Evaluation of Calibration Accuracy with HPS (HongIk Polarimetric Scatterometer) System for Multi-Bands & Multi-Polarizations Ji-Hwan Hwang*, Soon-Gu Kwon, and Yisok Oh Electronic Information and Communication Eng., HongIk University Contents 1) Objectives 2) Configuration of HPS system 3) Innovative antenna system for L-band 4) System calibration & accuracy evaluation 5) concluding remarks 1

Objectives 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. 2 IGARSS 2011

3 Configuration 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

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 (2) Inclinometer sensor (2-axis) (1) Step motors (3ea) [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

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-circuit RF-cable 7m Pol. indicator Tx-path Rx-path

6 IGARSS 2011 GUI software controls whole measurement system: ex) inc. angle, time gating, Tx power level, automatic meas., system cal. Configuration : GUI & DAQ File header Full-pol. Measurement data Incidence angle

7 IGARSS 2011 Innovative Ant. System for L-band (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, (in progress). Waveguide taper waveguide T-junction V-pol. H-pol. Common mode port Waveguide taper V-pol. H-pol. Conducting post Total length = 75cm, Reduced total size to about 56% Type 1 OMT is minimized by newly designed compact T-junction

8 IGARSS 2011 Innovative Ant. System for L-band (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, 1.26 λ 0 aperture: 450 × 450mm 2 ) Proposed new OMT (length: 560mm, 2.35λ 0 ) [4] J. -H. Hwang, Y. Oh, “Compact OMT Using Single-Ridged Triangular Waveguide”, IEEE MWCL, [5] J. -H. Hwang, S.G. Kwon, Y. Oh, “orthomode transducer using waveguide with 4-splitted triangular cross section”, Patent in Korea, (in progress).

9 System calibration (Single Target 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 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. s 0 : theoretical [S 0 ] of ref. target (sphere) m 0 : measured [M 0 ] of ref. target (sphere) m u : measured [M u ] of test target (C.R.) s pg : calibrated [S] of test target (C.R.) Mie exact solution 2DTST (2D target scanning technique) This method offers ‘well-aligned’ data

10 IGARSS 2011 System calibration (2D Target 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 Concept view of gird system for 2DTST Max point (-1,1) ~ boresight Scan resolution ~1˚

IGARSS 2011 System calibration (multi-bands/full-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 freq. [GHz] X-Band freq. [GHz] C-Band freq. [GHz] RCS,  p q [dBsm] L-Band theory un-cal. cal. s vv s vh s hv s hh After calibration, effective isolation levels improve more than about 10dB

freq. [GHz] freq. [GHz] freq. [GHz] Rasiduals [dB] X-BandC-BandL-Band IGARSS 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. : RMS norm of residuals Evaluation of Cal. accuracy

13 IGARSS 2011 Evaluation 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. *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.

14 IGARSS 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 remarks Thanks, do you have any questions??

15 IGARSS 2011 Appendix 1. (off-center error rate) 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. where, ‘corr.’ is correlation coefficient of centered and off-centered meas. data arrays of ref. target.

16 IGARSS 2011 Appendix 2. : Backsacttering coef.(σ˚) in tidal flat + pattern cal. [M 0 ] C-band, Oyster field Mud area C-band: oyster field X-band: mud area X-band, Oyster field Mud area