Interferometric Evaluation of Sentinel-1A TOPS data

N. Yague-Martinez, F. Rodriguez Gonzalez, R. Brcic, R. Shau

Remote Sensing Technology Institute. DLR, Germany FRINGE 2015 WORKSHOP ESA–ESRIN | Frascati, Italy

ESRIN/Contract No. 4000109669/13/I-AM

ESTEC/Contract No. 4000111074/14/NL/MP/lf

Overview

• Sentinel-1 TOPS IW mode

• Interferometric examples

• Integrated Wide Area Processor. InSAR Processing Chain • Spectral shift filtering

• Burst synchronization evaluation • Coregistration

• ESD estimator • Along-track shifts evaluation

• Slices mosaicking

• Conclusions

> Interferometric Evaluation of Sentinel-1A TOPS data > N. Yague-Martinez • FRINGE 2015 > 23/03/2015 www.DLR.de • Chart 2

Sentinel-1 TOPS IW mode

• Terrain Observation by Progressive Scan • S1 Interferometric Wide Swath (IWS) mode • Range Coverage: 250 km • SLC data available in slices of approx. 200 km length • Resolution: 5 m (rg) x 20 m (az) • Three subswaths: IW1, IW2, IW3

www.DLR.de • Chart 3 > Interferometric Evaluation of Sentinel-1A TOPS data > N. Yague-Martinez • FRINGE 2015 > 23/03/2015

Sentinel-1 – IW TOPS, Genoa

• Acquisition lies over north-west Italy • Elevation reaches 2000 m • Urban areas, plains, forested mountains

DEM Corrected Interferometric Coherence and Phase

Master Date 19-08-2014 Slave Date 07-08-2014 (12 days) Mode IW Resolution 4.5 m x 20.9 m (Burst 1, Beam 1) Extension 249 km x 179 km Polarisation VV Orbit Direction Ascending Effective Baseline 121.4 m avg. Height of Ambiguity 128.5 m avg. Incidence Angle 30.5° – 45.9° (15.4°) Average Coherence 0.17

Sentinel-1 – IW TOPS, Spain

• Zaragoza, Aragón, Spain. • Coherence-drop • Heavy rains in August 2014, AEMET

DEM Corrected Interferometric Coherence and Phase

Master Date 19-08-2014 Slave Date 31-08-2014 (12 days) Mode IW Resolution 4.5 m x 20.9 m (Burst 1, Beam 1) Extension 249 km x 179 km Polarisation VV Orbit Direction Descending Effective Baseline 49.0 m avg. Height of Ambiguity 317.4 m avg. Incidence Angle 30.5° – 45.9° (15.4°)

Integrated Wide Area Processor (IWAP)

• A multi-mode multi-sensor PS-InSAR processor • Based on TMSP, ITP, PSI-GENESIS • Flexible modular approach • Automated • Multi-threading and parallel processing

TOPS Mode - three level hierarchical structure

Slice Level Mosaics (interferogram, amplitude)

Subswath Level Mosaics (interferogram, amplitude)

Burst Level SLCs and Valid Pixel Masks

(interferogram, amplitude)

OrderFile

1: processorcall

4: productdelivery

Logging,Annotation,Visualization

InSAR Stack

PSI Results

Visual Interfaces

2:interactionrequest

3:user interaction

TOPS bursts spectral properties

Tfocussed

Tburst

Tdwell

• Tdwell << Tburst

(Tdwell: integration time for a point target) • Azimuth resolution

worse due to steering of the antenna. Resolution controlled by Tdwell.

• Time-varying

spectrum

𝑓𝑓

𝑡𝑡

InSAR Processing Chain – Burst level

“Slice Level” Coregistration

Mosaicking at sub-swath or slice level + Quality Control (ESD)

• Consideration of the time-varying Doppler centroid necessary.

• Spectral shift filtering • Range

• Compensation of wavenumber shift (due to perpendicular baseline).

• Azimuth • Similar principle to Spotlight mode:

De-ramping / Re-ramping

Burst synchronization: Mutual Along-track position

1st pass 2nd pass 𝑓𝑓

𝑡𝑡

Burst synchronization: Pointing accuracy / TZDS

𝑓𝑓

𝑡𝑡

1st pass 2nd pass

Descending

Stack 10 IWS images (03/10/2014 – 31/01/2015)

Master: 02/12/2014

Ascending

Stack 9 IWS images (06/10/2014 – 22/01/2015)

Master: 05/12/2014

Common Doppler Bandwidth Evaluation

Descending

Ascending

Common Doppler Bandwidth Evaluation

S1 IW. Doppler Bandwidth

Subswath IW1 IW2 IW3

Doppler BW

327 Hz 313 Hz 314 Hz

Common Doppler Bandwidth Evaluation (percentage) %

Ascending

Descending

No significant coherence loss if no-filtering in azimuth performed.

Analysis on more datasets on-going.

InSAR phase error due to an azimuth misregistration1, :

Time-varying spectrum of TOPS bursts!

Coregistration requirements

Δϕburst = 2πΔ𝑓𝑓𝑑𝑑𝑐𝑐Δ𝑡𝑡

1 R. Scheiber, A. Moreira. Coregistration of Interferometric SAR Images using Spectral Diversity", IEEE Transactions on Geoscience and Remote Sensing, 2000

S1 TOPS IW mode Azimuth resolution 20 m

Azimuth pixel spacing 14.1 m

Needed Azimuth co-registration

accuracy*

~0.001 pixel

(1.4 cm)

*Allowing = 1/100 cycle = 3.6º Δϕburst

Δ𝑡𝑡

Coregistration error effect. Salar de Uyuni

250 km

Artificial azimuth shift of 0.05 pixels (for demonstration)

Fine azimuth coregistration

Coregistration Workflow

“Slice Level” Coregistration

Mosaicking at sub-swath or slice level + Quality Control (ESD)

Coregistration

1P.Prats-Iraola, R. Scheiber, S. Wollstadt, A. Reigber “TOPS Interferometry with TerraSAR-X", IEEE Trans. Geosci. Remote Sens., vol. 50, no. 8, pp. 3179 -3188. 2012

Restituted Precise

Accuracy from Specs. 10 cm 2D (1-sigma) 5 cm 3D (1-sigma)

Expected AT InSAR accuracy 10 cm (1-sigma) 4.08 cm (1-sigma)

• Geometric prediction with external DEM and orbit information

• Range: Linear correction to account for orbital errors / geodynamic effects • Incoherent Cross Correlation (ICC)

• Azimuth: Rigid shift correction to account for orbital timing error / geodynamic effects

• Enhanced Spectral Diversity (ESD)1 -> achieves fine azimuth coregistration requirement.

• Orbit sources:

• Annotated in L1 Product / Restituted Orbit / Precise Orbit

• Basic ESD relation

• Apply pixel-wise to burst overlaps within subswath

• ESD phase ambiguity band

• ESD can be applied directly after geometric coregistration if Precise / Restituted Orbits are used (Ambiguity band is solved).

Coregistration: ESD estimator

𝜙𝜙𝐸𝐸𝐸𝐸𝐸𝐸,𝑝𝑝 = 2𝜋𝜋Δ𝑓𝑓𝑝𝑝𝑜𝑜𝑜𝑜𝑜𝑜Δ𝑦𝑦𝑓𝑓𝑎𝑎𝑎𝑎

Δ𝑦𝑦� = argminΔ𝑦𝑦

atan�𝑒𝑒𝑗𝑗 𝜙𝜙𝐸𝐸𝐸𝐸𝐸𝐸,𝑝𝑝−2𝜋𝜋Δ𝑓𝑓𝑝𝑝𝑜𝑜𝑜𝑜𝑜𝑜Δ𝑦𝑦𝑓𝑓𝑎𝑎𝑎𝑎

𝑝𝑝

Δ𝑦𝑦: az. shift (pix); 𝑓𝑓𝑎𝑎𝑎𝑎: SLC az. sampling freq.; Δ𝑓𝑓𝑝𝑝𝑜𝑜𝑜𝑜𝑜𝑜: freq. diff for each pixel in overlap area

IW1 IW2 IW3

< Δ𝑓𝑓𝑝𝑝𝑜𝑜𝑜𝑜𝑜𝑜 > 4814.25 Hz 4044.80 4267.22

Amb. Band ± 0.71 m ± 0.85 m ± 0.80 m

𝑡𝑡

𝑓𝑓

Δ𝑓𝑓𝑝𝑝𝑜𝑜𝑜𝑜𝑜𝑜

Along-tracks shifts evaluation

• Analysed orbits: • Restituted • Precise

• Temporal analysis • Use two stacks of acquisitions over Mexico City. • Analysis of the residual azimuth shift over time.

• Spatial Analysis • Use of a datatake over Germany with six slices. • Analysis of the residual azimuth shift along azimuth.

Along-tracks shifts evaluation: Temporal analysis

Std dev Along-track shift (InSAR)

REST. PREC.

Measured 2.52 cm 4.42 cm

From Specs 10 cm 4.08 cm

Std dev Along-track shift (InSAR)

REST. PREC.

Measured 3.35 cm 3.81 cm

From Specs 10 cm 4.08 cm

Necessary to perform a fine refinement of the geometric shifts.

Solid Earth Tides considered Ascending

Descending

Along-tracks shifts evaluation: Spatial analysis

Master Date 21-12-2014 Slave Date 02-01-2015 (12 days) Mode IW Resolution 5.3 m x 20.71 m (Burst 1, Beam 1) Range Extension 250 km Azimuth Extension 1000 km (6 slices) Polarisation VV Orbit Direction Descending Effective Baseline 48.1 m avg. Height of Ambiguity 263.1 m avg.

Along-tracks shifts evaluation: Spatial analysis

Std dev REST.

Std dev PREC.

7.4 mm 9.5 mm

Good spatial stability allows to retrieve timing offset from one slice, being applicable to the rest of slices.

• An L0 datatake packaged as L1 slice products (IW mode)

• All slices are processed with the same parameters on a common grid

• IW slice products were interferometrically processed using IWAP and then mosaicked

• Varying mean height between slices for FEP calculation phase jumps InSAR phase set consistently mean height.

• Could also mosaic L1 slice products and

then perform InSAR processing datatake level coregistration

Slice Mosaicking

Map data: Google, Landsat

6/7 slices, Datatakes 001F25 001BC9

All slices Datatakes 001FC1 001C20

Conclusions

• IWAP InSAR processing chain presented. • Uses a combination of ICC and ESD for fine coregistration

• S1A analysed data presents very good burst synchronization. Azimuth

spectral shift filtering necessary? More analysis on-going.

• Necessary to refine azimuth geometric shifts.

• Stability of along-track shifts within a DT (6 slices analysed) allows to retrieve orbital timing offset from one slice (if enough coherence) and apply it to the rest of slices.

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