Sahel pcb

SAHEL-CUBESAT: An Experimental Educational CubeSat

Project work in progress

This project aims to develop a complete experimental CubeSat for educational purposes. It is based on an STM32H7 MCU for data handling and uses two CC1200 radios for communication. The software is developed using a Zephyr-like RTOS.

Table of Contents

• Hardware

  • Structure
  • Electronics
    • Radio and OBC PCB
    • Solar Panel and Battery PCB

• Software

  • System Logic
  • Implementation

• Ground Station Communication

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Hardware

Structure

The structure was designed according to the specifications using Onshape. All printable components can be found in this directory.

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Electronics

KiCad is used for the electronic design. The hardware is divided into three parts. The first part includes the radio communication system and the OBC (On-Board Computer) for the CubeSat. The second part consists of a board integrating several sensors and a solar MPPT controller IC.

Radio and OBC PCB : Available in: hardware/sahel_pcb_main

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Components

MCU

STM32H7B0VBTx
High-performance Arm Cortex-M7 MCU with DSP and DP-FPU, 128 KB Flash, 1,376 KB SRAM, up to 280 MHz.

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FRAM — FM24CL64B

Sensors

Accelerometer, Gyroscope, and Magnetometer: LSM9DS1
Barometer: BMP388

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RTC & Temperature Sensor

RTC: RV-8803-C7
TEMP: TMP100

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CAN Interface: TCAN334G

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RS-485 Interface: ISL3170E

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Radio Interface: CC1200

The CC1200 device is a fully integrated single-chip radio transceiver designed for high performance at very low-power and low-voltage operation in cost-effective wireless systems frequency bands at 164–192 MHz, 274–320 MHz, 410–480 MHz, and 820–960 MHz.

UHF(TX/RX) 420-470 MHz

VHF(TX/RX) 164-192 MHz

Solar Panel and Battery PCB

In this project, the BQ24650 solar charger is used together with an INA219A current sensor for photovoltaic (IPV) monitoring.

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Battery Pack & Protection

The system uses four RS PRO Li-Ion batteries configured in a 2S2P arrangement with the following specifications:

Standard Discharge Conditions

The battery protection and monitoring system is based on the BQ76907 battery monitor IC and an INA219A current sensor.

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Software

The software is developed using Zephyr RTOS.
The middleware architecture is described in the Software Documentation.

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System Logic

We are developing the system based on Zephyr RTOS, using a multi-threaded architecture combined with Zephyr zbus for internal communication.

The application is structured around multiple independent threads that communicate through the zbus message bus system.

Key concepts:

• Each module acts as a zbus publisher and/or subscriber
• Data is exchanged through well-defined channels
• This avoids tight coupling between components
• Improves scalability and modularity of the system

Benefits of using zbus:

• Decoupled architecture between threads
• Clean publish/subscribe communication model
• Easier debugging and tracing of data flow
• Better maintainability for complex systems

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Ground Station Communication

The ground station software is not yet implemented.
The current plan is to develop it using either:

• Qt-based applications (for a custom GUI control station)
• or existing SDR software tools for signal reception and analysis

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Communication Mechanism

The communication between the system and the ground station is designed around a structured and layered protocol stack.

1. Physical & Link Layer

The system uses the AX.25 protocol as the primary data link layer.

AX.25 provides:

• reliable packet framing
• addressing capability (callsigns)
• error detection (CRC)
• compatibility with amateur radio / SDR systems

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2. Packet Structure

All exchanged data is encapsulated in AX.25 frames containing:

• payload data
• message type identifier
• optional sequence number
• checksum (handled by AX.25 layer)

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3. Command System

The ground station can send commands to the embedded system.

Commands are structured as:

• command ID
• optional parameters
• target module (if applicable)

Examples of command types:

• system control (start / stop / reset)
• configuration update
• request sensor data
• debug / diagnostics requests

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4. Internal Routing (zbus integration)

Incoming and outgoing messages are bridged into the internal zbus architecture.

Flow:

• AX.25 frame received → decoded → converted to internal message
• message published on zbus channel
• relevant modules subscribe and react

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5. Response System

The system responds using structured messages:

• ACK / NACK for command validation
• telemetry data packets
• event notifications
• status reports

All responses follow the same pipeline: zbus → encoder → AX.25 → ground station

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6. Design Goals

This architecture is designed to ensure:

• robustness over noisy RF links
• modular separation between communication and logic
• easy integration with SDR tools
• future compatibility with different ground station implementations