General information:   https://ifg.tugraz.at/downloads/ocean-tides
Format description at: https://ftp.tugraz.at/outgoing/ITSG/oceanAndAtmosphericTides/README.pdf

The ocean tide atlas, called GOT5.6, is
the latest solution in a series beginning with the work described in:

E J O Schrama and R Ray, Journal of Geophysical Research, v.99, p 24799, 1994.
and
R Ray, A global ocean tide model from Topex/Poseidon altimetry: GOT99.2,
   NASA Tech Memo 209478, 58 pages, Sept. 1999.
and
R Ray, Journal of Geophysical Research, 118, 4570-4584, 2013.

Converted into spherical harmonics by Torsten Mayer-Guerr