Hatanaka RINEX compression scheme.
RINEX data is commonly archived using a Hatanaka compression scheme,
the ssssdddh.yyd.Z i.e. the d.Z files.

To uncompress Hatanaka RINEX files get the latest version from...
ftp://terras.gsi.go.jp/software

You will leave NGS and be directed to terras.gsi.go.jp if the above link is selected.