 are generally used because they give the maximum amount of forward scatter; nevertheless, the amount of energy that is scattered forward is extremely small, and the losses in the scatter volume are on the order of 50 to 70 dB in addition to the free-space loss. Large short-term variations following the Rayleigh law also occur, and therefore quadruple or multiple diversity is almost always used. This is achieved by employing two antennas, horizontal and vertical polarization, different scatter volumes (angle diversity), and different frequencies. Space and polarization diversity generally yield the best results. Tropospheric scatter can provide communications up to 500 km, but paths as long as 800 km have been operated. Meteor scatter communications make use of ionized trails of meteors burning up on entering the Earth's atmosphere. This is not such a rare event as might be thought, and there is a steady shower of material entering the Earth's atmosphere, although with random and predictable variations. Meteors that leave usable trails have diameters between 0.2 mm and 2 mm, and the trails, which last for around half a second to several seconds, occur at a height of about 120 km. They can extend up to 50 km in length, although 15 km is a typical value. The number of particles entering the Earth's atmosphere is inversely proportional to size, SO larger meteors occur too infrequently to be of use, while the numerous very small parti- cles do not generate sufficient ionization. The variation in the number of usable ionized trails occurs on a daily basis, with more occurring during daylight as the Earth's rotations tend to sweep up more meteors than at night. There are also predictable showers occurring throughout the year. Meteor scatter communications can be established for paths from 200 to 2000 km, but information has to be transmitted in bursts when a link is established, SO the medium cannot be used for services such as speech. Radio waves are reflected from the trails by different mechanisms depending on the d