Introduction to the special issue on the BeiDou navigation system

China's BeiDou Navigation Satellite System (BDS) has evolved from a demonstration system (BDS-1) to a regional navigation satellite system (BDS-2) and has finally entered the phase of a global navigation system (BDS-3). In 2018, the BDS-3 implementation achieved historic milestones. A baseline constellation with 18 MEO satellites and one GEO satellite has been established. The BDS has the ability to provide fundamental positioning, navigation, and timing (PNT) services to global users. The current BDS features some distinctive design and unique service capabilities. First, the BDS provides a regional message communication service and a global short message communication service with 1000 and 40 Chinese characters at a time, respectively. Second, the BDS also provides a global search and rescue (SAR) service, a satellite-based augmentation service (SBAS), as well as a regional precise point positioning (PPP) service. This special issue of the Institute of Navigation's journal, NAVIGATION, provides a collection of papers presenting research achievements in the BDS over the last 2 years. It covers essential fields necessary to ensure the successful operation of BDS, such as constellation design, operational and control system design and construction, frequency and signal design, updated coordinate reference system and time system, satellite orbit determination, ionospheric model, and software receiver. Papers in this special issue present the development status of the BDS from several perspectives and introduce the main performance and progress to readers, which is helpful for researchers in the field of satellite navigation and beneficial to GNSS terminal manufacturers as well. The publication of the issue also provides an opportunity to promote users' understanding and applications of the BDS. We hope that readers find useful information in this issue, and we look forward to sharing our continued success in future BDS development.