# First Results from Sentinel-1A InSAR over Australia: Application to the Perth Basin
## 4 InSAR Datasets and Methods
### Reduction of Error Sources  
Following unwrapping, the interferograms contain residual unwrapping errors that appear as 2\(\pi\) phase jumps, particularly in regions of low coherence. These errors are identified under the knowledge that the phase field is conservative, i.e., for acquisitions at Epochs 1, 2 and 3, the phase of interferogram \(\phi_{1,3}\) is equal to the sum of the phase of interferograms \(\phi_{1,2}\) and \(\phi_{2,3}\)[56]. Consecutive loops of interferograms were therefore used to isolate and mask any phase unwrapping errors.  
The resulting interferogram phase contains contributions from various noise sources, most notably changes in atmospheric conditions (pressure, temperature, water vapour content) between acquisitions, causing artefacts that are typically a few cm in magnitude [13; 57]. The atmospheric contribution to phase is commonly divided into stratified and turbulent components [13]. The long-wavelength (>10 km), stratified component includes phase signals that correlate with topography due to vertical stratification of the atmosphere [58]. In the Perth Basin, these artefacts are most likely to occur across the \(\sim\)200-m elevation of the Darling Scarp in the east of the TerraSAR-X and Sentinel-1A scenes. The short-wavelength (<10 km) turbulent component is attributed to lateral changes in the distribution of water vapour over short time intervals, which results in spatially- and temporally-random phase patterns. Whilst these turbulent features may be reduced by temporal averaging [59], the stratified component is likely to have a seasonal dependence that can bias deformation estimates [58] and should therefore be estimated and removed.  
There are various approaches for correcting atmospheric artefacts in interferograms (for examples, see [PERSON] et al. [60] and the references therein), including empirical methods that utilise statistical properties of the data, most commonly the relationship between interferogram phase and elevation, to estimate the stratified contribution [61]. The success of this approach typically relies upon prior knowledge of either the location or spatial wavelength of deformation [62], of which we have neither in the Perth Basin. Predictive methods use external datasets, such as large-scale weather models [58; 63] or GPS [64; 65], to calculate and remove atmospheric artefacts. There are only two public-domain continuously operating GPS receivers in the Perth Basin (Figure 2), which is insufficient to use the latter approach. Instead, we estimate the stratified component of the atmospheric error using the ERA-Interim (ERA-I) global atmospheric reanalysis. This is a freely available tool from the European Centre for Medium-Range Weather Forecasts and has been used successfully to correct interferograms in a variety of atmospheric settings [58; 60; 66; 67]. ERA-I provides estimates of temperature, geopotential height and relative humidity at 37 pressure levels between 100 hPa and 1 hPa, on a 75 km \(\times\) 75 kmglobal grid, at six-hourly intervals [68; 69; 70]. We select the model cycle closest to the SAR acquisition time for each satellite (within 1 h) and combine the model outputs with a digital surface model using the method described in [PERSON] et al. [66] and [PERSON] et al. [67] to produce 2D maps of atmospheric delay. Differential maps of atmospheric delay are then subtracted from the interferograms.  
The final step of error correction is to empirically estimate and remove the best-fitting planar phase ramp representing the orbital error contribution to phase and any residual long wavelength atmospheric signals [13; 56; 71].