Sentinel-1 Constellation SAR Interferometry Performance Verification

Dirk Geudtner1, Francisco Ceba Vega1, Pau Prats2 , Nestor Yaguee-Martinez2 , Francesco de Zan3, Helko Breit3, Andrea Monti Guarnieri4, Yngvar Larsen5, Itziar Barat1, Cecillia Mezzera1, Ian Shurmer6 and Ramon Torres1

1 ESA ESTEC 2 DLR, Microwaves and Radar Institute 3 DLR, Remote Sensing Technology Institute 4 Politecnico Di Milano 5 Northern Research Institute (Norut) 6 ESA ESOC ESA UNCLASSIFIED - For Official Use Outline • Introduction − Sentinel-1 Constellation Mission Status − Overview of SAR Imaging Modes • Results from the Sentinel-1B Commissioning Phase • Azimuth Spectral Alignment Impact on common − Burst synchronization Doppler bandwidth − Difference in mean Doppler Centroid Frequency (InSAR) • Sentinel-1 Orbital Tube and InSAR Baseline • Demonstration of cross S-1A/S-1B InSAR Capability for Surface Deformation Mapping

ESA UNCLASSIFIED - For Official Use ESA Slide 2 Sentinel-1 Constellation Mission Status

• Constellation of two SAR C-band (5.405 GHz) satellites (A & B units) • Sentinel-1A launched on 3 April, 2014 & Sentinel-1B on 25 April, 2016 • Near-Polar, sun-synchronous (dawn-dusk) orbit at 698 km • 12-day repeat cycle (each satellite), 6 days for the constellation • Systematic SAR data acquisition using a predefined observation scenario • More than 10 TB of data products daily (specification of 3 TB) • 3 X-band Ground stations (Svalbard, Matera, Maspalomas) + upcoming 4th core station in Inuvik, Canada • Use of European Data Relay System (EDRS) provides complementary acquisition of Sentinel-1 data based on optical data link to GEO satellite + Ka-band downlink − increased coverage + quasi-real time (QRT) observation capabilities outside Europe • Open & free data access (Raw, SLC, GRD, L2 (OCN) data products) ESA UNCLASSIFIED - For Official Use ESA Slide 3 Sentinel-1 SAR Imaging Modes

• SAR Instrument provides 4 exclusive SAR modes with different resolution and coverage • Interferometric Wide Swath (IW) mode for land & coastal area • Extra Wide Swath (EW) mode for sea-ice monitoring and maritime surveillance • StripMap (SM) for volcanic islands and emergency situations • Wave (WV) mode is continuously operated over open ocean

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

ESA UNCLASSIFIED - For Official Use ESA Slide 4 4 Sentinel-1 Constellation Observation Scenario & Revisit Frequency and Coverage

ESA UNCLASSIFIED - For Official Use ESA Slide 5 S-1B & S-1A InSAR Verification

Repeat-pass Sentinel-1A & B cross-InSAR using Interferometric Wide Swath (IW) data pairs worked on the ‘spot’ Þ Formation of InSAR data pairs with 6-day interval • SAR instrument phase stability (repeat orbit cycles)

• Compatibility of S-1A & S-1B SAR instruments S-1A image: acquired on 10 Jun., • Satellite on-board timing and GNSS solution to 2016 S-1B image: acquired on 16 Jun. support position-tagged commanding (OPS angle) 2016, shortly after S-1B reached its designated orbital node • Accurate orbit control (orbital tube) phased 180° with S-1A Burst • Mission Planning system using burst cycle time synchronization grid points for datatake start time estimation

ESA UNCLASSIFIED - For Official Use ESA Slide 6 Burst Synchronization Verification

• Estimation of synchronization for S-1A/S-1B and S-1B/S-1B IW mode InSAR Scene and long Datatake (DT) pairs: • Orbital state vectors (POD) S-1A/-1B IW Scene pair S-1B/-1B IW Scene pair • Annotated raw start azimuth time (sensing time) of the bursts

Scene Burst Synchronization Variation: 0.15ms

S-1A/-1B IW DT pair S-1B/-1B IW DT pair

Datatake Burst Synchronization Variation:1ms

ESA UNCLASSIFIED - For Official Use ESA Slide 7 Burst (Mis) Synchronization vs Doppler Centroid Difference & Common Doppler Bandwidth

frequency k Burst (mis) Synchronization < 3ms a repeat-pass burst .123: Doppler rate due to antenna steering

krot target at center another target at center . : FM rate of first burst of second burst / 5 : Burst timing offset Df time 678 T del_shift

same target in second burst .123 time-freq ue nc y ∆" = . 5 line of target #$%&_()*+, / 678 in both bursts . − . dt / 123

Tdel <1 Resulting relative shift of Doppler spectra < 6Hz Loss in common Doppler bandwidth < 3%

IW1 IW2 IW3

PBW [Hz] 327 313 314

ESA UNCLASSIFIED - For Official Use ESA Slide 8 Mean Doppler Centroid Frequency Difference

S-1B ;<= centered around 100 Hz due to spacecraft attitude/ SAR antenna pointing offset (yaw & pitch) 300 S1A 200 S1B

100 S-1B/S-1B InSAR pairs 0 S-1B: "45 = 112 Hz

-100 "45 = 108 Hz Doppler centroid [Hz] -200

-300 Jan 2015 May 2015 Sep 2015 Jan 2016 May 2016 Sep 2016 k frequency a repeat-pass burst S-1A/S-1B InSAR pairs k rot another target at center of second burst S-1A: "45 = 25 Hz time-freq ue nc y ./ ∆"45 line of target ∆" = ∆" = in both bursts #$%_'()*+ 45 S-1B: "45 = 100 Hz ./ − .123 6

DfDC

Df fDC_shift ∆ " = 75 Hz time 6: TOPS conversion factor 45 ∆ "45_89:#3 = 24 Hz

target at center of first burst

t0 dt ESA UNCLASSIFIED - For Official Use ESA Slide 9 Effective Doppler Centroid Frequency Difference & Common Doppler Bandwidth S-1 A & S-1B Doppler Centroid Frequency Doppler Centroid Difference

Doppler Common Bandwidth

Processed Bandwidth (PBW) azimuth for IW mode: 314 Hz

ESA UNCLASSIFIED - For Official Use ESA Slide 10 S1-B Doppler Spectra before & after Pitch & Yaw Attitude Adjustment S-1B attitude adjustment on Sept. 8th, 2016, applying a positive Yaw correction of +0.034 deg. and negative Pitch rotation of -0.018 deg.

S-1B: before & after S-1B: after S-1A & S-1B: after

!"#$%": # = 100 Hz *#+"%: # = 27 Hz # = 27 Hz # = 24 Hz () () () () S−1A: #() = 36 Hz S−1B: #() = - 3 Hz

∆ #()_./012 = 13 Hz Þ ~ 4 % of PWB ESA UNCLASSIFIED - For Official Use ESA Slide 11 Sentinel-1 Orbital Tube and InSAR Baseline

• Sentinel-1 A & B are kept within an Orbital Tube around a Reference Mission Orbit (RMO) • During Sentinel-1A Commissioning: Relaxation of Ground-track dead-band to 120m Þ Orbital Tube radius of better than 100 (rms)

S-1A/S-1B perp. Baseline for IW and EW data stacks < 100m

200 200 S1A S1A S1B S1B 100 100

0 0 Perp. baseline [m]

-100 Perp. baseline [m] -100

-200 -200 Resonance between orbit repeat cycle Jan 2015 May 2015 Sep 2015 Jan 2016 May 2016 Sep 2016 Jan 2015 May 2015 Sep 2015 Jan 2016 May 2016 Sep 2016 and Moon perturbation (14 days) ESA UNCLASSIFIED - For Official Use ESA Slide 12 Sentinel-1A & B Orbit Maintenance

S1A

S1B

ESA UNCLASSIFIED - For Official Use ESA Slide 13 S-1A/S-1B Cross-InSAR Deformation Mapping: Italy Earthquake • M 6.2 central Italy earthquake on 24 August 2016 at 03:36:32 CEST • Sentinel-1A and Sentinel-1B IW data pairs acquired on 20 & 26 Aug. and 21 & 27 Aug. 2016 for generation of coseismic differential interferograms

effective baseline: 28.1 m mean Doppler frequency: 110 Hz (S1B) & 54 Hz (S1A)

ESA UNCLASSIFIED - For Official Use burst mis-synchronization: 3.12 ms ESA Slide 14 Conclusions

• Systematic data acquisition using the same SAR imaging mode ( IW mode), enables the build-up of long data time series for continuous observations with equidistant and short time intervals (interferogram stacks)

• Demonstration of Sentinel-1A/Sentinel-1B cross-InSAR capability Þ Formation of InSAR data pairs with 6-day intervals Þ Compatibility of both SAR instruments • Small orbital tube with R < 100m (rms) provides small InSAR baselines Þ Differential InSAR for surface deformation monitoring

• Accurate TOPS burst synchronization and small Doppler centroid differences Þ Large common Doppler bandwidth = optimal azimuth spectral alignment

Þ Excellent performance for wide-area (250km) InSAR mapping

ESA UNCLASSIFIED - For Official Use ESA Slide 15