Correlating Synthetic Aperture Radar (CoSAR) DIVYA CHALLA

SAR  Scan the side-looking images in Remote Sensing Applications.  2 dimensional imaging radar.  An end-to-end system: antenna transmitter, receiver, a data collection and signal processing stages.  Range and phase histories are saved to generate images.

SAR Imaging  Image from all coherent phase information.  SAR – high processing gain because of coherent summation of correlated responses of radar.  In raw data, signal energy is spread out in range and azimuth direction and in SAR focusing we collect dispersed energy into single pixel.  In range, spread out by duration of linear FM chirp.  In azimuth, signal spread out by length of period it is illuminated by synthetic aperture.

AZIMUTH RESOLUTION

Coherence and Non-coherence: Coherence: The target is static during the observation Non-coherence: Motion of scatterers between observations produces changing phases Ex: windblown vegetation Continuous change of water surface Precipitation effects Manmade effects

SAR for decorrelating surfaces  The acquisition times are short for SAR.  With SAR, we are assuming that the scene is static during our observation time.  For decorrelating surfaces, the coherent time is very less (<100ms)  So, this is a limitation on azimuth resolution achievable by a SAR.  However, resolution can be improved by generating large number of looks, but for higher orbits, as angular velocity with respect to target decreases azimuth resolution will degrade further to that of RAR.  A new concept named CoSAR where 2 receivers are operated with a relative motion in azimuth direction.

CoSAR  It has 2 receivers for a single transmitter both in relative motion and position in azimuth direction.  Generates high resolution image of some statistical properties like NRCS or the first Doppler moment.  Also provide information of cross-track interferometric phase, from which winds, ocean currents could be derived.  The received echoes combined and spatial autocorrelation estimated.  Spatial Correlation estimates combined to get high resolution image.

CoSAR system with 2 receivers moving in the opposite azimuth direction

CoSAR signal model

Resolution of CoSAR

Evaluation of iso-doppler Radar operating wavelength, λ 2 cm Position (x; y; z) -23,578 km; -20,050 km; 22,254 km Velocity (vx; vy; vz) 0 m/s; 0 m/s; 0 m/s Airborne Receiver1 Position (x,y,z) 0 km; 0 km; 4.35 km Velocity (vx, vy, vz) 0 m/s; 300 m/s; 0 m/s Airborne Receiver2 Position (x,y,z) 0 km; 8.35 km; 4.35 km Velocity (vx, vy, vz) 0 m/s; 300 m/s; 0 m/s

Geostationary CoSAR mission To monitor ocean winds and currents, a mission using CoSAR.

TO DO  More study on CoSAR  Practical analysis.

REFERENCES  Lopez-Dekker, P.; Rodriguez-Cassola, M.; De Zan, F.; Krieger, G.; Moreira, A. "Correlating Synthetic Aperture Radar (CoSAR)", IEEE Transactions on Geoscience and Remote Sensing, Volume: 54, Issue: 4, April  Lopez-Dekker, P.; Rodriguez-Cassola, M.; De Zan, F.; Krieger, “Correlating SAR (CoSAR): Concept, performance analysis, and mission concepts”, IEEE Transactions on Geoscience and Remote Sensing, 2013.