Vaisala RS-41 SGP Modification.

Viasala RS-41

Radiosondes are light weight sensor packages that are attached to weather balloons.
They transmit live RF weather telemetry down to earth as they rise.
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.

Another way to repurpose radiosondes is 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.

Hardware Details

Vaisala RS-41 Details:

The radiosonde RS41-SG was introduced by Vaisala in 2013.
On Vaisala's website it is possible to find very detailed specifications of the RS41-SG.

Needed Stuff

    RS-41  SGP                  

USB-TTL converter.
Some wires.
Soldering tools.

RS-41 Connector

RS-41 Connector:


1 - GND

2 - Uart3 Rx

3 - Uart3 Tx

4 - PB1 * (10k + cap + 10k)

5 - Vcc (Boost out) 5.0V

6  VBAT 3.3V

7 - RST



10- GND

ST-Link STM32 Connection

For this modification, we did shorten the platine by cutting of just before the 4 tiny connection holes.

Connect to ST-LINK STM32:


RS41 ----- ST-LINK


Pin 1 ----- GND

Pin 8 ----- SWCLK

Pin 9 ----- SWDIO = SWID



Download ST-LINK STM32 Software

Download Hex file for flashing from

Real flight reports GPS outage for shorts periods during flight if unwanted please use:

Software from SQ9MDD

Settings and Config for this software must be written before flashing no terminal connection possible.

Program and Flashing                                                      

Connect RS-41 and power on.

st link

Load hex file and program RS-41.

This message can happen:

Can not read memory!

Read out protection is activated

Disable Read Out Protection and retry.

In the target menu/option bytes/ disable bytes read out protection.

Terminal Connection

Disconnect RS-41 and ST-LINK STM32.

Connect to TTL-USB converter.

USB - TTL converter:

RS41 ------ USB-TTL

Pin 1 ----- GND

Pin 2 ----- TxD

Pin 3 ----- RxD

Pin 6 ----- 3.3V


Build together ->

Connect with USB cable to computer.

Select COM Port (most probaly Prolific driver is needed on Windows) .

Start (hyper)terminal like Putty and connect to serial COM Port and use 9600 Baud.

Power on RS-41, Welcome message should appear:


$$$$$$ STM32 RTTY & APRS tracker by OM3BC ...

Allowed commands (not case sensitive):

BUTTON ON / OFF - use button to turn off / this parameter specifies whether the push button can be used to turn off the radiosonde or not.
LEDs on / off - use LEDs / you can save energy when you do not use LEDs. After switching on, the LEDs always work, but when set to OFF, they automatically turn off after 10 minutes of operation.
POWER n - rf power n = 1 to 7 (7 is max.) / output power setting 0 = smallest, 7 = maximum power (approx. 40 mW).
APRSFRQ n - n = aprs frequency in kHz / APRS frequency (recommended frequency is 432,500 MHz)
RTTYFRQ n - n = rtty frequency in kHz / RTTY frequency. This frequency is also valid for CW identification.
APRSCALL string - aprs callsign (up to 6 characters)
RTTYCALL string - rtty callsign (up to 15 characters)
CWIDMESS string - cw message (up to 25 characters)
RTTY on / off - send rtty message
HOLDOFF n - n = time between two rtty messages in seconds
BAUD n - n = rtty baudrate
DBITS n - n = rtty databits (7 or 8)
SBITS n - n = rtty stop bits (1 or 2)
SHIFT n - n = 1,2,3,4 1 = 270, 2 = 540, 3 = 810, 4 = 1080 Hz
TEMP on / off - send temperature in rtty messages / (The value is the temperature of the radio chip, not the environment.)
ALT on / off - send altitude in rtty & aprs messages
SPEED on / off - send speed in rtty messages
COURSE ON / OFF - send course in rtty messages
UBAT on / off - send battery voltage in rtty messages
USYS on / off - send system voltage in rtty messages / This is a constant value for testing only.
SAT on / off - the number of GPS satellites heard in rtty messages
APRS on / off - send aprs messages
SPEEDCOURSE on / off - send speed and course in aprs messages
SYMBOL string - symbol from aprs symbol table (2 characters) / two characters that determine how the radiosonde appears on the website.
SSID n - aprs ssid n = 1 to 15 /
the caller ID of the APRS.
TXD n - Tx delay n = 10 to 500
MICE on/off - send coded aprs messages in mic-e format
TELEMETRY on/off - send telemetry data in aprs messages
APRS_EVERY n - time between aprs messages is n x holdoff / APRS packages are not required to be given too often. This parameter specifies the time between the two packets.
TAIL_EVERY n - time between tail text is n x aprs time / frequency of transmission of the attached information text (comment field).
TTEXT string - tail text (up to 100 characters) / attached information text (comment).
CWID on/off - send cwid messages
CWID_EVERY n - time between cw messages is n x holdoff
CW_SPEED n n = the CW identification speed in WPM.
IGATE on/off - monitoring aprs message via UART / after the parameter has been enabled, the radiosonde sends a text through the serial port that the iGate can link to.
NMEA on/off - send MNEA GPGGA and GPRMC messages via UART / If the parameter has been enabled, the radiosonde sends standard NMEA GPGGA and GPRMC text via the serial port.
DISP - shows the set parameters.
SERCOM n - speed of serial communication port (n = 300 to 115200)
DEF - set parameters to default values.
SAVE - save parameters to flash

If you need help, you can use the HELP or the ? command.


RS-41 Sending APRS Decode with Soundmodem.


RS-41 Sending RTTY Decode with FLdigi.


APRS Position showing in APRSIS32.


Connect it with Solar panels like PE2BZ did and launch it :)




UZ7HO SoundModem

FLdigi Appimage

FLdigi Windows

ST-LINK STM32 / USB-TTL Converter can be found on Ali/Ebay.