AN OVERVIEW ON POLARIMETRIC RADAR CALIBRATION Kamal Sarabandi

AN OVERVIEW ON POLARIMETRIC RADAR CALIBRATION Kamal Sarabandi Radiation Laboratory Department of Electrical Engineering and Computer Science University of Michigan, Ann Arbor, Michigan 48109 Outline: q. Introduction/Motivation q. Calibration using point targets q. Calibration using distributed targets q. Point calibration targets q. Truck mounted scatterometers IEEE GRSS Workshop on Microwave Remote Sensing, November 23, 2019

INTRODUCTION Radar Remote Sensing of Earth → For many application quantitative radar backscatter statistics of terrestrial targets are needed. → To retrieve the biophysical parameters of target →Successful inversion requires a large input vector space Radar polarimetry Radar interferometry Multifrequency, multi-temporal data Or combination of the above

INTRODUCTION

MEASUREMENTS OF SCATTERING METRIX • Direct measurement of scattering matrix is not possible due to distortions introduced by the polarimetric radar • Simplified block diagram of a polarimetric radar

CALIBRATION USING POINT TARGETS

Single Target Calibration Technique Sarabandi, K. , and F. T. Ulaby, “A Convenient Technique for Polarimetric Calibration of Single-Antenna Radar Systems, ”IEEE Transactions on Geoscience and Remote Sensing, vol. 28, no. 6, pp. 1022 -1033, November 1990.

q. The radar can range gate around distance r so q Basically, the short-range reflections and double-bounces between the antenna and the target are gated out. q From the resulting uncoupled equations q But

MEASURED SCATTERING MATRIX OF A 6” SPHERE

6” SPHERE Cross-Pol

VERTICAL CYLINDER

Point Calibration Targets • Calibration targets for imaging radars can be categorized into: 1. Passive targets ― Advantage: Stable and reliable ― Disadvantage: Large and physical dimensions 2. Active targets ― Advantage: Relatively small in size ― Disadvantage: Unstable because of high gain amplifiers • Requirements: 1. Large RCS 2. Known S with high accuracy 3. Insensitive to alignments (orientation insensitive)

Coherent Incoherent

Pentagonal Corner Reflector • At boresight only yellow area contribute to the RCS • Reflection from ground does not interact with the target Effect of the Soil Ground Plane

Design of New Optimum Corner Reflectors for SAR Calibration Sarabandi’s design fabricated and employed by Alaskan SAR Facility.

Sarabandi, K. , “Calibration of a Polarimetric Synthetic Aperture Radar Using a Known Distributed Target, ” IEEE Transactions on Geoscience and Remote Sensing, vol. 32, no. 3, pp. 575 -582, May 1994.

4 X 1 4 X 4 4 X 1

• The random process is ergodic, i. e. the statistical and spatial averages are identical.

Shuttle Radar Imaging L- and C-band Calibration The Shuttle Imaging Radar Mission April and October 1994 Payload: L-, C-, and Xband synthetic aperture radars Sarabandi, K. , L. E. Pierce, Y. Oh, M. C. Dobson, A. Freeman, and P. Dubois, “Cross Calibration Experiment Using JPL AIRSAR and Truck-Mounted Polarimetric Scatterometer, ” IEEE Transactions on Geoscience and Remote Sensing, vol. 32, no. 5, pp. 975 -985, September 1994.

Shuttle Imaging Radar SIR-C/X-SAR Sarabandi, K. , L. Pierce, M. C. Dobson, F. T. Ulaby, J. Stiles, T. C. Chiu, R. De Roo, R. Hartikka, A. Zambetti, and A. Freeman, “Polarimetric Calibration of SIR-C Using Point and Distributed Targets, ” IEEE Transactions on Geoscience and Remote Sensing, vol. 33, no. 4, pp. 858 -866, July 1995. Freeman, A. , M. Alves, B. Chapman, J. Cruz, Y. Kim, S. Shaffer, J. Sun, E. Turner, and K. Sarabandi, “SIR-C Data Quality and Calibration Results, ” IEEE Transactions on Geoscience and Remote Sensing, vol. 33, no. 4, pp. 848 -857, July 1995.

Shuttle Radar Imaging L- and C-band Calibration Experimental Procedure • Calibration Site: Raco • Truck measurements: immediately after SIR-C flight • Target: A grass field • More than 50 acres The distributed calibration target

Deployment of Point Targets • Approach - Deployment of point targets * Trihedral corner reflectors (2. 4 m, 1. 07 m) * Active radar calibrators - - SAPARC (L- and C- band) - - Recirculating PARC (Cband) - Polarimetric measurement of a distributed target using the L/C/X POLARSCAT Point targets used in SIR-C experiment

Polarimetric Scatterometer Measurements • A calibrated truck-mounted scatterometer can be used to measure the backscatter of a large area of a rough surface (plowed field or grass field). • The area must be sampled to include at least 100 pixels. • Measurements should be at the same incidence angle and concurrent (shortly after) the SAR overflight.

Measured Radar Cross Section Coefficient of distributed Target as a Function of SIR-C Overpass Incidence angle

Measured Radar Cross Section Coefficient of distributed Target as a Function of SIR-C Overpass SIR-C Incidence angle

Trihedral Corner Reflectors Active Radar Calibrator 8’-Trihedral

SIRC-C Flight Direction The white color indicates that there are equal power in VV, HH, and VH channels The ringing observed is due to the delay line in SAPARC Trihedral is shown yellow: equal power in VV and HH and no VH

Channel Distortion of SIR-C L-band SAR Extracted from Calibrated Distributed Target Radiometric Cal. Const. =2. 57 Cross calibration comparison: Showing calibration based on point targets are under-estimating by less than 1 d. B for co-pol And about 2 d. B for crosspol.

Comparison of Phase Difference Statistics

Plot of phase difference pdf for different “Degree of Correlation” and “Mean Phase Difference” values. Comparison of “Degree of Correlation” phase-difference statistics for SIR-C L-band using point and distributed calibration Sarabandi, K. , “Derivation of Phase Statistics from the Mueller Matrix, ” Radio Science, vol. 27, no. 5, pp. 553 -560, September – October 1992.

Comparison of Co-Pol Mean Phase Difference for SIR-C L-band using point and distributed calibration methods. Comparison of Co-Pol Mean Phase Difference for SIR-C L-band for different targets using distributed calibration algorithm and point targets.

Variation of Trihedral Point Targets in a Calibrated radar Scene