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.

Mechanical Frame

CubeSat Assembly with PCBs

Complete CubeSat Assembly

Hardware
- Structure
- Electronics
  - Radio and OBC PCB
  - Solar Panel and Battery PCB
Software
- System Logic
- Implementation
Ground Station Communication

Hardware

Structure

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

Mechanical faces X+ X- Y+ Y-

Mechanical faces Z+ Z-

Solar panel

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

Architectural Diagram of the Radio and OBC PCB

Radio and OBC PCB

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.

FRAM — FM24CL64B

Parameter	Value
Memory	64-Kbit (8K × 8)
I2C Address	`0b1010000`
SDA (I2C3)	PC9
SCL (I2C3)	PA8
Write Protect	PC5

Sensors

Accelerometer, Gyroscope, and Magnetometer: LSM9DS1
Barometer: BMP388

Parameter	Value
I2C Address (ACC/GYRO)	`0b1101011`
I2C Address (MAG)	`0b0011110`
I2C Address (BARO)	`0b1110111`
SDA (I2C4)	PD13
SCL (I2C4)	PD12
INT_ACC/GYRO	PE8
DRDY_MAG	PE7
DRDY_BARO	PB2

RTC & Temperature Sensor

RTC: RV-8803-C7
TEMP: TMP100

Parameter	Value
RTC I2C Address	`0b0110010`
TEMP I2C Address	`0b1001000`
SDA (I2C1)	PB7
SCL (I2C1)	PB6
RTC_INT	PB5
RTC_EVI	PB4

CAN Interface: TCAN334G

Signal	Pin
CAN1_TX	PD1
CAN1_RX	PD0
CAN_SHDN	PA15
CAN_STBY	PD2

RS-485 Interface: ISL3170E

Signal	Pin
UART2_RX	PD6
UART2_TX	PD5
UART2_EN	PD4

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

Signal	Pin
UHF_SCK(SPI1)	PA5
UHF_MISO(SPI1)	P6
UHF_MOSI(SPI1)	P7
UHF_CS(SPI1)	PA4
UHF_GPIO2	PC5
UHF_GPIO3	PC4

VHF(TX/RX) 164-192 MHz

Signal	Pin
UHF_SCK(SPI4)	PE12
UHF_MISO(SPI4)	PE13
UHF_MOSI(SPI4)	PE14
UHF_CS(SPI4)	PE11
UHF_GPIO2	PB10
UHF_GPIO3	PE14

Solar Panel and Battery PCB

Solar charger PCB

Battery charger PCB

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

Parameter	Value
Input Voltage (Vin)	9V – 24V
MPPT Voltage (VMPPT)	9V
Charge Voltage	8.2V
Charge Current	1.2A
Precharge Current	125mA
Termination Current	125mA
INA219A I2C Address	`0b1000010`
SDA (I2C4)	PD13
SCL (I2C4)	PD12

Battery Pack & Protection

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

Parameter	Value
Nominal Voltage	3.60V
Capacity	3500mAh
Maximum Charge Voltage	4.20V
Energy	12.60Wh
Operating Voltage	4.20V – 2.50V
Cut-Off Voltage	2.50V
Standard Charge Method	CC-CV
Standard Charge Current	0.2C (0.7A)
Standard Charge Cut-Off Current	0.02C (0.07A)
Maximum Continuous Charge Current	1.0C (3.5A)

Standard Discharge Conditions

Temperature Range	Discharge Current
0°C – 10°C	0.1C (0.35A)
10°C – 20°C	0.2C (0.7A)
20°C – 45°C	0.5C (1.75A)

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

Parameter	Value
I2C Address (BQ76907)	`0b0001000` (default)
I2C Address (INA219A)	`0b1000000`
SDA (I2C1)	PB7
SCL (I2C1)	PB6
Battery Alert	PE2

Software

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

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

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

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

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)

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

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

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

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