Reprogramming Vaisala RS-41 Radiosondes to Transmit APRS, RTTY, CW in the Ham or ISM Bands

Radiosondes are light weight sensor packages that are attached to weather balloons. They transmit live RF weather telemetry down to earth as they rise. With an RTL-SDR and appropriate antenna it can be possible to decode this telemetry. One related hobby that a few people enjoy is radiosonde chasing, which is tracking and collecting radiosondes once they have fallen back to the earth. Some people collect them as trophies, and others like to repurpose them. For example in this previous post we've seen how some radiosondes can be repurposed into L-band antennas for RTL-SDR's.

Another way to repurpose radiosondes has recently been submitted to us by regular contributor 'happysat' who wrote in and let us know that it is actually possible to reprogram the commonly used Vaisala RS-41 radiosondes into being able to transmit ham radio APRS, RTTY or CW mode signals in the ISM or ham bands. The initial hack was first performed by SQ5RWU, and then OM3BC who managed to create easier to use software that could reflash the radiosondes internal firmware via the serial port on the radiosonde. This hack could be useful for any ham requiring a cheap transmitter for their own high altitude balloon experiments.

Happysat ended up testing this software with some RS-41 radiosondes that he had, and managed to receive some generated signals with an RTL-SDR of his. Some photos that he's submitted are shown at the end of this post.

In addition to the above, happysat also wanted to mention his other radiosonde re-purposing project which was turning a DFM-06 and DFM-09 into a functional GPS unit that could be used for navigation when connected to a laptop, or to sync time on PCs.

Transmit APRS
PE2BZ's Modded RS41 with Solar Panels
RS41 Programmer
Transmit APRS PE2BZ's Modded RS41 with Solar Panels RS41 Programmer
Notify of

Inline Feedbacks
View all comments
Paul Wagner

So all/most of the sensors will still work with the custom firmware? Because the TELEMETRY command isn´t really clear to me. I want tot reprogram a RS-41-SGP, but it wouldn´t make much sense if the dataset isnt´t complete (PTU and GPS signals sent). Does anyone know for sure?


The DFM09 is obsolete there is another chip on the Sonde and i cannot connect over uart at this time.
There comes data scrap, the new chip is JN3 Flash. I had no time for this Sonde at the moment, but in a few days i will see what i can do with the GPS Chip and the Datasheets. The quick and dirty way has just become a chance,
for now The Software save Problem is first.


I had tested it just quick with 5 Sondes i can programm it and connect via UART. I give all data from me in ram and then i wrote SAVE but here are the problem, it doesent save “cannot save parameters” but why?
does anyone know the clue?



As i know, the RS-41 mainly used in Europe, in my country (Hungary) pretty sure. The output much less as specified, in practice appr. 80mW. The local HAM paperpress in hungary, detailed article these sondes, besides my never found a falled . . . Maybe sometimes !


I imagine one could go directly to Vaisala sales, but is there a secondary market / retailer for these?
Radiosondes seem like a rather niche market.


If you get into sonde chasing you’ll end up with a shelf full of them pretty quickly…


Do they launch RS41’s in the US? I live near a launch site and see signals on 1680mhz that i can decode with sondemonitor.