The geometry of radar bounce meteor observing. Credit: NASA.
When meteors strike the Earth's atmosphere at speeds of tens of thousands of kilometers per hour they ionize the air in their path. These luminous ionized trails are not only visually striking – they also reflect radio waves. During a shower like the Leonids, radio signals from TV stations, radar facilities, and AM/FM transmitters are constantly bouncing off meteor trails. The echoes can be heard around the world.
Most meteor echo enthusiasts listen for reflections of distant radio transmitters at frequencies between 40 MHz and 100 MHz. These are the best bands for such work because weakly ionized meteor trails reflect signals most efficiently at lower frequencies, below 100 MHz.