Linearly polarized HF waves change their polarization due to the total electron content between ground transmitter and the rocket payload (Faraday rotation). Differentiation with respect to altitude yields electron densities. Four frequencies will be used to assure good sensitivity at low altitudes on the one hand, and coverage up to apogee on the other. The absolute values are insensitive to payload charging or aerodynamic effects, but the height resolution is usually not better than 1 km.