The DSSP program was designed by Wolfgang Kabsch and Chris Sander to
standardize secondary structure assignment. DSSP is a database of
secondary structure assignments (and much more) for all protein
entries in the Protein Data Bank (PDB).

DSSP is also the program that calculates DSSP entries from PDB entries.
DSSP does not predict secondary structure.

If you use DSSP, please quote:
Dictionary of protein secondary structure: pattern recognition of
hydrogen-bonded and geometrical features. Kabsch W, Sander C,
Biopolymers. 1983 22 2577-2637

A series of PDB related databases for everyday needs. Joosten RP, Te
Beek TAH, Krieger E, Hekkelman ML, Hooft RWW, Schneider R, Sander C,
Vriend G, NAR 2010