sentinel-1 in-orbit calibration and performance...

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Sentinel-1 In-orbit Calibration and Performance Verification

Paul Snoeij Dirk Geudtner Ignacio Navas-Traver Mike Brown European Space Agency


Sentinel–1 Key Facts - 1

• Constellation of two satellites (A & B units)

• Near-Polar sun-synchronous (dawn-dusk) orbit at 693 km mean altitude

• Both S-1 satellites are in the same orbit (180 deg. phased in orbit)

• 12 days repeat cycle (1 satellite), 6 days for the constellation

• 7 years design life time with consumables for 12 years

• Launch of Sentinel-1A May 2013 followed by Sentinel-1B 18 months later

• Prime contractor: Thales Alenia Space, Italy • Instrument responsible: EADS Astrium, Germany


Sentinel–1 Key Facts - 2

• C-Band SAR instrument operates at centre frequency of 5.405 GHz

• On-board data storage capacity (mass memory) of 1400 Gbit

• Two X-band RF channels for data downlink with 2 X 260 Mbps

• On-board data compression using Flexible Dynamic Block Adaptive Quantization

• Optical Communication Payload for data transfer via laser link with the GEO European Data Relay Satellite (ERDS) system


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46o Sentinel-1 SAR can be operated in 4 exclusive measurement modes with different resolution and coverage:

Sentinel-1 SAR Acquisition Modes

Polarisation schemes for IW, EW and SM: - single polarisation: HH or VV - dual polarisation: HH+HV or VV+VH For Wave mode: HH or VV

IW and WV main modes of operations TOPS operation for IW and EW modes


CSAR ASAR / ERS Orbit: 12 d (06:00 LTDN) 6 day repeat S-1A and S-1B ASAR: 35 d (10:00 LTDN) Maximum (continuous) imaging per orbit: 25 minutes much better than ERS and ASAR Stripmap Mode (SM) Swath: 80 km comparable to ERS and ASAR

Resolution: 5x5 m (Az, Rg, 1L) better than ASAR capability

Extra-Wide Swath Mode (EW) - TOPS ASAR - ScanSAR

Swath: 400 km same as ASAR

Resolution: 40x20 (Az, Rg, 1L) better than ASAR capability

Wave (WV) - Leapfrog

Swath: 20x20 km at 100 km interval better than ASAR

Resolution: 5x5 m (Az, Rg, 1L) better than ASAR

Interferometric Wideswath Mode (IW) - TOPS NEW BASELINE MODE Swath: 250 km Resolution: 20x5 m (Az, Rg, 1L) comparable to ERS Sensitivity, Ambiguity and Radiometric Performance comparable to ASAR No separate AP mode but dual-pol on receive capability in all imaging modes without performance reduction


Sentinel-1 Commissioning Phase Objective

End-to-End System performance verification and calibration

– Check-out of spacecraft and verification of ground system interfaces

– In-orbit verification of instrument performance and calibration

– In-orbit calibration activities comprise: – Internal instrument calibration (monitoring of drift effects and compensation)

– Pointing calibration (using data acquired over rainforest and transponder sites)

– Geometric calibration (pixel location)

– Antenna model verification

– Radiometric calibration (absolute and stability)

– Polarimetric calibration (channel imbalance, gain, phase)

– Interferometric verification including burst synchronization and InSAR phase stability

– Level 0 and Level 1b SAR product verification (i.e. w.r.t. SAR instrument performance)


Sentinel-1 Platform Commissioning Objective and Main Activities

Verify and optimize the performance of the platform and predict the

end-of-life performance

Main activities: – Verification of each subsystem (i.e., AVS, TT&C, Thermal, Power, Propulsion)

– Data buffer and memory areas verification; memory dumps

– Verification during the deployment phase

– SMU and MilBus check-out

– AOCS characterization and optimization/evaluation of AOCS sensors – STR (basic mode transitions; consistent attitude generation) – POD parameters tuning (using GPS), – Reaction wheel de-saturation threshold tuning

– RCT firing calibration before Orbital Correction Maneuvers

– S-band check-out (TT&C)

– X-band check-out (PDHT will be switched on)


Sentinel-1 Ground System Interface Verification

– X-Band Acquisition Verification (data reception)

– FOS-PDGS Interfaces Verification

– PDGS-CPAF Interfaces Verification

– Ka-Band Acquisition Verification (data reception via EDRS)

– Interface verification between Sentinel-1 GS and EDRS/AlphaSat GS


Sentinel-1 Orbital Tube and InSAR Baseline

– Satellite will be kept within an Orbital Tube around a Reference Mission Orbit (RMO)

– Orbital Tube radius (statistical) is 50 m (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


Sentinel-1 Attitude Steering Modes

Roll-steering mode

–Sensor altitude changes around the orbit

–Introduction of additional satellite roll angle depending on latitude to maintain a quasi “constant” slant range

–Advantages:

– Single PRF around orbit per swath or subswath (except for S5 (S5-N and S5-S)

– Fixed set of constant Elevation antenna beam patterns at Hmin = 697.6 km off-Nadir = 30.25

at Hmax = 725.8 km off-Nadir = 28.65

Total zero-Doppler steering mode

–Yaw and pitch adjustments around the orbit to account for Earth rotation effect

–Provides Doppler centroid at 0 Hz


TOPS Burst Synchronization

– TOP interferogram generation requires burst synchronization of repeat-pass datatakes

– TOPS burst duration for:

– EW: 0.54 s (worst case)

– IW : 0.82 s (worst case)

– Requirement for Burst Synchronization: ≤ 5ms

– Event Control Code (ECC) for datatake (Measurement Mode)

– Instrument schedule execution is using Orbit (on-board) Position Schedule (OPS) commanding based upon location

– Location for OPS is not specified by lat. and long. (e.g. WGS 84), but by means of OPS angle (angle in orbital plane between Reference plane and desired location on orbit


Sentinel-1 Instrument Verification & Calibration Requirements

– Radiometric accuracy better than 1 dB (3)

– Radiometric stability better than 0.5 dB (3)

– Antenna Model accuracy of 0.2 dB (3) for absolute gain

– Phase error over 10 min DT better than 5

– Inter-channel Phase accuracy better than 15

– Antenna Pointing accuracy better than 0.01

– Pixel Localization 2.5m (3)

– End-to-End System verification and Calibration shall be completed within 3-months Commissioning Phase (Challenge!)

3 months


Antenna Pointing Determination Objectives and Methods

– Verification of Antenna deployment

– Correct antenna pointing is required for illuminating the swath with maximum signal power (design purpose of the antenna pattern)

– Misalignment effects – Mechanical Antenna miss-pointing (antenna coordinate frame to AOCS

alignment)

– Electrical Antenna miss-pointing (deviation between electrical and mechanical pointing)

– AOCS (satellite) miss-pointing

– Pointing in Elevation – Use of 2-way notch pattern acquired over Rainforest site and comparison

with Antenna Model pattern (apply for different polarization)

– Pointing in Azimuth – Use of 1-way notch pattern acquired over transponder and comparison with

Antenna Model pattern (only at same lat. Possible)

– Use of SM data in at high, low, and zero squint angle for TOPS

– Doppler centroid estimation from SAR data and attitude (round orbit)


Antenna Model Verification

Elevation Antenna Pattern

– Analysis of 2-way pattern (gamma profile) acquired over Rainforest

– Gain offset between different sub-swath for IW and EW

Azimuth Antenna Pattern

– Analysis of 1-way pattern acquired over Transponder sites

– Analysis of 2-way pattern using range compressed data acquired over homogeneous site (e.g. rainforest)


Sentinel-1 Calibration Concept

Pre-launch activities

– Instrument RF on-ground characterization

– Antenna on-ground characterization

– Antenna model validation

In-orbit calibration activities during Commissioning Phase

– Internal instrument calibration including stability monitoring based on analysis of calibration pulses and TRM characterization using PCC techniques

– External calibration using rainforest and reference targets

Operational phase

– Long-term instrument monitoring based on internal calibration

– Use of Antenna model

– Recheck of calibration for S-1B

– Analysis of SAR data statistics


Sentinel-1 Transponder

– Main function of the transponder is to act as a very stable target with a sufficiently large RCS (70 dBm2)

– 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.


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.


Transponder deployment

Cross over region of beams in ascending and descending swaths

As many sub-swaths / beams as possible

Three transponders close to ESTEC

Easy deployment and maintenance


Sentinel-1 Commissioning Phase Scenario Current timeline consist of acquisitions over: • Transponder areas for geometric and

radiometric calibration, and antenna model verification

• Lake area for NESZ measurement • Rub Al Khali Desert under

investigation as secondary NESZ area

• Rainforest for geometric calibration and antenna model verification

• High altitude flat area for coherence verification

• Additional site for independent verification

• Long data takes (25 minutes) for all modes


Sentinel-1 Commissioning Phase Calibration and Performance Analysis Facility (CPAF)

Set of data analysis tool boxes used by the Commissioning Phase Team


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