# Comparing Sentinel-2A and Landsat 7 and 8 Using Surface Reflectance over Australia
## 1 Introduction  
The Sentinel satellite constellation of the European Space Agency (ESA) includes several satellite families, as part of their Copernicus programme of Earth observation. The Sentinel-2 family carries a single instrument called the Multi Spectral Instrument (MSI), which is a high-resolution optical instrument, with spectral characteristics similar to both the SPOT (Satellite Pour l'Observation de la Terre) and Landsat families [1]. It improves on these by having higher spatial resolution than Landsat, and more spectral bands and greater revisit frequency than either. The long term plan is to launch and maintain a series of Sentinel-2 satellites (named 2A, 2B, etc. ), over many years, with Sentinel-2A launched on 23 June 2015, and Sentinel-2B launched on 7 March 2017. They each have a planned lifetime of seven years, and further launches are planned with the same instrument in later years. The MSI instrument has a total of 13 spectral bands, at different spatial resolutions. There are four visible and near infra-red bands at 10 m pixel size, roughly corresponding to equivalent Landsat Thematic Mapper (TM) bands, and six near infra-red bands and short wave infra-red bands at 20 m pixel size. The orbit and swath width mean that the nominal revisit time for a single satellite is 10 days. When two satellites are operating, they will be out of phase so that the nominal revisit time over both satellites will be five days [1] (subject to the ESA's acquisition scheduling). Areas that are in the overlap between swaths will obviously be revisited more often. The Landsat family of satellites has provided a long record of earth observation stretching over 40 years, and much existing work has been developed from their data [2, 3]. In order to build on this record, it will be useful to be able to combine imagery acquired by the two families of instruments,and perhaps more importantly to transfer thresholds and fitted models trained on Landsat data for use with the higher resolution imagery of Sentinel-2. This raises the question of how comparable the different sensors are, and what impact the difference would have on models of biophysical quantities. [PERSON] and [PERSON] [4] explored these questions in a small study of five image pairs, and pointed to the need for larger studies. [PERSON] et al. [5] presented work on a complete system for producing surface reflectance and a number of biophysical products from Sentinel-2, and included some comparison of surface reflectance with that generally available for Landsat-8 (the United States Geological Survey Climate Data Record product), for six test sites in Europe. Their results show that the agreement between the two products is good, reporting Root Mean Square error values on the order of 0.03 reflectance units, and this speaks well of the systems these workers have set up. However, since the Sentinel-2 and Landsat-8 products used were created using different surface reflectance transformation algorithms, it is not possible to assess the extent to which the differences are a direct result of the sensor responses or the differences in processing methods. NASA have released documentation for a combined product merging Landsat-8 and Sentinel-2 imagery, called the Harmonized Landsat-8 Sentinel-2 Product (HLS). In creating this product, they have also explored the comparability of these two sensors, and published linear adjustment functions to transform Sentinel-2 reflectance to Landsat-8 reflectance [6]. These adjustment functions were based on hyperspectral spectra taken from 500 sites in a global dataset, combined with the spectral response functions of the two sensors. Since Landsat-5 failed in late 2011, there is no opportunity to acquire Sentinel-2 imagery coincident with that. However, both Landsat-7 and Landsat-8 are still operational and there is now a considerable period in which data have been acquired from Sentinel-2A, Landsat-7 and Landsat-8.