A high-resolution radio echo sounder operating at a frequency of 840 MHz has been developed for sounding of small and medium-sized polar glaciers and ice caps. The sounder uses a compact, high-gain antenna which improves the system performance, suppresses valley wall echoes and simplifies operation from light aircraft. Successful field trials were carried out on the Rusty, Trapridge and Hazard Glaciers, Yukon Territory, Canada. Results of airborne surveys compare well with ice depths obtained from earlier ground-based soundings on the Rusty and Trapridge Glaciers. The maximum ice thickness encountered was 200 m on Hazard Glacier. Owing to the high operating frequency, random scattering from inhomogeneities within the ice is a major cause of signal degradation. For this reason the sounder cannot penetrate great thicknesses of temperate or debris-rich ice. Spatial averaging, an immediate result of operating from a moving platform, reduces the effects of back-scattered "clutter. " Results of ground-based tests on the Hazard Glacier yield a value for ftan 8 = 0.26 at -50C, in agreement with predicted values. The total received power and the echo details have both been found to be very sensitive to small (<<I0 cm) changes in antennae position. Large fluctuations in power, caused by roughness at or near the ice/air surface, prevented using single coverage data to detect birefringence in glacier ice. The results also indicate that the standard photographic records should be replaced by a recording medium capable of storing more precise and accessible data. A storage medium such as magnetic tape should not degrade the radar data, and would at the same time relieve a data processing burden.