Welcome to IDL, Deepjyoti! The MRDFITS function appears to be a routine from NASA's Astrolib IDL library: https://www.harrisgeospatial.com/docs/mrdfits.html You can find more information about how to download this library here if you have not done so already: https://idlastro.gsfc.nasa.gov/ Once you get the module downloaded onto your system, you must add the entire directory structure to your IDL Path. For more information on IDL Path Management, go here: https://www.l3harrisgeospatial.com/Support/Self-Help-Tools/Help-Articles/Help-Articles-Detail/ArtMID/10220/ArticleID/16156/Quick-tips-for-customizing-your-IDL-program-search-path
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