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

Ethiopia GPS Network - ADD0-Addis Ababa Observatory P.S.
Start: 16-Feb-1999
End: 04-Jul-2000
Bilham, Roger G., 2002, Ethiopia GPS Network - ADD0-Addis Ababa Observatory P.S., UNAVCO, GPS/GNSS Observations Dataset, 
https://doi.org/10.7283/T5QF8R56