A meteoroid is a particle of debris in the Solar System. The visible phenomenon due to the fly of a meteoroid through the atmosphere is called a meteor. If a meteor reaches the ground, then it is called a meteorite.
Meteors may occur in showers, like the Perseids or the Leonids, or at random (sporadic meteors) when they are not associated with a determined single event. Meteor showers happen when the Earth passes through a trail of comet particles.
When a small meteoroid (< 10-3g) enters the atmosphere, it does not produce light, but it collides with the molecules of the atmosphere and remove one or more electrons from them. This creates an electron trail on which a radio signal can be reflected and then can be detected by a radio receiver. A meteor detected by this way is called a radio meteor.
The main principle of forward scattering is illustrated in figure below. A radio receiver is tuned on the frequency emitted by a transmitter. Direct transmission is impossible because of the curvature of the Earth. When a meteor enters the atmosphere, its trail may reflect the radio signal from the transmitter to the receiver.
An important characteristic of meteor observation by forward scattering is that the reflection is specular. It means that the meteor acts like a long mirror on which reflection occurs at one location (specular point). The consequence is that data computed from the observations only apply to this particular part of the trail.
Profiles of the Receiving Signal
The features of the receiving signal essentially depend on the ion density in the meteor trail. Two cases are mainly discussed: the underdense trails, which correspond to very low ion densities, and the overdense trails, which correspond to very high ion densities.
Underdense meteors correspond to faint meteors. They are the most observed. Underdense echoes are very short (no more than a few tenths of second) and their typical profile is illustrated in this figure, on which the X axis represents time and the Y axis represents power.
Overdense meteors are rarer because they correspond to bigger meteoroids. Echoes can last for one up to several seconds. A typical signal is showed in the next figure, on which the X axis represents time and the Y axis represents power.
Benefits of Radio detection
Radio observation is done 24/7, independently of the weather conditions and during daylight. This leads to discover diurnal meteor showers.
Fainted meteors cannot be detected by visual observations because they don't emit light. Both methods of observation are therefore complementary.
Complete information about forward scattering and meteor science in general can be found on the website of the International Meteor Organization (IMO).