# Comparing Sentinel-2A and Landsat 7 and 8 Using Surface Reflectance over Australia
## 2 Methods  
### Comparison Dataset  
The differing orbits and swath widths of Landsat and Sentinel-2 mean that it is possible to find many instances in which the two satellites observe the same ground locations on the same day. Since both satellites are sun-synchronous with mid-morning overpass time, these pairs of observations are generally only minutes apart, and it can therefore be assumed that there is no change in the land surface occurring between the two observations. Atmospheric conditions are also unlikely to have changed significantly between the two acquisitions. This implies that any differences found between the two observations are due solely to the differences in the sensors, or the observation conditions, rather than a change in the target.  
It is important to note that the orbits and swath widths of the two instruments can imply significant differences in view angle. These paired observations could involve a nadir observation from one instrument paired with an edge-of-swath observation from the other instrument. Hence, there is also potential for considerable variation in atmospheric path length, and variation in the effects of the Bi-direction Reflectance Distribution Function (BRDF), and so the differences in observation conditions could be of some importance.  
In order to minimize differences due to observation conditions, the imagery for both sensors was standardized to surface reflectance at a standard observation geometry, using the methods described by [PERSON] et al. [8]. This transforms the imagery by correcting for atmospheric effects and the effects of the Bidirectional Reflectance Distribution Function (BRDF), and presents surface reflectance at nadir view and a solar zenith angle of 45\({}^{\circ}\). The spectral response functions supplied by the ESA for Sentinel-2A were added to the atmospheric correction code (Second Simulation of the Satellite Signal in the Solar Spectrum, known as 6S [9]), so that the identical radiometric processing was used for all sensors. Aerosol optical thickness (AOT) in Australia tends to be very low by global standards, and is assumed to be a constant low value of 0.05 [10; 11]. While this value would not be realistic in many other parts of the world, this is not important to the comparisons at hand. The important point is that the same value is used for all sensors.  
It is then assumed that any remaining difference between observations is due solely to the difference in the sensor responses to the pixels in question.  
The bands used for comparison were those that correspond most closely between the different instruments. The correspondence is given in Table 1. While the Sentinel-2 Near Infra-Red (NIR) band 8A has a spectral response which corresponds more closely to that of Landsat-8 OLI, this band is available at only 20 m pixel size, and so it is not likely that this would be the band in major use for studies working at 10 m resolution. In addition, the Landsat-7 ETM+ NIR band is wider, and covers roughly the same window as the Sentinel-2 band 8. Thus, for these reasons, only Sentinel-2 band 8 was considered for NIR correspondence.  
A random selection of points was chosen across a large part of continental Australia. These were chosen on the basis of the tiling used by the ESA for distribution of Sentinel-2 data, with a random selection of unmasked points within each tile contributing to the whole dataset, to ensure an even distribution of points across the landscape. From this set of points, one third were set aside as validation, while the rest were used for fitting of the prediction models described below. All of the comparisons shown in Section 4 were carried out using the validation dataset.  
The 10 m and 20 m Sentinel-2 bands were resampled to match the 30 m pixels of the corresponding Landsat imagery, using the \"average\" resampling method provided in the gdalwarp software (version 1.11.5) [12].