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

MAGNET - HOLT-HOLT P.S.
Start: 13-Apr-2006
End: 16-Oct-2010
Blewitt, Geoff, Hammond, William C., Kreemer, Corne, 2009, MAGNET - HOLT-HOLT P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T58050SH