The first thing depends on what you do and what resolution you need. Secondly, you can decide which satellite im you can choose. Among them, domestic environmental satellites and cebers satellites are free. Quyc can be downloaded from the websites of Satellite Environment Application Center of the Ministry of Environmental Protection and China Resources Satellite Application Center. In addition, the Landsat data and modis data before 2000 are also free. In official website, you can spend money to buy other mo data, especially high score data.