Repurposing Vaisala RS41 radiosondes for amateur radio high-altitude balloon tracking

Repurposing radiosondes for amateur radio high-altitude balloon tracking was a presentation I gave at VUSHF2022, the 42nd Nordic V/U/SHF meeting held between 26th and 29th of May 2022 at Kristalliranta, Säkylä, Finland. The presentation describes the hardware of Vaisala RS41 radiosondes, discusses why they are useful for amateur radio hobbyists and, finally, gives instructions on how to find your own radiosonde - an STM32 microcontroller board fallen from the sky! The presentation slides are available here for download.

Mikael OH3BHX during the presentation at VUSHF2022 (photo: Lasse OH3HZB)

Download the presentation slides as a PDF file.

With the new RS41ng amateur radio firmware I’ve developed, the RS41 radiosondes can transmit APRS, Horus 4FSK and CW modes without any hardware modifications. The recently developed Horus 4FSK mode performs very well and is specifically designed for tracking high-altitude balloons. Additionally, the firmware supports use of a BMP280 atmospheric sensors for sensor readings and a Si5351 clock generator to transmit on HF/VHF frequencies.

Web resources about reprogramming and tracking RS41 radiosondes:

Open-source amateur radio firmware for RS41
- RS41ng - more advanced firmware by Mikael OH3BHX: https://github.com/mikaelnousiainen/RS41ng
- RS41HUP - the original firmware: https://github.com/darksidelemm/RS41HUP
Tracking RS41 radiosondes to hunt and get one
- https://sondehub.org/
Setting up a receiver to track RS41 radiosondes
- https://github.com/projecthorus/radiosonde_auto_rx
- https://github.com/dl9rdz/rdz_ttgo_sonde
Useful sites about high-altitude balloons
- Project Horus / Amateur Radio Experimenters Group (AREG / VK5ARG)
  - Posts about latest flights and experiments: https://www.areg.org.au/archives/category/activities/project-horus
  - Older Project Horus posts: http://www.projecthorus.org/
- Dave Akerman: http://www.daveakerman.com/
- Overlook Horizons (check out their YouTube channel): https://www.overlookhorizon.com/

Here’s an embedded Tweet with a video showing a reprogrammed RS41 radiosonde transmitting multiple modes:

Added CW (morse) support to the RS41 firmware too! Now it's a quite feature-complete tracker for short balloon flights.#hamradio #hamr #hab #rs41 #cw #morsecode #aprs #4fsk pic.twitter.com/awTJEeuecS
— Mikael Nousiainen (@mikaelnou) August 14, 2021

You can also follow me on Twitter at @mikaelnou where I will tweet about radiosondes, high-altitude balloons and amateur radio in general!