teor scatter relies upon the fact that meteors are constantly entering the earth's atmosphere. It is estimated that about 75 million enter every day. The vast majority of them are small, and do not produce the characteristic visible trail in the sky. In fact most meteors are only about the size of a grain of sand and any that are an inch across are considered to be large. The meteors enter the atmosphere at speeds of up to 75 km/ second and as the atmosphere becomes more dense they burn up, usually at heights of around 80 km. The heat generated from the friction from the air causes the atoms on the surface of the meteor to vaporize. The atoms become ionized and in view of the speeds, leave a trail of positively charged ions and negatively charged atoms. The trails do not normally last for very long. As the density of the air is relatively high, the electrons and ions are able to recombine quickly. As Radio waves and propagation 49 Meteor Meteor trail Weak signal after reflection Transmitter Receiver Figure 2.27 Meteor scatter link a result the trails normally only last for a second or SO. However, the level of ionization is very high and they are able to reflect radio waves up to frequencies of 100 MHz and more. While the level of ionization is very high, the area which can reflect signals is very small, and only a small amount of energy is reflected. Despite this there is just enough for a sensitive receiver to receive. The meteors come from the sun, and there are two main types. Most enter the atmosphere on a random basis, while others are from meteor showers. The showers occur at specific times of the year and occur as the earth passes through areas around the sun where there is a large amount of debris. A wide range of frequencies can support meteor scatter communica- tions, although at lower frequencies signals suffer from attenuation in the D layer of the ionosphere. Also if frequencies in the HF portion of the spectrum are used then there is the possibility of propagation by reflection from the ionosphere.