A GNSS uses satellite positioning techniques to provide users with accurate and timely navigation information. A global positioning system is a subset of a global navigation satellite system because a GPS system need only provide the ability to determine position information. A GNSS must include:
Real time navigation information
It must be possible to generate real-time navigation information fast enough for safe navigation. The TRANSIT satellite system required several satellite passes to determine a position. Because an aircraft can't sit still for several hours, TRANSIT was not sufficient for aircraft navigation. Both the NAVSTAR and GLONASS systems provide near-instantaneous position (close enough.)
Autonomous integrity checking
There must be some way for the user to determine the accuracy of the navigation solution in a timely fashion. It would be bad for an airplane to hit the ground in front of a runway because a GPS satellite is transmitting bad information. Generally this problem is solved by having the aircraft view a large enough number of satellites that it can calculate several sets of redundant positions, comparing them against each other. Bad solutions can be detected and thrown out. At this time, neither the NAVSTAR or GLONASS systems have enough satellites to do this. The only way to do it is to use both NAVSTAR and GLONASS satellites at the same time.
Accuracy sufficient for safe navigation.
The U.S. Department of Transportation has determined that the most stringent requirements for aircraft landing are 4.1 meters horizontal accuracy and 0.6 meters vertical accuracy. There is no way for either NAVSTAR or GLONASS to provide this accuracy without external aids. Therefore, a GNSS using NAVSTAR and GLONASS must include these aids.