: Origin Angle Beyond the SAR Mode, CryoSat operates also in SARin mode

b

rclosest

2

)(sin 01 sin0 closestrx

x0

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

SARin Mode Application No SARin Mode over

Mediterranean Sea

The James Bay in CanadaCovered in SARin

A series of small Islands across track the satellite pass

• 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

THANKS FOR YOUR ATTENTION

salvatore.dinardo@esa.int

Conclusions & Way Forward