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

El Salvador GPS Network - SNJE-Volcan Santa Ana P.S.
Start: 19-Mar-2007
End: 31-May-2021
DeMets, Charles, 2007, El Salvador GPS Network - SNJE-Volcan Santa Ana P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T55B00KF