GNSS or Global Navigation Satellite System is the generic term for a constellation of satellites that provide time and location data to receivers on earth.
The most common forms of GNSS are GPS and GLONASS. There are however many more different types including Beidou and Galileo and regional system IRNSS and QZSS.
Each of these constellations effectively provides the same service, however, signal strength may differ based on the location of the receiver and number of satellites in the constellation.
For Tekron clocks to effectively and accurately receive time we recommend that the GNSS receivers have minimum four satellites in view to acquire time and location and at least one satellite to maintain the time for even more satellites to choose from we also recommend that GPS and GLONASS are used (GPS + GLONASS in the configuration software)
If you would like a more detailed look at how GNSS works and how Tekron is able to use GNSS to provide timing, check out this article.