MULTI-CHANNEL SAR EXPERIMENTS FROM THE SPACE AND FROM GROUND: POTENTIAL EVOLUTION OF PRESENT GENERATION SPACEBORNE SAR

Transcription

1 3 nd International Workshop on Science and Applications of SAR Polarimetry and Polarimetric Interferometry POLinSAR 2007 January 25, 2007 ESA/ESRIN Frascati, Italy MULTI-CHANNEL SAR EXPERIMENTS FROM THE SPACE AND FROM GROUND: POTENTIAL EVOLUTION OF PRESENT GENERATION SPACEBORNE SAR P. Lombardo, M. Sedehi, F. Colone

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

3 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 3 RX channels 2 RX channels 6 RX channels d el d az P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 3

4 Potentialities of M-SAR from the space 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: exact positioning of strong moving targets in the high resolution SAR images; MULTIPOL: increased monitoring capabilities by collecting fully polarimetric images; ATI: sea current/wave monitoring by along-track SAR interferometry; SAR-ECCM: protection of the SAR sensor from e.m. interferences by implementing antenna-based (Electronic Counter Counter Measure) techniques; MB-BSAR: BSAR: Using a passive multi-channel antenna allows bistatic MTI, bistatic RELOC, EXTENDED IMAGING and SAR-ECCM. Multistatic SAR: The combination of an active illuminator with one passive receivers flying in close formation will yield a bistatic SAR; Already presented with reference to COSMO- SkyMed at POLinSAR 2005 P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 4

5 MTI & RELOC MTI MTI allows to detect low RCS slow ground moving target. Key point of the technique is the discrimination of the target from the surface clutter echoes which is: - strong due to the low incidence angles of the satellite observation geometry. - spectrally spread duer to the high platform speed. RELOC Exact positioning and imaging of moving targets in high resolution on SAR images. Applicable to objects with high reflectivity. The effect can be the radial motion component leads to compensated for, erroneous displacements if accurately estimated The radial velocity estimation accuracy is proportional to the temporal baseline obtained with two independent RX channels with the largest possible along-track displacement. P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 5

6 MULTIPOL & ATI MULTIPOL When using an antenna BFN allowing only a single receiving channel (either H or V), multiple sub- apertures can be used to receive in both linear polarization (one in each sub-aperture). The availability of polarimetric SAR data allows a relevant improvement in target recognition and classification for: man-made made object; land scenario. ATI Sea current/wave monitoring by ATI-SAR can be obtained by using two independent receiving channels with the largest possible horizontal along-track baseline. P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 6

7 SAR-ECCM 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 SAR pulse transmitted power is penalized by 2-way propagation losses, while interference power has only 1-way propagation losses 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 The use of antenna nulling techniques shall cancel the effects of interferences over the collected SAR data, thus fundamentally allowing a normal operation despite the presence of the continuous wave disturbance signals. 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 interference P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 7

8 SAR Vulnerability The vulnerability measure of a SAR is defined as the imaged area where interference denies information acquisition Signal-to-Noise Ratio No interference present Along-track (Km) Signal-to-Interference Ratio (50dBW EIRP interference) Incident angle (degree) Along-track (Km) Denied area SINR<3dB SINR<10dB SINR<10dB SINR<3dB Incident angle (degree) Yellow colored areas have a SINR<10dB are (new target acquisition capability is denied); Red 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 P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 8

9 SAR Vulnerability Reduction Signal-to-Interference Ratio Denied area Antenna nulling 1 aux antenna Antenna nulling 2 aux antennas 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. P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 9

10 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 M i Passive Ω SAT Active SAT P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 10

11 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 spacetime 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. P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 11

12 Extended Imaging Extended Imaging, performing a coherent processing of two RX signals, allows an improvement of the image geometric characteristics: 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 direction in order to increase the illuminated zone in range direction. P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 12

13 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 produced by multichannel systems real time: data immediately available. P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 13

14 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 groundbased 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 P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 14

15 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. 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-PolSAR SS-Multistatic SAR SS-MB-BSAR SS-BSAR-ECCM SS-BDInSAR P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 15

16 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 multifrequency polarimetric images. P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 16

17 SS- Bistatic Polarimetric SAR Experiment The use of a dual-polarization receiver for the ground station allows to: test the polarimetric capability for both: Dual polarization configuration; Full polarization configuration (requires dual polarization capability for the illuminator). analyze the improvement in target classification achievable using polarimetric SAR images. P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 17

18 SS- Multi-static SAR Experiments The use of a several ground station receivers yields Space-Surface Surface Multi- static SAR (SS-MSAR), allowing to: understand bi & multi static reflectivity for different classes of targets; use multi-angle target characteristics to obtain high performance classification. use multi-static reflection to invert bio-physical parameters of the observed scene, which might cumulate or replace polarization in POLINSAR measurements P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 18

19 SS- Multi-Beam Bistatic SAR Experiments The use of multiple apertures connected to multiple channels for the ground based receiver allows to: test the clutter cancellation, slow moving target detection and relocation capability; test and compare several multi-beam antenna configuration, using real measured performance; P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 19

20 SS- Bistatic SAR ECCM Experiment The use of a multi-channel receiver for the ground station allows to: test the vulnerability of a SAR sensor to electro-magnetic interference; test the interference cancellation capability for a SAR system using an antenna based ECCM technique; analyze the impact of the antenna nulling processing on the quality of focused image (e.g. peak loss, image resolution, SideLobe Level, ); P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 20

21 SS- Bistatic Differential InSAR Experiment The use of Differential Interferometry for a ground based receiver gives a potentiality for local monitoring of temporal evolution of Earth surface deformation such as: subsidence movement slope stability land slide glacier movement The information is available at every revisiting time of the satellites constellation without scheduling a specific mission. Practically achievable vertical baseline is an issue P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 21

22 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). P. Lombardo, M. Sedehi, F. Colone, Multi-channel SAR experiments from the space and from ground 22