TETRA BlueStation is a libre stack for a TETRA base station. TETRA is the radio system used by many emergency services: voice, text, and IP data to handheld and fixed radios. Commercial base stations cost tens of thousands of euros and lock operators into a vendor.
The stack aims at something you can actually run — on about €200 of hardware — so surplus TETRA handsets can be reused for resilient wireless communication. Code: MidnightBlueLabs/tetra-bluestation (Midnight Blue).
Project page (NLnet): TETRA BlueStation.