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The combination of continuous Global Navigation Satellite System (GNSS) measurements over a sparse network of points covering Australia with relatively low frequency but high spatial density observations from Interferometric Synthetic Aperture Radar (InSAR) is fundamental to the new geodetic reference frame being developed for Australia. Recognising the economic importance of improved positional accuracy and the potential for geodetic tools to contribute to an understanding of energy related issues, the Australian Government has funded an innovative regional geodetic network of GNSS survey marks and co-located radar corner reflectors. This new network has been installed in the Surat Basin, Queensland where regional subsidence is expected due to significant resource extraction from the subsurface. In this contribution we present initial observations of the a-priori line-of-sight height error derived from corner reflector response in TerraSAR-X, Sentinel-1A, RADARSAT-2 and ALOS-2 SAR imagery of the Surat Basin. 1. COMBINATION OF GNSS AND INSAR Permanently deployed corner reflectors allow precise ground movements to be extracted from SAR acquisitions as persistent scatterers with stable phase characteristics at known positions [1, 2]. Since the signal to noise ratio from a resolution cell containing a corner reflector will be much greater than that from a resolution cell containing natural scatterers, the persistent scatterer signals can be used to validate the deformation signal measured spatially in SAR scenes using conventional differential InSAR time series techniques [3, 4]. The co-location of corner reflectors with GNSS survey marks will ensure SAR-derived deformation products can be tied to (and help constrain via deformation models) the local geodetic reference frame. 2. SURAT BASIN GEODETIC NETWORK The increasing demand for energy in Australia has led to increased exploitation of unconventional coal seam gas reserves, particularly in the Surat and Galilee basins in Queensland. CSG production began in the Surat Basin in 2006 and reserves are currently being produced by several operators. Predictions of the magnitude of subsidence based on poroelastic modelling and gas production rates indicate subsidence on the order of a decimetre may be occurring [5]. Due to this expected signal of around 100 km wavelength, we have deployed a new permanent geodetic network over the northern Surat Basin in Queensland (Fig. 1) [6]. The network consists of 65 GNSS survey mark sites, with a subset of 40 sites having a co-located radar corner reflector. This subset of the geodetic network covers a region of approximately 20,000 km 2 in the vicinity of the towns of Dalby, Miles and Chinchilla. Permanent installation of the full geodetic network was completed in November 2014. Annual 7-day occupation campaign GNSS measurements on the survey marks and local ties between survey marks and corner reflectors are planned. Figure 1. Distribution of 65 new geodetic sites in southern Queensland. Also plotted are land parcels designated as petroleum leases for production and exploration permits for petroleum, coal seam gas wells and other petroleum wells (as of 11 Dec 2014). 3. CORNER REFLECTOR DESIGN We sought a single target design that would provide a bright, yet stable response in XCand L-band SAR imagery data and that would be stable over long (nominally decadal) time periods for permanent deployment in the landscape. A passive corner reflector was chosen over an active transponder for deployment in remote Queensland. Once deployed, corner reflectors are comparatively autonomous since transponders would require regular site visits for maintenance, continuity of power supply and transmission licencing over long time periods. Since suitable transponders [e.g. 7] are not currently commercially available, the design  Proc. ‘Fringe 2015 Workshop’, Frascati, Italy 23–27 March 2015 (ESA SP-731, May 2015) and manufacture costs are also much less for a corner reflector. Furthermore, a corner reflector can more easily meet our requirement for use with different radar frequencies. A triangular trihedral corner reflector design was chosen because of the simplicity of manufacture, long term structural rigidity, relative stability for large radar cross section and a three decibel beam width of approximately 40 degrees. This last fact ensures that the permanently deployed corner reflectors can be aligned in an average boresight orientation for all orbiting SAR sensors of interest (on either an ascending or descending orbital pass) yet the signal to noise ratio of the radar response for each SAR sensor will still be high and stable. Since radar response is target size and radar frequency dependent we recognised that a single design would require a compromise for some radar frequency bands. We undertook a prototyping exercise to establish the most appropriate size of triangular trihedral corner reflector [8]. We manufactured 18 prototypes ranging in size from 1.0 m to 2.5 m inner leg dimension. The radar cross section (RCS) at a range of viewing angles was characterised at a ground radar reflection range. Following this characterisation, the prototypes were deployed in a temporary network near to Canberra in early 2014. During this deployment the radar response of the prototypes was tested with Xand C-band imagery from TerraSAR-X, COSMO-SkyMed, RADARSAT-2 and RISAT-1. Unfortunately the temporary deployment pre-dated the launch of the new L-band SAR satellite, ALOS-2. Results from these tests led to a corner reflector of 1.5 m inner leg length being chosen for the permanent Surat Basin geodetic network (Fig. 2) [8]. The three 2.0 m and three 2.5 m prototype corner reflectors were permanently re-deployed in the Surat Basin geodetic network. A secondary aim of the permanently deployed corner reflectors is that they provide the international space community with a reliable means to perform ongoing radiometric, geometric, and impulse response measurements for calibration of SAR sensors on spaceborne or airborne platforms [9]. To date they have been used by ISRO, e-GEOS, JAXA and ESA to calibrate and/or validate RISAT-1, COSMO-SkyMed, ALOS-2 and Sentinel-1A SAR data. In the Surat Basin geodetic network, most corner reflectors are installed on the soil profile of the sedimentary basin; one of the forty corner reflectors was coupled to bedrock at the ground surface. The foundation for each corner reflector, depicted in Fig. 2, comprises a suspended two metre square slab supported by four concrete pillars each of 3.0 metre length. This foundation was designed to mitigate the impact of signals originating from seasonal swelling in the upper soil layers. Upon installation, all forty corner reflectors were aligned for ascending passes of orbiting SAR satellites. The boresight azimuth and elevation angles for ALOS2, RADARSAT-2 and Sentinel-1A were calculated for each position, and an average of these orientations was used when aligning each corner reflector. Corner reflector positions and current alignments are given in [6]. Figure 2: a) Engineering drawing of the 1.5 m triangular trihedral corner reflector (side-on view) and concrete foundation design. Dimensions in mm. b) 1.5 m corner reflector installed at Site 26. Concrete slab dimension is 2.0 m square. Most sites have a plastic fence to protect the corner reflector from livestock

Corner reflectors as the tie between InSAR and GNSS measurements: Case study of resource extraction in Australia