The ionized meteor trails are classified into two types due to the electron density of the trail. They are underdense trails and overdense trails. The two types will be discussed in the following sections.

1.3.1 Underdense trail

The underdense trails are formed by micrometeors with masses from 1 Exp-5 g to 1 Exp-3 g. The electron density of such trails is less than (1 E 14) e/m [7,9].

The underdense trails which are invisible to the unaided eye, occur most frequently, and are useful for communication. The average waiting time for each trail (assuming 2000 W transmitter power and 13 dB antenna gain) is on the order of at most several minutes. The trial survive for 0.5 to 1 s [9].

1.3.2 Overdense trail

If the electron density of the trail is so large (q >1 E 14 e/m) the trail is said to be overdense trail [7,9]. The overdense trails occur much less frequently than the underdense trails. The waiting times on the order of ( 20 ) minute, but due to their high reflectivity and long life-time( several seconds ) they are potentially very useful for communication purposes[9].