ship receiver / GPS / GLONASS
There are two satellite positioning networks which orbit the earth: the Global Positioning System (GPS) and the Global Navigation Satellite System (GLONASS). The GPS network was commissioned in 1993 and the GLONASS in 1996.
The constellations are similar in that both require at least four satellites to be in view at least 10° over the horizon to calculate a fix. Both require at least five satellites to determine if the fix calculated is correct with a 99.9% certainty of its stated accuracy which is the system's integrity. If it finds fault with the integrity of a satellite, it must acquire a sixth satellite to replace the one that failed the integrity test.
The constellations are different in that the GPS accuracy is better than 20 meters for 95% of the time, but this accuracy is denied by a practice called Selected Availability (SA). This has been done for security reasons. This degraded position information is called the Standard Positioning Service (SPS). The stated accuracy is 100 meters for 95% of the time.
The GLONASS accuracy is 8 meters for 95% of the time. The system is not degraded.