KEYWORDS: Radar, Calibration, Ka band, Data processing, Signal to noise ratio, Doppler effect, Reflectivity, Antennas, Signal attenuation, Data conversion
Data processing, calibration, and quality evaluation are critical elements for successful airborne radar missions. For a downward-looking airborne radar, the usage of ground as a calibration target has been discussed before but not completely analyzed for precipitation measurement missions at Ka-band. In this study, the team performed data analysis and calibration modeling for the Millimeter-Wave Airborne Radar for Learning and Education (MARBLE), which was developed as a recent undergraduate team effort beginning in 2016. Millimeter-wave radar missions for MARBLE include precipitation measurement and terrain remote sensing through vertical profiling. To achieve these mission goals, the team used multiple time- and spectrum-domain processing methods on the ground return data collected from 2018 NASA ER- 2 engineering calibration flights. Some of the algorithms include spectrum analysis with various CPI arrangement and multi-lag processing to enhance signal-to-noise ratio (SNR). Doppler calibration based on aircraft platform motion and orientation is also considered. Useful results are obtained from ground power calibration as well as Doppler estimation. In addition, multiple ground calibration tests with actual weather results are incorporated to supplement the airborne measurements after some hardware checking and improvement. Based on the reasonable outcomes from the calibration measurements, a new high-altitude flight campaign for precipitation measurement is being planned for 2020.
Supported by NASA’s Undergraduate Student Instrumentation Project (USIP), OU-IART worked with Garmin International Inc. to develop a novel, low C-SWaP, high-altitude storm-chasing radar, called MARBLE. The radar operates at 35 GHz (Ka-band), and the concept of operation is flying over the top of a convective storm to measure the reflectivity and vertical motion of the clouds. The system design is extremely simple and low-cost, which uses single-stage up-down conversion between Ka-Band operation and the X-band “base radar transceiver”. The radar is very easy to operate by a pilot, through a simple switch. Laboratory tests and ground verifications were performed, and the system airworthiness has been validated. The systems developments started in spring 2016, and the initial engineering validation flights were performed in Sep 2018, using a NASA high-altitude aircraft (ER-2). Initial flight tests show that MARBLE can operate in high-altitude environments and obtain return signatures from various targets. Further data processing is ongoing to evaluate the effectiveness of precipitation observations. In addition, the design, development and testing process successfully served the educational goals of the program.
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