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

Ethiopia GPS Network - ADD1-Addis Ababa Observatory P.S.
Start: 01-Aug-1997
End: 21-Jul-2007
Bilham, Roger G., 1998, Ethiopia GPS Network - ADD1-Addis Ababa Observatory P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T5KP80G9