Top 3 qgroundcontrol Open-Source Projects

PX4-Autopilot

26 7,762 9.9 C++

PX4 Autopilot Software

Project mention: Kalman Filter Tutorial: Kalman Filter from the Ground Up | news.ycombinator.com | 2023-10-14

They are widely used in narrow circles :)
Two of (arguably the best) open source RC aircraft flight controllers (ArduPilot and PX4) are using extended Kalman filters in their state estimators (essentially sensor fusion that provides attitude/position estimate):
https://github.com/ArduPilot/ardupilot/tree/master/libraries...
https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/e...
I'm not that familiar with cleanflight/betaflight/inav scene to know what the FPV racer flight controllers use.

DroneBridge

5 685 0.0 C

DroneBridge is a system based on the WifiBroadcast approach. A bidirectional digital radio link between two endpoints is established using standard WiFi hardware and a custom protocol. DroneBridge is optimized for use in UAV applications and is a complete system. It is intended be a real alternative to other similar systems, such as DJI Lightbridge or OcuSync.
InfluxDB

www.influxdata.com featured

Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
ESP32

4 333 9.0 C

DroneBridge for ESP32. A secure & transparent telemetry link with support for WiFi and ESP-NOW. Supporting MAVLink, MSP, LTM or any other protocol

NOTE: The open source projects on this list are ordered by number of github stars. The number of mentions indicates repo mentiontions in the last 12 Months or since we started tracking (Dec 2020).