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Various networks (e.g. USCRN, SNOTEL, BIEBRZA_S-1, WEGENERNET, FMI, HOBE, RSMN) show in 2017 for many stations odd, most likely wrong, values for the soil temperature in the winter period, while being flagged with "G" as "Good". Also the soil moisture values are then not correctly flagged, as missing the "D01"-flag.
The problem I faced is visible as consecutive measurements with the same value for the soil temperature. E.g. the value 0.100, or -0.100, or 0.35000 or 1.2000, etc. lasting for days/weeks/months. I found also alternating patterns (e.g. 0.20/0.30, or -0.1/0.1) that are most likely to be not the temperature signal.