Interesting project, can't wait to help ESA out a bit. Seems like the phone will just track the satellites via GPS, so it's not even that battery intensive. Overnight left charging could provide a lot of data. The fact that there's a leaderboard makes it even more gamified.
Don't phones use a combination of various global positioning systems these days, including glonass and galileo?
Actually how much detailed GPS information can phones access? Most 'common' apps will use the wider 'location services', which combine GNSS data with things like known wifi points and 3/4/5g radio towers to provide better accuracy.
If you install this app, you'll see exactly what GPS data your phone is picking up.
https://github.com/barbeau/gpstest
I can see GPS, Galileo, Chinese, Russian, and Japanese ones.