Use of USGS 30 arc second DEM in Grass

[Kazunori Inoue]

Dear Grass Users
I want to use USGS DEM 30 second data in Grass 4.1.5 for Linux  I have not
found a command to import it. If it can be converted to ascii file I think
it can be imported to Grass. As you know a USGS DEM 30 second file covers a
very vast area, so that I would like to import a part of one DEM file 
In this case, I think a binary file (for example w100n40.dem) has to be
converted to ascii file and its data structure should be understood. 
Please let me know what should I do for my purpose? What should I learn at
first?

Thank you

Kazunori Inoue