independent verification of the sentinel-1 system calibration · 2019. 2. 14. · • shape of the...

www.DLR.de/HR • VG 1 <CEOS 2013-10, S-1A CAL Scenario> • <Satellite-SAR-Systems> • <Calibration> • <Schwerdt> www.DLR.de/HR • VG 1 <Title> • <Satellite-SAR-Systems> • <Calibration> • <Schwerdt>

Independent Verification of the Sentinel-1 System Calibration

Marco Schwerdt, Kersten Schmidt, Gabriel Castellanos, Nuria Tous Ramon, Björn Döring, Pau Prats, Manfred Zink ASAR/CEOS Workshop 2013

www.DLR.de/HR • VG 2 <CEOS 2013-10, S-1A CAL Scenario> • <Satellite-SAR-Systems> • <Calibration> • <Schwerdt>

Challenge to calibrate Sentinel-1

Strategy / Recommendations

• Successful Execution of all Calibration Activities

• Delivery of Calibrated SAR Data Products as soon as possible

DLR’s Calibration Scenario for Sentinel-1 • S1-coverage across the DLR’s calibration field

• Calibration Procedures (Absolute Radiometric Calibration, Antenna Pointing, Geometric Calibration, Antenna Model Verification, Internal Calibration)

In-Orbit Calibration Plan

End-to-End Radiometric Calibration Budget

Conclusion

Overview


• 4 different modes operated at least over 10 Years

• Dual polarization

• Wide range of swath positions (18°-47°)

• Tight performance in all Modes

1.0dB (3σ)

Commissioning Phase 3 Months

Efficient Calibration

Strategy

Chart by Astrium GmbH (Germany)

Challenge to calibrate Sentinel-1


Internal Calibration Facility • compensate for drift effects by internal calibration pulses • derive actual settings of the TRMs by pulse coded technique (PCC) for

tuning/optimizing the antenna model

Antenna Model • shift most of the antenna characterization from the CP to pre-launch activities

=> shift the effort from space to ground

• provide all reference patterns for radiometric correction of the SAR data • derive the actual pointing of the antenna

Goal • reduce the calibration effort during the commissioning phase

• ensure stable and reliable operation of a precise SAR system over a period > 10 years

only a few selected beams have to be really measured

• short duration of 3 months CP • reasonable effort for long term system

monitoring

Calibration Strategy I


Relative radiometric calibration of all SAR data products has to be already performed by applying:

• internal calibration for drift compensation

• shape of the antenna patterns

• gain offset between different beams

Then, absolute radiometric calibration can be performed by measuring the whole Sentinel-1 system against reference targets independent of both:

• the target position within the swath and

• the beam and mode being operated.

Thus, one absolute calibration factor is valid for all operation modes

Minimum number of measurements required against reference targets is defined by the end-to-end system calibration budget:

• worst case parameters across all modes are applied because measurements are performed independent of beam and mode

Hence, the end-to-end system calibration budget for one specific mode will be better because not all worst cases are combined by one mode.

derived by the antenna model

Calibration Strategy II


High Radiometric Accuracy 1dB (3σ) • Measuring at least one beam of each mode

• Against 3 reference targets deployed within the swath

• Measuring at least one beam with low, one with mid and one with high incidence angle

• Measuring at least one beam in both transmit polarisations

• Absolute radiometric reference knowledge by 2 different types of reference targets (transponder and corner reflector)

• Execute equal number of measurements for asc and des passes

Tight Schedule of 3 months CP • Co-/cross polar receiving channels should be measured simultaneously

• Test site within crossover area of ascending and descending swathes

Recommendations/Rules for Beam & Site Selection

Which beams are suitable?


SM2 asc Day 10

SM1 des Day 5

• 2 x low • 2 x mid • 2 x high

IW1 des Day 12

IW3 asc Day 8

SM5 des Day 7

EW3 asc Day 3

WV1 des Day 5

incidence angle

DLR Oberpfaffenhofen

suitable Test Site for all

External Calibration Measurements

S1A Coverage across DLR Cal-Field

• Selected beams

• Asc and des passes for low, mid and high incidence angles within a repeat cycle

SM1, SM2, IW1, EW3, SM5, IW3 SM1 SM1, SM2 SM1, SM2, IW1 SM1, SM2, IW1, EW3 SM1, SM2, IW1, EW3, SM5 SM1, SM2, IW1, EW3, SM5, IW3


SM2 asc Day 10

SM1 des Day 5


IW1 des Day 12

IW3 asc Day 8

SM5 des Day 7

EW3 asc Day 3

WV1 des Day 5

WV1 asc Day 10

incidence angle

• Selected beams

• Ascending and descending passes for low, mid and high incidence angles

• Transponder test site D39, D40 and D41 is driven by modes enabling the co- and cross polar channel

SM1 SM1, SM2 SM1, SM2, IW1 SM1, SM2, IW1, EW3 SM1, SM2, IW1, EW3, SM5 SM1, SM2, IW1, EW3, SM5, IW3

D39 x D40

x D41

x DLR

Oberpfaffenhofen



SM2 asc Day 10

SM1 des Day 5


IW1 des Day 12

IW3 asc Day 8

SM5 des Day 7

EW3 asc Day 3

85 km

D38 x

WV1 des Day 5

WV1 asc Day 10

incidence angle

• Selected beams

• Ascending and descending passes for low, mid and high incidence angles

• Transponder test site D39, D40 and D41 is driven by modes enabling the co- and cross polar channel

SM1 SM1, SM2 SM1, SM2, IW1 SM1, SM2, IW1, EW3 SM1, SM2, IW1, EW3, SM5 SM1, SM2, IW1, EW3, SM5, IW3 SM1, SM2, IW1, EW3, SM5, IW3 and WV1

D39 x D40

x D41

x

at least 4 Targets per Pass

• D39, D40, D41 Transponder • D38, D42, D43 Corner Reflector

DLR Oberpfaffenhofen


D43 x D42

x


Schwerdt, Folie 10

Beam / Mode

Day within Repeat Cycle Test Site

4.21 day (des) D38 - D41

9.68 day (asc) D38 - D40

SM2 9.68 day (asc) D39 - D43

SM3 11.20 day (des) D39 - D43

SM4 2.68 day (asc) D40 - D43

SM5 6.20 day (des) D39 - D42

6.20. day (des) D38 - D40

7.69 day (asc) D38 - D42SM6

SM1

Beam / Mode


4.21 day (des) D38 - D41

9.68 day (asc) D38 - D43

EW2 11.20 day (des) D40 - D43

EW3 2.68 day (asc) D39 - D43

EW4 6.20 day (des) D39 - D42

EW5 7.69 day (asc) D40 - D42

4.21 day (des) D38 - D39

9.68 day (asc) D39 - D40

WV2 2.68 day (asc) D41 - D42

EW1

WV1 Black ≥ 4 Targets per Pass Blue = 3 Targets per Pass (except for WV 2 Targets)

• 2.68 day (asc) IW1, IW2, SM4, EW3, WV2

• 4.21 day (des) SM1, EW1, WV1

• 6.20 day (des) IW2, IW3, SM5, SM6, EW4

• 7.69 day (asc) IW3, SM6, EW5

• 9.68 day (asc) SM1, SM2, EW1, WV1

• 11.20 day (des) IW1, SM3, EW2

Beam / Mode


11.20 day (des) D38 - D42

2.68 day (asc) D38 - D40

6.20. day (des) D41 - D43

2.68 day (asc) D41 - D43

6.20. day (des) D38 - D40

7.69 day (asc) D38 - D41

IW1

IW3

IW2

6 Passes within

1 Repeat Cycle

S1A-Coverage over DLR Cal-Field Ref Orbit S1A_TEST_MPL_ORBRES_20130814T120000_20130827T120000_1004.EOF


Pre- Phase

Launch T 0 T 4 T 3

Radiometric Calibration

Antenna Model Verification

Antenna Pointing

Rainforest Cal Targets

GeoCal

T 2 T 1

End of CP

Internal Calibration

T 5 1. 2. 3. 4. 5. 6. 7.

6.5 Repeat Cycles => 2.5 Months

In-Orbit Calibration Plan


Absolute Radiometric Calibration

Beam / Mode 4. Cycle 5. Cycle 6. cycle

11.20 day (des) 11.20 day (des) 11.20 day (des)

- -

- -

7.69 day (asc) 7.69 day (asc) 7.69 day (asc)

SM1 4.21 day (des) 4.21 day (des) -

SM2 9.68 day (asc) 9.68 day (asc) -

SM5 6.20. day (des) 6.20. day (des) 6.20. day (des)

EW3 2.68 day (asc) 2.68 day (asc) 2.68 day (asc)

- - 4.21 day (des)

- - 9.68 day (asc)WM1

IW1

IW3

7 beams (IW1, IW3, SM1, SM2, SM5, EW3) within 1 repeat cycle

2 passes per beam

Equal number of passes for asc/des

4 targets for IW3 (asc), SM1 (des) 5 targets for IW1 (des), SM2 (asc), SM5 (des), EW3 (asc) 2 targets for WV1 (asc/des)

At least one beam in both transmit polarisations modes (grey boxes)

=> within 3 repeat cycles 100 measurements

sufficiently to be compliant with the end-to-end system

calibration budget in all modes (see VG 18)


Schwerdt, Folie 13

Rainforest

2-way elevation pattern with notch on receive

different orbits (attitude control check)

Ground Receiver Unit

1-way notch-pattern in azimuth

2 acquisitions at low, mid and high incidence angle

1 notch-beam in second polarisation

lowANP1

ANP2H

D38 - D41

D39 - D43

4.21 (des)

9.68 (asc)

mid ANP3 H D39 - D43 11.20 (des)

mid ANP4 V D40 - D43 2.68 (asc)

highANP5

ANP6H

D39 - D42

D38 - D42

6.20 (des)

7.69 (asc)

not critical with respect to the schedule

• 2 passes (asc/des) for low, mid and high inc. angle (3)

• 3 ground receiver units per pass

18 measurements within 1 repeat cycle

Antenna Pointing


Schwerdt, Folie 14

Rainforest (Elevation Pattern)

2-way pattern, 4 acquisitions per selected beam

3 StripMap beams (low, mid, high inc. angle)

Gain offset between different beams by IW / EW

not critical with respect to the schedule

• 2 passes (asc/des) for low, mid, high inc. angle

• 2-3 ground receiver units per pass

• also applicable for absolute Rad-Cal

16 measurements per Pol within 2 repeat cycle

Ground Receiver Unit (Azimuth Pattern)

1-way azimuth pattern, 2 acquisitions per Pol at

low, mid, high inc. angle (S1/S2, IW1, IW3)

For mid inc. angle (IW1) both transmit polarisations

IncidenceAngle

Beam Test Sites 2. Cycle (H-Pol) 3. Cycle (V-Pol)

lowSM1

SM2

D38 - D41

D39 - D43

4.21 (des)

9.68 (asc)

4.21 (des)

9.68 (asc)

mid IW1D38 - D42

D38 - D40

11.20 (des) (1. cycle)

2.68 (asc) (2. cycle)

11.20 (des) (1. cycle)

2.68 (asc) (2. cycle)

high IW3D38 - D40

D38 - D41

6.20 (des)

7.69 (asc)

6.20 (des)

7.69 (asc)

Geometric Calibration

can be performed simultaneously

Imaging mode

Antenna Model Verification


Next Generation of DLR’s Reference Targets

designed for Sentinel-1

Remote controlled

Corner Reflector

Remote controlled Transponder


Complete electronic is covered by the housing including also the antennas

Remote controlled

Coherent chirp detection and flexible backscatter matrix (HH, VV, HV, VH)

Center Frequency 5.405 GHz

Bandwidth 100 MHz

RCS 60 dBm²

Radiometric Stability ≤ 0.1 dB (1σ)

Absolute Radiometric Accuracy 0.2 dB (1σ)

DLR’s Transponder Specification

⇒ Verified by real measurements campaigns using RADARSAT-2

⇒ Thanks to MDA for supporting our activities


First RADARSAT-2 image of new DLR transponder

• 8 successful acquisitions using RadarSAT-2 executed in April 2013

• absolute radiometric accuracy of 0.2 dB (1σ)


DLR

Passes per Repeat Cycle 6

Beams for AbsCal 7

Passes per Beam (H/V) 2 / 1

Targets per Pass 4 - 5

Targets Accuracy 0.6 dB (3σ)

End-to-end System Budget S-1A (3σ)

H (co/cross) 0.94 dB / 0.97 dB

V (co/cross) 0.96 dB / 0.99 dB

Compliant with the strong requirement of 1 dB (3σ) end-to-end system budget in all operation modes

S-1 System Budget by DLR‘s Calibration Scenario


Conclusion Efficient calibration strategy has been developed for Sentinel-1 based on:

• PCC method (developed by DLR and successfully applied by TerraSAR-X) providing the actual setting of individual T/R modules

• precise antenna model providing antenna patterns and beam-to-beam gain offsets

• only 1 absolute calibration factor for all operation modes

Based on different rules/recommendations a set of 7 beams (IW1, IW3, SM1, SM2, SM5, EW3, WV1) has been established for measurements against at least 4 reference targets per pass

The in-orbit calibration plan for DLR’s complementary calibration campaign with a test site of 3 transponders and 3 corner reflectors deployed in South Germany is compliant with

• the end-to-end system calibration budget in all modes: < 1.0 dB (3σ) and

• the schedule of the commissioning phase of 3 months

Next generation of the DLR’s reference targets (transponders and corner reflectors) was designed and developed for the Sentinel-1A mission

independent verification of the sentinel-1 system calibration · 2019. 2. 14. · • shape of the...

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