Dirk Geudtner1, Pau Prats2, Nestor Yaguee-Martinez2, Andrea Monti Guarnieri3 , Itziar Barat1, Björn Rommen1 and Ramón Torres1

Sentinel-1A SAR Interferometry Verification

1ESA ESTEC 2DLR, Microwave and Radar Institute 3Politecnico Di Milano

2

Sentinel-1 SAR Imaging Modes

• SAR Instrument provides 4 exclusive SAR modes with different resolution and coverage

• Polarisation schemes for IW, EW & SM: single pol: HH or VV dual pol: HH+HV or VV+VH

• Wave mode (WV): HH or VV

• SAR duty cycle per orbit: up to 25 min in any imaging mode up to 74 min in Wave mode

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Sentinel-1 SAR TOPS Mode

TOPS (Terrain Observation with Progressive Scans in azimuth) for Sentinel-1 Interferometric Wide Swath (IW) and Extra Wide Swath (EW) modes

• ScanSAR-type beam steering in elevation to provide large swath width (IW: 250km and EW: 400km) • Antenna beam is steered along azimuth from aft to the fore at a constant rate

• Sentinel-1 IW TOPS mode parameters: ±0.6°azimuth scanning at Pulse Repetition Interval with step size of 1.6 mdeg.

All targets are observed by the entire azimuth antenna pattern eliminating scalloping effect in ScanSAR imagery Constant SNR and azimuth ambiguities Reduction of azimuth resolution due to decrease in dwell time

Sentinel-1A IW dual-pol image, acquired over Namibia

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Sentinel-1A IW TOPS InSAR

S-1A IW interferogram of data pair acquired 7-19 August, 2014 (2π height = 128.82m)

Verification of: • SAR instrument phase stability • Satellite on-board timing and GNSS solution

to support position-tagged commanding (OPS angle)

• Mission Planning system using TOPS cycle time grid points for datatake start time estimation

• Accurate orbit control (orbital tube)

Burst synchronization

1200

km

250km

Image courtesy, DLR-IMF

Repeat-pass TOPS InSAR using Interferometric Wide Swath (IW) data pairs worked on the ‘spot’

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Datatake Start Time Estimation for Burst Synchronization – Position-tag Commanding

Image courtesy, DLR-IMF

First imaging PRI techo

Calculation of OPS angle αstart_plan based on : - S-1A Reference orbit - use of an orbital point grid based on burst cycle time

• Data acquisition (repeat orbit cycle) over the same ground location uses on On-board Position Schedule execution (OPS) based on Orbit Position angle (instead of timing)

αstart_plan

PVT

(on-board GPS)

using SAR mode LUT

Instrument executes measurement according to tstart

Spacecraft Avionics converts (on-board) planned OPS angle (αstart_plan) to time (tstart) by analytical propagation of GPS PVT data

OPS angle

~20 s

αPVT

tstart

∆𝛼𝛼

time ∆𝑡𝑡

techo

using actual S-1A orbit position

Advantage: more accurate DT start time estimation no need for precise orbit prediction or frequent update of on-board command queue

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Verification of S-1A IW DT Start Time Estimation for Burst Synchronization

Image courtesy, DLR-IMF

• 498 IW measurements between Aug. 7th, 2014 (Reference Orbit reached) and Sept. 6th, 2014

• Measured performance includes: - Accuracy of the on-board conversion from OPS angles to instrument start time - Accuracy of instrument in achieving the requested start time

Average = 1.32 ms Std dev = 1.28 ms

Duration of IW bursts: IW1: 0.8s IW2: 1.06s IW3: 0.83s

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Burst Azimuth Spectral Alignment

Burst Timing Mis-synchronization: Antenna Mis-pointing (squint):

another target at center of second burst

target at center of first burst

δt

∆fT del shift

krot

ka frequency

time

Tdel

time-frequency line of target in both bursts

same target in second burst

repeat-pass burst

time-frequency line of target in both bursts

t0

another target at center of second burst

target at center of first burst

δt

∆ffDC shift

krot

ka frequency

time

∆fDC

repeat-pass burst

∆𝑓𝑓𝑇𝑇𝑑𝑑𝑑𝑑𝑑𝑑_𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = 𝑘𝑘𝑎𝑎 𝑘𝑘𝑟𝑟𝑟𝑟𝑟𝑟

𝑘𝑘𝑎𝑎 − 𝑘𝑘𝑟𝑟𝑟𝑟𝑟𝑟 𝑇𝑇𝑑𝑑𝑑𝑑𝑑𝑑 TOPS:

∆𝑓𝑓𝑇𝑇𝑑𝑑𝑑𝑑𝑑𝑑_𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = 𝑘𝑘𝑎𝑎𝑇𝑇𝑑𝑑𝑑𝑑𝑑𝑑 ScanSAR: ∆𝑓𝑓𝑓𝑓𝐷𝐷𝐷𝐷_𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 =

𝑘𝑘𝑎𝑎𝑘𝑘𝑎𝑎 − 𝑘𝑘𝑟𝑟𝑟𝑟𝑟𝑟

∆𝑓𝑓𝐷𝐷𝐷𝐷 =∆𝑓𝑓𝐷𝐷𝐷𝐷𝛼𝛼

⇒TOPS is more robust than ScanSAR

𝑇𝑇𝑑𝑑𝑑𝑑𝑑𝑑 ∆𝑓𝑓𝐷𝐷𝐷𝐷

<1

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Sentinel-1A Burst Synchronization Results

48 InSAR product pairs • 28 ascending geometry • 20 descending geometry • 46 in IW mode • 2 in EW mode

Burst (along-track) synchronization< 2.83ms

Ascending Descending

Mean AT mismatch [ms] 1.17 2.83

Stdev AT mismatch [ms] 1.71 1.97

Estimation of along-track burst synchronization: • Orbital state vectors (POD, restituted orbits) • Annotated raw start azimuth time (sensing

time) of the bursts • Fine Co-registration using cross-correlation

and ESD techniques

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Sentinel-1A Burst Spectral Alignment Results

Mean Doppler Centroid difference < 20 Hz due to stable attitude and antenna pointing

Common Doppler bandwidth > 95%

-200 -150 -100 -50 0 50 100 150 200-15

-10

-5

0

5

10

15

20

25

30

35

Azimuth frequency [Hz]

Spec

tral in

tens

ity [d

B]

SlaveMaster

Satellite Pitch angle adjustment of 0.025 deg in Oct. 2014

⇒ Reduced mean Doppler centroids

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Sentinel-1A Orbital Tube

• Reference orbit was reached on August 7th, 2014 • Satellite is kept within an Orbital Tube around a Reference Mission Orbit (RMO) • Initially specified Orbital Tube radius of 50 (rms) ⇒ equivalent to Ground-track dead-band of 60m

RMS of Orbital Tube w.r.t. Reference Orbit expressed in baseline coordinates

after adjustment of orbit Eccentricity to improve along-track baseline

• During S-1A Commissioning: Relaxation of Ground-track dead-band to 120m

⇒ Orbital Tube radius of better than 100 (rms)

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Sentinel-1A Orbital InSAR Baseline

Perpendicular Baseline < 150m

worst case orbital configuration

-250,00

-200,00

-150,00

-100,00

-50,00

0,00

50,00

100,00

150,00

200,00

1 9 17 25 33 41 49 57 65 73 81 89 97 105

Perpendicular baseline [m]

S-1A Commissioning results

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Sentinel-1A SM Mode D-InSAR Earthquake Surface Deformation Mapping Demonstration of Differential and Multi-Aperture (Squint) SAR Interferometry

M6.0 South Napa Valley earthquake on August 24th, 2014

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

D-InSAR MAI (MS-InSAR)

Use of Stripmap (SM-1) data pairs acquired on August 7th and 31st, 2014

Pass 2(shortly after)

Pass 1(pre-event)

dispdr

incθ

x∆

z∆

BPass 2(shortly after)

Pass 1(pre-event)

dispdr

incθ

x∆

z∆

BPass 2(shortly after)

Pass 1(pre-event)

dispdr

incθ

x∆

z∆

B

Image courtesy: DLR-IMF

Image courtesy: Andrea Monti Guarnieri, POLIMI

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Sentinel-1A IW Mode D-InSAR Earthquake Surface Deformation Mapping

Images courtesy: Contains Copernicus data (2015)/ESA/DLR Microwaves and Radar Institute/GFZ/e-GEOS/INGV–ESA SEOM INSARAP study

M7.8 Nepal earthquake on April 25th, 2015 Sentinel-1A IW (TOPS) mode acquisitions on 17 & 29 April, 2015

M8.3 Chile earthquake on Sept. 16th, 2015 Sentinel-1A IW (TOPS) mode acquisitions on 24 August & 17 September, 2015

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Sentinel-1A Observation Scenario Tectonic and Volcanic Areas

BLUE: Acquisitions in IW dual pol mode, VV+VH polarisation, every 12 days ascending and descending

BLACK: Acquisitions in IW mode,

VV polarisation, every 12 days ascending or descending; repeat on the same track every 24 days

Stripmap mode (SM) acquisitions

over selected small volcanic islands

Increased sampling density over

supersites outside Europe About one third of global landmass

regularly covered, based on this acquisition strategy

• All Land and Ice masses systematically provided as IW SLC data products

• Includes all global tectonic/volcanic areas • About 1.4 TB of IW SLC data available daily

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Elevation Antenna Pattern Phase Correction in IPF v2.43 New IPF version 2.43 (since end of March 2015) applies a complex Elevation Antenna Pattern (EAP) correction for inter-channel phase correction in support of polarimetry applications Previous versions of the IPF did not correct for the EAP phase ⇒ Interferogram between SLC data generated by previous and current IPF (2.43) version shows phase ramps in range, and eventually also phase jumps between sub-swaths.

range

azim

uth

Images courtesy P. Prats, DLR

EAP is annotated in SLC product (in complex format for the new IPF version)

Proposed Correction: Calculation of complex EAP for SLC data processed before end of March 2015, as outlined in TN by PDGS

Alternative Correction: Undo EAP phase correction of SLC products, processed since March 2015 using the annotated complex EAP

IPF 2.36 vs IPF 2.43 (EAP phase removed)

Similar Issue with proposed Bistatic Correction in the IPF which implies introducing a range-dependent azimuth shift

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Conclusions

• Using the same SAR imaging mode (instrument settings), e.g. IW mode, facilitates the build-up of long data time series for continuous observations with equidistant and short time intervals (interferogram stacks)

• Sentinel-1 acquires systematically and provide routinely SAR data for operational monitoring tasks for Copernicus (GMES) and national EO services

• Sentinel-1 A & B will fly in the same orbital plane with 180 deg. phased in orbit, each with12-day repeat orbit cycle ⇒ Formation of InSAR data pairs having time intervals of 6-days • Small orbital tube with radius of < 100m (rms) provides small InSAR baselines

• TOPS burst synchronization and small Doppler centroid differences ⇒ enable wide-area SAR Interferometry Coherent Change Detection monitoring • Collaboration with Canada’s RADARSAT Constellation Mission (RCM) to facilitate multi-satellite SAR monitoring ⇒ requires harmonization of data acquisition strategies

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Backup Slides

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Sentinel-1 Orbital Tube and InSAR Baseline

• S-1A & B Satellites will be kept within an Orbital Tube around a Reference Mission Orbit (RMO) • Orbital Tube radius with <100m (rms) • Orbit control is achieved by applying across-track dead-band control at the most Northern point and Ascending Note crossing

• Sentinel-1 A & B will fly in the same orbital plane with 180 deg. phased in orbit • 12-day repeat orbit cycle for each satellite • Formation of SAR interferometry (InSAR) data pairs having time intervals of 6-days