A low-cost, open-source communications platform designed for beyond visual line of sight (BVLOS) operation of UAVs and ground-based robotic systems. Built around the Raspberry Pi Compute Module 5, the platform uses a customised PCIe backplane to support interchangeable communications modules including cellular modems and high-speed breakouts without the subscription costs or proprietary lock-in of existing solutions.
Current BVLOS solutions, commercial platforms like the Elsight Halo or high-end options from Silvus and Doodle Labs, are either cloud-subscription dependent or oriented toward defence procurement budgets. This project bridges the gap between hobbyist hardware and serious communications infrastructure using commercial off-the-shelf (COTS) and hobbyist components.
The platform was developed and validated as part of a BEng Electrical and Electronic Engineering project at the University of Southampton.
The system is composed of five PCBs that connect through a shared backplane, these are only the ones developed for this project, you could expand and add to this ecosystem as you see fit. Imagination is the limit (and the laws of physics)
- Power Supply Module — Wide-input (6.2–80V) synchronous buck converter based on the LM5146, with polarity-agnostic inputs, dual power-ORing, and TVS transient protection. Rated 5V at 10A (50W continuous).
- Compute Module — Carrier board for the Raspberry Pi CM5, exposing USB 3, Ethernet, GPIO expansion, and high-density PCIe interfaces.
- Cellular Modem Module — M.2 B-Key breakout supporting modems such as the Quectel RM520-GL, with onboard SIM card management, low-ESR decoupling, and an active-low modem reset circuit.
- Breakout Module — General-purpose PCIe-to-header breakout for integrating additional peripherals or custom radio hardware.
- Backplane — Passive interconnect using PCIe edge connectors. Supports active or passive configuration depending on system requirements.
Each module connects via a custom-keyed PCIe connector chosen for its current capacity, locking mechanism, and wide availability compared to alternatives such as M.2 NGFF.
Modules are keyed as shown below, with power connections consistent across all connector types and GPIO/high-speed lanes varying by module role, use the following as a rough design guideline should you want to implement your own boards:
| Connector | Role |
|---|---|
| PCIe x1 | Power module |
| PCIe x2 | Cellular / breakout modules |
| PCIe x3 | Compute module |
| x1 | x2 | x3 |
|---|---|---|
 |
 |
 |
The PSU is designed to handle the transient loads generated by cellular modems under maximum transmit, the Quectel RM520-GL draws up to 1512 mA at 3.7V, and ripple exceeding 100 mV will trigger an automatic brown out. The design addresses this through:
- Low-ESR MLCC capacitor networks at the modem supply rail
- LM5146-based buck converter with BSC072N08NS5 power MOSFETs
- CISPR 25 Class 5 input filtering
- Full-bridge Schottky rectifier providing polarity-agnostic and power-ORing operation on dual inputs
Measured efficiency across a 10–60V input range and thermal characterisation data are included in the results documentation.
While this was the power supply designed as part of this project, the way the backplane has been designed is that should a more efficient vesion
The platform runs OpenWRT as its base operating system, chosen for its minimal boot overhead, flexible package management, and suitability for embedded communications roles. Pre-built image configurations can be used for instant deployment without requiring Linux expertise.
Cellular data logging during field testing was handled via MATLAB, with GPS, RTT, and signal quality data correlated and visualised against mapped tower locations.
This is an initial hardware revision. The platform has been tested in both laboratory conditions and a real-world BVLOS scenario. Known issues from revision 1 are documented in the errata, covering:
- Onboard Ethernet trace routing under the Hirose connector
- Secondary USB 3 data line path via backplane
- Crowbar circuit behaviour
- Voltage ripple under certain load conditions
A revision 2 design addressing these is in progress.
/hardware
/kicad # KiCad files for the entire project
/plots # PCB plots
/gerbers # Fabrication outputs
/firmware
/openwrt # OpenWRT image configurations and package lists
/software
/cellular-logging # GPS + cellular data logging scripts (MATLAB)
/docs
/results.md # Power supply efficiency, thermal, and cellular test results
/errata.md # Known issues and workarounds
/bom.md # Bill of materials with COTS part references
Hardware fabrication files are located in /hardware/gerbers. The design uses standard 1.6mm four-layer PCBs and is compatible with common PCB manufacturers (JLCPCB, PCBWay, etc.).
For software setup, refer to the OpenWRT configuration in /firmware/openwrt. A pre-built image can be flashed directly to the CM5, for other compute boards for complete instructions see the OpenWRT repository here.
A full BOM with part numbers, suppliers, and approximate costs will be available in docs/bom.md. The design prioritises component availability and flexability, all critical components can be sourced through standard distributors (Farnell, Mouser, DigiKey) or can be substituted entirely dependent on usecase.
Contributions are welcome, and encoraged! The next steps I will be persuing in this project are towards:
- Additional communications module designs (802.11ah/af, LoRa, SDR breakouts)
- OpenWRT package configurations and deployment tooling
- Automated module detection via the I2C GPIO expander
- Improved crowbar and voltage protection circuitry
Please open an issue before starting significant hardware changes to avoid duplicating work in progress.
Hardware designs are released under CERN OHL-S v2. Software is released under MIT. See individual files for details.
Developed at the University of Southampton, School of Electronics and Computer Science, under the supervision of Dr Ivan Ling. Thanks to Mohammad Soorati for hardware access and involvement in the XPRIZE Wildfire Competition, Joshua Curry for cellular communications expertise, and Ryan Gupta, James Thompson, and Luis Sentis at UT Austin for real-world testing as part of the Flare-X team.