Design and Development of an Amplitude Leveling Subsystem for FM Radars

Abstract

To better understand contributions of large ice sheets to sea level rise, remote sensing radars are used to measure relevant characteristics. The Center for Remote Sensing of Ice Sheets (CReSIS) at the University of Kansas has been developing ultra wideband radars to measure the surface elevation of polar ice sheets, near-surface internal layers in polar firn, and thickness of snow cover on sea ice. There is a need for an amplitude leveling subsystem for these radars to achieve constant transmit power since amplitude distortions degrade range sidelobe performance of these radars. A closed-loop amplitude leveling subsystem for frequency-modulated radars is designed, constructed and tested. This system uses a coupler and power detector to sample transmit power and feedback a control voltage to a variable-gain amplifier that controls the amplitude of the transmit signal. The closed-loop system is able to decrease amplitude variation to ±0.72 dB. Results are presented, and sources of error are analyzed for this system. Measurements of required control voltage versus frequency are presented for an open-loop system that does not use the coupler or power detector. These two systems are compared, and recommendations are given for future work.