Ionospheric absorption with a 38 MHz ROACH-based array
Abstract
Due to the presence of free electrons and ions among the many neutral particles high in the Earth's atmosphere (the ionosphere), signal perturbations from outer space can be detected at ground level throughout the day. These perturbations are caused mainly by Thomson scattering and collision of electrons with neutral particles. These mechanisms transfer the energy from the signal into kinetic energy of the neutral gas. The signal absorption caused by this phenomenon is proportional to the electron density in the ionosphere at any given height. The total ionospheric opacity can be determined with a passive receiver on the 30--45 MHz frequency range by measuring the intensity of cosmic noise which has an antenna temperature between 1~kK and 20~kK over this frequency range. This work is about the development and implementation of a measuring system which can help to characterize the absorption of the cosmic noise signals at low- and mid-latitudes. The system includes a 16 dipole antenna array, with a central frequency of 38~MHz and a 10~MHz bandwidth. The antenna is followed by an analog processing system that routes the signals to a set of analog-to-digital converters. The analog processing system has two components: a filter and an amplifier. A total gain of around 90~dB over a bandwidth of 12~MHz relative to the central frequency is achieved. The implementation consists of using surface-mount technology components. The signals from the 16 elements are fed to a beamforming system implemented in an FPGA-based ROACH digital platform, which essentially works like a spatial filter, improving the angular resolution of the system from approximately 120~degrees for a single dipole to 25~degrees for the full array. This, in turn, allows for the possibility to change the pointing direction of the receiving pattern, the implementation of multi-beam reception and the spectral analysis over the band of interest.
- Publication:
-
Boletin de la Asociacion Argentina de Astronomia La Plata Argentina
- Pub Date:
- August 2020
- Bibcode:
- 2020BAAA...61C..86B
- Keywords:
-
- radio continuum: general;
- opacity;
- instrumentation: miscellaneous