BASTA (Bistatic Radar System for Atmospheric Studies) is a horizontally-staring cloud radarand was mounted on the right-hand side of the ATR42. BASTA is a 1 W bistatic FMCW (Frequency Modulated Continuous Wave) 95 GHz Doppler cloud radar developed from the ground-based BASTA system. It was used in an aircraft for the first time during EUREC4A, with two antennas of 20 cm (0.95° beamwidth) installed in back lateral windows of the ATR. The radar was operated in two modes, one after the other, at 12.5 m and 25 m range resolutions with 0.5 and 1 s time resolutions respectively. It led to a measurement in one mode every 1.5 s. The maximum range was 12 km with an ambiguous velocity of 9.85 m.s−1 for both modes. The minimum detection range is about 80 m from the aircraft due to coupling between the two antennas.
The calibration of the radar has been derived from other field campaigns and confirmed using in-situ data. The sensitivity of the radar is estimated at around -35 dBZ at 1 km. Level 2 data are the most elaborated product, the two modes being combined to optimise the advantages of each range resolution. Within the first 250 meters, the 12.5 m mode is used while the 25 m mode covers the rest of the profile. The combined reflectivity and Doppler profiles are available every 1.5 s. The radar gates are geolocated in latitude, longitude and altitude in order to derive maps. The reflectivity is corrected for gaseous attenuation using co-located information from dropsonde temperature, humidity and pressure. A parameterisation of liquid attenuation for both cloud and precipitation as a function of reflectivity was derived thanks to in-situ data and applied to correct reflectivity for liquid attenuation. The corrected reflectivity is then used to distinguish cloud areas from drizzle or rain. The radar Doppler velocity is corrected for aircraft motion and folding using gate-to-gate correction.