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

PBO GPS Network - P687-MSH_Tube1_WA2004 P.S.
Start: 16-Oct-2004
End: 13-Jul-2022
UNAVCO Community, 2004, PBO GPS Network - P687-MSH_Tube1_WA2004 P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T5R20Z9Z