BDS-3 SAR service and initial performance

Search and rescue (SAR) service is one of the major services provided by BDS-3 system. Six BDS-3 MEOSAR payloads are optimally distributed on three orbits and work normally. BDS-3 system uniquely adopts the architecture of Regional Up-Link Stations and crosslinks between satellites to provide return link service (RLS) for distressed people worldwide. BDS-3 B2b signal is designed to support the return link message (RLM) broadcasting, positioning, navigation and timing. A variable-length message type of RLM is specially designed to enrich SAR operation information categories and improve SAR operation efficiency. The performance of MEOSAR, RLS and positioning is evaluated through numerical simulation and preliminary in-orbit test. According to the results, the global availability is over 98.79%, with only six BDS MEOSAR payloads. The test indexes of BDS MEOSAR payloads meet the COSPAS-SARSAT MEOSAR space segment commissioning standard requirements. BDS RLS time delay is less than 2 min, which demonstrates the efficiency of BDS RLS architecture. And B2b positioning accuracy is better than 10 m.