ACTOS is a measurement platform caried as an external load below a helicopter for studying the atmospheric boundary layer. The relatively low flight speed of a helicopter allows for high-resolution in-situ measurements in both time and space, down to the centimeter range. ACTOS is designed for measurements under cloud-free conditions as well as for operations in cloudy atmospheres. ACTOS’s operational conditions include measurement flights from the ground up to altitudes of approximately 3,000 m under non-icing conditions. Operation within clouds is thus possible provided they have sufficiently large gaps to allow safe passage. In this process, ACTOS is guided on a tether approximately 150 m long beneath the helicopter, allowing the measurement platform to descend into the cloud from above while the helicopter moves safely above the cloud.

ACTOS is equipped with a comprehensive set of instruments for measuring fundamental meteorological parameters related to dynamics and thermodynamics, as well as aerosol and cloud properties (see “Instrumentation”). The measurement platform operates autonomously, with its own power supply and internal data acquisition; a Wi-Fi connection to the helicopter is used exclusively for monitoring purposes, as well as for coordinating and controlling the measurement flights. The system is designed to be modular and flexible, allowing the instrumentation to be adapted to the specific scientific objectives of each experiment.

The measurement platform also comes with a comprehensive logistical infrastructure, including a specially equipped shipping container for transport, storage, and safe operation during campaigns. As a consequence, ACTOS is designed and ready for use worldwide.