From METOP ASCAT to METOP-SG SCA: Science Needs over Land

Wolfgang Wagner, Sebastian Hahn, Stefano Elefante, Mariette Vreugdenhil

Department of Geodesy and Geoinformation (GEO)Vienna University of Technology (TU Wien)

http://www.geo.tuwien.ac.at/

European C-Band Scatterometer Series

AMI Scatterometer

Frequency: 5.3 GHzPolarisation: VV

Resolution: 50 kmDaily coverage: <40%

SatellitesERS-1: 1991-2000ERS-2: 1995-2011

METOP ASCAT

Frequency: 5.255 GHzPolarisation: VV

Resolution: 25 kmDaily coverage: 82%

SatellitesMETOP-A: 2006 ongoingMETOP-B: 2012 ongoingMETOP-C: 2018

METOP-SG SCA

Frequency: 5.355 GHzPolarisation: VV + VH + HH

Resolution: ~12.5 kmDaily coverage: ~88%

SatellitesMETOP-SG-B1: 2022 METOP-SG-B2: 2030

SCA Research & Development Needs

General• Use of VH and HH to improve backscatter models over land• Exploitation of improved spatial resolution and temporal sampling

Improved vegetation modelling by using VH/VV and other polarisation indices to complement information content of slope

• In radiative transfer vegetation is modelled using at least to parameters: optical depth and scattering albedo

• Goal: Self-standing vegetation products & improved soil moisture retrievals Characterisation of sub-surface scattering

• Robust identification of occurrence of phenomenon• Goal: Soil mapping & improved soil moisture retrievals

Other R&D Needs• Freeze/thaw monitoring in NRT• Detection of inundation and dynamic water bodies• Use of backscatter echoes stemming only from soil moisture sensitive areas

How to Advance Beyond the State of Art?

Knowledge about C-band backscatter still dominated by insights gained from experimental studies using only few, intermittent SAR images

• Focus on (spatial) details may distort one’s understanding of the “big picture”

How to overcome this?• Analysis of dense & long time series of existing satellite data over sufficiently

large/diverse regions– ASCAT: from 2018 three satellites in orbit– Sentinel-1: VV+VH combination

• Experimental ranging radar systems– Airborne– Ground-based

• Theoretical modelling

Incomplete and partly wrong understanding of vegetation and soil scattering phenomena

Hydrological Open Air Laboratory (HOAL)

A hydrological observatory for interdisciplinary research in Petzenkirchen, Austria. Aerial picture of the HOAL viewing North, courtesy of Alexander Eder.

ASCAT Soil Moisture over HOAL

Current operational H-SAF product: Seasonal biases (summers too wet)

Improved vegetation correction: Elimination of seasonal biases

Sentinel-1 VH Sensitivity to Vegetation Cover

RGB composite of VH backscatter at April, May and June, including land cover and Sampling Units.

Sentinel-1 CR = VH/VV for Different Crop Types

Backscatter Dynamics over Agriculture and Forests

Dostálová et al. (2017) Annual seasonality in Sentinel-1 signal for forest mapping and forest type classification, submitted.

Airborne Nadir-Looking Ku-Band Profiling Radar

Tomoradar built and operated by FGIPiermattei et al. (2017) An analysis of Ku-band profiling radar observations of boreal forest, in prep.

ASCAT over An Nafud Desert

Presented @ IGARSS’12

Tomographic Profiling

At the GB-SAR Microwave Measurement Facility C-band VV tomographic profiling (TP) measurements were collected of trihedral and a pile of cobble stones

Morrison (2013) Mapping Subsurface Archaeology with SAR, Archaeological Prospection, 20, 149–160.

Example C-band TP image of a soil troughTP image showing the presence of various features in the soil volume. The soil surface is at 150cm.

Soil Surface Scattering

Existing scattering models, e.g. IEM, assume a hard boundary between the air (with ε0) and soil (with εsoil)

Recent research has demonstrated the importance of considering an air-soil transition zone

• Allows simulating a linear relationship between backscatter (in dB) and soil moisture content

• Effective “soil surface roughness” dependent on soil moisture

Schneeberger et al. (2004) Topsoil structure influencing soil water retrieval by microwave radiometry, Vadose Zone Journal, 3(4), 1169-1179.

Air-to-Soil Transition Model

RT1 Scattering Model

Radiative transfer model for bi- and monostatic scattering Generalised phase functions for modelling surface-volume interactions Available on GitHub: https://github.com/TUW-GEO/rt1

Quast, R., W. Wagner (2016) An analytical solution for first-order scattering in bistatic radiative transfer interaction problems of layered media, Applied Optics, 55(20), 5379-5386.

𝐼𝐼𝑖𝑖𝑖𝑖𝑖𝑖 ∝ �⋯ 𝒫𝒫𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 � 𝒫𝒫𝑣𝑣𝑣𝑣𝑣𝑣 𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑

Vegetation Scattering

Soil Scattering

Outlook

ASCAT and Sentinel-1 allow studying C-band co-pol and cross-pol backscatter in preparation for SCA

Scientific prospects• Unravelling the effects of vegetation structure and water content, soil surface

roughness and sub-surface scattering on backscatter Potential synergistic products

• Soil moisture, vegetation (optical depth, water content, phenology), freeze/thaw, etc.

ESCAT+ASCAT+SCA → High stability climate data records 1991-2045

AcknowledgementsEUMETSAT: H-SAF CDOP3H2020: Advanced SAR (Grant #606971)ESA: CCI Soil Moisture Phase 2Austrian Space Application Programme: “VegetationDynamics”Austrian Science Fund: Vienna Doctoral Programme on Water Resources Systems