Level gauging including an approximation of a distance to the surface of a product kept in a tank. The approximation is determined by relating an amplitude of a first harmonic of an IF signal with an amplitude of a second harmonic of the IF signal.Basically, each harmonics represents a given distance range. By determining the received power in two or more harmonics, and correlating them to each other, the distance may be estimated. Depending on modulation and other parameters the distance dependence may be very different, and may be selected to suit the application.