Michele

AFSK - High-Altitude Balloon Challenge

🛰️ AFSK High‑Altitude Balloon Challenge

Bachelor’s Thesis – Electronics & Telecommunications (2013‑2014)

Designing and building an APRS‑based telemetry stack to track a stratospheric balloon at ~40 km altitude.

Project Overview

The AFSK High‑Altitude Balloon Challenge was an interdisciplinary endeavour combining embedded electronics, amateur‑radio communications, and atmospheric science. The goal was to launch a 10 cm cube payload to the stratosphere, transmit its live GPS coordinates over APRS (Automatic Packet Reporting System), and recover the balloon after burst.

Although regulatory constraints halted the actual launch, the payload hardware and firmware were fully realised and later repurposed as an educational kit for STEM workshops on RF telemetry.

Mission Objectives

  • Real‑time tracking of altitude, latitude, longitude and temperature up to ~40 km.
  • Operate entirely within amateur‑radio VHF legal limits (144.800 MHz, 10 mW EIRP).
  • Achieve an airtime autonomy of ≥ 4 hours with Li‑Po batteries at −50 °C.
  • Provide an open‑source reference design for students and hobbyists.

System Architecture

┌──────────────┐        1200 baud AFSK          ┌──────────────┐
│   Payload    │  ···························▶ │    Ground    │
│ (Cube + RF)  │                             │ (SDR + PC)   │
└──────┬───────┘                               └─────┬────────┘
       │ GPS / I²C                                  │ USB
       ▼                                           ▼
┌──────────────┐                          ┌──────────────────┐
│   NEO‑6 GPS  │                          │ Digipeater / IGate│
└──────────────┘                          └──────────────────┘

Data path: NEO‑6 GPS → Arduino Nano → AX.25 framing → AFSK modulator → VHF transmitter → SDR/receiver → Dire Wolf → Xastir.

Hardware Design

ModuleComponent(s)
MicrocontrollerArduino Nano (ATmega328P @ 16 MHz)
RF ChainRadiometrix HX1 144.800 MHz FM transmitter, LPF, ¼‑wave whip
ModulationCustom 1200 baud Bell‑202 AFSK generation via DAC + RC filter
Positioningu‑blox NEO‑6M GPS (UART, 1 Hz)
Power2 × 18650 Li‑Ion + TPS61090 boost, NTC for battery temp
PCB2‑layer FR‑4, KiCad 5, 50 × 50 mm

Firmware & Protocols

  • Language: Arduino C++ (AVR‑GCC) – written for ≤ 16 kB flash.

  • APRS Stack:

    • NMEA parsing ($GPGGA, $GPRMC).
    • AX.25 UI frame construction with CRC‑16.
    • NRZ‑I bit‑stuffing and RZ shaping.
    • 1200 baud AFSK tone generation (1200 Hz ≈ ‘mark’, 2200 Hz ≈ ‘space’).

  • Power Management: Deep‑sleep between beacons (adjustable 30–120 s).

  • Reliability: FEC via duplicate frames every 5th beacon; watchdog at 4 s.

Ground Segment

  1. Receiver: RTL‑SDR v3 + quarter‑wave ground‑plane.
  2. Modem: Dire Wolf v1.7 in KISS TNC mode.
  3. Mapping: Xastir or YAAC for live map.
  4. Optional IGate: Forward frames to APRS‑IS for global tracking.

Testing & Validation

  • Thermal Chamber: −50 °C to +60 °C, 3 h cycles – battery & oscillator drift.
  • Vibration: 15–55 Hz sweep, 5 g – connector and solder‑joint integrity.
  • Range Tests: Ground‑to‑ground LOS @ 1 mW → > 35 km decode using hilltop.
  • EMC: Conducted emission ≤ −36 dBm @ 144 MHz (CISPR‑11 class B).

Key Outcomes & Lessons Learned

  • Hands‑on APRS/AX.25: From bit‑stuffing to CRC‑16 and NRZ‑I.
  • Low‑temperature design: Li‑Ion derating and quartz drift mitigation.
  • Modular pedagogy: Payload now serves as a plug‑and‑play classroom demo, where students demodulate prerecorded AFSK and plot flight paths.

Future Work

  • Integrate LoRa at 433 MHz as an alternative downlink.
  • Add environmental sensors (pressure, humidity) and compress via Mic‑E.
  • Port firmware to STM32 for higher duty‑cycle and SD logging.

Made with curiosity and a bit of static.