Subsystems

Overview

The FSW interfaces with satellite hardware through dedicated subsystem modules in the subsystems/ directory. Each module encapsulates communication with a specific hardware component.

subsystems/
├── environmental.py   # RP2040 sensor MCU (UART)
├── obc.py             # On-board computer functions (psutil)
├── payload.py         # Camera capture and image handling
├── solar.py           # INA219 solar panel sensors (I2C)
├── eps.py             # ATmega328PB EPS (bit-bang UART)
└── retransmit.py      # Packet retransmission from .chunked files

Environmental Subsystem

File: subsystems/environmental.py

Manages communication with the RP2040 sensor MCU via hardware UART.

Connection Parameters:

Parameter Value
Port /dev/serial0
Baud Rate 9600
Timeout 1 second

Class: SerialConnection

The send_command() method handles all RP2040 communication:

  1. Flushes input/output buffers
  2. Sends command as uppercase string + \n
  3. Waits 50ms for processing
  4. Reads response with 2-second timeout
  5. Parses response using ast.literal_eval()
  6. Retries up to 3 times on failure

Fire-and-forget commands (MORSE, RESET) skip the response wait.

Data Flow:

Pi Zero                          RP2040
  │                                │
  ├──── "GET_GYRO\n" ────────────►│
  │                                ├── Read BNO055 sensor
  │◄──── "[1.23, -0.45, 0.78]\n" ─┤
  │                                │
  ├──── ast.literal_eval() ───►  [1.23, -0.45, 0.78]

Placeholder: UART wiring between Pi Zero and RP2040

On-Board Computer (OBC)

File: subsystems/obc.py

Provides system information using psutil and manages image file operations.

Functions:

Function Returns Description
get_processes() string Comma-separated list of running process names
get_cpu_usage() float CPU utilization percentage
get_ram_usage() float RAM utilization percentage
get_disk_usage() float Disk utilization percentage
get_hostname() bytes Hostname padded/truncated to 11 bytes
list_files(dir) string Comma-separated file listing. Defaults to capture
reboot() — Executes sudo reboot

Image Processing Pipeline:

send_image(filename, chunk_size)
  └── process_all_chunks(filename, chunk_size)
        ├── base64_encode_chunks()  ← Generator: reads raw file in chunks
        ├── Format: [base64_data][total:04d][current:04d]
        ├── Writes .chunked file for retransmission
        └── Returns list of packet strings

Chunk Sizes by Radio:

Radio Raw Chunk Base64 + Metadata Total Payload
RFM69 33 bytes 44 chars + 8 digits ~52 bytes
RFM95 168 bytes 224 chars + 8 digits ~232 bytes

Payload Camera

File: subsystems/payload.py

Class: PayloadCamera

Captures photos using the Raspberry Pi camera module and prepares them for transmission.

Capture Process:

  1. Execute rpicam-still -q 40 -o capture/{count}.jpg
  2. Gzip compress the JPEG
  3. Create .chunked file for transmission

Placeholder: Sample captured image in ground station

Solar Panel Monitoring

File: subsystems/solar.py

Reads voltage and current from four INA219 current sensors on the I2C bus.

Sensor Mapping:

I2C Address Panel Physical Location
0x40 X- Negative X face
0x41 X+ Positive X face
0x44 Y+ Positive Y face
0x45 Y- Negative Y face

Configuration: 12-bit ADC resolution, 16V bus voltage range.

Return Format: 8 float values — [V1, I1, V2, I2, V3, I3, V4, I4]

If a sensor is unavailable, it returns 0.000 for both voltage and current.

Placeholder: Solar panel INA219 sensor wiring

Electrical Power System (EPS)

File: subsystems/eps.py

Communicates with the ATmega328PB EPS controller via bit-banged UART using pigpio.

Connection Parameters:

Parameter Value
TX GPIO 27
RX GPIO 22
Baud Rate 9600
Library pigpio (software UART)

Protocol:

Commands are sent as &BMS [COMMAND]\n. Responses are comma-separated values parsed as [int, int, int, int, int, float] representing error code, 4 channel states, and battery voltage.

Pi Zero                        ATmega328PB
  │                                │
  ├── "&BMS STATUS\n" ────────────►│
  │◄── "0,1,1,0,1,3.72\n" ────────┤
  │                                │
  └── Parse: error=0, ch1=ON, ch2=ON, ch3=OFF, ch4=ON, batt=3.72V

Channel Control:

EPS CH1,1    → Turn channel 1 ON
EPS CH3,0    → Turn channel 3 OFF

Retransmission

File: subsystems/retransmit.py

Reads specific line numbers from .chunked files to support packet retransmission. When the ground station detects missing packets after an image transfer, it sends a RETRANSMIT command with the filename and packet numbers.

RETRANSMIT 5.jpg.gz.chunked 3 7 15 22

The retransmit module reads lines 3, 7, 15, and 22 from the .chunked file and returns them as individual packets.