In accordance with the annual maintenance plan for the BeiDou Navigation Satellite System (BDS), an upgrade and maintenance operation of on-orbit BDS-3 satellites will be carried out from 31 May 2021.  According to the maintenance process, a single satellite will be set to be unusable for some period of time. During the upgrade and maintenance operation, the service performance indicators of BDS are in line with the " BeiDou Navigation Satellite System Open Service Performance Standard".