separation of echoes) in the raw data to about 0.6 m, insufficient for profiling ice in most temperate regions. Such low frequencies generally limit the vertical resolution (i.e. Lawrence Seaway, but were limited by helicopter clutter and an inability to penetrate frazil formations, most likely due to their high water content. Reference Batson, Batson, Shen and RugglesBatson and others (1984, Reference Batson, Batson, Shen and Hung) have extensively profiled sections of the St. Previous airborne ice profiling with short-pulse radar has generally been confined to pulses centered at frequencies below about 250 MHz ( Reference DeanDean, 1977 Kovacs and Morey, 1978 Reference Rossiter, Rossiter, Snellen, Butt and RidingsRossiter and others, 1980), with some of the work aimed at profiling sea-ice thickness. However, only in the last few years have UHF-frequency antennae been available that are capable of producing the required resolution for ice-thickness monitoring. A prime candidate for fulfilling this requirement is short-pulse (also known as ground-penetrating) radar, which was commercially developed in the early 1970s. Such a system could also aid in hydrologic studies of winter channel formation and sediment transport. Ice-thickness data are needed for winter navigation and for prediction of damage during break-up. The minimum ice thickness that could be resolved from the raw data was about 0.2 m with the 600 MHz antenna and less than 0.15 m with the 900 MHz antenna.Īn accurate remote system is needed for rapidly profiling ice thickness on rivers. Areas of incoherent bottom scattering investigated by drilling revealed highly variable ice conditions, including frazil ice. Rough-surface scattering was always followed by the appearance of bottom scattering but, in many cases, including a hanging-wall formation of solid frazil ice, bottom scattering occurred beneath coherent, smooth-surface reflections. All cases of incoherent surface returns (scatter) occurred over ice rubble. On the Yukon River, coinciding temporally coherent surface and bottom reflections were associated with solid ice and smooth surfaces. Detailed surface investigations on the Tanana River revealed good correlations of echo delay with solid ice depth, but an insensitivity to frazil-ice depth due to its high water content. The most readily interpretable data were acquired when the ice or snow surface was smooth. Data were acquired in Alaska over lakes (as a proving exercise) and two rivers, whose conditions varied from open water to over 1.5 m of solid ice with numerous frazil-ice formations. Clutter from the helicopter offered minimal interference with the ice data. The antenna packages were mounted 1.2 m off the skid of a small helicopter whose speed and altitude were varied from about 1.8 to 9 m/s and 3 to 12 m. The ice-thickness profiling performance of a helicopter-mounted short-pulse radar operating at approximate center frequencies of 600 and 900 MHz was assessed.