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. 1st pulse 2nd pulse Antenna aperture Antenna position at the 1st pulse Tx Phase Center Fore antenna Aft antenna Antenna position at the 2nd pulse Trx Phase Center Rx Phase Center ： wavelength ： signal amplitude

SAR System The 10cm resolution Ku-band airborne SAR system (by Mitsubishi Electric Co.) SAR Acquisition parameters parameters value Center frequency 16.45GHz Transmitted signal bandwidth 600MHz Peak transmitted power 300W Pulse Repetition Frequency 4500Hz Polarization VV Effective baseline 0.2m Incidence angle 60deg 40cm

Test site : Asahi Reef near Cape Irago, Aichi Prefecture, Japan © Japan Coast Guard (W1064) Flight path A Flight path B Flight path C Asahi Reef Tidal Current

We used a GPS-equipped handmade float to observe the “ground truth.” GPS Logger (DG-100) Pole Sinker

Ground Truth Sea surface current speed was 0.7m/s～0.9m/s during the flight test N 1st trial 2nd trial 3rd trial Time Sea surface current speed 1st trial 12:37:18～12:38:23 0.5 m/s = 0.9 knot 2nd trial 14:02:18～14:03:18 0.7 m/s = 1.4 knot 3rd trial 15:01:48～15:02:53 0.9 m/s = 1.7 knot Flight test was conducted during this period

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. 1st trial 2nd trial 3rd trial 12：00 13：00 14：00 15：00 © Japan Coast Guard

perpendicular to the current 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 (1.4km) A B C Tidal Current N Range (340m) SAR image (Resolution: 0.6m) Tidal Current ATI interferogram (Resolution: 35m) rad

Results Flight Path A : average cross range velocity was estimated to be 0.95m/s parameters value platform velocity 100 m/s Effective baseline 0.2 m wavelength 0.018 m off-nadir angle 60° ATI phase 1.15 rad vGND = 0.95[m/s] Azimuth (1.4km) A B C Tidal Current N Range (340m) SAR image (Resolution: 0.6m) Radial velocity map (Resolution: 35m) m/s

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 range A B C Tidal Current Flight Path C azimuth range

Results Flight Path B & C : the two images have been registered using the buoy signal. N azimuth range range azimuth 　Flight Path B: SAR image (0.6m res.) 　Flight Path C: SAR image (0.6m res.) rad rad Flight Path B: Interferogram （10m res.） Flight Path C: Interferogram （10m 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 = -0．64m/s] Estimated sea surface current speed：0.82m/s Matches well with the ground truth (0.7m/s～0.9m/s ). Spatial resolution : 10m x 10m

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.