Sentinel-1 Calibration and Performance

Sentinel-1 Calibration and Performance Paul Snoeij Evert Attema Björn Rommen Nicolas Floury Berthyl Duesmann Malcolm Davidson Ramon Torres European Space Agency

Sentinel-1 Mission Objectives Component of EU & ESA s Global Monitoring for Environment and Security Programme (GMES) GMES Core Services: GMES Marine Services GMES Land Monitoring Services GMES Emergency Response Services GMES Atmospheric Monitoring Services GMES Security Services GMES Downstream services

Sentinel-1 Programme Status Thales Alenia Space Italia as Prime Contractor EADS Astrium GmbH as Instrument Responsible First (S-1A) satellite launch 2012, S-1B 18 months later

Main Principles of Sentinel Data Policy Sentinel Data Policy = full and open access to the Sentinels L1 and L2 data to all users Free for systematic & on-line distribution, cost of reproduction for media or nonstandard products Obligatory registration of ALL users, via simple on-line procedure, through signature of Terms and Conditions, defining respective responsibilities of data provider and user (copyright, reference to data provider, disclaimer for ESA liability) conditions for onward distribution of data Terms and Conditions to be established between ESA and EC

Sentinel-1 System Space Segment A constellation of two satellites nominal lifetime in orbit of 7 years (consumables for 12) Global coverage Near-Polar Sun-Synchronous dusk-dawn orbit @ 693km. Repeat Cycle 12 days A second satellite in the same orbit but with a different Mean Anomaly C-Band Synthetic Aperture Radar Payload Ground Segment Mission operations for a system of satellites over a period of 20 years S-Band station (Kiruna proposed), with a back-up for S/C contingencies Downlink currently assumes three X-Band receiving stations

Sentinel-1 System Sentinel-1 has two main operational modes, the Interferometric Wide Swath mode and the Wave mode, that: satisfies most currently known service requirements avoids conflicts and preserves revisit performance provides robustness and reliability of service simplifies mission planning & decreases operational costs satisfies also tomorrow s requests by building up a consistent long-term archive However Mutually exclusive modes are provided for continuity reasons (w.r.t. ERS & Envisat) and for accommodation of emerging user requirements Two other mutually exclusive dual polarisation modes are provided

Sentinel-1 Performance Requirements Mode Access Angle Single Look Resolution Swath Width Polarisation Interferometric Wide Swath Wave mode > 25 deg. 23 deg. and 36.5 deg. Range 5 m Azimuth 20 m Range 5 m Azimuth 5 m > 250 km HH+HV or VV +VH > 20 x 20 km Vignettes at 100 km HH or VV Main modes intervals Strip Map 20-45 deg. Range 5 m Azimuth 5 m > 80 km HH+HV or VV +VH Extra Wide Swath > 20 deg. Range 20 m Azimuth 40 m > 400 km HH+HV or VV +VH For All Modes Radiometric accuracy (3 σ) Noise Equivalent Sigma Zero Point Target Ambiguity Ratio 1 db -22 db -25 db Distributed Target Ambiguity Ratio -22 db

Sentinel-1 SAR Modes Flight Direc4on Sub- Satellite Track Orbit Height ~700 km 20 0 200 25 0 23 0 36.5 0 Wave Mode 45 0 Strip Map Mode Extra Wide Swath Mode Interferometric Wide Swath Mode

Instrument Accommodation on Spacecraft SAR Electronic Subsystem (SES) on S/C SES Panel SAR Antenna Subsystem (SAS) Aperture : 12.3 m x 0.84 m, 14 Tiles each with 20 dual polarized resonant waveguide arrays (5 SAS Panels) 280 dual polarised T/R modules

Sentinel-1 Satellite system sizing and performance Satellite Features Overall Launch Mass Main Body Dimensions Operational lifetime Propellant lifetime Operative autonomy Attitude Profile Attitude accuracy and knowledge 2298 kg (incl. 135 kg prop.) 3400 x 1340 x 1340 mm 7.25 years 12 years 96 h Performance Parameters Geo-Centric and Geodetic, 3 axes stabilized. Nominal Attitude: Right Looking Continuous steering in Yaw Pitch and Roll Accuracy: 0.01 each axis. Knowledge: better than 0.003 each axis Orbit Knowledge accuracy Propulsion Power TT&C On-Bard Storage Capacity Data Downlink 10 m (3 sigma) each axis real-time processing. GPS data allow 5 cm in postprocessing on ground. Mono-propellant, 6(orbit) + 8 (attitude) thrusters ~5980W SAW generated Power (EOL). 240 Ah Li-Ion battery. @ 65V S-Band, zenith/nadir antennas: 64 kbps U/L and 128/2048 kbps (switchable) D/L TLM Data 1410Gbit @EOL X-Band (8.025 8.400GHz). 2 x 260Mbps (useful data rate) 8PSK, 4D-TCM Satellite Reliability & Availability Reliability : 0.74 @ 7.25years. Availability :0.983

C-band SAR comparison Mission/mode Sentinel-1 / Interferometric Wide-swath Sentinel-1 / Strip Map Sentinel-1 / Extra Wide-swath Envisat / ASAR Wide Swath Azimuth Resolution (m) Ground Range Resolution (m) No. of Looks (N) NESZ (db) Swath Width (km) PI Abs. 20 5 1-22 250 2300 4.4 5 5 1-22 80 1472 2.8 40 20 1-22 400 1301 2.5 150 150 11.5-23.5 400 937 1.8 Envisat / Image 5 20 1-22 80 736 1.4 ERS-2 / Image 5 20 1-20 80 526 1.0 PI relative to ERS The performance indicator (PI) is derived by dividing the swath width (in km) by the radiometric resolution of a 150x150 m product with a backscatter level of -20dB. A large swath width in combination with a good radiometric resolution will yield a high value for the PI.

In-orbit External Calibration

Sentinel-1 Transponder Main function of the transponder is to act as a very stable target with a sufficiently large RCS The transponder will also function as a receiver for the azimuth antenna pattern. The azimuth pattern receiver mode involves detection and measurement of the amplitudes of received SAR pulses. This mode will confirm the expected azimuth beam pattern for the C-SAR phased array. Pointing can be derived using an azimuth notch pattern on transmit.

CSAR and Transponder Requirements Parameter CSAR Requirement Transponders Requirement Maximum Point Target Radar Cross Section 75 dbm 2 70 dbm 2 Transmit polarisation CSAR Receive polarisation Transponder Transmit polarisation Transponder Receive polarisation CSAR H or V H and V H or V Both H and V H to V imbalance 15 degrees Amplitude < 0.05 db; Phase < 5 deg Radiometric accuracy 1.0 db (3σ) 0.1 db (3σ) Radiometric stability 0.5 db (3σ) 0.1 db (3σ) Time Delay Accuracy of the antenna pattern estimation or accuracy of the transponder receiver mode Dynamic range of the receiver mode Pixel localisation 0.1 db within the swath 1.0 db at -20 db level with respect to the maximum 0.2 db of absolute gain 2.5-10 m (3σ) depending on the mode Adjustable from 1.0 µs to 1000 µs in increments of 0.01 µs 0.05 db in the main lobe of the received azimuth pattern relative to the peak value 0.5 db at -20 db level with respect to the peak value of the main lobe of the received azimuth pattern The dynamic range shall be sufficient to reconstruct the azimuth pattern of the Sentinel-1 SAR antenna down to a side lobe level of 40 db. 1 m (3σ)

Transponder Architecture A single antenna for both reception and transmission A microwave transceiver to frequency shift signals Digital Signal Processor for the programmable delay on the signal, to fine tune the gain to achieve a 76 dbm2 radar cross section (RCS), and to apply a transponder compensation filter. Support subsystems a control and data storage computer; a GPS Clock for UTC synchronization; a Pan-Tilt unit for orienting the antenna boresight towards the expected satellite overpass location; support for external communications, enabling full remote control and a data download capacity; a power supply; an environment control system.

Selected Sites in The Netherlands Cross over region of beams in ascending and descending swaths As many subswaths / beams as possible Three transponders close to ESTEC Easy deployment and maintenance

Transponder measurements Orbit IW1 IW2 IW3 WV1 WV2 EW1 EW2 EW3 EW4 EW5 SM1 SM2 SM3 SM4 SM5 SM6 Cycles D8 K EKN K E 4 A15 N E EK N E K N 6 D37 N E KN E K E 4 A88 E K K K N K N 5 D110 K E E KN E KN E 5 A117 N N 2 A161 EK E K E K 4 Total 3 3 3 2 2 6 3 2 4 2 3 2 3 2 3 2 9 4 17 15 E = ESTEC K = KNMI N = NLR

Sentinel-1 radiometric accuracy Requirement: 1dB radiometric accuracy (3σ) Mode SM IW EW WV Absolute radiometric accuracy, co-polar (db, 3σ) 0.75 0.77 0.90 0.92 Absolute radiometric accuracy, cross-polar (db, 3σ) 0.76 0.78 0.91 NA

Mission Performance within GMES Services Example 1: Ship Detection Smaller ships 20m 10m

Example 2: Security Dyke Monitoring using Radar Interferometry Estimated sea-level rise impacts in the Netherlands and Germany. 0.65 m (dark blue) and 1.3 m (light blue) (ESA/EAPRS/De Montfort Univ.) Courtesy Hansje Brinker BV http://www.hansjebrinker.net

Example 3: Land cover classification Based on multi-temporal radar scene acquisitions and multi-temporal metrics 75.000 km² - 10 frames (IS 1 IS 3) 4 land cover classes (agriculture + grassland aggregated) accuracy assessment using 50 geocoded Quick Bird Snapshots from Google Earth (ca. 500 reference points)

Summary of the Level 2 product performance prediction S1 Level-2 Product Resolution Performance Units Subsidence Rate 5 x 5/20 m2 1.3/1.3 mm/year Land Cover Classification (2 db contrast ) Forest Non-Forest Classification 30 x 30 m2 90/75 % correct classification 30 x 30 m2 98.6/75 % correct classification Soil Moisture 100 x 100 m2 0.8/1.2 volume % Flood Mapping 30 x 30 m2 93/79 % correct classification Snow Cover Classification 30 x 30 m2 80/75 % correct classification Ship Detection 5 x 20 m2 20/40 ship length (m) Sea Surface Wind Speed 100 x 100 m2 0.4/0.8 m/s Sea Surface Currents 5 Hz 30/30 cm/s

Conclusions Sentinel-1 mission introduced Sentinel-1 data products maintain data quality of ESA s previous SAR missions (ERS-1/-2, ENVISAT ASAR) Continuity in performance for geophysical products secured In response to user needs substantial improvements expected in Revisit frequency Coverage Timeliness and reliability of service The same mission plan for every cycle, no need for acquisition requests In-orbit calibration performance guarantees 1 db (3 sigma) radiometric accuracy