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

UNAVCO Polar MRI South Pole SPRESO GPS Network - SPR1
Start: 02-Feb-2007
End: 27-Nov-2007
Johns, Bjorn, 2007, UNAVCO Polar MRI South Pole SPRESO GPS Network - SPR1, UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T5251GDQ