Concept Design and Feasibility Studies for a Ka-band, UAS-based Cloud Sensing Radar

Ground and airborne radars operating at Ka or higher frequency bands have been important for the detection, analysis and classification of both microphysical cloud particles and larger size hydrometeors. The relatively recent development and expanding use of unmanned aircraft platforms has enabled larger scale, higher resolution and innovative process for cloud and hydrometeor observations, which also enables us for potentially the first time to achieve better understanding of the couplings and interactions that must be present from the upper microscale to the mesoscale with regards to determine the cloud microphysical processes (including aerosol interactions) key to cloud development, such as microphysical-dynamical-radiative feedback loops. The OU team is collaborating with University of North Dakota for developing a novel Ka band radar sensor which has the small SWaP that can be mounted on a medium-size UAV platform, which can fly to a much closer range to the cloud systems than existing remote sensors. The FMCW (frequency-modulated continuous wave) waveform and baseband radar scheme is used for the first time for this application, and integrated circuit implementations are being developed to meet the SWaP requirements. Knowledge-based framework regarding to different particles in the backend is also being developed to support a broad range of scientific studies.

Detailed link budget analysis regarding to different types of targets is provided at the initial stage. It could be used as guideline for system design and the baseline of performance expectations. It was shown that if the airborne radar antenna has a beamwidth of 4.6/15 degrees (Horizontal/Vertical) and a gain at 30 dB, and the FMCW transceiver operates at 35 GHz with 50 MHz ramping bandwidth, 20 MHz LPF cutoff frequency and 1 ms ramping time, 50 m range resolution is achievable at 1km range, with a minimum detectable reflectivity at 0 dBZ and 10 Watt transmitted power. It is sufficient to detect hazard large particles in cloud (i.e. ice) and hydrometeors which are larger than 0.1 mm.

A laboratory prototype of this radar system is being developed at OU's Radar Innovations Laboratory. The prototype system is designed to transmit and receive at 35.2 GHz frequency using light-weight, patched array antennas. These antennas are dual polarized, and offers high gain and very low side lobe levels. A baseband FMCW board is applied, which operates at 2.4 GHz carrier frequency. A PLL (Phase Locked Loop) is used to generate LO signal which supports up-conversions using multipliers and mixers to generate the 35.2 GHz transmit signal. The returned signal due to scattering and absorption of radar signals by fog droplets and particles (if present) is down converted back to the 2.4 GHz band. The processed baseband signal is used to study the reflectivity and backscattering to determine the particle size, visibility and other related factors. Initial laboratory test results show that the transceiver chain is working as expected, while higher transmit power is needed to achieve a desired standoff observation distance. Integration of the key hardware elements into UAV payload is being performed.