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

PBO GPS Network - P299-Duckworth_CN2007 P.S.
Start: 08-Jun-2007
End: 16-Feb-2021
UNAVCO Community, 2007, PBO GPS Network - P299-Duckworth_CN2007 P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T5GX48JG