firelink

Flight computer and ground station for the Firelink avionics flown on the Scarecrow and Banshee rockets (MACH 24/25).

• Flight computer (C++, Raspberry Pi Zero 2 W): pre-flight WiFi configuration page, web arming, high-rate sensor logging and LoRa telemetry. Lives at the repo root (src/, include/).
• Ground station (Python, macOS/Linux laptop): receives the LoRa downlink from a second HAT over USB, shows a live terminal dashboard + web map, logs everything, exports GPX/KML. Lives in groundstation/.
• The radio contract between them is documented in docs/telemetry_protocol.md.

Hardware

Device	Bus	Address / pin (defaults, all configurable)
GTop PA1010D GPS	I2C	/dev/i2c-1 @ 0x10
LSM6DS3/33 IMU	I2C	0x6A
BME280 env sensor	I2C	0x76
Gravity Geiger counter (SEN0463)	GPIO pulse	BCM 17
Buzzer	GPIO	BCM 4
Waveshare SX1262 868 MHz LoRa HAT	UART + GPIO	/dev/serial0, M0/M1 = BCM 22/27

How it works

1. Power on → firelink.service starts the flight binary, the Pi's WiFi access point (firelink) comes up, sensors self-test, GPS begins its warm-up.
2. Pre-flight → connect a phone/laptop to the Pi's WiFi and open http://10.42.0.1. The page shows live sensor status and lets you edit settings (sampling rates, radio channel, GPIO pins, ...), recalibrate pad pressure, and arm the rocket.
3. Armed → arming requires typing the 4-digit code freshly generated on the web page each load (anti-accidental-arm interlock). Once armed, all sensors stream to timestamped CSVs under data/<session>/ (columns match firelink-py so existing post_flight/ analysis keeps working) and GPS + flight telemetry is transmitted over LoRa. While waiting on the pad the CSVs are written at a decimated pad rate (rec.prelaunch_hz); when a launch is detected, the last rec.launch_buffer_s seconds are back-filled at full rate from a RAM ring buffer and recording continues at full rate, so the launch is never lost and long pad holds don't bloat the files. Every row is timestamped, so the pad-rate → full-rate boundary is visible in the row spacing (and events.log records the moment explicitly).
4. Landed → detected automatically (launch must first be latched, then IMU and baro quiet for land.window_s seconds); the buzzer plays, CSV recording drops to rec.landed_hz, and the radio drops to a low-rate GPS beacon for recovery. Walk up, rejoin the WiFi, disarm, and download the CSVs straight from the page.
5. Meshtastic → the portable Meshtastic node flown in the rocket is a fully independent position beacon (own power + RF); watch it in the usual Meshtastic app.

Pi setup (once)

sudo apt update && sudo apt install -y libgpiod-dev gpiod i2c-tools
sudo raspi-config   # Interface Options: enable I2C; enable serial port,
                    # DISABLE serial login shell
bash system/setup_ap.sh                       # creates WiFi AP "firelink"
sudo cp system/firelink.service /etc/systemd/system/
sudo systemctl enable --now firelink.service  # start at boot

Copy firelink.conf.example to firelink.conf to tweak persisted settings (or just use the web page once it's running).

Note: the buzzer/Geiger/LoRa GPIO code uses the libgpiod v1 API. Raspberry Pi OS Bullseye ships v1; on Bookworm install the v1 compatibility package or build libgpiod 1.6 from source.

Build & run

make            # builds ./firelink
make test       # host-side unit tests (no Pi hardware needed)
sudo ./firelink [firelink.conf]

or with CMake:

cmake -B build && cmake --build build

Ground station

See groundstation/README.md. Quick version:

cd groundstation
python3 -m venv .venv && source .venv/bin/activate
pip install -r requirements.txt
python -m firelink_gs --port /dev/tty.usbserial-XXXX --web

You can try it without any hardware: python tools/fake_transmitter.py creates a virtual serial port broadcasting a synthetic flight — point --port at it.

Repository layout

src/, include/        flight computer (C++)
  sensors/            gps, imu, bme280, geiger, nmea parser
  telemetry/          LoRa UART driver, 48-byte packet codec
  web/                embedded HTTP server + control page
  outputs/            buzzer (GPIO + PWM melodies)
groundstation/        Python ground station + test tools
system/               systemd unit + WiFi AP setup
tests/                host-runnable C++ tests (packet, nmea, config)
docs/                 telemetry protocol spec