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

GPS Soil Moisture Network - CHLL-CHLL_NCAR_C02014 P.S.
Start: 18-Dec-2013
End: 14-Nov-2022
Braun, John, 2014, GPS Soil Moisture Network - CHLL-CHLL_NCAR_C02014 P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T50V89X9