MULTI-CHANNEL SAR EXPERIMENTS FROM THE SPACE AND …

MULTIMULTI--CHANNEL SAR CHANNEL SAR EXPERIMENTS FROM THE SPACE EXPERIMENTS FROM THE SPACE

AND FROM GROUND: POTENTIAL AND FROM GROUND: POTENTIAL EVOLUTION OF EVOLUTION OF PRESENT PRESENT

GENERATION SPACEBORNE SARGENERATION SPACEBORNE SAR

P. Lombardo, M. Sedehi, F. Colone

3nd International Workshop on Science and Applications of SAR Polarimetry and Polarimetric Interferometry

POLinSAR 2007January 25, 2007 – ESA/ESRIN Frascati, Italy

P. Lombardo, M. Sedehi, F. Colone, “Multi-channel SAR experiments from the space and from ground“ 2

Present spaceborne SAR generation

The generation of spaceborne SAR under present development (COSMO-SkyMed, TerraSAR-X, RadarSat,…) is characterized by few innovative features:

large active phased array antenna;metric resolution;polarimetric capabilities;multiple imaging modes (SPOT, STRIPMAP, SCANSAR).

TerraSAR-X


Multichannel evolution of present generation SAR

Large active phased array antenna have the potentialities to be split into multiple sub-apertures.If the multiple sub-apertures are connected to parallel fully coherent receiving channels…

… a multi-channel SAR (M-SAR) can be obtained.

Single-channel

2 RX channels

3 RX channels

6 RX channels

daz

d el


Potentialities of M-SAR from the space

MTI:MTI: clutter cancellation and detection of ground slowly moving targets applying Space-Time Adaptive Processing (STAP) on the multi-channel set of collected radar data;RELOC:RELOC: exact positioning of strong moving targets in the high resolution SAR images;MULTIPOL:MULTIPOL: increased monitoring capabilities by collecting fully polarimetric images;ATI:ATI: sea current/wave monitoring by along-track SAR interferometry;SARSAR--ECCM:ECCM: protection of the SAR sensor from e.m. interferences byimplementing antenna-based (Electronic Counter Counter Measure) techniques;MBMB--BSAR:BSAR: Using a passive multi-channel antenna allows bistaticMTI, bistatic RELOC, EXTENDED IMAGING and SAR-ECCM. MultistaticMultistatic SAR:SAR: The combination of an active illuminator with one passive receivers flying in close formation will yield a bistatic SAR;

Already Already presented with presented with

reference to reference to COSMOCOSMO--SkyMedSkyMed at at POLinSARPOLinSAR

20052005


MTI & RELOC

MTI allows to detect low RCS slow ground moving MTI allows to detect low RCS slow ground moving target. Key point of the technique is the target. Key point of the technique is the discrimination of the target from the surface clutter discrimination of the target from the surface clutter echoes which is:echoes which is:-- strong due to the low incidence angles of the strong due to the low incidence angles of the satellite observation geometry.satellite observation geometry.-- spectrally spread spectrally spread duerduer to the high platform speed.to the high platform speed.

MTIMTI

Exact positioning and imaging of moving targets in high resolutiExact positioning and imaging of moving targets in high resolution SAR images. on SAR images. Applicable to objects with high reflectivity.Applicable to objects with high reflectivity.

The radial velocity estimation accuracy is proportional to the temporal baseline obtained with two independent RX channels with the largest possible along-track displacement.

• the radial motion component leads to erroneous displacements

The effect can be compensated for,

if accurately estimated

RELOCRELOC


MULTIPOL & ATI

MULTIPOLMULTIPOLWhen using an antenna BFN allowing only a single When using an antenna BFN allowing only a single receiving channel (either H or V), multiple subreceiving channel (either H or V), multiple sub--apertures can be used to receive in both linear apertures can be used to receive in both linear polarization (one in each subpolarization (one in each sub--aperture).aperture).

The availability of The availability of polarimetricpolarimetric SAR data allows a SAR data allows a relevant improvement in target recognition and relevant improvement in target recognition and classification for:classification for:

•• manman--made object;made object;•• land scenario.land scenario.

Sea current/wave monitoring by Sea current/wave monitoring by ATIATI--SAR can be obtained by using SAR can be obtained by using two independent receiving channels two independent receiving channels with the largest possible horizontal with the largest possible horizontal alongalong--track baseline.track baseline.

ATIATI


SAR-ECCM

even if the interference transmitted power is not high and it is received by

the side-lobes of the SAR antenna, it can deny the SAR imaging capability

For imaging radars, the effect of a wideband noise-like interference is to mask the scene visible in the imaged area with a high uniform noise level

interference

The SAR pulse transmitted power is penalized by 2-way propagation

losses, while interference power has only 1-way propagation losses

Imaging capability of a SAR could be seriously limited or denied by an electromagnetic interference signal impinging on the antenna array during the synthetic aperture

The use of antenna The use of antenna nullingnulling techniques shall techniques shall cancel the effects of interferences over the cancel the effects of interferences over the collected SAR data, thus fundamentally collected SAR data, thus fundamentally allowing a normal operation despite the allowing a normal operation despite the

presence of the continuous wave presence of the continuous wave disturbance signals.disturbance signals.


SAR Vulnerability

Incident angle (degree)

Alo

ng-tr

ack

(Km

)

20 25 30 35 40 45 50 55-20

-15

-10

-5

0

5

10

15

20

-30

-25

-20

-15

-10

-5

0

5

10

15

20

Incident angle (degree)

Alo

ng-tr

ack

(Km

)

20 25 30 35 40 45 50 55-20

-15

-10

-5

0

5

10

15

20SINR<3dB

SINR<10dB

SINR<3dB

SINR<10dB

The vulnerability measure of a SAR is defined as the imaged areawhere interference denies information acquisition

Signal-to-Noise RatioNo interference present

Signal-to-Interference Ratio(50dBW EIRP interference) Denied area

Yellow coloredYellow colored areas have a SINR<10dB are (new target acquisition capability is denied);Red coloredRed colored areas have SINR<3dB (confirmation of an already acquired target capability is denied).

An interference located inside the access area can limit the image capability of a space-borne SAR for most of the range access area and for a long azimuth area


SAR Vulnerability Reduction

Antenna Antenna nullingnulling

1 aux antenna1 aux antenna

Antenna Antenna nullingnulling

2 aux antennas2 aux antennas

Signal-to-Interference Ratio Denied area

Results show that interference suppression capabilities is important especially when operating in hostile areas or in high density RF systems areas,

where potential RF spurious transmissions in the SAR band are expected.


Bistatic Multi-channel SAR

The combination of an active illuminator with one passive receivers flying in close formation can yield a bistatic SAR.Looking at the same scene from different aspect angles has advantages in a wide-range of remote sensing applications. Examples are:

(i) improved joint classification using, both, mono and bistatic SAR data;(ii) single-pass cross-track interferometry;(iii) along-track interferometry;(iv) polarimetric interferometry imaging.

A passive multi-channel antenna allows to test multi-channel potentialities:

(i) MTI; (ii) RELOC; (iii) EXTENDED IMAGING; (iv) SAR-ECCM.

Moving target detection, relocation and along-track interferometry applications requires a limited distance between the two satellites to avoid decorrelation.

This imposes constraints to the SAR bistatic operation∆Ω

i

∆M Active

SAT

Passive SAT


Bistatic MTI & RELOC

Doppler frequency is not enough to separate slow targets from clutter. The high satellite speed implies a wide clutter spectrum, so that only fast moving targets can be detected. This also requires high PRF values.

Splitting the SAR antenna into multiple sub-apertures allows to use space-time information to achieve a narrower notch, to perform clutter cancellation before target detection.

In particular, the multi-channel sensor is expected to allow:

• MTI: Clutter cancellation and detection of slowly moving ground targets for surveillance by applying space-time adaptive processing (STAP) techniques to the multi-channel set of collected radar data; • RELOC: Exact positioning and imaging of strong moving targets in the high resolution SAR images by using a spatial discrimination capability to compensate for the erroneous displacements due to the motion component.

Potential interest for surveillance in both military and civil protection applications.


Extended Imaging

• Resolution: Using the same PRF of the single-channel case, it is possible to improve the azimuth resolution without reducing the range swath extension. The TX antenna has an azimuth aperture augmented with respect of the usual single-channel case in order to extend the azimuth footprint. The received signal is acquired by the two parallel RX-channels each one relative to a half side of the receiving antenna.

• Swath extension: It is possible to reduce the value of PRF(regarding the single-channel case), thus allowing an improvement of the unambiguous zone, without reducing the azimuth resolution. The TX antenna has the same pattern aperture of a usual single-channel system in azimuth direction, whereas the pattern aperture is augmented in elevation directionin order to increase the illuminated zone in range direction.

Extended Imaging, performing a coherent processing of two RX signals, allows an improvement of the image geometric characteristics:


M-SAR experiments from the ground (1/2)

Space-Surface BSAR (SS-BSAR) consists of a non-geostationary space-borne radar transmitter and a stationary ground based receiver

The use of a space-borne SAR as illuminator allows:to generate a sufficiently high power density on Earth’s surface;to exploit a favorable pulse waveform (e.g. wideband);platform motion required for SAR

The use of a ground based receiver allows:low cost experimentation of single &

multichannel techniques (only passive antenna with multiple sub-apertures with no space qualification)

no data-link/data storage limitations, with the high amount of data producedby multichannel systems

real time: data immediately available.


M-SAR experiments from the ground (2/2)

SS-BSAR imaging parameters:• Range resolution depends on the illuminator transmitted frequency bandwidth• Azimuth resolution is twice the space-borne one

-Received power is dominated by the ground-based receiver, which is in the short range (while transmitted power is for a full space-based system)

-Resolution is determined by the space-based platform, which determines the maximum synthetic aperture (without TX beam steering)

- the required PRF is set by the narrow footprint of the ground-based receiver, which allows only to a small portion of the ground clutter to reflect radiation, thus reducing the frequency bandwidth


Experiments from the ground

SS-BSAR can be a “low cost” solution to test several SAR applications of potential interest for both military and civil users.

SS-Multistatic SAR

Different experiments can be carried on using one or more ground receivers:Polarimetric SAR (SS-PolBSAR) Multi-static SAR (SS-Multistatic SAR);Multi-Beam SAR (SS-MB-BSAR);SAR ECCM (SS-BSAR-ECCM);Bistatic Differential Interferometry SAR (SS-BDInSAR);

SS-MB-BSAR SS-BSAR-ECCM SS-BDInSARSS-PolSAR


Flexibility of M-SAR experiments from the ground

Therefore:

-Power is not a problem: also a reduced power could be transmitted by the TX

-The PRF of the TX is higher than required to properly sample the spectrum of the echoes from the small footprint of the ground-based receiver

In consequence:

- widening the TX antenna beam gives no problems in terms of power level and PRF, while it provides larger synthetic apertures without steering the TX antenna and thus higher azimuth resolution without steering the TX beam. This can be obtained by either using a tapering function or switching off some antenna elements during TX.

- transmitting alternately on multiple polarizations or frequencies (using globally the same PRF of the standard space-base radar, but for each polarization/frequency only a fraction of this PRF), provides multifrequencypolarimetric images.


SS- Bistatic Polarimetric SAR Experiment

The use of a dualThe use of a dual--polarization receiver for the ground station polarization receiver for the ground station allows to:allows to:

test the test the polarimetricpolarimetric capability for both:capability for both:

Dual polarization configuration;Dual polarization configuration;

Full polarization configuration (requires dual Full polarization configuration (requires dual polarization capability for the illuminator).polarization capability for the illuminator).

analyze the improvement in target classification analyze the improvement in target classification achievable using achievable using polarimetricpolarimetric SAR images. SAR images.


SS- Multi-static SAR Experiments

The use of a several ground station The use of a several ground station receivers yields Spacereceivers yields Space--Surface MultiSurface Multi--static SAR (SSstatic SAR (SS--MSAR), allowing to:MSAR), allowing to:

understand bi & multi understand bi & multi ––static static reflectivity for different classes of reflectivity for different classes of targets;targets;

use multiuse multi--angle target angle target characteristics to obtain high characteristics to obtain high performance classification.performance classification.

use multiuse multi--static reflection to static reflection to invert bioinvert bio--physical parameters of physical parameters of the observed scene, which might the observed scene, which might cumulate or replace polarization in cumulate or replace polarization in POLINSAR measurements POLINSAR measurements


SS- Multi-Beam Bistatic SAR Experiments

The use of multiple apertures connected to The use of multiple apertures connected to multiple channels for the ground based multiple channels for the ground based receiver allows to:receiver allows to:

test the clutter cancellation, slow test the clutter cancellation, slow moving target detection and relocation moving target detection and relocation capability;capability;

test and compare several multitest and compare several multi--beam beam antenna configuration, using real antenna configuration, using real measured performance;measured performance;


SS- Bistatic SAR ECCM Experiment

The use of a multiThe use of a multi--channel receiver for the channel receiver for the ground station allows to:ground station allows to:

test the vulnerability of a SAR sensor to test the vulnerability of a SAR sensor to electroelectro--magnetic interference;magnetic interference;

test the interference cancellation test the interference cancellation capability for a SAR system using an capability for a SAR system using an antenna based ECCM technique;antenna based ECCM technique;

analyze the impact of the antenna analyze the impact of the antenna nullingnulling processing on the quality of processing on the quality of focused image (e.g. peak loss, image focused image (e.g. peak loss, image resolution, resolution, SideLobeSideLobe Level,Level,……););


SS- Bistatic Differential InSAR Experiment

The use of Differential The use of Differential InterferometryInterferometry for a for a ground based receiver gives a potentiality ground based receiver gives a potentiality for local monitoring of temporal evolution for local monitoring of temporal evolution of Earth surface deformation such as:of Earth surface deformation such as:

subsidencesubsidence movementmovementslopeslope stabilitystabilityland slideland slideglacierglacier movementmovement

The information is available at every The information is available at every revisiting time of the satellites constellation revisiting time of the satellites constellation without scheduling a specific mission.without scheduling a specific mission.

Practically achievable vertical baseline is an Practically achievable vertical baseline is an issueissue


Conclusions

The potential evolution of the modern spaceborne SAR has been discussed.

Potential multi-channel evolution from space and medium term test:split the phased array antenna of SAR in multiple sub-apertures to obtain a

multi-channel SAR;use multi-channel receivers on a passive receiver carried by a mini-satellite to

test multi-channel SAR capability, using present generation SAR as illuminators;develop or multiple additional only single/multiple passive receivers to obtain

bistatic/multistatic SAR.

Experiments from ground to validate multi-channel and multistatic SAR potentialities. Use SS-MSAR as a “low cost” solution to carry on different experiments:

• Polarimetric SAR (SS-PolBSAR);• Multi-static SAR (SS-MSAR);• Multi-Beam SAR (MB-SS-BSAR);• SAR ECCM (SS-BSAR-ECCM);• Bistatic Differential Interferometry SAR (SS-BDInSAR).

MULTI-CHANNEL SAR EXPERIMENTS FROM THE SPACE AND …

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