Calibration with trihedral corner reflectors: a case study using satellite-based X, C and L- band frequency Synthetic Aperture Radar data over Queensland, Australia Medhavy Thankappan1, Matthew Garthwaite1, Peter Meadows2, Nuno Miranda3, Adrian Schubert4 and David Small4

1Geoscience Australia, Canberra, Australia 2BAE Systems Applied Intelligence, Essex, United Kingdom 3European Space Agency, Frascati, Italy 4 Remote Sensing Laboratories, Department of Geography, University of Zurich, Switzerland

Acknowledgments

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

Gunning Grazing Company

Australian Geophysical Observing System (AGOS) 4 CORS GNSS stations Repository of SAR data

GNSS instrumentation pool Robotic GNSS antenna calibration facility

Geodetic network

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

• Four sizes, 1.0 m 1.5 m, 2.0 m and 2.5 m

• Experiment with three surface finishes in the 1.5 m size

• 18 prototypes in total

Prototype Reflectors

Untreated Aluminium Powder Coated Perforated Aluminium

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

Characterisation of Reflectors

Used the DSTO outdoor radar ground reflection range in Adelaide, SA in June 2013

Performed tests at X-band and C-band

Tested all twelve 1.0 m and 1.5 m reflectors

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

CR Characterisation Results

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

• Consistent results for C-band • Less consistent for X-band

1.0 m

Plain sheet

Pow

der-coated

Mesh

Gunning Grazing Company, Dec 2013 – May 2014

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

Field Deployment for Evaluation (Gunning Site)

SAR Data Acquisitions for Evaluation

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

Clutter Intensity: Temporal Changes

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

LOS Height Error

<0.5 mm at C-band

<0.1 mm at X-band

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

RCS vs Clutter Intensity

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

http://www.ga.gov.au/metadata-gateway/metadata/record/82751/

CR Design for AGOS (Queensland Array)

The 1.5 m triangular trihedral corner reflector was chosen for final deployment in the Surat Basin; 34 installed.

Three 2.0 m and three 2.5 m prototypes also installed.

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

Permanent Location of AGOS CRs

65 sites. 40 with co-located radar corner reflector

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

CR Positions

Positions of all CR sites obtained with RTK

Mean 2σ uncertainty of 2 cm horizontal and 4 cm vertical

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

CR Coordinates

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

http://www.tandfonline.com/doi/full/10.1080/08120099.2015.1040073#.Vim8lvmqpBc

Link to paper with details of CR coordinates

Signal-to-Clutter Ratio

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

# SAR images # reflectors imaged Mean SCR Std. Dev. ~ Height Error (mm)

TerraSAR-X 11 11 49.57 3.19 0.005 Sentinel-1A 3 38 32.00 2.85 0.194

RADARSAT-2 8 40 36.18 2.17 0.122 ALOS-2 11 36 29.86 6.14 4.641

Sentinel 1A: Radiometric Calibration

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

• Orientation of the CR array is fixed currently not specifically set for S1A • Elevation and azimuth offset between the CR and SAR boresight angles

IW

S1A_IW_GRDH_1SDV_20141217T084036_20141217T084101_003756_0047A9_3129.SAFE

Results for VV

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

VV

VV

• Stability of individual CRs has been assessed (separately for VV and HH pol):

• All IW acquisitions are from the same track – some variability, especially for orbit 5788. Normalising each product reduces the IW STD from 0.37 to 0.21 dB:

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

• The S1 relative RCS offset is due to the elevation pointing of the CRs (theoretical RCS curve for a trihedral curve assumes that the CR Azimuth offset is zero):

An azimuth offset of 2° leads to a reduction in RCS of just 0.02dB.

VV

VV

• IW and SM standard deviation

Results for VV - contd.

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

• HH pol relative RCS:

• Normalising relative RCS. No change in IW STD:

HH

HH

Results for HH

HH and VV

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

• The IW standard deviation:

N.B. 2.5m CRs are 3, 5 & 14; 2.0m CRs are 4, 8 & 9

HH and VV

Sentinel-1A Geometric Calibration

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

ALOS2: L-band Calibration

Six 2.5 m and 2.0 m CR being used as ongoing calibration targets by JAXA for their L-band SAR mission ALOS-2

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

Summary

• An array of 40 CRs of 1.5, 2.0 and 2.5m sizes deployed in Queensland, Australia; some CRs individually characterised for RCS; designed to support SAR calibration

• Ongoing data acquisition over array with X, C and L-band SAR systems for calibration

• Sentinel-1A radiometric calibration (IW and SM): stability of CRs assessed for VV and HH; RCS variability attributed to CR elevation / azimuth offsets with boresight

• Sentinel-1A geometric calibration IW and SM: overall SM scatter similar to IW SLC scatter plot; some observed anomalies being investigated

• The 2.0m / 2.5m CRs are optimally oriented for ongoing calibration of ALOS-2; flexibility to support specific missions

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

Thank you

CEOS SAR Workshop 2015, ESA-ESTEC, Netherlands 27-29 October 2015

medhavy.thankappan@ga.gov.au