Sensitivity time control

Abstract: A weather radar incorporating a sensitivity time control modifying circuit for adaptively increasing the gain of the IF amplifier as a function of precipitation rate; thereby increasing the range of the radar through the precipitation and identifying on the display those regions in which the radar might otherwise inaccurately indicate the precipitation rate. Reflections from the precipitation are received by the radar antenna coupled to a mixer wherefrom an IF signal is coupled to the IF amplifier wherein it is amplified in accordance with a gain that is determined by the modified sensitivity time control signal.

Abstract: A new short-pulse digital profiling radar system that operates at lower frequencies than most ice radars used in polar regions to date has been designed and built by the U.S. Geological Survey. The transmitter is an avalanche transistor pulser which drives a resistively loaded dipole transmitting antenna. A similar, but separate antenna is connected to the receiver. The receiver has adjustable sensitivity time control (STC) of as much as 60 dB to compensate for attenuation and geometric spreading factors. A fiber-optic cable is used to transmit both control signals and data. The data-acquisition and display system incorporates very high-speed digitizing and signal averaging, real-time profile display, and data storage on standard computer nine-track magnetic tape. The system was successfully used on Ice Stream B in West Antarctica at centre frequencies of 1, 2, 4, 8, and 12.5 MHz. Bottom-return signal-to-noise ratios of more than 40 dB were obtained at 2 MHz through 800 m of ice. Convoluted internal surfaces not related to present bottom topography were resolved within the ice streams and anomalous strong reflections or “bright spots” were identified near the base of the ice. At present, there is no satisfactory glaciological explanation for either of these observations.

Abstract: A modern coherent Antarctic radar depth sounder for probing the ice sheets of Antarctica and Greenland has been designed and developed by the University of Kansas. It was successfully tested during the austral summers of 1987 and 1988 at Downstream Β and Upstream B, Antarctica. Ground-based measurements were made with the radar in a mobile hut hauled by a Sno-cat in 1987 and in a Spryte vehicle in 1988. The coherent Antarctic radar depth sounder (CARDS) is an unfocussed synthetic-aperture chirp radar where the along-track resolution is improved by extensive coherent integration. Surface acoustic wave (SAW) devices are used to implement pulse expansion and compression. A common stable oscillator for the transmitter and the receiver establishes coherency. The system signal-to-noise ratio is enhanced by pulse compression and coherent integration. Antennas for the ground-based measurements are configured with an array of eight dipole elements, four active and four passive, the latter acting as reflectors. The aircraft antennas also consist of four active elements hung underneath the two wings. The wings serve as reflectors. A PC facilitates system control and data recording on a high-density recorder. Α-scope plots of selected records allow frequent field checks on system performance. More descriptive display facilities have been incorporated in the latest version of the system. The radar transmits 60 ns, 20 W peak power at 150 MHz. The number of coherent integrations is selectable up to 64 k. The system is capable of 5 m range resolution and 5 km range in ice. A programmable sensitivity time control (STC) increases the receiver dynamic range. System parameters such as pulse-repetition frequency, number of integrations, and display modes can be chosen during field operations by user-friendly software. This paper describes the design and field operations of the system. Some results of the 1987 operations at Downstream Β are presented.