RS Auto_RX JSON Formats

auto_rx uses rs1729's RS radiosonde decoders to decode radio telemetry, with the decoded telemetry passed into auto_rx as JSON objects, which are then interpreted. In the default use-cases, auto_rx starts up the decoders itself, and reads the JSON data from the decoder's stdout. In 'UDP mode', auto_rx listens for JSON objects via UDP packets, on port 50000.

This page attempts to describe the format of the JSON objects which are passed between the RS decoders and auto_rx. Note that this is different to the 'payload summary' JSON format emitted by auto_rx.

Overall Format Description

Telemetry is passed as JSON objects, with a range of mandatory and optional fields.

Mandatory Fields

These fields represent the bare minimum amount of data for a sonde position to be plotted on a map, enabling recovery.

Field	Type	Description	Special Cases
id	string	Radiosonde Serial Number	iMet sondes do not provide a serial number, and so the ID field is just set to 'iMet'. auto_rx generates a serial number based on launch time and transmit frequency. DFM sondes do not regularly transmit their serial number, and so before the serial number is observed the RS decoder will output an invalid serial number, resulting in auto_rx ignoring the packet. Ideally this number should be unique over the entire flight, but for some sondes this isn't always the case.
frame	int	Frame Number	The overflow point for the frame numbers varies between sonde types. Some sondes (M10/M20) do not transmit a frame number at all, and so a unix timestamp (seconds since epoch) is provided in its place.
datetime	string	Date/Time from the sonde's GPS. Usually as YYYY-MM-DDTHH:MM:SS.SSSZ format.	auto_rx uses the dateutil parse function to decode the time, and so is fairly tolerant to variations in time format. iMet sondes do not transmit date, and so only time (HH:MM:SSZ) is provided. auto_rx adds on the date portion.
lat	float	Latitude (decimal degrees)
lon	float	Longitude (decimal degrees)
alt	float	Altitude (metres)

Optional Fields

Field	Type	Description	Default Value	Special Cases
type	string	Radiosonde type provided by the decoder	Based on the decoder which was started.
subtype	string	If known, the specific model of radiosonde being decoded. Mainly used	None	RS41 subtypes are provided as a string, DFM subtypes are provided as a hex code, decoded here.
temp	float	Measured Temperature (deg C)	-273.0
humidity	float	Measured Relative Humidity (%)	-1
pressure	float	Measured Pressure (hPa)	-1
vel_h	float	Horizontal Velocity (m/s)	0.0
vel_v	float	Vertical Velocity (m/s)	0.0
heading	float	Heading (degrees True)	0.0
sats	int	Number of SVs used in position solution	0
batt	float	Battery Voltage (volts)	0.0
bt	int	RS41 burst-timer value, in seconds	None
encrypted	bool	Flag to indicate if incoming telemetry is encrypted (e.g. RS41-SGM)	None	If this is set to True, the current frequency is locked out for 2 hours.
freq	int	Centre frequency of the radiosonde signal, supplied in kHz. If supplied, this will be used instead of any frequency information auto_rx may already have.

Radiosonde Types and known Subtypes

Type Code	Name	Decoder	Subtypes
RS41	Vaisala RS41	rs41mod	RS41-SG, RS41-SGP (Pressure Sensor), RS41-SGM (Military)
RS92	Vaisala RS92	rs92mod
DFM	Graw DFM06/DFM09/DFM17	dfm09mod	DFM06, DFM09, DFM17
M10	Meteomodem M10	m10mod
M20	Meteomodem M20	mXXmod
IMET	Intermet iMet-1AB / iMet-4	imet1rs_dft
MK2LMS	Lockheed Martin Sippican LMS6 - 1680 MHz Variant	mk2a_lms1680
LMS6	Lockheed Martin Sippican LMS6 - 400 MHz Variant	lms6Xmod	LMS6, LMSX (possible new replacement for the LMS6)
MEISEI	Meisei iMS100 / RS11G	meisei100mod	IMS100, RS11G

auto_rx 'UDP Mode' Configuration

In this mode, auto_rx listens for JSON data in the above described format on UDP port 50000 (one JSON object per UDP packet). This is useful if you want to run the decoders separately, or experiment with running multiple decoders at once.

Entering this mode require a few configuration adjustments, and then adjustments to how auto_rx is started.

Within your station.cfg file, you will need to set your SDR device ID to 'TCP001', e.g.:

# Individual SDR Settings. 
[sdr_1]
# Device Index / Serial
#	If using a single RTLSDR, set this value to 0
#	If using multiple SDRs, you MUST allocate each SDR a unique serial number using rtl_eeprom
#	i.e. to set the serial number of a (single) connected RTLSDR: rtl_eeprom -s 00000002
#	Then set the device_idx below to 00000002, and repeat for the other [sdr_n] sections below
#
#   For the special case when auto_rx is used entirely with a JSON-via-UDP input, set the below to TCP001
#   This will bypass all RTLSDR checks and allow auto_rx to continue running.
device_idx = TCP001

All other settings are configured as normal.

Then, to start up auto_rx in UDP mode, run it with:

$ python3 auto_rx.py -m UDP -f 403.000

Note that the frequency value will be used when sending data off to SondeHub. Don't worry too much about it though, as this value is overridden if any incoming UDP data contains a 'freq' field (which it should).

Starting up decoders outside of auto_rx

The following provides an example of starting up a decode chain outside of auto_rx, and feeding the output data from the decoder into auto_rx via netcat:

rtl_fm -p 0 -d 0 -M raw -F9 -s 48000 -f 401488000 2>/dev/null |./fsk_demod --cs16 -b 1200 -u 22800 -s 2 48000 4800 - - | ./rs41mod --ptu2 --json --softin -i --jsn_cfq 401500000 | nc -u localhost 50000

Breaking this apart a little, we have a few commands piped together:

• rtl_fm, producing signed 16-bit IQ output (the 'raw' option), at 48 kHz sample rate, tuned 12 kHz below the expected sonde centre frequency (401.5 MHz)
• fsk_demod, configured to accept that IQ input (--cs16), and look for a 4800 baud signal between +1200 and +22800 Hz. The demod is configured to output soft-symbols (floating point values between -1.0 and +1.0) via the -s option.
• rs41mod, which accepts the soft-symbols (--softin -i) calculates PTU data (pressure/temp/humidity, via the --ptu2 flag), and outputs JSON data to stdout (--json). We also tell the decoder what the centre frequency of the incoming signal is (--jsn_cfg 401500000) so it can include this in the UDP packets.

Performing a Radiosonde Detection

To know which decoder to start up, you need to know what radiosonde type you are listening to! Sometimes you will know this in advance (e.g. scheduled launches from a known launch site), other times not so much (which is why auto_rx exists in the first place...). You can use the dft_detect utility to determine what you are listening to:

$ rtl_fm -p 0 -d 0 -M raw -F9 -s 48000 -f 401500000 2>/dev/null |./dft_detect -t 5 --iq --bw 20 --dc - 48000 16
RS41: 0.9874 , +313.3Hz

In this example there is a RS41 within the receiver passband, with a centre frequency at ~+313 Hz. The 0.9874 number is a 'correlation score', indicating how well the detector was able to match the RS41 header to the received signal.

While the rtl_fm argument is pretty similar to what is used in the RS41 decoding example earlier, note that we have tuned to the centre frequency of the radiosonde signal, not the -12 kHz offset frequency we used to decode.

The argument to dft_detect are as follows:

• -t 5 - Only run the detection processes for 5 seconds. This is long enough to reliably detect any radiosonde type.
• --iq - Accept IQ input
• --bw 20 - Filter the input to a 20 kHz bandwidth.
• --dc - Apply a DC-blocking filter to the input.
• - - Take input from stdin
• 48000 16 - Informs the decoder that the input sample rate is 48 kHz, with 16-bit samples.