Sentinel-1 Transponder Performance

Paul Snoeij Bjorn Rommen Ignacio Navas-Traver Dirk Geudtner European Space Agency

Harry Jackson Steppes House Services

D’Arcy Hart Dean Rowsell Trevor Stuber Muthu Gandi C-Core

400 km

400 km

250 km

200

km

100

km

80 Km

23o

36o

S1

S6 Stripmap Mode (SM)

5m * 5m

Interferometric Wide

Swath Mode (IW)

20m * 5m

Extra Wide

Swath Mode (EW)

azimuth 40m * range 20m

Wave Mode (WV)

5m * 5m

19o

46o

30o

45o

19o

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 and Transponder Requirements Parameter CSAR Requirement Transponders Requirement Maximum Point Target Radar Cross Section 75 dBm2 70 dBm2 as seen by the satellite 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 (3σ) 0.1 dB (3σ) Radiometric stability 0.5 dB (3σ) 0.1 dB (3σ) Time Delay Adjustable from 1.0 µs to 1000 µs in

increments of 0.01 µs 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 (3σ) depending on the mode

1 m (3σ)

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 stores the CSAR transmitted signal at selectable sampling rate for the azimuth antenna pattern reconstruction.

–  The receiver mode involves detection and measurement of the amplitudes of received SAR pulses producing an antenna pattern as quick-look output.

–  S-1 pointing can be derived using an azimuth notch pattern on transmit.

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.

Design Description, GMES Sentinel-1 Transponder TN-02 European Space Agency (ESTEC) Report/Proposal no: R-09-090-734 vers. 2.0 December 17, 2010

3-73

Figure 3-39: Perspective View of Full Assembly

Transceiver - 1

–  The receiver module is a super heterodyne design

–  The first filter prevents out-of-band interferers from saturating the LNA, the second filter provide noise reject at IF, and the third filter provides an anti-aliasing and mixer image rejection function prior to the ADC

–  Gain control is achieved through a DAT

Transponder - Receiver

Ref 1.9 dBm Att 30 dB

Offset 1.9 dB

A

LVL

EXT

Center 75 MHz Span 150 MHz15 MHz/

*

3DB

RBW 300 kHzVBW 1 MHzSWT 5 ms

1 PKMAXH

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

SWP 10 of 10

1

Marker 1 [T1 ] -0.32 dBm 75.000000000 MHz

2

Delta 2 [T1 ] -1.07 dB -65.700000000 MHz

3Delta 3 [T1 ] -1.03 dB 49.200000000 MHz

Date: 26.APR.2013 14:00:51

Receiver Frequency Response

Transponder Rx Gain Stability

0 2 4 6 8 10 12 14 165.8

5.82

5.84

5.86

5.88

5.9

Puls

e Av

erag

e Po

wer (

dBm

)

RX Gain Stability Testing

Time (min)0 2 4 6 8 10 12 14 16

24.7

24.8

24.9

25

25.1

25.2

Sens

or T

empe

ratu

re (d

eg C

)

Receiver Gain Stability Testing

Transceiver - 2

Transceiver - 3

–  The transmitter module is a direct conversion design utilizing a single sideband (quadrature) modulator, that accepts balanced and low-pass filtered I and Q outputs from the two-channel DAC to up-convert the signal while rejecting the unwanted sideband.

–  Digital equalization of I and Q channels minimizes undesired artifacts produced by the modulator.

–  Gain control is provided by a digitally programmable attenuator (DAT).

Transponder Tx Linearity

-30 -25 -20 -15 -10 -5 0 5 1034.4

34.5

34.6

34.7

34.8

34.9

35

35.1

35.2

35.3

35.4

X: -2.188Y: 35.28

TX Linearity Testing

TX Input (dBm)

TX G

ain

(dB)

X: -5.193Y: 35.27

X: -12.21Y: 35.29X: -22.16

Y: 35.26

Transmitter Linearity Testing

Transponder

Ref 1.1 dBm Att 50 dB

A

Offset -15.5 dB

LVL

EXT

Center 5.405 GHz Span 150 MHz15 MHz/

3DB

RBW 3 MHzVBW 10 MHzSWT 20 ms

1 PKMAXH

EXREF

-18

-16

-14

-12

-10

-8

-6

-4

-2

01

Marker 1 [T1 ] -0.19 dBm 5.405000000 GHz

2

Delta 2 [T1 ] -1.04 dB -47.100000000 MHz3

Delta 3 [T1 ] -1.01 dB 48.300000000 MHz

Date: 29.APR.2013 12:06:46

Transmitter Frequency Response

Ref 10 dBm Att 40 dB

*

Offset 1.9 dB

A

LVL

3DB

RBW 300 kHzVBW 1 MHzSWT 5 ms

Center 75 MHz Span 150 MHz15 MHz/

EXT

1 PKMAXH

-10

-8

-6

-4

-2

0

2

4

6

8

10

1

Marker 1 [T1 ] 7.82 dBm 75.000000000 MHz

2

Delta 2 [T1 ] -1.48 dB -50.000000000 MHz

3

Delta 3 [T1 ] -2.87 dB 50.000000000 MHz

Date: 29.APR.2013 12:34:02

Transceiver Frequency Response

Transceiver - 4

–  A circulator is used to multiplex transmitted and received signals to and from the antenna.

–  A low noise amplifier (LNA) in the receive path minimizes the over-all noise figure of the transponder.

–  A switchable loop-back path is provided with an integrated attenuator to facilitate the transponder internal calibration.

Transponder

Transponder Cabinet

–  Communication to the Host Computer through a TCP-IP Ethernet interface.

–  Power is provided from the shelter-contained power supply

Transponder

Transponder SAR Calibration Mode

© MPB

•  The entire SAR calibration sequence is expected to take a maximum of 15 minutes; most of this time (10 minutes) is spent warming up the transponder in Pre-Op.

•  The Pre-Op mode allows the transponder hardware to warm-up and initialize to prepare for internal calibration of the system.

•  During Pre-Op Mode hardware devices are powered and/or initialized, and the system will warm-up. A warm-up time of 10 minutes is sufficient to configure all system components and ensure their temperature stability.

•  After the warm up a system diagnostic is run and results are stored in the transponder database.

•  Next the transponder transitions to Internal Calibration.

Transponder Internal Calibration Mode

© MPB

•  Closing loopback switches connects the internal loopback path so that signals transmitted are streamed to the receiver through the loopback attenuator.

•  The processor’s FIFO is initially prefilled with the loopback signal. The hardware trigger is asserted and held for a period longer than loopback signal length to flush the FIFO data.

•  Transponder power amplifier is warmed up using expected PRF and pulse width to generate a pulse train that is streamed through loopback path to simulate conditions of transponder during SAR calibration.

•  A compensation filter is computed using transmitted and received digital signals, correcting for effects due to difference between loopback signal path and normal operation signal path.

•  The compensation filter coefficients are used to program processing platform filter which effectively calibrates the transponder.

Transponder Measurement Mode

© MPB

•  Prior to measurement mode, all necessary parameters (gain block, pulse delay, decimation rate, filter coefficients) are initialized.

•  The server waits until the satellite arrival is imminent and asserts the hardware trigger to initiate ADC and DAC conversion.

•  The trigger assertion simultaneously invokes the GPS time stamp trigger and FPGA data streaming.

•  The server also streams decimated SAR pulse samples via the FPGA processing platform bus interface.

Calibrating the Calibrator - 1

Calibrating the Calibrator - 2

Absolute calibration of the transponder will be determined on a test range using a flat metal plate of known radar cross section

–  A pulse is transmitted from the transponder to the plate which is then reflected back and received.

–  This received pulse is recorded and used to determine the system transfer function during external calibration.

–  The external loop gain (twice the free space loss and the RCS of the disk) determines the RCS of the transponder during external calibration.

–  The difference of the internal transfer function during the external calibration with the operational settings will transfer the RCS of the transponder from external calibration to operational value of 76 dB m^2.

Estimated error budget

Source of Error Absolute Error Stability

(dB (3σ)) (dB (3σ)) SAR Calibration: Antenna Pointing 0.025 Polarization Mismatch at 45° 0.058 Antenna Thermal 0.06 Electronics Thermal 0.001 Waveguide Attenuator Thermal 0.054 Waveguide Circulator Thermal 0.011 Other Waveguide Thermal 0.048 External Calibration: Target Plate Flatness 0.006 Target Plate Diameter 0.002 Target Plate Thermal 0.01 Target Plate Pointing 0.01 Antenna Pointing 0.004 Range Measurement 0.003 Target Illumination 0.01 Clutter 0.05 Multipath Correction 0.04 Total Calibration Error: 0.092 0.096

Thank you for your attention

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