Meteor Echoes – 26th March 2010

At our March meeting Allan Bell gave us a talk about Meteor Echoes, in which he explained how radio (and radar) could be used to watch for meteors, and how meteor trails could be put to other uses. Meteors produce trails of ionised gas which may persist for several minutes, and these will reflect radio signals if the geometry is good. Ideally, a powerful VHF transmitter needs to be located outside line-of-sight reception distance, so that the receiving station will only receive a signal when a meteor trail is in a suitable location. In the past a transmitter in Gdansk transmitting on 70MHz served well, but while it is still used at times, it no longer transmits all night. A VHF beacon GB3ANG transmits continuously from Scotland, which satisfies the over-the-horizon requirement for observers in the South of England, but it only emits ten watts, so a large antenna is needed if it is to be used for meteor observations. In the USA meteor observers have used VHF TV and NASA satellite radar system signals for meteor observations. Even if the geometry is good, not all signals come via reflection from meteor trails. Atmospheric and Ionospheric conditions can also give over-the-horizon reception of VHF signals, and if a transmitter is strong enough then reflections off satellites can also be heard. Allan rounded off his talk by explaining that meteor echoes can be used for more than simply counting meteors. They can also be used to hold short over-the-horizon conversations on VHR radio, and there are a number of scientific stations using dedicated transmitters and arrays of receivers to determine the atmospheric conditions at meteor altitude much more economically than by using sounding rockets.