Learning Objective 062.06.01.02.24: State that agreements have been concluded between the appropriate agencies for the compatibility and interoperability by any approved user of NAVSTAR and GLONASS systems.

Learning Objective 062.06.01.02.25: State that the different GNSSs use different data with respect to reference systems, orbital data, and navigation services.

NAVSTAR/GPS (created by the USA) and GLONASS (created by the USSR/Russia) are both GNSS systems, and have most recently been joined by Galileo, the European version, and Bei Dou, the Chinese version. They are all separate systems, providing roughly the same service to users. They can all be used in most parts of the globe (usually not the extreme polar regions due to satellite availability and orbital tracks).

Their interoperability is varied, and there are effectively two separate levels of "interoperability"

• Interoperability at a system level
  • Where two systems can be used separately and the final results compared (usually requiring some sort of mathematical translation layer).
• Interoperability at a signal level
  • A higher level of compatibility, where the signals from the satellites to the receivers are similar enough to use different systems in the same way for a single position calculation.

GPS and GLONASS have fundamentally different frequencies and reference frames (WGS-84 vs PZ-90), so they are not signal interoperable, but are interoperable at a system level. This is the reason that, if you've used a non-aviation Garmin device, you might have seen that you can choose to use GPS only or GPS + GLONASS, where both systems work independently and the outputs are compared using a mathematical translation layer. This can prove to have higher availability, integrity and possibly accuracy than GPS alone due to the 2 separate systems.

Galileo and GPS, on the other hand, have certain frequencies where they are signal interoperable, and therefore can be used to assist the accuracy or integrity of the other system, though not on all frequencies (this part is outside of the LOs).

Therefore, for this question, we are happy to say that the GPS and GLONASS systems are compatible (given this particular wording) and that their functions (i.e. their outputs) are interoperable. It's quite close to the edge of becoming incorrect with this wording, but feedback indicates it is correct. The option B about EU regulations says only GPS is compatible where we believe Galileo is also operational now within EASA.