Sea Surface Current Measurement WithKu-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 OrganizationMitsubishi 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)

2nd pulse1st pulse

Antenna aperture

1R t

1 0

10

exp

4

s a j

R t

2 1R t R t R

Antenna position at the 1st pulse

Antenna position at the 2nd pulseTrx Phase

Center

Tx PhaseCenter

Rx PhaseCenter

Fore antennaAft antenna

： wavelengtha： signal amplitude

0

10

exp

4 4

ss a j

R t R

Signal phase difference in the fore and aft antenna corresponds tothe target cross track velocity

Signal phase difference in the fore and aft antenna corresponds tothe 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.

SAR System

SAR Acquisition parameters

The 10cm resolution Ku-band airborne SAR system (by Mitsubishi Electric Co.)The 10cm resolution Ku-band airborne SAR system (by Mitsubishi Electric Co.)

40cm

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

© Japan Coast Guard (W1064)

Test Site

Tidal Current

Flight path A

Flight path B

Flight path C

Asahi Reef

Test site : Asahi Reef near Cape Irago, Aichi Prefecture, JapanTest site : Asahi Reef near Cape Irago, Aichi Prefecture, Japan

Ground Truth

We used a GPS-equipped handmade float to observe the “ground truth.” We used a GPS-equipped handmade float to observe the “ground truth.”

Sinker

Pole

FloatGPS Logger (DG-100)

Ground Truth

Sea surface current speed was 0.7m/s ～ 0.9m/s during the flight testSea 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

Ground Truth

The “ground truth” acquired by the handmade float is valid. The “ground truth” acquired by the handmade float is valid.

12： 00 13： 00 14： 00 15： 00

© Japan Coast Guard

• 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

Results

Flight Path A : speed of the current has been measured using the flight pathperpendicular to the current

Flight Path A : speed of the current has been measured using the flight pathperpendicular to the current

SAR image (Resolution: 0.6m)

rad

Azimuth (1.4km)R

ange (340m

)

ATI interferogram (Resolution: 35m)

N

Tidal Current

AB

C

Tidal 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.

Results

Flight Path A : average cross range velocity was estimated to be 0.95m/sFlight Path A : average cross range velocity was estimated to be 0.95m/s

Radial velocity map (Resolution: 35m)

SAR image (Resolution: 0.6m)

m/s

Azimuth (1.4km)R

ange (340m

)

N AB

C

Tidal Current

sin

1

4

baseline

pATIGND B

Vv

vGND = 0.95[m/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

pV

baselineB

ATI

Results

Flight Path B & C : Two perpendicular ATI data sets have been acquiredin rapid succession (10 minutes interval).

Flight Path B & C : Two perpendicular ATI data sets have been acquiredin rapid succession (10 minutes interval).

Flight Path Bazimuth

range

Flight Path C

azimuth

range

AB

C

Tidal Current

• 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.

Results

Flight Path B & C : the two images have been registered using the buoy signal.Flight Path B & C : the two images have been registered using the buoy signal.

　 Flight Path C: SAR image (0.6m res.)

rad rad

N

Flight Path C: Interferogram （ 10m res. ）

azimuth

range azimuth

range

　 Flight Path B: SAR image (0.6m res.)

Flight Path B: Interferogram （ 10m res. ）

Results

V_N = 0 ． 51 [m/s]

V_E = -0 ． 64m/s]

Mean velocity

Estimated sea surface current speed ： 0.82m/s

Matches well with the ground truth (0.7m/s ～ 0.9m/s ).

N

Spatial resolution : 10m x 10m

Flight Path B & C : Two-dimensional sea surface current map can be obtained by combing two perpendicular ATI data sets in rapid succession.

Flight Path B & C : Two-dimensional sea surface current map can be obtained by combing two perpendicular ATI data sets in rapid succession.

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.