Evaluation of Calibration Accuracy with HPS Evaluation of Calibration Accuracy with HPS (HongIk Polarimetric Scatterometer)(HongIk Polarimetric Scatterometer)

System for Multi-Bands & Multi-PolarizationsSystem for Multi-Bands & Multi-Polarizations

Ji-Hwan Hwang*, Soon-Gu Kwon, and Yisok OhElectronic Information and Communication Eng., HongIk University

Ji-hwan_hwang@mail.hongik.ac.kr; yisokoh@hongik.ac.kr

Contents 1) Objectives 2) Configuration of HPS system 3) Innovative antenna system for L-band 4) System calibration & accuracy evaluation 5) concluding remarks

1

ObjectivesObjectives

• For a comparison study on earth observation with satellite SAR system at various frequency bands, requirements of multi-bands / polarizations ground based scatterometer continuously are increasing.

• the existing HPS(Hongik Polarimetric Scatterometer) system basically support L-,C-,X-bands and full-polarizations (vv-, vh-, hv-, hh-pol.), however, those are separated system.

• this study shows configurations of 1)integrated system for multi-bands with single platform and evaluates an accuracy of 2)system calibration in the field experiments.

2IGARSS 2011

3IGARSS 2011

Configuration of HPS systemConfiguration of HPS system

1) Boom structure (8m) : movable platform 2) Head part : it’s consist of sensor (OMT+Ant.) and

incident angle control

3) Sub-circuit : this circuit achieves two functions of

frequency converting and switching for full-polarization

4) DAQ, Graphic User Interface : HPS is basically network analyzer

based system, Agilent 8753E and controlled by GUI

[1] P. O’Neill, et al, “Survey of L-Band Tower and Airborne Sensor System Relevant to Upcoming Soil Moisture Missions,” IEEE Newsletter Geosci. Remote Sensing, Jun. 2009.

4

• To implement the motion control(θ,Ф), head part is composed of 3

step motors and inclinometer sensor. Incident angle (θ) : tolerance 0.2°

IGARSS 2011

Configuration : Head Part Configuration : Head Part

(2) Inclinometer sensor (2-axis)

(1) Step motors

(3ea)

MOVE Ang.(Amove=An-Ain)

Meas. Ang. (A(n+1))

Input Inc.angle (Ain)

No

YesSet position (destination)

Meas. Curr. Ang.

(An), n=0

(n+1)

Meas. Ang. (An), n=1

Amove

[2] J.-H. Hwang, S.-M. Park, S.-G. Kwon, Y. Oh, “Study on the calibration of a full-polarimetric scatterometer system at X- band (in Korean)”, KIEES, Apr. 2010.

5

• Switching circuit offers full-polarimetric signal without mode change of network

analyzer (Agilent 8753E) and switch the path for each bands• Frequency conversion and amplifier circuit only for X-band minimize signal

distortion and path loss through 7m RF cable : (9.65GHz to 1.25GHz, gain 32dB

p1dB 20dBm)

IGARSS 2011

Configuration : sub-circuitConfiguration : sub-circuit

RF-cable 7m

Pol. indicator

H-p

ol.

V-p

ol.

P1

P2

Circ#1

Circ#2

SW#1

J1

J2

SW#2

J1

J2

8.4GHz11.5dBm

+12V

+12V

+12VMIXTX

MIXRX

DIV

ISOTX

ISORX

BPF PA

LNA

LO

•Freq. Conversion (X-band) •Pol. Selection

Bypass for L- & C-bands

SW#3

J2

J1

SW#4

J1

J2

SW#5

J2

J1

SW#6

J2

J1

Net

wo

rk a

nal

yzer

An

tenn

a + O

MT

Tx-path

Rx-path

6IGARSS 2011

• GUI software controls whole measurement system:

ex) inc. angle, time gating, Tx power level, automatic meas., system cal.

Configuration : GUI & DAQ Configuration : GUI & DAQ

File header

Full-pol. Measurement data

Incidence angle

7IGARSS 2011

Innovative Ant. System for L-band Innovative Ant. System for L-band (type 1)(type 1)

• the existing ant. system (horn ant. + OMT) for L-band is too bulky to be installed in HPS platform. So, we suggested two types of compact OMT.

[3] J. -H. Hwang, S.G. Kwon, Y. Oh, “orthomode transducer using trapezoidal waveguide”, Patent in Korea, 10-2010-0090749 (in progress).

Waveguide taper

waveguideT-junctionV-pol.

H-pol.

Common mode port

< Commercial class 1 OMT >

Waveguide taperV-pol.

H-pol.Conducting

post

Total length =75cm,

< OMT with Compact T-junction>

Reduced total size to about 56%

• Type 1 OMT is minimized by newly designed compact T-junction

8IGARSS 2011

Innovative Ant. System for L-band Innovative Ant. System for L-band (type 2)(type 2)

• Total length of new OMT is about 86cm (commercial OMT 1.35m)• It achieved reducing the OMT size to about 62% and also keeping the comparable performance with the same class OMT structure

Stepped-horn antenna(length: 300mm, 1.26λ 0

aperture: 450× 450mm2)

Proposed new OMT(length: 560mm, 2.35λ 0)

[4] J. -H. Hwang, Y. Oh, “Compact OMT Using Single-Ridged Triangular Waveguide”, IEEE MWCL, 2011.[5] J. -H. Hwang, S.G. Kwon, Y. Oh, “orthomode transducer using waveguide with 4-splitted triangular cross section”, Patent in Korea, 10-2010-0091816 (in progress).

9

System calibration (Single Target System calibration (Single Target Cal. Tech.)Cal. Tech.)

IGARSS 2011

[5] K. Sarabandi, and F. T. Ulaby, “A convenient technique for polarimetric calibration of single-antenna radar systems,” IEEE Trans. Geosci. Remote Sensing, Nov. 1990.

IGARSS 2011

• HPS system was calibrated using STCT at each frequency bands.• to calibrate HPS system, measured data of ref. and test targets are needed,

and we can calculate the calibrated scattering matrix of test target.

2 2 2 012 21 11 222 0 0 0 02

12 21 11 22

2 2 2 012 21 22 112 0 0 0 02

12 21 22 11

2 2 212 21 11 222 0 0 02

12 21 11

12 1

1

12 1

1

2 2 11

u u u u

vv

u u u u

hh

u u u

vh

m m m ms C C C s

m m m mC

m m m ms C C C s

m m m mC

m m m mCs C C C

m m mC

0022

2 2 2 021 12 11 222 0 0 0 02

21 12 11 22

0 012 210 011 22

2 2 11

1 , 1 1

u

u u u u

hv

sm

m m m mCs C C C s

m m m mC

m ma C a

m m a

s0 : theoretical [S0] of ref. target (sphere)

m0 : measured [M0] of ref. target (sphere)

mu : measured [Mu] of test target (C.R.)

spg : calibrated [S] of test target (C.R.)

Mie exact solution

2DTST (2D target scanning technique)

This method offers ‘well-aligned’ data

10IGARSS 2011

System calibration (2D Target System calibration (2D Target Scanning Tech.)Scanning Tech.)• 2DTST: this technique can measure full-pol. freq. responses of ref. / test

targets and graphically choose the ‘well-aligned’ center data.

[4] J. -H. Hwang, S. -M. Park, Y. Oh, “Calibration Accuracy Enhancement in the Field Experiment with a Ground-Based Scatterometer”, IGARSS 2010, Aug. 2010.

• Concept view of gird system for 2DTST

-100

10

-100

10-40

-20

0

[deg] [deg]

No

rm. P

atte

rn V

V-p

ol [

dB

]

-100

10

-100

10-40

-20

0

[deg] [deg]

No

rm. P

atte

rn H

H-p

ol [

dB

]

-100

10

-100

10-100

0

100

[deg] [deg]Ph

ase

diff

ere

nce

, hh

-vv

[de

g]

-100

10

-100

10-100

0

100

[deg] [deg]Ph

ase

diff

ere

nce

, vh

-vv

[de

g]

Max point (-1,1) ~ boresight

Scan resolution ~1˚

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System calibration (multi-bands/full-System calibration (multi-bands/full-pol.)pol.)

11

• These results are calibrated Full-pol. freq. responses using STCT and 2DTST• to evaluate calibration accuracy, conducting sphere of 30cm diameter and

30/45cm TCR were used as REF. and test target, respectively.

9.4 9.5 9.6 9.7 9.8 9.9-40

-30

-20

-10

0

10

freq. [GHz]

X-Band

5.1 5.2 5.3 5.4 5.5-40

-30

-20

-10

0

10

freq. [GHz]

C-Band

1 1.1 1.2 1.3 1.4 1.5-40

-30

-20

-10

0

10

freq. [GHz]

RCS,

pq

[dBs

m]

L-Band

theoryun-cal. cal.s

vvs

vhs

hvs

hh

After calibration, effective isolation levels improve more than about 10dB

9.4 9.5 9.6 9.7 9.8 9.9-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

freq. [GHz]

5.1 5.2 5.3 5.4 5.5-1.5

-1

-0.5

0

0.5

1

freq. [GHz]

1 1.1 1.2 1.3 1.4 1.5-6

-5

-4

-3

-2

-1

0

1

freq. [GHz]

Rasi

dual

s [d

B]

X-BandC-BandL-Band

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Cal. resid.un-cal. resid.

Cal. accuracy~ 0.3dB

~ 0.4dB

~ 0.3dB

• Calibration accuracy can be defined to RMS value of the norm of residuals,

which is a degree of similarity between two data arrays.

: : ..

#of data points

pq theory pq measnorm M MCal accuracy

: RMS norm of residuals

Evaluation of Cal. accuracyEvaluation of Cal. accuracy

13IGARSS 2011

Evaluation of Cal. accuracyEvaluation of Cal. accuracy

• These results show the spatial changes of calibration accuracy

depending on the degree of mis-alignment between antenna and test target.• we can assign ‘Reliability zone’, which is ‘well-aligned’ region to

guarantee the calibration accuracy of 0.5dB in field experiments.

0 5 10 15 20 25 300

10

20

30

40

50

60

Off-Center Angle [deg]

Cal.E

rror

: mag

[dB]

vv: [dB]

hh: [dB]hh-vv: [deg]

10

20

30

40

50

60

0 5 10 15 20 25 300

10

20

30

40

50

60

Off-Center Angle [deg]

vv: [dB]

hh: [dB]

h

10

20

30

40

50

60

h-vv: [deg]

0 5 10 150

10

20

30

40

50

60

Off-Center Angle [deg]

vv: [dB]

hh: [dB]

hh-vv: [deg]

Phase e

rror [deg]

10

20

30

40

50

60

X-Band C-Band L-Band

Reliability zone ~ 6.5° ~ 5° ~ 3°

*Note: It depends on meas. antenna radiation pattern: HPBW (29°/35°, 25°/29°, 12°/13°), E-/H-. Phase error does not exceed about 7˚ in all frequency bands.

14IGARSS 2011

• HPS system for multi-bands & multi-polarizations integrated to single-platform and, especially, the newly designed antenna + OMT system for L-band is applied. • To calibrate scatterometer system, STCT and automatic 2DTST were used, and these results agreed well with theoretical RCS.

• ‘Reliability Zone’, to guarantee 0.5dB calibration accuracy, are 6.5˚, 5˚, and 3˚ in L-,C-,X-bands, respectively.

• This ground-based HPS system will be continuously used for comparison study of satellite SAR system as a test-bed.

Concluding remarksConcluding remarks

Thanks, do you have any questions??

15IGARSS 2011

Appendix 1. (off-center error rate)Appendix 1. (off-center error rate)

0 10 20 30 400

20

40

60

80

100

Off-Center Angle [deg]

Err

or r

ate

[%]

Threshold of 5% error rate~3

# of samples = 441eaAVG: full-pol.

• to verify the off-center error caused by mis-alignment, we measured

surroundings from the center of ref. target. • we assume that the well-aligned center data, maximum position between

antenna and target, have highest accuracy and reliability.

Error rate (1 .) 100 (%)corr

where,

‘corr.’ is correlation coefficient of

centered and off-centered meas.

data arrays of ref. target.

< e.g., Off-center errors of X-band >

16IGARSS 2011

Appendix 2. Appendix 2. : Backsacttering coef.(: Backsacttering coef.(σσ˚) in tidal flat + pattern ˚) in tidal flat + pattern cal.cal. [M [M00]]

C-band, 2010.08.16

10 20 30 40 50 60 70-40

-30

-20

-10

0

10

Incidance angle [deg]

0 pq [d

B]

oysterC1

6Aug2010.txt

0vv

0vh

0hv

0hh

10 20 30 40 50 60 70-40

-30

-20

-10

0

10

Incidance angle [deg]

0 pq [d

B]

mudC1

6Aug2010.txt

0vv

0vh

0hv

0hh

Oyster field Mud area

10 20 30 40 50 60 70-40

-30

-20

-10

0

10

Incidance angle [deg]

0 pq [d

B]

oysterX1

7Aug2010.txt

0vv

0vh

0hv

0hh

10 20 30 40 50 60 70-40

-30

-20

-10

0

10

Incidance angle [deg]

0 pq [d

B]

mudX1

7Aug2010.txt

0vv

0vh

0hv

0hh

C-band: oyster field

X-band: mud areaX-band, 2010.08.17

Oyster field Mud area