This dataset contains raw observation data (Level 0) of dropsondes released from the High Altitude Long Range aircraft HALO over the Arctic Ocean and North Atlantic during 6 research flights of the ASCCI campaign in March 2025 (including a scientific test flight in February 2025). The raw data are stored in one CSV file per sonde (72 in total) that contains the decoded raw signals (time, GPS position, meteorological variables) recorded by the corresponding receiver channel of the KITsonde system. The file name indicates the start date and time of the decoded signal (before the release time), the number of the receiver channel (ch), and the unique 8-digit sonde identifier.