With a choice of satellite systems from the different countries, which one shall I enable in my device, for better performance such as time to first fix (TTFF), and battery usage?
GNSS Constellations:
| GPS | US |
| QZSS | Japan |
| BEIDOU | China |
| GALILEO | EU |
| GLONASS | Russia |
| IRNSS | India |
The more satellites enabled do not help that much in better TTFF, but it does help is difficult environments where there are obstructions such as device casing and/or tall buildings, limited parts of the sky are visible. Generally GPS and GLONASS are the most established and give the best coverage. As a general rule, use as many constellations as you can for better reliability/accuracy, with priority to GPS + GLONASS.
If you are looking for better overall fix/power consumption compromise, then you can consider only using GPS + GLONASS.
A particular constellation may have better coverage depending on your country/region. Australia is one of the few countries where many different constellations are visible. To see the actual near real-time trace and satellite paths:
https://www.gnssplanning.com/#/settings
Use this settings page to set your location, click Apply, then click world view tab. Note even the Japanese QZSS flies over Australia.