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

GPS Soil Moisture Network - SPOL-SPOL_CGPS_2014wr P.S.
Start: 10-Jan-2014
End: 04-Mar-2015
Braun, John, 2014, GPS Soil Moisture Network - SPOL-SPOL_CGPS_2014wr P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T5W37TGS