earth moon earth (RF technique) (EME)
Earth-Moon-Earth( EME) is a radio transmission technique in which the moon is used as a reflector and reflects the radio signal emitted by the earth back to earth.
Since radio waves in the VHF and UHF ranges are neither space waves nor ground waves, i.e. there are no reflective layers in the earth's atmosphere to transmit the radio signals beyond line-of-sight, the EME link is used in professional radio technology and in amateur radio for radio signals in the 2-m band and in the 70-cm band. This involves transmitting a highly focused, powerful radio signal to the moon. However, the moon reflects it diffusely, so that only a fraction of the transmitted signal can be received back on earth. The receiver must therefore be able to receive and amplify signals with the lowest possible field strength.
The method works only with etremly bundled and tracked antennas and a very high radiation power, because the lunar surface represents only a fraction in relation to the celestial surface. The transit time of the signal from the earth to the moon and back is between 2.5 s and 3 s, depending on the distance to the moon.