Ground-based HF to VHF radar calibration by scattering off a stratospheric balloon

Abstract. The knowledge of radar system properties is crucial for judging radar performance and correctly interpreting received raw and analysed data. The most critical part of that is the radiation pattern, which is often very difficult to measure for atmospheric/ionospheric sounding radars as the beam pointing direction is often almost vertical. Another fundamental aspect of multi-receiver radars is the accurate knowledge of receiver phases, which are a prerequisite for interferometry applications. Both topics present challenges for large-aperture radars, and are even more difficult for those operating in the lower highfrequency band. On May 28th 2024 the stratospheric balloon mission HELIX was launched with its payload at the Esrange Space Center, which later flew very close the Norwegian island Andøya. This drifting structure was seen on three co-located radar systems covering the lowermost HF (3.17 MHz) and the lower VHF (32.55 MHz and 53.5 MHz) region. The detected backscatter was used to perform active phase calibration of the VHF system, which allowed the verification of the target positioning by interferometry for all three radars. Furthermore, it enabled the general confirmation of the two-way radiation pattern of all the systems.