iMet-54 Receiver / Decoder

ESP32-based receiver and decoder for iMet-54 radiosondes (weather balloons), using the CC1101 433MHz transceiver module. Captures live telemetry (GPS position, temperature, humidity) and serves it via a built-in WiFi web interface.

No SDR required works with cheap CC1101 modules. Also includes a transmitter clone for testing without a real radiosonde.

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Hardware Required

Component	Specs / Notes
MCU	ESP32 (DevKit v1 or similar)
RF Module	CC1101 (ELECHOUSE or compatible)
Antenna	402 MHz tuned (¼-wave-18.7 cm)
Power	5V USB or 3.3V regulator

Wiring (VSPI)

CC1101 Pin	ESP32 GPIO	Function
SCK	18	SPI Clock
MISO	19	SPI Data In
MOSI	23	SPI Data Out
CS	5	Chip Select
GDO0	4	Serial Data Output
GDO2	2	Serial Clock Output
VCC	3.3V	Power
GND	GND	Ground

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Two Modes: Receiver + Transmitter

This repo contains two sketches use whichever fits your needs:

Sketch	Purpose	Use Case
iMet-54_RX.ino	Receiver / Decoder	Track real iMet-54 sondes live
iMet-54_TX.ino	Transmitter / Clone	Test your receiver without a real sonde

TX Mode

The transmitter lets you simulate a radiosonde for bench testing:

• Configurable frequency, GPS position, time, temperature, humidity
• Transmits one valid iMet-54 frame per second
• Web config portal at 192.168.4.1

To use:

1. Flash iMet-54_TX.ino to a second ESP32+CC1101
2. Connect to WiFi AP iMet54-Config / imet1234
3. Set your test parameters (freq, lat/lon, temp, etc.)
4. Start the receiver on the same frequency — frames should appear instantly

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Quick Start

1. Install libraries via Arduino Library Manager:

   • ELECHOUSE_CC1101_SRC_DRV
   • Built-in: WiFi, WebServer, Preferences (ESP32 core)

2. Upload iMet-54_RX.ino to your ESP32.

3. Connect to WiFi AP:

   • SSID: iMet54-RX
   • Password: imet1234

4. Open http://192.168.4.1 in your browser.

5. Start RX — default frequency is 402.000 MHz. Use the RSSI Scan to find your local sonde if needed.

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Features

Live Decoding

• GFSK @ 4800 baud, 8N1 serial framing
• Sync word: 0x24 0x24 0x24 0x24 0x42
• Hamming[8,4] FEC with error correction tracking
• Dual CRC validation (custom LFSR + CRC32/802.3)

Web Dashboard

• Real-time SSE frame streaming
• RSSI spectrum scan with click-to-tune
• GPS track display (lat/lon/alt)
• PTU data: temperature, relative humidity, sensor diagnostics
• Export: CSV, KML
• Console log with colour-coded hex dump

Data Fields

Field	Bytes	Format	Notes
Serial Number	0x00–0x03	uint32	Decimal string
GPS Time	0x04–0x07	int32	HHMMSS.mmm
Latitude	0x08–0x0B	int32	DDMM.mmmm × 1e6 → decimal degrees
Longitude	0x0C–0x0F	int32	DDDMM.mmmm × 1e6 → decimal degrees
Altitude	0x10–0x13	int32	0.1 m units
Temperature	0x1C–0x1F	float32	°C
RH Raw	0x20–0x23	float32	Sensor raw value
RH Temp	0x24–0x27	float32	°C
Status	0x2A–0x2B	uint16	Status word
CRC32	0x34–0x37	uint32	CRC32/802.3

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Protocol Details

Frame Encoding Pipeline

Raw RF → 8N1 de-framing → 1728 bits
       → De-interleave (8×8 transpose per block)
       → Hamming[8,4] decode (216 codewords → 108 bytes)
       → CRC check → Parse fields

GPS Coordinate Conversion

raw = DDMM.mmmm × 1,000,000   (big-endian int32)
deg = trunc(raw / 1e6)
min = frac(raw / 1e6) × 100
decimal_degrees = deg + (min / 60)

Example: 0xFE7A2BF6 = -25,547,786 → -25.91298°

Humidity Calculation

Uses Hyland-Wexler saturation vapour pressure formula:

RH% = (RH_raw × SVP(T_rh_sensor)) / SVP(T_air)

Clamped to 0–100%.

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Architecture

Task	Core	Priority	Purpose
loop()	1	1	Web server, SSE push, scan trigger
decode_task	0	2	Bit sync, frame decode, ring buffer drain
rx_isr()	Any	ISR	Sample GDO0 on GDO2 rising edge

• Ring buffer: 16 kbit circular queue (ISR → decoder)
• Frame buffer: 100-frame circular store (auto-overwrite oldest)

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API Endpoints

Endpoint	Method	Description
/	GET	Main dashboard (HTML)
/events	GET	SSE stream (JSON frames)
/status	GET	JSON: {decoding, freq, frames, scanning}
/set_freq?f=402.000	GET	Start RX on frequency (MHz)
/stop_rx	GET	Stop RX, idle radio
/scan_start?start=399&stop=402&step=50	GET	Start RSSI scan (kHz step)
/scan_data	GET	JSON scan results: {running, points:[{f, r}]}

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Troubleshooting

Problem	Likely Cause	Fix
No frames decoded	Wrong frequency	Run RSSI scan, check local sonde frequency
CRC always [NO]	Weak signal / interference	Check antenna, move away from noise sources
ECC errors high	Marginal signal	Improve antenna, check coax
Web UI won't load	Not connected to AP	Join iMet54-RX WiFi network first
Serial output garbled	Baud mismatch	Set serial monitor to 115200

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Credits

• Protocol reverse-engineered and verified against rs1729/RS reference implementation (imet54mod.c).
• Hamming tables, CRC polynomials, and sample frames cross-checked for bit-exact compatibility.

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License

MIT — use at your own risk for educational and research purposes. Not certified for operational meteorological use.