Forward Scatter: a General Overview

The general principle of meteor observation by forward scattering of radio waves off meteor trails is easy to understand. It is illustrated in Figure 1. A VHF radio receiver (30-100 MHz) is located at a large distance (about 500-2000 km) from a transmitter at the same frequency. Direct radio contact is impossible due to the curvature of the Earth. When a meteor enters the atmosphere, its trail may reflect the radio waves from the transmitter to the receiver. At the receiver, where the signal of the transmitter is normally not received, the transmission can then be received for a moment, as long as the meteor trail is present. Such reflections last from a tenth of a second to a few minutes.

Figure 1 - The principle of forward scatter observation of meteors. Signals from a radio transmitter are scattered to a receiver by a meteor trail, thus enabling the detection of the meteor.

This observation method became popular among amateur astronomers in the middle 1980's and is a variant of the radar observation method employed by professional astronomers since the mid forties. Meteor scatter is also used to communicate in the VHF band over big distances.

Radio observations can provide a lot of information on meteors, but a good theory is necessary for a correct interpretation of the recordings. The classical theory for radio meteor observations was developed mainly in the 1940's and 1950's. The main emphasis then was on backscatter (radar) observations. Forward scattering received much less attention, since the theory and interpretation of the observations are severely complicated by a more complex geometry. Later on, more general numerical approaches to the scattering problem have been used, like the full wave theory, ray tracing techniques, and the long wavelength approximation.