1. 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

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

3. 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.

4. 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

5. 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

6. 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

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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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.）

14. 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

15. 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.