beaconDB is a drop-in replacement for MLS, which uses the same format request that’s used by Mozilla’s Ichnaea.

The source code is available on Codeberg: https://codeberg.org/beacondb/beacondb

You can contribute to the project by using an app like NeoStumbler (GitHub) or Tower Collector (GitHub) to submit location reports. NeoStumbler does Wi-Fi, Bluetooth and GSM, while Tower Collector can only do GSM cell towers. Both are FOSS and available on F-Droid.

It is also recommended by the GrapheneOS project: https://grapheneos.social/@GrapheneOS/112759509558471713

https://grapheneos.org/articles/positon-location-service

Just keep in mind that it’s still in relatively early development, which is why it really needs contributions.