I'm just trying to build this app with zero knowledge in Java and Android.
You can use an external GPS/GNSS receiver as same as internal one. Connect external Bluetooth Global Navigation Satellite System (GNSS) receivers, using the GPS, GLONASS, Galileo, Beidou or QZSS system, to your Android device.
Main Screen
About
Settings
Radar Interface: Each satellites have prefix in name
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G: GPS
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R: GLONASS
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E: Galileo
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S: SBS(WAAS, MSAS, EGNOS, etc)
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B: Beidou/Compass
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Q: QZSS
BlueGnss4Droid is licensed by GPLv3 and later
Copyright 2014 Hiroshi Miura miurahr@linux.com
Copyright 2014 Bluetooth GNSS for OSM Mappers Project
Copyright 2010-2013, Herbert von Broeuschmeul
Copyright 2010-2013, BluetoothGPS4Droid Project