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Model Specific Notes
This page provides information for specific radiosonde models.
These sondes are known to drift in frequency throughout their flight. This may cause auto_rx to lose lock on the signal, or sometimes detect a single radiosonde on adjacent 10 kHz channels.
The use of the newer experimental decoders (which have better drift handling ability) may assist with decoding of these sondes.
iMet sondes do not transmit their serial number within their telemetry packets. This makes life somewhat difficult, as we need a unique identifier for the various mapping systems (sondehub.org / APRS / Chasemapper) to be able to distinguish between different radiosondes, and for multiple stations to contribute positions for the same radiosonde.
As a way around this issue, auto_rx generates a unique ID for each received iMet sonde by using:
- The reported time and telemetry sequence number to calculate the power-on time of the sonde;
- and the transmit frequency of the sonde, quantised to the nearest 100 kHz.
These two bits of information are concatenated, and a SHA256 hash is calculated. The last 8 characters of the hash are used for the Sondehub ID, and the last 5 for the APRS callsign. The code that calculates the ID is available here.
Warning - iMet-1 sondes increment their frame counter twice every second, vs once per second for iMet-4 sondes. This needs to be detected and compensated for when calculating the power-on time. Refer here for the algorithm used.