Tutorial Radiosonde

Tutorial: Radiosonde

What it is. A radiosonde is a small weather instrument carried up by a balloon. Weather services launch them once or twice a day from stations around the world. During the flight, which lasts about two hours and reaches 25 to 35 km height, the sonde sends position, altitude, temperature and other weather data. When the balloon bursts, the sonde falls back. OnAir decodes the sondes of several makers, plots the tracks on a map, and estimates where each one will land, so you can try to find it afterwards.

What you need

• Radio. The mode asks for 8 Msps with a channel width of 6 MHz; the lowest rate that works is 2 Msps. At a lower rate you cover less of the 400 to 406 MHz band: At 8 Msps the whole 400 to 406 MHz band is not covered, so centre the capture on the frequency your local station uses (the tuner's tooltip explains). Every sonde inside the captured width is found and decoded at the same time, up to 8. An RTL-SDR at 2.4 Msps covers part of the band; a HackRF, Airspy or SDRplay covers more. See Supported-radios.
• Antenna. A 400 MHz antenna: a quarter-wave ground plane, a Yagi pointed up, or a small discone. Outdoors, high, with a clear sky. A 400 MHz LNA and filter helps with weak sondes.
• Frequencies from the repository's frequency database. The radiosonde band 400.15 to 406 MHz (meteorological aids, WMO allocation); the mode default is 403 MHz. Older Vaisala and Meteolabor sondes use the 1680 MHz band (1675 to 1683 MHz); the tuner offers it as 1680 MHz (LMS6).

Before you start

• Where and when: sondes are launched from upper-air stations on a fixed schedule, usually twice a day around 00 and 12 UTC (the exact times depend on the station). The sonde is in sight of your antenna for much of the flight when it is high.
• Gain: moderate. Keep the capture centred on where sondes are active in your region.
• Offset: a sonde's own oscillator adds a few kHz of frequency error; the receiver follows it.

Step by step

1. Mode list, group Weather & science, entry Balloons, 403 MHz.
2. In the tuner, Band: choose 400 to 406 MHz (403.0), 1680 MHz (LMS6), or type a custom frequency. At 8 Msps you may not cover the whole band, so centre on the frequency your local station uses (the Carriers found in the band list shows what the search found).
3. Set CHANNELS (how many sondes are followed at once, 1 to 8) and press Start.
4. Watch the Map and Table tabs.

What you should see

A carrier appears in the spectrum, then a row in the Table with the sonde type and serial. Status lamps show IQ, Carrier, Decoding and Position. The status line shows "n active of n", the number of carriers and frames ok / total. Frames start to fail below roughly 9 dB signal-to-noise in a 10 kHz bandwidth.

Reading the screen

• Map. The track of each sonde, its current position, a dashed line and a cross for the predicted landing when it is descending. fit and follow keep it in view.
• Table. Columns Type, Serial, Alt m, MHz, Temp, Sats, SNR dB, Frames, Heard. Click a row for all fields, altitude and temperature plots, and calibration progress: some sondes need to send their calibration table once before temperatures are right.
• Landing. "landing (estimate)" and "landing from you" give the predicted spot and distance; copy landing position copies it. copy position copies the current position.
• Carriers found in the band. A list of Frequency, Level dB, Channel, Sonde.
• Decoded types in this version: RS41, RS92, DFM (DFM-06, -09, -17), M10, M20, and LMS6 on 1680 MHz.

Options and settings

• CHANNELS: the number of sondes followed at once; each costs a little CPU.
• clear list: forgets the sondes heard so far (the receiver keeps following the carriers).
• Test signal: simulated sondes with types (RS41, DFM, M10, M20, RS92, LMS6), count, drift east and SNR.

Troubleshooting

• No carrier. Check the band you tuned and the time (launches are few); check the antenna.
• Carrier but no frames. Weak signal or wrong type; look at the SNR.
• Temperature wrong. Wait for calibration to finish.
• A sonde disappears. Low altitude behind hills; the track ends there. Use the landing estimate.
• Overload. Lower the gain; strong pager and TETRA signals near 400 MHz.
• Wrong sonde frequency. Use the local station's frequency; there is no single value.

Tips

If you want to recover a sonde: copy the landing position, drive near, and look for it. Ask the landowner before entering private land.

Legal and privacy notes

Radiosonde transmissions are public, unencrypted meteorological telemetry. Receiving is generally legal. Positions can be shared on Live-map only if you opt in. See Legal and Privacy. Never touch a sonde that fell on a power line or into a hazardous place.

Under the hood

Each maker (Vaisala RS41 and RS92, Graw DFM, Meteomodem M10 and M20 (check), LMS6) uses its own modulation, framing and error protection; frames carry serial, time, GPS-derived position, velocity and measurements. WMO allocates 400.15 to 406 MHz and 1675 to 1683 MHz to meteorological aids.

Related

Mode-Radiosonde, Live-map, Tutorial-Weather-satellite-images, Troubleshooting