Sea Surface Current Measurement With KuBand SAR AlongTrack

Sea Surface Current Measurement With Ku-Band SAR Along-Track Interferometry Nobuo Kumagae, Kazuo Kawamura, Kenji Tatsumi, Masatada Furuhata, Masayoshi Tsuchida, Masao Tsuji, Tomoya Yamaoka, Kei Suwa Japan Resources Observation System and Utilization Organization Mitsubishi Electric Corporation

Outline • • • Introduction SAR-ATI The SAR System Test Site & The Ground Truth Results and Discussion Conclusion

Introduction • Applications of sea surface current map – Ecological studies – Coastal surveillance – Validation of coastal tide and current models • SAR-ATI (Along Track Interferometry) for sea surface current map – SAR-ATI measures the line-of-sight component of the surface velocity. – Two-dimensional sea surface current map can be obtained by combing two perpendicular ATI data sets in rapid succession.

SAR ATI (Along Track Interferometry) Signal phase difference in the fore and aft antenna corresponds to the target cross track velocity • Two receive antennas are aligned along track (fore and aft antennas). • At the first pulse the signal is received by the fore antenna, and at the second pulse the signal is received by the aft antenna. • By the time aft antenna observes, the moving target would move and cause the phase difference. 1 st pulse 2 nd pulse Antenna aperture Tx Phase Center Fore antenna Antenna position at the 1 st pulse Antenna position at the 2 nd pulse Trx Phase Center Rx Phase Center ： wavelength ： signal amplitude Aft antenna

SAR System The 10 cm resolution Ku-band airborne SAR system (by Mitsubishi Electric Co. ) SAR Acquisition parameters Center frequency value 16. 45 GHz Transmitted signal bandwidth 600 MHz 40 cm Peak transmitted power 300 W Pulse Repetition Frequency 4500 Hz Polarization VV Effective baseline 0. 2 m Incidence angle 60 deg

Test Site Test site : Asahi Reef near Cape Irago, Aichi Prefecture, Japan Flight path A Flight path B Asahi Reef Ti da l Cu rr en t Flight path C © Japan Coast Guard (W 1064)

Ground Truth We used a GPS-equipped handmade float to observe the “ground truth. ” 0) 0 1 t og a L o Fl PS G r ge G D ( ole

Ground Truth Sea surface current speed was 0. 7 m/s～ 0. 9 m/s during the flight test N 1 st trial 2 nd trial 3 rd trial Flight test was conducted during this period Time Sea surface current speed 1 st trial 12: 37: 18～ 12: 38: 23 0. 5 m/s = 0. 9 knot 2 nd trial 14: 02: 18～ 14: 03: 18 0. 7 m/s = 1. 4 knot 3 rd trial 15: 01: 48～ 15: 02: 53 0. 9 m/s = 1. 7 knot

Ground Truth The “ground truth” acquired by the handmade float is valid. • The “ground truth” acquired by the handmade float is consistent with the sea surface current data published by the Japan coast guard. 1 st trial 12： 00 13： 00 2 nd trial 3 rd trial 14： 00 15： 00 © Japan Coast Guard

Results Flight Path A : speed of the current has been measured using the flight path perpendicular to the current • Flight Path A is perpendicular to the sea surface current. • The phase error has been corrected using the pixels in the ground area. • Average ATI phase is 1. 15 rad. Azimuth (1. 4 km) Range (340 m) N A B SAR image (Resolution: 0. 6 m) l da Ti t n re ur C ATI interferogram (Resolution: 35 m) rad C

Results Flight Path A : average cross range velocity was estimated to be 0. 95 m/s parameters v. GND = 0. 95[m/s] Azimuth (1. 4 km) Range (340 m) N value platform velocity 100 m/s Effective baseline 0. 2 m wavelength 0. 018 m off-nadir angle 60° ATI phase 1. 15 rad A B SAR image (Resolution: 0. 6 m) l da Ti t n re ur C Radial velocity map (Resolution: 35 m) m/s C

Results Flight Path B & C : Two perpendicular ATI data sets have been acquired in rapid succession (10 minutes interval). • Two ATI data have been acquired from two perpendicular flight paths. • Time interval between Flight Path B and C are 10 minutes. • A buoy is included in the both images for the image registration. Flight Path B azimuth A range B l da t n re ur C azimuth Ti Flight Path C range C

Results Flight Path B & C : the two images have been registered using the buoy signal. N range azimuth 　Flight Path B: SAR image (0. 6 m res. ) rad Flight Path B: Interferogram （10 m res. ） 　Flight Path C: SAR image (0. 6 m res. ) rad Flight Path C: Interferogram （10 m res. ）

Results Flight Path B & C : Two-dimensional sea surface current map can be obtained by combing two perpendicular ATI data sets in rapid succession. N Mean velocity V_N = 0．51 [m/s] V_E = -0．64 m/s] Estimated sea surface current speed： 0. 82 m/s Matches well with the ground truth (0. 7 m/s～ 0. 9 m/s ). Spatial resolution : 10 m x 10 m

Conclusion • We have successfully demonstrated that the along track interferometry SAR is a valuable tool for the sea surface current mapping. • The correspondence with the GPS record of sea surface current proves the validity of ATI SAR technique for the sea surface current mapping.