BeiDou Navigation Satellite System:

In-mid 2015, China started the build-up of the third generation BeiDou system (BDS-3) in the global coverage constellation. The first BDS-3 satellite was launched 30 September 2015. As of March 2016, 4 BDS-3 in-orbit validation satellites have been launched.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". Which system did China start building in March of 2017?
unanswerable