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

Yellowstone Hotspot GPS Network - HAMG-Hammer P.S.
Start: 23-May-2006
End: 31-Dec-2011
Smith, Robert B., 1999, Yellowstone Hotspot GPS Network - HAMG-Hammer P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T5NV9GH8