International audienceThe role of the Absolute Scalar Magnetometer (ASM) in the European Space Agency (ESA) Swarm mission is to deliver absolute measurements of the magnetic field's strength for science investigations and in-flight calibration of the Vector Field Magnetometer (VFM). However, the ASM instrument can also simultaneously deliver vector measurements with no impact on the magnetometer's scalar performance, using a so-called vector mode. This vector mode has been continuously operated since the beginning of the mission, except for short periods of time during commissioning. Since both scalar and vector measurements are perfectly synchronous and spatially coherent, a direct assessment of the ASM vector performance can then be carried out at instrument level without need to correct for the various magnetic perturbations generated by the satellites. After a brief description of the instrument's operating principles, a thorough analysis of the instrument's behavior is presented, as well as a characterization of its environment in flight, using an alternative high sampling rate (burst) scalar mode that could be run a few days during commissioning. The ASM vector calibration process is next detailed, with some emphasis on its sensitivity to operational conditions. Finally, the evolution of the instrument's performance during the first year of the mission is presented and discussed in view of the mission's performance requirements for vector measurements
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