The libhorus FSK modem used by Horus Binary supports the following signal types:
Horus Binary v1 Mode (4-FSK)
Horus Binary v1 packets take the form:
<preamble><unique word><payload>
where
<preamble> = 0x1B1B1B1B
<unique word> = 0x2424
The payload consists of a 22-byte long binary packet, encoded with a Golay (23,12) code, and then interleaved and scrambled, for a total encoded length of 43 bytes. The binary packet format is available here, and the golay-encoding/interleaving/scrambling is performed by horus_l2_encode_packet.
At the start of a packet is a Payload ID (one byte). A lookup table for payload IDs is located here. If you are going to fly your own payload using this mode, you must get a payload ID allocated for your use. This can be done by submitting an issue or a pull request to this repository, or e-mailing me at vk5qi (at) rfhead.net
Packets are then transmitted using 4FSK modulation, at 100 baud. Symbols are encoded 'naturally', not grey-coded, i.e. symbol 0 = 00, 1 = 01, 2 = 10, 3 = 11. The spacing of the four tones is not critical - as long as they are fully contained within the receiver passband the modem will find and demodulate the signal.
A worked example for generating and encoding these packets is available in the RS41HUP repository.
Horus Binary v2 Mode
The v2 mode works exactly the same way as the v1 mode, though with a 32-byte long packet.
RTTY (UKHAS-Standard Sentences)
UKHAS-standard telemetry sentences sent via RTTY can also be decoded by this modem, with slightly better performance than fldigi. This telemetry takes the general form:
$$CALLSIGN,frame_counter,HH:MM:SS,latitude,longitude,altitude,other,fields,here*CRC16\n
Note the use of at least two (2) '' symbols at the start of the sentence. This is used as a 'unique word' for packet detection, and must be present. Other quantities of ''s will result in the packet not being detected.
Only RTTY telemetry with the following parameters are supported:
- Baud Rate: Default 100, though this can be set arbitrarily with the --rate command-line argument.
- Tone Spacing: 150 to ~1 kHz will work
- Encoding:
rtty7n1= ASCII 7N1 (7-bit ASCII, no parity, 1 stop bits)rtty7n2=ASCII 7N2 (7-bit ASCII, no parity, 2 stop bits)rtty8n2=ASCII 8N2 (8-bit ASCII, no parity, 2 stop bits)
- CRC: CRC16-CCITT (as big-endian hex at the end of the sentence)
To enable RTTY demodulation, you will need to replace the MODE definition with rtty7n2 (or the desired mode) in the supplied startup scripts.
Note: The RTTY demodulator in horusdemodlib does not operate on a byte-by-byte basis like fldigi. Updates are only provided by the modem when a header and CRC are detected.
When parsing telemetry for uploading to SondeHub-Amateur, only the basic fields shown above (up to altitude) are parsed.
Usage - Horus Demod
The horus_demod binary accepts 48khz 16-bit signed-integer samples via stdin, and can decode either RTTY or the MFSK (binary) packets. Successfuly decoded packets are output via stdout, and debug information is provided via stderr. Refer to the GQRX RX Guide for dependency and compilation instructions.
Suitable audio inputs could be from a sound card input, or from a SDR receiver application such as GQRX.
The horusdemodlib.uploader python module will accept decoded packets from horus_demod, and upload them to the HabHub tracker, for display on a map. Uploading to Habitat can be inhibited using the --noupload option.
We can string these applications together in the command shell using 'pipes', as follows:
Demodulating from a Sound Card
$ sox -d -r 48k -c 1 -t s16 - | ./horus_demod -m binary -g --stats=5 - - | python -m horusdemodlib.uploader
The above command records from the default sound device.
Demodulating using rtl_fm
This assumes you want to use an rtl-sdr dongle on a headless Linux machine.
rtl_fm -M raw -F9 -s 48000 -p 0 -f 434645000 | ./horus_demod -q -m binary -g --stats=5 --fsk_lower=1000 --fsk_upper=10000 - - | python -m horusdemodlib.uploader
Tune 5 kHz below the expected centre frequency (i.e. 434.650 MHz - 5000 Hz = 434.645000 MHz = 434645000 Hz, as above), and make sure your dongle has a known PPM adjustment.
Note the use of the --fsk_lower and --fsk_upper options. These set the limits of the FSK demodulators internal frequency estimator, and has an effect similar to a band-pass filter. The modem will only detect signals within this range. These options take frequencies in Hz, and these must be positive frequencies.
Note that there is a start_rtlfm.sh script provided which does the calculations for the above.