The following information obtained from UNAVCO's online DOI system:
(https://www.unavco.org/data/doi/search/search.html)

Popocatepetl GPS Network - UIGF-UNAM P.S.
Start: 13-May-1996
End: 01-Aug-1997
Cabral-Cano, Enrique, 1997, Popocatepetl GPS Network - UIGF-UNAM P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T58G8J17