I am a nigerian, that has interest in becoming remote sensing expert, but i want to start my PhD in field but i'm afraid of the availability of data?

What kind of data are you interested in? Landsat is certainly a very useful series of satellites which provides free multi-spectral satellite data from 1972 onwards in the Visible, NIR, SWIR, TIR and Panchromatic wavelength bands. But Landsat is not the only one, there are many other freely available options . I have provided some answers to similar questions asked in RG earlier. The links are attached. This will help you to find many freely available options like ASTER, SAR Data, High Resolution Data, Multi-spectral and Hyperspectral Data. Additionally I have a few suggestions for your question related to PhD.

I think the best way to approach your question would be to first:

Decide what your specific research interests are in a particular domain. Here are some examples provided by EROS USGS : - Agriculture forecasting, Agricultural management/production/conservation, Alternative energy exploration/development, Assessments and taxation, Biodiversity conservation, Climate science/change, Coastal science/monitoring/management, Cryospheric science, Cultural resource management/anthropology/archaeology, Defense/national security, Ecological/ecosystem science/monitoring, Education: School,university/college, Emergency/disaster management, Energy /metals/minerals exploration/extraction/development, Engineering/construction/surveying, Environmental regulation, Fish and wildlife science/management, Fire science/management, Forest science/management, Geology, Hazard insurance, Humanitarian aid, Law enforcement, Land use/land cover change, Public health, Range/grassland science/management, Real estate/property management, Recreation science/management, Rural planning and development, Software development, Telecommunications, Technical training, Transportation, Urban planning and development Urbanization (e.g., growth, sprawl), Utilities Water resources to name a few

Once you have decided what your research interests are based on your academic and professional background and specific skills sets, you can then look for appropriate tools to solve the research problem that you have or want to tackle in you PhD. There are many tools available, Satellite Data is only one of the tools.

Look for specific universities, researchers and research institutes which are active in Remote Sensing and with whom you would like to work with during your PhD. Collaboration is key which provides a setting for academic, professional and personal development.

I hope these few suggestions might help you.

https://www.researchgate.net/post/What_are_the_different_sites_for_downloading_best_resolution_below_20_m_for_free_remote_sensing_imagery

https://www.researchgate.net/post/Where_can_I_download_free_RADAR_data

https://www.researchgate.net/post/Can_anybody_provide_me_the_sources_of_R_G_B_NIR_band_satellite_images_of_urban_areas_having_resolution_less_than_5_m

https://www.researchgate.net/post/How_can_I_get_free_hyperspectral_data_apart_from_Usgs_EO-1_Hyperion

https://www.researchgate.net/post/I_want_to_pursue_Phd_in_remote_sensing_Can_anyone_tell_me_which_institute_is_best_What_are_the_scholarships_available

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