panelsar: calibration and performance validation of the...

1 CEOS conference, 28 Oct 2015

PanelSAR: Calibration and Performance validation of the first FMCW Spaceborne SAR CEOS conference, 27-29 Oct 2015

Peter Hoogeboom TU Delft Max Pastena, Ernesto Imbembo SSBV Matern Otten TNO [email protected]


Contents

• Introduction PanelSAR • Calibration strategy

• Antenna model • Internal calibration • External calibration

• Conclusions


PanelSAR

• PanelSAR is a modular small size X-band FMCW SAR, utilizing one (iFMCW)

or more (full FMCW, bistatic and single pass interferometry) platforms.

• TU Delft (Prof. Hanssen and Prof. Hoogeboom) is Principal Investigator.

• ESA controls the project through the PRODEX program.

• SSBV builds the instrument. The design is by SSBV, TNO, TUD.

• The industrial goal is to enable very low cost radar data for high resolution

high repetition rate interferometry. This requires reduced mission cost

through reduced Size, Weight and Power (SWAP).

• PanelSAR is geared to small satellite platforms (< 500 kg).

• Nominal operation altitude 580 km, nominal resolution 4 meter, 10-20 km

stripmap, 50-80 km ScanSAR, spotlight mode down to 1 meter resolution.


FMCW SAR

• Advantages vs pulse radar: • Continuous transmission • Low peak power • Low output frequencies • Easy timing • Low complexity

Time

Frequency ∆t fb

Transmitted signal

Received signal

Ts

B

Transmit antenna

RF waveform

Local oscillator

IF amplifier

RF amplifier Waveform generator

Mixer IF filter

Radar back-end

A/D converter Receive antenna

FMCW timing relies on short ranges!


FMCW SAR split over two satellites


Interrupted (i)FMCW SAR

• Single antenna for transmit and receive • Interrupted long chirp transmissions • No transmission during receive windows • Transmit duty cycle < 50% • PanelSAR uses an active array antenna with 108 modules

Transmit/ Receive antenna

RF waveform

Local oscillator

IF amplifier

RF amplifier Waveform generator

Mixer IF filter

Radar back-end

A/D converter

Circulator


Instrument performance specs for 580km 1 day repeat orbit

Observation mode

Parameter FMCW mode (2 satellites)

iFMCW mode (single satellite)

StripMap Resolution (Az. x Ground range) 1.7 x 2.0 m 1.7 x 3.0 m Incidence angle 30° 30° Instantaneous bandwidth 150 MHz 100 MHz Swath 9.3 km 9.1 km Worst case AASR / RASR -24.8 / -27.8 dB -24.9 / -27.4 dB Worst case NESZ -19.9 dB -18.6 dB ScanSAR Resolution (Az. x Ground range) 15.3 x 3.0 m 15.3 x 6.0 m Incidence angle 30° 30° Instantaneous bandwidth 100 MHz 50 MHz Swath 69.3 km 68.8 km Worst case AASR / RASR -21.1 / -24.0 dB -21.1 / -23.7 dB Worst case NESZ -20.7 dB -20.7 dB SpotSAR Resolution (Az. x Ground range) 1.0 x 1.0 m 1.0 x 2.2 m Spot size (Az. x Ground range) 6.1 x 5.5 km 5.7 x 8.1 km Incidence angle 30° 20° Instantaneous bandwidth 300 MHz 200 MHz Worst case AASR / RASR -24.6 / -26.0 dB -24.5 / -27.8 dB Worst case NESZ -19.1 dB -18.2 dB


Antenna configuration

1 T/R module, 4 SWG’s

Element: 3 tiles

Panel: 3 elements (1x1 m)

1 m

3 m

Total antenna: 12 x 9 phase centers (el x az) Electronic steering capability +/- 1 deg az, +/- 5 deg el Tile: 4 T/R

assemblies


Panel RF architecture

• The RF system of each panel consists of one

transceiver and 3 elements (2 shown)

• Each element has TX (red, active) and RX

(green, passive) feed networks to three tiles

• Tiles are composed of TRM assemblies with

four TRMs and slotted waveguides

• Calibration is required for the transceiver, all

RF paths in the elements and TRMs

• Transceiver signals from the panel are

digitally interfaced to the central electronics,

the 3 panel signals can be made available

separately in the instrument output data

stream

Digital out to Central Elec-tronics

Panel


Transceiver electronics

• The transceiver on each panel consists of passive and active components

and therefore requires calibration

• A total pass through calibration of transmit and receive chain is

accomplished by a calibration loop at TRM level in the elements

Elements


TR module

• TX and RX chain have their own attenuator/ phase shifter to avoid ultrafast switching

• Each waveguide has its own amplifier in transmit (only 1 is shown)

• In receive each waveguide has its own LNA

• Noise figure of the TRM about 3.7 dB

• T/R module has a separate calibration branch and a pass through branch

To waveguide

Calibration in/out

Receive out

Transmit in

Pass through branch


Element layout and calibration approach

The elements consist of

• TRM’s allow to calibrate the

total RF path up to the TRM

or including the TRM

• Each tile has 4TRMs and 16

waveguides that are fully

characterized on ground

before flight to create an

antenna model

• TRMs can be calibrated in

phase and gain on RX and TX

Calibration interface


Internal calibration modes

• The TRM supports 4 calibration settings: • TXRX pass through (for transceiver calibration)

• Inhibit (TRM deselected)

• TX path calibration

• RX path calibration

• On each panel calibration can be performed of:

A. The complete RF path through transceiver and combiner splitter networks up to the TRMs;

B. The path of A plus the TRM transmit path of all selected TRMs (from 1 to all)

C. The path of A plus the TRM receive path of all selected TRMs (from 1 to all)


Transceiver calibration

Transceiver calibration

• The attenuation of att 3+5 is 68 dB

• The output level is -56 dBm

• Switch TRM SW 4 can be used to bypass the TRM


TRM Transmit calibration

• The calibration signal passes through the TX chain of the TRM

• Is coupled to the calibration branch using a coupler

• Inserted in the RX chain just after the TRMs receive hardware

• The attenuation of the att. 4+5 is 50 dB

• The output power is about -77 dBm


TRM Receive calibration

• The calibration signal passes through the calibration branch of the TRM

• Is coupled to the receive network of the TRM

• Fed via the normal receive path to the transceiver

• The attenuation of the att. 3+4 is 82 dB

• The output power is about -82 dBm


External calibration

• Needed for instrument performance verification • Will be performed with active or passive calibration devices

such as transponders and corner reflectors • Rainforest observation • Interferometric analysis and performance estimation


Conclusions

• In the Netherlands the PanelSAR system is realised under the PRODEX program. A small FMCW SAR will be built for this scientific program TU Delft is Principal Investigator.

• FMCW SAR seems feasible and provides a useful balance between performance, SWAP (Size, Weight, Power) and cost.

• Specific hardware implementation for internal calibration is provided.

• External calibration will be exercised.


Questions… ???

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