RS41ng - Amateur radio firmware for Vaisala RS41 radiosonde

NOTE: This firmware is a work in progress and some features might not work as expected yet!

This is a custom, amateur radio-oriented firmware for Vaisala RS41 radiosondes. Some of the code is based on an earlier RS41 firmware project called RS41HUP, but most of it has been rewritten from scratch. The Horus 4FSK code has been adapted from the darksidelemm fork of RS41HUP.

What are the Vaisala RS41 radiosondes and how can I get one?

The RS41 radiosondes are used extensively for atmospheric sounding by the meteorological institutes in various countries and thus easily available to be collected once they land, an activity called radiosonde hunting: see YouTube presentation about Tracking and Chasing Weather Balloons by Andreas Spiess or Chasing Radiosonde Weather Balloons used in Meteorology for Fun by Mark VK5QI for more details!

You can track radiosondes without any additional equipment either via SondeHub or radiosondy.info that both use an existing network of receiver stations. Alternatively, you can set up your own radiosonde receiver station.

For your own receiver station, you will need:

A cheap software-defined radio USB dongle, such as an RTL-SDR
An antenna suitable for receiving the 400 MHz radiosonde band transmissions. Antennas for the 70 cm amateur radio band usually work fine!
Radiosonde tracker software: common choices are RS41 Tracker for Windows and radiosonde_auto_rx for Linux / Raspberry Pi.

What can I do with an RS41 radiosonde?

The Vaisala RS41 radiosondes uses an off-the-shelf STM32F100C8 32-bit microcontroller, which can be reprogrammed using an ST-LINK v2 programmer or a smaller [ST-LINK v2 USB dongle]((https://www.adafruit.com/product/2548).

The RS41 hardware can be programmed to transmit different kinds of RF modulations (morse code, APRS and different FSK modulations) on the 70 cm (~433 MHz) amateur radio band. The radiosonde contains a UBX-G6010 GPS chip, so it can be used as a tracker device, e.g. for high-altitude balloons.

The RS41ng firmware is just one example of what can be achieved with the RS41 hardware!

Why does the RS41ng firmware exist?

The motivation to develop this firmware is to provide a clean, customizable and modular codebase for developing RS41 radiosonde-based experiments.

See the feature list below.

Features

The main features the RS41ng firmware are:

Support for multiple transmission modes:
- Standard 1200-baud APRS
- Horus 4FSK v1 and v2 modes that has improved performance compared to APRS or RTTY
- Morse code (CW)
- JT65/JT9/JT4/FT8/WSPR/FSQ digital modes on HF/VHF amateur radio bands using an external Si5351 clock generator connected to the external I²C bus
Support for transmitting multiple modes consecutively with custom, rotating comment messages (see config.c)
Support for GPS-based scheduling is available for transmission modes that require specific timing for transmissions
Support for custom sensors via the external I²C bus
Enhanced support for the internal Si4032 radio transmitter via PWM-based tone generation (and ultimately DMA-based symbol timing, if possible)
Extensibility to allow easy addition of new transmission modes and new sensors

Transmission modes

On the internal Si4032 transmitter:

APRS (1200 baud)
Horus 4FSK v1 and v2 (100 baud)
Morse code (CW)

On an external Si5351 clock generator connected to the external I²C bus of the RS41 radiosonde:

Horus 4FSK v1 and v2 (50 baud, because the Si5351 frequency changes are slow)
JT65/JT9/JT4/FT8/WSPR/FSQ mode beacon transmissions using the JTEncode library. I've decoded FT8, WSPR and FSQ modes successfully.
- GPS-based scheduling is available for modes that require specific timing for transmissions
Morse code (CW)

Notes about APRS

Bell 202 frequencies are generated via hardware PWM, but the symbol timing is created in a loop with delay
There is also code available to use DMA transfers for symbol timing to achieve greater accuracy, but I have not been able to get the timings working correctly

Notes about Horus 4FSK

The Horus 4FSK v1 and v2 modes have significantly improved performance compared to APRS or RTTY.
Use horus-gui software to receive the 4FSK mode and to submit packets to Habhub high-altitude balloon tracking platform.
See horus-gui installation and usage instructions and horusdemodlib library that is responsible for demodulating the signal.

External sensors

It is possible to connect external sensors to the I²C bus of the RS41 radiosonde.

The following sensors are currently supported:

Bosch BMP280 barometric pressure / temperature / humidity sensor

Sensor driver code contributions are welcome!

Planned features

Continuous transmission mode for Horus 4FSK
Configurable transmission frequencies and schedules based on location / altitude
Support for more I²C sensors
RTTY on both Si4032 (70 cm, non-standard shift) and Si5351 (HF + 2m) with configurable shift
Investigate possibility to implement 1200 bps Bell 202 modulation (and possibly also 300 bps Bell 103 modulation) for APRS using Si5351, this requires special handling to make Si5351 change frequency quickly
- See: etherkit/Si5351Arduino#22

Configuring the firmware

Configure your amateur radio call sign, transmission schedule (time sync), transmit frequencies and transmission mode parameters in config.h
Set up transmitted message templates in config.c, depending on the modes you use

Building the firmware

Software requirements:

GNU GCC toolchain version 8.3.0 or higher for cross-compiling the firmware for the ARM Cortex-M3 architecture (arm-none-eabi-gcc)
CMake version 3.6 or higher for building the firmware
OpenOCD version 0.10.0 or higher for flashing the firmware

On a Red Hat/Fedora Linux installation, the following packages can be installed:

dnf install arm-none-eabi-gcc-cs arm-none-eabi-gcc-cs-c++ arm-none-eabi-binutils-cs arm-none-eabi-newlib cmake openocd

Steps to build the firmware

Install the required software dependencies listed above
Build the firmware using the following commands
```
mkdir build
cd build
cmake ..
make
```
The firmware will be stored in file build/src/RS41ng.elf

Flashing the firmware

Hardware requirements:

A working Vaisala RS41 radiosonde :)
An ST-LINK v2 programmer for the STM32 microcontroller in the RS41 radiosonde.
- These smaller ST-LINK v2 USB dongles also work well.
- Partco sells the ST-LINK v2 USB dongles in Finland

The pinout of the RS41 connector (by DF8OE) is the following:

______________________|           |______________________
|                                                       |
|   1           2           3           4           5   |
|                                                       |
|   6           7           8           9          10   |
|_______________________________________________________|

(View from the bottom of the sonde, pay attention to the gap in the connector)

1 - SWDIO (PA13)
2 - RST
3 - MCU switched 3.3V out to external device / Vcc (Boost out) 5.0V -- This pin powers the device via 3.3V from an ST-LINK programmer dongle
4 - I2C2_SCL (PB10) / UART3 TX -- This is the external I²C port clock pin for Si5351 and sensors
5 - GND
6 - GND
7 - SWCLK (PA14)
8 - +U_Battery / VBAT 3.3V
9 - +VDD_MCU / PB1 * (10k + cap + 10k)
10 - I2C2_SDA (PB11) / UART3 RX -- This is the external I²C port data pin for Si5351 and sensors

Steps to flash the firmware

Remove batteries from the sonde
Connect an ST-LINK v2 programmer dongle to the sonde via the following pins:

SWDIO -> Pin 1
SWCLK -> Pin 7
GND -> Pin 5
3.3V -> Pin 3

Unlock the flash protection - needed only before reprogramming the sonde for the first time

openocd -f ./openocd_rs41.cfg -c "init; halt; flash protect 0 0 31 off; exit"

Flash the firmware

openocd -f ./openocd_rs41.cfg -c "program build/src/RS41ng.elf verify reset exit"

Power cycle the sonde to start running the new firmware

Developing / debugging the firmware

It is possible to receive log messages from the firmware program and to perform debugging of the firmware using GNU GDB.

Also, please note that Red Hat/Fedora do not provide GDB for ARM architectures, so you will need to manually download and install GDB from ARM GNU GCC toolchain.

Semihosting allows the firmware to send log messages via special system calls to OpenOCD, so that you can get real-time feedback and debug output from the application.

Semihosting has to be disabled when the RS41 radiosonde is not connected to the STM32 programmer dongle, otherwise the firmware will not run.

Steps to start firmware debugging and semihosting

Connect the RS41 radiosonde to a computer via the STM32 ST-LINK programmer dongle
Enable semihosting and logging in config.h by uncommenting the following lines
```
#define SEMIHOSTING_ENABLE
#define LOGGING_ENABLE
```
Start OpenOCD and leave it running in the background
```
openocd -f ./openocd_rs41.cfg
```
Start ARM GDB
```
arm-none-eabi-gdb
```

Connect GDB to OpenOCD for flashing and debugging (assumes you are in the build directory with Makefiles from CMake ready for build)

target remote localhost:3333
monitor arm semihosting enable
make
load src/RS41ng.elf
monitor reset halt
continue # this command runs the firmware

OpenOCD will output log messages from the firmware and GDB can be used to interrupt and inspect the firmware program.

To load debugging symbols for settings breakpoints and to perform more detailed inspection, use command file src/RS41ng.elf.

Si4032 Bell FSK modulation hack for APRS

The idea behind the APRS / Bell 202 modulation implementation is based on RS41HUP project and its "ancestors" and I'm describing it here, since it has not been documented elsewhere.

The Si4032 chip seems to support only a very specific type of FSK, where one can define two frequencies (on a 625 Hz granularity) that can be toggled by a register change. Because of the granularity, this mechanism cannot be directly used to generate Bell 202 tones.

The way Bell 202 AFSK is implemented for Si4032 is kind of a hack, where the code toggles these two frequencies at a rate of 1200 and 2200 Hz, which produces the two Bell 202 tones even though the actual frequencies are something else.

Additionally, the timing of 1200/2200 Hz was done in RS41HUP by using experimentally determined delays and by disabling all interrupts, so they won't interfere with the timings.

I have attempted to implement Bell 202 frequency generation using hardware DMA and PWM, but have failed to generate correct symbol rate that other APRS equipment are able to decode. I have tried to decode the DMA-based modulation with some tools intended for debugging APRS and while some bytes are decoded correctly every once in a while, the timings are mostly off for some unknown reason.

Currently, the Bell 202 modulation implementation uses hardware PWM to generate the individual tone frequencies, but the symbol timing is created in a loop with delay that was chosen carefully via experiments.

Debugging APRS

Here are some tools and command-line examples to receive and debug APRS messages using an SDR receiver. There are examples for using both rx_tools and rtl-sdr tools to interface with the SDR receiver. The example commands assume you are using an RTL-SDR dongle, but rx_fm (from rx_tools) supports other types of devices too, as it's based on SoapySDR.

Dire Wolf

Dire Wolf can decode APRS (and lots of other digital modes) from audio streams.

rx_tools:

rx_fm -f 432500000 -M fm -s 250000 -r 48000 -g 22 -d driver=rtlsdr - | direwolf -n 1 -D 1 -r 48000 -b 16 -

rtl-sdr:

rtl_fm -f 432500000 -M fm -s 250k -r 48000 -g 22 - | direwolf -n 1 -D 1 -r 48000 -b 16 -

SigPlay

SigPlay is a set of tools for DSP and signal processing. SigPlay also includes a command-line tool to decode and print out raw data from Bell 202 encoding, which is really useful, as it allows you to see the bytes that actually get transmitted -- even if the packet is not a valid APRS packet!