What is the definition of PSTD?

who can express the PSTD?
I am face a difculty to read the paper from MTT-s about PSTD.
and the other question:may the pstd apply to analsys the microstrip circuit?

i think PSTD is not a very versatile for microstrip simulation
since you can't handle PEC in PSTD
do you think so ?