Re: 4FSK at GPSL

Mark Jessop

Just as a bit of background information here - I developed this mode ('Horus Binary') a few years ago, as an attempt at producing a HAB telemetry mode that:
- Used a modem which was very robust,
- ... but would still provide a reasonable position update rate (every few seconds)
- ... did not require any proprietary modem ICs (e.g. LoRa), and
- ... would be completely open source.

I worked with David Rowe ( on this, and we ended up settling on 4-FSK, which was relatively easy to generate via a variety of means. We started out using RadioMetrix modules modulated using an ADC on an Arduino, but eventually moved to using the Si4032 transceiver IC found on the RS41 radiosondes as our reference platform. The occupied bandwidth of the 'reference' implementation is ~ 1kHz, which is mainly set by the minimum frequency shift possible on the Si4032 (270 Hz). In most cases the transmit power used is only 25mW.

The modem uses 4-FSK at 100 baud, with a binary telemetry format utilising Golay(23,12) forward-error-correction. The demodulator is available as graphical user interface utility ('Horus-GUI') which runs on Windows, Linux, and OSX, and also as a command-line utility perfect for 'headless' decoding.
At this baud rate, the modem is *extremely* robust. Reception of a 25mW transmitter is easily possible at ranges > 200km with nothing more than a RTLSDR and a 1/4-wave mag-mount, giving it a very low barrier to entry.

All the information on the mode is available here:
The current 'reference implementation' uses re-purposed Vaisala RS41 radiosondes, and software here:

Mark VK5QI

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