Detection of Meteors by RADAR

1. Introduction

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Why develop a RADAR system to detect Meteor Trails?

Traditionally meteors have been studied optically, initially by counting the number of visible trails per minute or hour and logging the intensity and direction from which they seem to arrive. Latterly, sensitive film was able to be used with all sky camera lenses to collect data for subsequent analysis. In recent years sensitive video cameras have been brought to bear to record and store meteor trail images. During World War 2 and the development of RADAR, it was noticed that meteors could generate 'false echoes' and subsequent adaptation of war surplus equipment led to considerable work on the detection and analysis of meteors. The RADAR detection of the ionised trails left by meteors is relatively straight forward and a great deal of information can be deduced from the details of the return signal. The analysis of the sometimes complex returns is quite challenging and much effort has been devoted to determining the statistics of meteor numbers, mass, trajectories and the manner in which ionisation trails develop, break up and decay. The RADAR techniques employed by professional observers has generated a huge amount of information on these topics and today much is understood that could not be deduced from optical or photographic means alone. Amateur RADAR observations of meteors has developed over the years and is well within the capability of amateur radio enthusiasts, for example, who regularly use meteor trails for long range communications (if only for brief periods).

How amateurs can set up a Meteor Radar.

Some experienced radio amateurs are capable of building a complete RADAR system with both transmitter and receiver. Fortunately for most people it is not necessary to own and use a high power transmitter with all the issues that this involves. The existence of high power broadcast transmitters in various countries means that there is no lack of signals reaching the ionosphere and capable of interacting with meteor trails. Many amateurs make use of old VHF analogue TV transmitters, but with the advent of specialist very high power continuous wave (CW) transmitters in several countries including France and the US, it is now possible to use these as the basis of a Meteor Radar system.

Scope of this Article

This article is intended for amateur radio astronomers who wish to build a low cost receiving system to detect meteors using the French Space Surveillance Radar known as GRAVES.

In section 2 we examine the nature of meteor particles and their origins. The difference between sporadic and meteor showers is examined and the dates of major meteor showers through the year are listed.

In 2.3 the nature of meteor dust particles is discussed and the type of ionised trails they generate through ablation is examined. An example is given of the height distribution of meteor trails in the upper atmosphere.

In section 3 we look at the basics of pulsed RADAR and how a CW Doppler radar is different.

Section 4 explores the Graves RADAR in France and shows how the beam geometry intersects the lower ionosphere some 100km from the southern UK.

Section 5 gets to the heart of what is needed to construct a receiver with sufficient frequency stability and sensitivity to receive echoes from the Graves RADAR. It then deals with the nature of echoes received and their relationship to the physics of their generation.

Section 5.5 shows how automatic echo counting can be conducted with the aid of a computer and freely available software specially configured for this purpose. Also in this section, we show some amateur echo detection rate results and how the signal strengths of echoes follow a logarithmic distribution with numbers.

Section 6 is concerned with classification of echo types based on their endurance and frequency behaviour.

Section 7 explores the nature of echo fading diffraction.

In section 8 we look at meteor head echo velocity distribution and the diurnal variation in echo numbers together with an example of peak echo rates during a meteor shower and the long term variability of echo rates over several years.

Section 9 contains a number of conclusions and observations about the radar detection of meteors by amateurs.

Finally, we present data gathered by the author during the 2010 Orionid shower using the equipment previously described, showing that detection of up to a thousand meteors during a shower is possible with amateur equipment.