Receiving Data from Re-Programmed RS41 Radiosondes

At EMDRC (Eastern & Mountain District Radio Club), our primary payload for the High Altitude Balloon are re-programmed radiosondes. Radiosondes are battery-powered telemetry instrument packages that are carried into the atmosphere typically by a weather balloon primarily used for purposes of measuring altitude, pressure, temperature, relative humidity, wind (both speed and direction), and cosmic ray readings at high altitudes. One of the most popular type of radiosondes is the Vaisala RS41. Check out the insides of a Vaisala RS41 in the following video.

A look inside the Vaisala RS41 radiosonde

At EMDRC our High Altitude Balloon initiative will be using reprogrammed RS41 radiosondes to send telemetry back to the ground. These RS41 radiosondes have been flashed with the open-source RS41ng firmware and make use of the Horus Binary 4FSK mode written by David Rowe and Mark Jessop (VK5QI).

The Horus Binary provides a robust telemetry mode designed specifically for high-altitude balloon tracking, and allows for reliable reception at many hundreds of km range with a modest receiver setup.

If you’d like to be able to decode these signals first, you will need some kind of receiver for whatever frequency the telemetry is being transmitted on. Usually this is within the 434 MHz ISM / LIPD / 70cm band, but could be different.

You need a receiver capable of receiving Single-Sideband, in particular the ‘upper’ sideband or USB. This could be a conventional amateur radio transceiver (something like the IC-7000, IC-706, FT-817, the list goes on and on…), or a scanner (Icom IC-R10, Yupiteru MVT-7100, etc.).

If you don’t have a UHF SSB receiver lying around another option is use an SDR dongle like the RTL-SDR along with software such as Horus GUI on windows or the “horusdemodlib” if using the RTLSDR with the Raspberry Pi.

1. Horus Gui on Windows : The easiest way to demodulate telemetry is using the Horus-GUI software. A detailed guide for setting up the Horus-GUI can be found here –
2. Raspberry Pi Headless : There’s other options including running it with a Raspberry Pi headless–Raspberry-Pi-‘Headless’-RX-Guide

If you don’t want to go to the trouble of setting up a Raspberry Pi from scratch the EMDRC High Altitude Balloon team have created a ready-to-go Raspberry Pi image with all the necessary software setup and configured to decode telemetry. All you will need to do is download the image, flash it to an SD card, put it in a Raspberry Pi.

Here is a link to download the prepared image –

If you run into issues and would like some assistance please drop us a note at trevor at hack2 dot live.