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

Southern Basin and Range GPS Network - APEX-Apex P.S.
Start: 27-Mar-1999
End: 23-Jun-2009
Wernicke, Brian P., 1999, Southern Basin and Range GPS Network - APEX-Apex P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T5154FCP