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GNSS is satellite navigation system which covers whole globe. The primal GNSS is GPS in the USA, GLONASS in Russia, and GALILEO in EU.
RNSS is satellite navigation system which covers some specific regions. QZSS (Quasi Zenith Satellites System, Michibiki in Japanese) in Japan is one of the example of QZSS because it covers Asia and Oceania. Region.

If GNSS and RNSS have compatibility with GPS, it is relatively easy to use their signals as GPS in GPS processing software. With increase of GNSS/RNSS satellites, accuracy/precision of positioning is expected to be improved, especially in metropolitan area and moving platform (ship and buoy, for example) where not enough GPS observation is available. It is beneficial for estimates of tropospheric delay and other parameters.

GNSS: Global Navigation Satellite System
RNSS::Regional Navigation Satellite System

The table above shows total number of satellites, inclination, number of satellites x number of orbit plane, for each GNSS and QZSS. The inclination of GLONASS is larger than that of GLONASS and GALILEO, and it works to reduce no satellite area with only GPS in the northern sky (in the northern latitude). It helps to reduce representativeness error for space in ZTD estimates.

The sky map above show positions of GPS satellite (black solid circles, 9 satellites) and GLONASS satellite (red solid circles, 5 satellites) as snapshot. GLONASS satellites are in the northern sky (the top of the sky map is north direction).