Preliminary Test Flight of a Compact High Altitude Imager and Sounding Radiometer (CHAISR) Microwave Radiometers for Meteorological Observation from HALE UAV

Three compact microwave radiometers are developed for a lightweight solar-powered HALE UAV (High-Altitude, Long Endurance; Unmanned Aerial Vehicle) or pseudo-satellite. The platform aims to operate at UTLS, i.e. altitude of 16~20 km, where air becomes thin enough to prevent operation of a conventional fossil fuel engines. Despite atmospheric science community has long been attracted in its potential scientific and operational value as an observation platform, only limited opportunities were available. The payload aims to obtain vertical temperature profiles and column-averaged water vapour for entire troposphere where most weather system takes place. Given total weight (< 3 kg) and maximum power (< 50 W) constraints are not the only challenges for design of the CHAISR. It requires nominal operation in thermal range between −75 and +43 °C and from 1013 to 50 hPa. Along with optical cameras and in situ sensors in the CHASIR, three microwave radiometers with 16 channels from 18 to 60 GHz are to fit in 130 mm diameter and 290 mm length with total weight less than 1.5 kg for cross-track scan unit. Maximum power consumption of less than 15 W does not allow conventional internal blackbody calibration facility onboard, and alternative methods has been developed. This study represents calibration of miniature microwave radiometer followed by preliminary results of a series of test flights conducted in 2017. While CHAISR has reached 2/3 of its target altitude, data from test flight showed effective performance of tipping curve calibration with altitude as expected. On the contrary, pre-flight calibration with liquid nitrogen indicates there are rooms to improve method of lab-based characterisation of the CHAISR. The result suggests feasibility of in situ cold reference for microwave radiometer and better than 1°K of total RMSE can be achievable once accuracy of warm reference is available from noise diode or ambient temperature readings. Continuous improvement of quality of instrument is currently explored at the same time, researching way of improving current specification of microwave radiometer in CHAISR.