Meteor scatter or meteor burst communications

- a summary, overview or tutorial covering the basics of Meteor Scatter or Meteor Burst Communications, a form of radio signal propagation often used at VHF.

Meteor scatter or meteor burst communications use a form of radio communications system that is dependent on radio signals being scattered or reflected by meteor trails. Meteor scatter communications is a specialized form of propagation that can be successfully used for radio communications over paths that extend up to 15000 or 2000 km.

Meteor scatter or meteor burst communications are used for a number of applications on frequencies normally between about 40 and 150 MHz. They are used professionally for a number of data transfer applications, particularly when transferring data from remote unmanned sites to a base using a radio communications link. Nowadays using computer controlled systems, this form of radio communications can offer an effective alternative to other means, and especially where satellites may need to be used because of the cost. In other applications, radio hams use meteor scatter as a form of long distance VHF radio signal propagation.

Basics

Meteor scatter or meteor burst radio communications relies on the fact that meteors continually enter the Earth's atmosphere. As they do so they burn up leaving a trail of ionisation behind them. These trails which typically occur at altitudes between about 85 and 120 km can be used to "reflect" radio signals. In view of the fact that the ionisation trails left by the meteors are small, only minute amounts of the signal are reflected and this means that high powers coupled with sensitive receivers are often necessary.

Meteor scatter propagation uses the fact that vast numbers of meteors enter the Earth's atmosphere. It is estimated that around 10^12 meteors enter the atmosphere each day and these have a total weight of around 10^6 grams.

Fortunately for everyone living below, the vast majority of these meteors are small, and are typically only the size of a grain of sand. It is found that the number of meteors entering the atmosphere is inversely proportional to their size. For a ten fold reduction in size, there is a ten fold increase in the number entering the atmosphere over a given period of time. From this it can be seen that very few large ones enter the atmosphere. Although most are burnt up in the upper atmosphere, there are a very few that are sufficiently large to survive entering the atmosphere and reach the earth.

Meteor categories

It is possible to split the meteors entering the atmosphere into two categories. One category is those that are associated with meteor showers at particular times of the year. The other is the meteors that enter the atmosphere all the time that are known as sporadic meteors.

• Meteor showers:   It found that at specific times during the year, the number of meteors entering the atmosphere rises significantly as a result of meteor showers. They occur as the Earth's path passes through debris in its orbit around the Sun. Often these have been traced back to the passage of a comet. For some of the larger showers, the number of visible trails rise significantly allowing the casual observer to see a worth while of trails in an evening. Of the meteor showers, the Perseids shower in August is probably the best.
  
  Shower meteors are characterised by what is termed their radiant. This is the point in the sky from which they appear to originate. The radiant is usually identified by the name of the constellation or major star in the area of the sky from which they appear to come, and this name is usually given to the shower itself. Apart from the main showers, there are hundreds and possibly thousands of smaller showers that have been recorded, often by amateur observers.
  
• Sporadic Meteors:   The greatest number of meteors entering the atmosphere arises from sporadic meteors. These are the space debris that exists within the universe and in our solar system. The majority of this debris arises from the vast amounts of material that is thrown out by the Sun into the universe. Unlike the shower meteors they enter in all directions and they do not have a radiant.

Changes over the day

It is found that after meteor showers have been discounted, the density of space debris in the solar system is broadly constant, although there are some variations as described later. Despite this the rate at which meteors enter the atmosphere changes considerably over the course of a day. This results from effects associated with the rotation of the Earth. This occurs because the meteors are "swept up" as the Earth's atmosphere rotates into the sunrise, where the atmosphere forms the leading edge as the Earth moves round the Sun. and falls away as it rotates into the sunset. Similarly it falls away at sunset where the atmosphere forms the trailing edge. The same effect can be seen as an automobile is driven in rain, and the rain drops hit the front windscreen but very few hit the rear window.

This effect means that the minimum number of sporadic meteors enter the atmosphere at around 6pm, and the maximum number at around 6 am. Also, the ratio between the maximum and minimum is around 4:1, but the exact figure is dependent upon a number of factors including the latitude at which the measurement is taken being a maximum at the equator and a minimum at the poles.

There are other factors that affect the numbers of meteors entering the atmosphere. One is the season and there are two reasons to which this can be attributed:

• The first is that the density of space debris around the Earth's orbit is not uniform. The density is higher in the areas of the orbit that the earth passes through in June, July and August.
• The other reason is related to declination of the Earth's axis. There is a 22.5 degree tilt of the polar axis relative to the sun that gives rise to the different seasons, and as well as the seasonal variation in meteor rate. Those areas at right angles to the direction of travel will receive the most meteors, whereas those at a greater angle receive less.

These two effects have combine differently dependent upon the hemisphere. The maximum to minimum variation is accentuated in the northern hemisphere where the two effects add together. However it is minimised in the southern hemisphere where the two effects tend to cancel each other.

It is also found that the number of meteors entering the atmosphere changes with the sunspot cycle. The number of meteors rises to a peak around the trough of the sunspot cycle.

Meteor Trails

The meteor trails used by meteor scatter radio signal propagation form as the meteors enter the Earth's atmosphere. As the atmosphere becomes more dense, the meteors burn up as the friction from the rises. The meteors enter the atmosphere at speeds anywhere between about 10 and 80 kilometres a second and they normally burn up and form trails at altitudes ranging between 85 and 120 kilometres, dependent upon factors including the size, speed and angle of entry.

As the meteor enters the more dense areas of the atmosphere and heat starts to be generated as a result of the friction from the air, the meteor heats up to such a degree that the atoms vaporise, leaving a trail of positive ions and negative electrons. The trail that is formed is a very long thin parabola with the meteor at its head. Typically the trails are only a few metres wide, but they may be over 25 km long.

The level of ionisation in the meteor trail is very high. It is much higher than the level of ionisation generated by the Sun in the ionosphere. As a result the frequencies that can be affected are much higher than those normally experienced in the ionosphere. Often frequencies up to about 150 MHz can be reflected by these trails.

Meteor trails can be categorised into two categories according to the density of electrons. One type is termed "over dense", and the other "under dense". The point at which they change from one type to another is taken to be an electron density of 1 x 10^14 electrons per cubic meter. This actually corresponds to a critical frequency of 90 MHz. While the electron density is used to define the type of ionisation trail, it is actually the way in which a trail reacts that is of real importance.

The meteors that create the under dense trails are normally very small, often the size of a grain of sand. Those that generate the over dense trails are usually larger. Typically meteors have to have a mass larger than about 10^-3 grams with a radius of around 0.004 metres to create an over dense trail.

• Over dense trails:   These trails provide relatively "strong" reflections. Having a high electron density, signals do not completely enter over dense trails and they are "reflected". These reflections have a slow rise to the peak strength and a slow decay. Their overall duration is generally a few seconds, but during the period of the reflection the signal undergoes multi-path related effects that affect their performance for the very high data rate transmissions normally used for professional applications. They are less common than under dense traisl as they result from larger sized meteors.

• Under dense trails:   These meteor trails are ones that act in a very much different way to the over dense trails. Having a lower electron density, the signal penetrates the trail and it is scattered rather than being refracting it. In this way some of the signal is returned to earth. Again the portion of the signal that is returned to earth is very small and very efficient radio systems are required to be able to make use of them. The reflected signal typically rises to a peak strength in a few hundred microseconds and then decays. This may take between a few hundred milliseconds to as long as a few seconds. This decay is attributed to the spreading and diffusion of the trail's electrons.

Of the two types of meteor ionisation trail, it is normally the under dense ones are normally used for commercial communications. Over-dense ones are used for ham radio operations. The reason for different types being used is that the requirements for the two types of communications are somewhat different.

Frequencies

In common with other types of radio signal propagation, meteor scatter is frequency dependent. Reflected power levels as well as the burst duration are both affected by the frequency used. The levels of power returned reduce significantly with increasing frequency, as does the effective duration of the trail. As a result the maximum limit for meteor scatter operation is generally around 150 MHz, although some very dense trails have been known to affect frequencies as high as 500 MHz.

For the commercial systems that use the under dense trails the maximum frequency is somewhat lower, and the communications are often limited to a maximum frequency of about 50 MHz. Typically most operation takes place between about 40 and 50 MHz, although operation on lower frequencies would be possible. Below 30 MHz interference levels rise as a result of the increased number of signals resulting from ionospheric propagation.

Doppler shift

When using meteor scatter or meteor burst communications it is found that the signals that are received are subject to a Doppler shift. This arises because the point where the signal is reflected changes as the meteor moves forwards and new ionisation is created, and the trail behind it diffuses. This can give a shift in frequency of as much as 2 kHz on the higher frequency bands although it is correspondingly lower for the lower frequency bands.

Signal paths

Meteor scatter or meteor burst communication is able to support communication up to distances of around 2000 km. There is also a minimum distance that exists. This arises because the meteor trails are only able to reflect signals over small angle. Shorter distances required the signals to leave the transmitter antenna at a higher angle and therefore a much higher angle of reflection is needed. This factor limits the minimum range to about 500 km. The optimum distance is around 1000 km.

Meteor scatter summary

Meteor scatter or meteor burst communications is an interesting form of radio communications that can be used for medium data rate signals at the low end of the VHF spectrum. It is used occasionally for commercial data applications where real time communications are not required. A link is set up that looks for signal propagation via a meteor trail and when one is available the data is transmitted using this. The link remains dormant until the next one is detected. These links use the under dense trails. For ham radio applications most operation takes place during the periods of meteor showers. When signals can be heard, high speed Morse is normally used to transmit the required information.