Abstract: A system for monitoring the level of an RF signal having such a small amplitude that it must be amplified before it can be monitored. To determine the deviation of the amplifier gain from the desired value, an additional RF signal of relatively high amplitude is used and is attenuated by a predetermined amount before being applied to the amplifier. The levels of the attenuated and subsequently amplified RF signal are compared with the level of the unchanged signal. From the deviation of this ratio, the gain variation is determined.Alternatively, in lieu of gain control of the amplifier the gain variation may be mathematically offset in the digital instrumentation provided.The monitoring system is suitable for use in the so-called instrument-landing system (ILS) and in VOR and DVOR en-route navigation systems.