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

PBO GPS Network - P503-SuperHillsCS2007 P.S.
Start: 23-May-2007
End: 11-Jul-2022
UNAVCO Community, 2007, PBO GPS Network - P503-SuperHillsCS2007 P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T5MP5186