In order to study the formation and evolution of the charge layer created by a corona at the ground underneath thunderclouds, we have developed a new system of electric field detection. It is composed of five self-contained sensors designed to measure the ambient electric field at various altitudes between the surface and a tethered balloon that suspends the entire system of equipment. Each sensor can detect ambient field intensities up to 150 kV/m with an accuracy of 1.5% and without any detectable influence of a local corona effect on the measurement. A standard shutter field mill detects the surface electric field. The data collected aloft are telemetered via a 400-MHz frequency band. An adapted data processing system provides a real-time display of the multilevel evolution of the electric field within the first hundreds of meters above the ground. The digitized data can be used to display the instantaneous vertical profile of field intensity on various time scales. ¿ American Geophysical Union 1991