# First Results from Sentinel-1A InSAR over Australia: Application to the Perth Basin  
[PERSON]  
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precision [9], repeat surveys are labour intensive and are therefore only carried out over a limited area or after large time intervals. Continuous GPS provides measurements at high temporal resolution (\(\sim\)30 s), but only at discrete spatial locations. These geodetic methods therefore do not capture the full complexity of the deformation field in space and/or time, and additional time series of measurements are required over larger areas and at higher spatial and temporal resolutions.  
One such tool to achieve this is interferometric synthetic aperture radar (InSAR). Since the early 1990s, InSAR has become an increasingly popular method for measuring ground deformation over a range of spatial scales, from individual buildings or structures [10] to continents [11]. InSAR operates by comparing the phase of time-separated satellite radar images acquired over the same region (for further details, we refer the reader to [PERSON] et al. [12], [PERSON] [13], [PERSON] and [PERSON] [14], [PERSON] and [PERSON] [15]). Preliminary InSAR studies of the Perth Basin between 1992 and 1997 reported subsidence rates of up to 50 mm/y [16]; however, this was later disproved by [PERSON] [17], who found that average subsidence rates did not exceed \(\sim\)4 mm/y over this time.  
In April 2014, the European Space Agency (ESA) launched Sentinel-1A, the first half of a two-satellite constellation providing the most extensive and systematic coverage of SAR data to date [18] and the first mission to deliver data on a near-real-time, open-access basis. Sentinel-1A data have since been used to make observations of deformation that is centimetres-to-metres in magnitude, associated with earthquakes (e.g., Mw 7.8 Gorkha, Nepal: [19; 20; 21; 22]); volcanic eruptions (e.g., Fogo, Cape Verde: [23]); and the development of sink holes (e.g., Wink, Texas: [24]). However, the detection of small-magnitude displacements, such as subsidence of the Perth Basin, is reliant on longer time series of images.  
Mainland Australia was fully imaged during the initial phase of Sentinel-1A. However, only a limited spatial coverage of repeat images has subsequently been acquired. Repeated coverage is densest over Victoria and New South Wales, where to date, over 60 scenes have been acquired 10 times or more (Figure 1). In Western Australia, this level of repeat has only been achieved in two locations, including over the Perth Basin (Figure 1).  
In this study, we first review the previous use of InSAR over Australia, commenting on the availability of data and the relatively small number of peer-reviewed studies. We then describe an application of Sentinel-1A data to the Perth Basin, validating the observations with an independent TerraSAR-X InSAR survey over the same time period. In doing so, we carry out a reconnaissance investigation into the larger extent of the deformation field in the Perth Basin, whilst demonstrating the applicability of Sentinel-1A to measure small-magnitude deformation associated with changes in groundwater levels. Using these observations, we highlight the importance of ongoing time series of InSAR data to measure subsidence due to groundwater extraction throughout this population centre.