A free app for Android, by Konstantin Gründger.

OGN is a network of HF receivers maintained by gliding enthusiasts mainly in Europe which listen to signals of flarm devices and OGN trackers. The OGN app connects to the network and visualizes the data received. It can display information such as the current position, altitude, etc. It can also display information about the flarm device such as the device type, the country, etc.

You can see detailed information about the aircraft such as the registration number, the competition sign, and the model. If you click on the map, you can see the position of the aircraft.

You can set the distance and the area of the map.