A very strong surface duct was observed near San Nicolas Island, California, using a unique helicopter-based data acquisition system developed for making fine-scale measurements of radio refractivity in the lower marine troposphere. Such fine-scale measurements are used in a computer model (TEMPER) for predicting microwave propagation in the first 400 m above the ocean surface. The helicopter system makes measurements of atmospheric temperature, pressure, relative humidity, and radar altitude. These measurements are acquired by an on-board computer, which derives and displays real-time plots of modified refractivity versus altitude. Data is stored on floppy disks for later use in the TEMPER (Tropospheric Electromagnetic Parabolic Equation Routine) computer model for microwave propagation prediction. The helicopter measurements are presented and compared with TEMPER computer model microwave propagation predictions using this helicopter data and a standard atmosphere profile. ¿ American Geophysical Union 1992