KaRIn – the Ka-band Radar Interferometer on SWOT: Measurement Principle, Processing and Data Specificities

Roger Fjørtoft, Jean-Marc Gaudin, Nadine Pourthie, Christine Lion, Alain Mallet, Jean-Claude Souyris (CNES DCT/SI/AR)

Fifamè Koudogbo, Javier Duro, Patrick Ordoqui, Alain Arnaud (Altamira Information)

Christian Ruiz (CapGemini)

SWOTIGARSS 2010, Honolulu, Hawaii, 25-30 July 2010 2

Outline

■ Introduction Mission, satellite, KaRIn instrument

■Measurement principle Absolute vs. relative height restitution

■Processing LR (oceanography), HR (hydrology)

■Specificities of KaRIn images Ka-band, near-nadir

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Mission objectives and scientific requirements

■Mission SWOT (Surface Water and Ocean Topography) = innovative altimetry mission

improved spatiotemporal coverage for oceanography and high resolution altimetry data for hydrology.

KaRIn : Single pass Ka-band interferometric SAR system (JPL concept). Co-operation between NASA/JPL (USA) and CNES (France) Launch date: 2019 - 2020

■Main scientific requirements

Oceanography: Global coverage (<78°), sea surface height precision < 2 cm at ~1 km resolution LR mode

Hydrology: Global inventory of rivers > 100 m (50 m) and lakes > (250 m)2, height precision < 10 cm at average spacing 50 m, slope precision :1 cm/km HR mode

Introduction

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Xs

Ys Zs

■ Platform 1300 kg 1600W (orbit average) >21 m2 solar arrays High performance ACDS 4 Tb mass memory 655 Mbps X-Band TM

■ Payload (nominal configuration) Ka Band Radar Interferometer Ku/C Nadir Altimeter Water vapor radiometer POD suite: Doris + LRA + GPSP

SWOT satellite

Introduction

Preliminary figures fromthe CNES phase 0 study

10 m mast

■ Orbit 970 km 78° Non SSO 22 days repeat pass

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KaRIn

■ Ka-band Radar Interferometer : bistatic SAR (35.75 GHz 100 MHz)

■ 10 m mast

■ Near-nadir swaths (0.6-4.1°) on both sides of the track.

■ Monostatic mode improves the interferometric sensitivity by a factor of 2.

■ Intrinsic SAR resolution2 m x (70 – 10 m)

■ HR mode (hydrology): 4 m x (70 – 10 m)

■ LR mode (oceanography): 1 x 1 km2

■ Continuous acquisition

Vsat

(Altitude 970 km)

2 m70 – 10 m

20 km140 km

60km

60km

Introduction

SWOTIGARSS 2010, Honolulu, Hawaii, 25-30 July 2010 6

h

B

r

H

r1

r2

sin12 Brrr

2r

.2

arcsinB

cosrHh

Absolute height through SAR interferometry

B = horizontal baseline (mast length) = wavelengthH = satellite altitude (orbit)r1 = distance master antenna – target (time)

r2 = distance slave antenna – target (time) = unwrapped interferometric phase = incidence angle

’ = measured interferometric phase [0,2]

= n•2 + ’

n must be determined from auxiliary data

Measurement principle

A1 A2

P

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Challenges of absolute height restitution

■ KaRIn can restitute absolute height with auxiliary data : absolute height reference with accuracy within ± Ea/2 (Ea : altitude of ambiguity, ranging from 10 to 60 m)

Ocean (LR mode): mean sea surface and tide models, or nadir altimeter measurements

Hydrology (HR mode): DEM (SRTM or better)

■ Phase unwrapping on a pixel-by-pixel basis implies tropospheric correction throughout the swath.

■ Alternative solution: Phase unwrapping from reference points (e.g. DEM, similar to conventional SAR interferometry) tropospheric correction should no longer be necessary (current baseline for HR processing).

Measurement principle

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Processing steps

■SAR processing (range and azimuth compression)

■ Interferometric processing (co-registration, computation of interferometric phase and coherence)

■Restitution of “acquisition geometry” (geolocation, precise orbit determination, correction of roll, baseline variations, tropospheric delay, …)

■Extraction of geophysical parameters (water surface detection (HR only), computation of water surface heights, slopes etc.)

■Multitemporal analysis (medium and long term variations: flooding, floodplains, …)

Processing

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■ LR mode (oceanography) Unfocussed SAR processing Interferometric processing (including co-registration) Averaging/multilooking ( 1 km resolution) BAQ coding, transmission, decoding, reformatting Geolocation Calibration (roll and baseline variations, tropospheric corrections) Absolute height restitution SSH Estimation of SWH and wind speed, SSH slopes, 0

Resampling of all products to geographically fixed grid (1 km)

■ HR mode (hydrology) Pre-summation by factor 2 in azimuth direction BAQ coding, transmission and decoding, reformatting SAR processing Interferometric processing (including co-registration) Geolocation Detection of water surfaces (prior information slant range) Phase unwrapping/flattening, fit to existing DEM (calibration) Adaptive averaging within water bodies absolute heights, slopes etc. Resampling to triangular irregular network (TIN), 50 m average spacing Multitemporal analysis (on fixed grid)

Processing steps (preliminary)

Onboard processing

Ground processing

Onboard processing

Ground processing

> 1 Gb/s

0.2 Mb/s

> 1 Gb/s

300 Mb/s

Processing

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Unfocussed SAR processing

Reference: Full SAR processing + multilooking to ~100 m resolution

(Simulated SAR image based on DEM)

Unfocussed processing (LR mode)~100 m resolution + multilooking

(Barely visible degradation)

Unfocussed processing (burst mode)

(Clearly visible loss of details)

Processing

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Unfocussed SAR processing Assessment of impact on interferograms

Full SAR processing (reference)

Unfocussed processing (LR mode)

After further multilooking to the 1 km2 grid, the loss in height precision w.r.t. full SAR processing is about 1 mm (not yet optimized).

Unfocussed processing (burst mode)

Processing

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■Sub-optimal performance with simple methods due to speckle (and possibly limited water/land contrast)

Pixelwise K-Means, ML, SVM, …

■Spatial context and available prior data must be exploited

■Assessment of advanced methods : Active contours / snakes (update water surface boundaries starting

from existing mask) Narrow structure extraction (cf. road detection in SAR images) Markov chain and Markov random field classification methods Hybrid segmentation : edge detection, region merging, edge position

refinement …

Processing

Detection of water surfaces

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… w.r.t. existing spaceborne earth observation SAR systems

■ Ka-band (wavelength of only 8.6 mm) [compared to X-, C-, L-band] Less specular reflection Weaker penetration into vegetation, soil, snow,… Higher sensitivity to tropospheric conditions & rain A smaller baseline can be used for interferometry (10 m mast) Few reports on backscattering from natural surfaces, especially in

■ Near nadir (0.6-4.1° incidence) [typically 20-50° for spaceborne SAR] Strong layover Inversion of land/water radiometric contrast w.r.t. SAR (water > land) Strong relative incidence variation, implying strong/rapid range variation in

several key parameters (pixel size, altitude of ambiguity, orbital fringes, …)

■ R. Fjørtoft et al., “Specificities of Near-nadir Ka-band Interferometric SAR Imagery”, Proc. EUSAR 2010.

Specificities of KaRIn data

SWOTIGARSS 2010, Honolulu, Hawaii, 25-30 July 2010 14SAR images Coherence Interferogram

Simulation of SLC images

■ Radiometric simulator: Simulation of RCS for different surface types in various conditions (sensitivity studies, case studies)

Bare soil [Hybrid IEM-GO + Hallikainen/Dobson] Water surfaces [Hybrid IEM-GO + Meissner/Wentz] Vegetation (trees) [Rad. transf.+ Ulaby/El-Rayes] …

■ Geometric simulator Integrates results of radiometric simulator Geometric effects such as layover, shadow etc. Simulation of interferometric pairs of SLC images

DEMLand cover classes

EM models

Orbit file

a

r

Layover/ shadow mask

Studies of detection of water surfaces, absolute height restitution, …

Simulation

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Simulation of raw images

Simulation

ΔT = t0 – t

-1 = t

+1 – t

o = 1 / PRF

→V

t-1

to

t+1

RCSt-1

RCSto

RCSt+1

Stacking of all “raw images”

indexed by time

Focussing

Final RAW image

level 1 data

Study impact of moving water

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■ SWOT = NASA / JPL – CNES cooperation

■ Measurement principle (JPL concept) Ka-band near-nadir interferometric SAR (bistatic/monostatic, two swaths) Absolute height restitution in LR mode, relative in HR mode (current baseline)

■ Processing Onboard unfocussed SAR and interferometric processing in LR mode Automatic detection of water surfaces in HR mode

■ Specificities of Ka-band interferometric SAR

Few quantitative reports on Ka-band backscattering from natural surfaces Severe layover distortion (terrain slope often greater than look angle) Strong relative incidence angle variation, implying strong/rapid variation in several

parameters: range pixel size, altitude of ambiguity, orbital fringes, …

■ Outlook Extension of simulation activities Ground measurements and airborne campaigns Prototyping of processing chains

Summary and outlook

SWOTIGARSS 2010, Honolulu, Hawaii, 25-30 July 2010 17Thank you for your attention