What do I use to convert Northern EASE-Grid to Global EASE-Grid?

Assuming the data that were produced conform to the ML (25km) EASE-Grid definition, defined by the Ml.gpd and described in our Guide to EASE Grids, then you should be able to utilize the IDL tools that are distributed with our other EASE Grid geolocation tools.

The IDL programs "EASE_CONVERT" and "EASE_INVERSE" will translate to and from lat/lon and r/s coordinates for a given grid.

There is also an html-formatted help file, called "easeconv_help.html," that describes the IDL program interfaces in each geolocation tool's tarball.