Internal and External Calibration of the Sentinel-1 CSAR

We care for a safer world

Internal and External Calibration of the Sentinel-1 CSAR

Paul SnoeijEvert AttemaIgnacio Navas-TraverAllan ØstergaardRamon Torres

European Space Agency


Radar Design Drivers

• High Sensitivity (NESZ : -22 dB):

- Peak transmit power (280 units each 15W)

- Low loss waveguide radiators

• 1dB radiometric accuracy and 0.5dB stability (3σ):

- Extensive pre-flight characterization

- In-orbit Internal Calibration and TRM RF Characterization

- Stable non-nominal signal paths and waveguide radiators

• 5m ground range resolution:

- 100 MHz bandwidth

• High Sensitivity & Low Ambiguity Levels:

- 12.3 m by 0.84 m antenna aperture

• Dual Polarisation Capability:

- One Tx Chain and two parallel Rx Chains

• TOPS Mode (Azimuth Scan) and ScanSAR implementation:

-±0.8° azimuth scanning at PRI rate

-±12° elevation scanning and beam shaping capability


Sentinel-1 SAR Modes

Orbit Height~700 km

Flight Direction

Sub-Satellite Track

250

200

200

450230

36.50

Strip Map Mode

Wave Mode

Interferometric Wide Swath Mode

Extra Wide Swath Mode


Instrument Accommodation

SAR Antenna Subsystem (SAS)

Aperture : 12.3 m by 0.84 m

14 Tiles each with 20 H polarized and 20 V polarized slotted waveguide arrays (SWGA)

SAR Electronic Subsystem (SES) on

S/C SES Panel


Tile

Tile (14 tiles for complete SAR)

• H-pol: 20 radiators and TRMs

• V-pol: 20 radiators and TRMs

• Size : 880mm by 786mm by 37mm

• Centre Frequency : 5.405 GHz

• Bandwidth (-15 dB) : > 200 MHz

• Polarisation : Dual Pol. (H & V)

• Cross-Polarisation:<-35 dB

• Loss: < 0.5 dB

• Tile amplifier (TA) to provide the

required input and output signal levels

Complete SAR:

• H-pol: 14(az) by 20(el) = 280 SWGA

• V-pol: 14(az) by 20(el) = 280 SWGA


Transmit Receive Module (TRM)

Characteristics:1) Simultaneous HPA (Tx) and LNA (Rx) operation prevented by hardware.

2) Linear operation range (except for the PA) is required.

3) Nominal gain and phase quantization is 0.5 dB and 5.6° respectively.

4) Isolation and stability requirements.

5) Title amplifier (almost) equal to the TRM


RF Characterization (long term variations)

Goal and purpose

1) To measure the drift of the transmitter power independently for each TRM between data takes

2) To measure the drift of the receiver gain independently for each TRM between data takes

3) To detect and locate TRM failures.

4) Subsequently the antenna model will apply knowledge of the individual TRM drifts (failures) to

determine the Tx and Rx patterns.

5) Frequency of RF characterization measurement is in the order of once per orbit.

Calibration requirements

1) All nominal paths of the transmitted signal and of the received echoes shall be covered by

RF characterization signals (the paths are shared by the Internal Calibration).

2) High stability of non-nominal paths used by RF characterization signals.

3) S1 pulse length, bandwidth and duty cycle is used as baseline during RF characterization, phase is

determined by the PCC512 coding.

Technique and advantages of PCC512 (PCC32)

1) Phase coding using Hadamard matrices (sequences are powers of 2)

2) Hadamard sequence provides a system of linearly independent and orthogonal equations.

3) All TRM is operated under nominal load for the power supplies and thermal conditions.

4) To suppress leakage between Tx paths and Rx paths.



Calibration for Tx H-pol

Calibration for Tx V-pol

Cal signals, Paths and PCC commanding



Att

φ

HPA

Att

φ

LNA HPA

Att

φ

LNA

40dB

HPA

Att

φ

LNA

TRM4

TA

HPA

Att

φ

LNA HPA

Att

φ

LNA HPA

Att

φ

LNAHPA

Att

φ

LNA

EFE 1

SES RX H SES RX V

AZ BFN TX 1:14

AZ BFN RX H 14:1

AZ BFN RX V 14:1

EL BFN TX 1:10

EL BFN RX H 10:1

EL BFN RX V 10:1

Radiator H (1, … ,20)

Radiator V (1, … ,20)

9

9H

18V

9V

18H

Tile 2

·

·

·

Tile14

13

13V

13H

Tile 1

EFE 2

·

·

·

EFE10

TRM3 TRM2 TRM1

TRM4 TRM3 TRM2 TRM1


Antenna Model (long term variations)

Correction of a

HPA

Att

φ

LNA HPA

Att

φ

LNA HPA

Att

φ

LNAHPA

Att

φ

LNA

a280 b280ai bi

….….

a2 b2a1 b1

F280(u,v)

Fi(u,v)

F2(u,v) F1(u,v)

Commanded beam, a

Flight PCC512

RF Characterization

TXCal and RxCal

TRM drifts

and failures

Beam

computation

Embedded

sub-array

patterns,

On ground sub-array

pattern measurements

and complex alignment

-80 -60 -40 -20 0 20 40 60 80-14

-12

-10

-8

-6

-4

-2

0

2

elevation in deg.

pa

tte

rn in

dB

VP waveguide, f = 5.405GHz

el. 10

el. 20

-80 -60 -40 -20 0 20 40 60 80-14

-12

-10

-8

-6

-4

-2

0

2

elevation in deg.

pa

tte

rn in

dB

Hp waveguide, f = 5.405GHz

el. 10

el. 20

Mode, elevation (swath) and

azimuth (TOPS) pointing

dependent TX and RX

patterns,FSAR(u,v)

Fi(u,v)


Prediction of patterns

Directivity within beam:

0.05dB @ 3σ

Ground characterization:

Element patterns

Complex alignment

(Active and passive RL)

In-orbit monitoring:

RF Characterization

External Calibration


Internal Calibration (short term variations)

Goal and purpose

1) To track the fine variations of the transmitter power times the receiver gain product, PG,

during a data take.

2) Subsequent in the ground processing the complex PG will be applied for image correction.

Calibration requirements/justification

1) All nominal paths of the transmitted signal and of the received echoes shall be covered by internal

calibration signals.

2) High stability of non-nominal paths used by calibration signals.

3) Uniform illumination is used as baseline during Internal Calibration in place of the Tx and Rx

beams, as the phase alignment due to external free space propagation towards the swath

and correspondingly in the antenna on receive is not needed.

Purpose of PCC2

1) To suppress leakage between Tx paths and Rx paths.

TRM (EFE) limitations

1) Simultaneous Tx and Rx operation prevented by hardware for any given TRM.

2) Linear operation range (except for the PA) is required.

3) Nominal gain and phase quantization is 0.5 dB and 5.6° respectively.



Calibration for Tx H-pol

9 PRIs for cal

11 measurements

Calibration for Tx V-pol

9 PRIs for cal

11 measurements

Cal signals, Paths and PCC commanding


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 RequirementsParameter CSAR Requirement Transponders Requirement

Maximum Point Target Radar Cross Section 75 dBm2 70 dBm2

Transmit polarisation CSAR

Receive polarisation Transponder

H or V H or V

Transmit polarisation Transponder

Receive polarisation CSAR

H and V Both H and V

H to V imbalance 15 degrees Amplitude < 0.05 dB; Phase < 5 deg

Radiometric accuracy 1.0 dB (3s) 0.1 dB (3s)

Radiometric stability 0.5 dB (3s) 0.1 dB (3s)

Time Delay Adjustable from 1.0 ms to 1000 ms in

increments of 0.01 ms

Accuracy of the antenna pattern estimation or

accuracy of the transponder receiver mode

0.1 dB within the swath

1.0 dB at -20 dB level with

respect to the

maximum

0.2 dB of absolute gain

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

Dynamic range of the receiver mode 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.

Pixel localisation 2.5 - 10 m (3s) depending

on the mode

1 m (3s)


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


Selected Sites in The Netherlands

ESTEC

KNMI

(Royal

Netherlands

Meteorological

Institute)

NLR

(National

Aerospace

Laboratory)


Transponder measurements

E = ESTEC

K = KNMI

N = NLR

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


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


Thank you for your attention

Simulated Sentinel-1 Mosaic based on ASAR

Internal and External Calibration of the Sentinel-1 CSAR

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