(written from a Production point of view)
According to the shooting script for Star Trek: The Motion Picture, the lunar monitor relay was a Starfleet device on or near Earth's satellite Luna. One of these relays transmitted an image of V'ger in Earth orbit to the USS Enterprise. Later, this same relay or another one like it, transmitted an image of V'ger launching plasma energy bolts at the planet. The lines mentioning these devices was cut from the theatrical film.