obal coverage (and better coverage at the poles than GPS). Most GLONASS receivers also support a combined GPS+GLONASS dual mode for enhanced operation. The timescale for GLONASS is based on UTC(SU) from Moscow and uses Moscow Standard Time (MSK) of UTC + 3 hours, and like UTC, it also implements leap seconds. Russia has worked over the last few years to massively improve the alignment of UTC(SU) with UTC, SO that at the time of writing it is generally less than 3-4 ns. BeiDou Navigation Satellite System The Chinese government developed and launched the first generation of BeiDou as a regional sys- tem, using geostationary earth orbit (GEO) satellites, in 2000. They declared it operational in 2003 and decommissioned it in 2012. It was basically an experimental system. In 2006, the Chinese government officially announced the development of the second-generation GNSS called BeiDou-2 (also known as Compass), which ended up with 16 satellites launched. The constellation consisted of five GEO satellites, five inclined geosynchronous satellite orbit (IGSO) satellites, and four MEO satellites This system offered open PNT services with regional coverage to the Asia-Pacific region centered on China's longitude. In 2015, the Chinese government started working on a third-generation system called BeiDou-3, which has added a constellation of 24 MEO satellites in three orbital planes. This system offers global coverage with a constellation of 35 satellites (in three separate constellations in MEO, IGSO, and GEO orbits) and was declared to be operational in 2020. Most BeiDou receivers support a com- bined GPS+BeiDou dual mode for enhanced operation, and since BeiDou-2, the system also offers space-based augmentation capabilities (discussed in the upcoming section "Augmentation Systems Enhancing GNSS"). The BeiDou system takes its time, BeiDou time (BDT), from China's National Time Service Center (NTSC), with an epoch of 1 January 2006. BDT is a continuous timescale (no leap seconds) steered to within 50 ns of UTC. BeiDou broadca