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

MAGNET - SPRK-SPRK P.S.
Start: 18-Nov-2004
End: 06-Jul-2017
Blewitt, Geoff, Hammond, William C., Kreemer, Corne, 2009, MAGNET - SPRK-SPRK P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T5DV1H33