Meteor scatter communications has been used by ham radio VHF enthusiasts for many years. It enables radio communications contacts to be made over distances up to about 2000 km on the VHF bands, although it does require the use of high performance radio stations and some specialised operating techniques
Apart from being used in ham radio circles, meteor scatter communications is also used in professional and commercial radio communications applications. It provides a relatively low cost and reliable form of radio communication for medium distances provided that real time data transfer is not required. As such it is used for radio communications applications such as sending data from remote weather stations and for oil rigs where it provides a useful element of their data communications structure.
The use of meteor scatter communications, whether for ham radio or other applications, requires the use of specialised techniques as the meteor trails only last for a short time. The bursts of signal created by such trails are commonly referred to as "pings", due to their characteristic sound and these pings may only last for as little as a tenth of a second. Nevertheless this is sufficient to carry some information, although several pings may be needed to complete a contact. Although high speed Morse used to be popular, newer modes including WSJT with its FSK441 mode are now very popular.
Basics of meteor burst radio communications
Its basis of operation of meteor scatter communications is quite simple. Every day colossal numbers of meteors enter the Earth's atmosphere. It is estimated that around a million, million enter that atmosphere each day! Most of them are very small, and only the size of a small pebble, but even these are large enough to leave a visible trail lasting a second or so as they burn up. These meteor trails leave ionised particles that are able to reflect radio waves and provide a means of radio signal propagation that is useful to many radio users including radio amateurs.
Larger meteors also enter the atmosphere from time to time. Fortunately for us here below only very few of them make it through the atmosphere and there is only a small number of documented cases of people actually being hit.
There are two categories into which meteors can fall:
- Sporadic meteors
- Meteors from meteor showers
Meteors are considered in different ways as there are slight differences between them.
1 - Sporadic Meteors
By far the greatest number of meteors entering the atmosphere are from what are termed sporadic or random meteors. They arise from the general space debris found in space, and in the solar system, most of it is thought to emanate from the Sun.
The rate at which these meteors enter the atmosphere is not constant. This affects the way in which meteor burst radio communications occur. The variation is caused by a number of factors:
- It is found that space debris is not evenly distributed in the areas through which the orbit of the Earth passes. This means that the months between June and August produce a much greater number of random meteors than do the other months.
- There is also a change in line with the seasons. This results from the change in the attitude of the Earth over the course of a year. To see why this occurs, the analogy of a car travelling along in a rainstorm can be used. As any car driver or passenger will know far more rain hits the front windscreen than any of the other windows. As the seasons change the angle of tilt of the earth changes and different areas of the earth present a larger frontal area to the oncoming meteors. The result of this is that the Southern Hemisphere receives more meteors in March and the Northern Hemisphere has more in September.
- The time of day has a similar effect. The part of the Earth rotating into the sunrise receives far more meteors than the part rotating into the sunset. It can be seen that any meteors entering the Earth's atmosphere at the end of the day have to be travelling faster than the earth. Meteors entering around sunrise are swept up by the Earth's motion. The effect of this is very large showing a ratio of as much as 3:1.
- There is also a link to the eleven year sunspot cycle. It appears that the number of sporadic meteors reaches a peak around the bottom of the cycle and the minimum is around the peak of the cycle. In this way meteor scatter radio propagation from random meteors is at its best when HF radio communications are in the dip of the sunspot cycle, and vice versa.
Although it is possible to use the trails from random meteors to establish radio links at almost any time, it is helpful to understand why there is a considerable variation in the numbers at different times. Obviously this leads to a considerable change in the number of available meteor trails that can be used for radio links.
2 - Meteor Showers
Meteor showers provide a very significant increase in the number of meteor trails that can be used for radio communications. These showers arrive at fixed times of the year, appearing on an annual basis. Some are small, whereas others are much larger and can last for several days. There are hundreds or possibly even thousands of these showers. The smaller showers are not easy to distinguish, but some of the larger showers produce a spectacular display if their occurrence coincides with a clear night.
When a meteor shower is observed it will be seen that the meteors appear to come from a single point in the sky which is known as the "radiant". This is a perspective effect caused by the fact that all the particles enter the Earth's atmosphere parallel to one another. The radiant gives rise to the name of the shower - the Perseids shower has its radiant in the constellation of Perseus.
Showers are caused by groups of particles orbiting the Sun in an elliptical orbit. Usually they are associated with comets which leave their debris behind them. Although not all meteor showers have been linked to particular comets, it is thought that all showers come from this source.
Showers vary in intensity from one year to the next. This happens because the particles are not evenly spread around their orbit. One of the most reliable and constant showers is the Perseids shower, but even this one shows some significant variations from one year tot he next.
|Quadrantids||1 January||3 January||6 January|
|April Lyrids||19 April||21 April||24 April|
|Eta Aquarids||1 May||4 May||7 May|
|June Lyrids||10 June||15 June||21 June|
|Ophiuchids||17 June||20 June||26 June|
|Capricornids||10 July||26 July||15 August|
|Delta Aquarids||15 July||27 July||15 August|
|Pisces Australids||15 July||30 July||20 August|
|Alpha Capricornids||15 July||2 August||25 August|
|Iota Aquarids||15 July||6 August||25 August|
|Perseids||25 July||12 August||18 August|
|Orionids||16 October||21 October||26 October|
|Taurids||20 October||4 November||25 November|
|Cephids||7 November||9 November||11 November|
|Leonids||15 November||17 November||19 November|
|Geminids||7 December||14 December||15 December|
|Ursids||17 December||22 December||24 December|
Meteor trails used for meteor scatter radio communications
It is the meteor trails that the meteors entering the atmosphere leave that cause the radio signals to be reflected. The trails are formed as the meteors of all sizes enter the atmosphere. Entering at speeds of anywhere between 10 and 75 km per second they burn up at altitudes between 80 and 120 km - approximately around the same altitude as the E region or E layer in the ionosphere.
The trails are formed as the friction from the air causes the meteor to heat up to very high temperatures. The temperatures are sufficient to cause the atoms on the surface of the meteor to vaporise leaving a trail of positively charged ions and free electrons. The resulting trail is in the form of a long thin parabola with the meteor at its head. The trail will vary in size but as a rough estimate they can be up to 20 km long but only a few metres wide.
The levels of ionisation in the trails is very high, and is sufficient to reflect radio waves at very high frequencies - often as high as 150 MHz and sometimes more. The trails can be split into two categories - under-dense and over-dense. The dividing line is determined by the density of electrons left by the meteor - normally the dividing line is taken as 1 x 10^14 electrons per square metre.
- Over-dense meteor trails: It is the over-dense trails that have been traditionally used for amateur radio or ham radio communications with either high speed Morse or even SSB. The trails last for longer than the under-dense ones, and this makes it feasible to transfer data using Morse or occasionally SSB. However the trails are less frequent than the under-dense ones because they require larger meteors to produce them and generally they are only experienced at or near the peak of major meteor showers. Additionally, the reflections they produce sometimes have large signal strength variations as well as producing some multipath effects that can cause problems with some forms of high-speed data transmission. Nevertheless these trails are ideal for ham radio operation.
- Under-dense meteor trails: Under-dense trails usually rise to a peak in a few hundred microseconds and then gradually fade away. They may only lasts a few tenths of a second whilst others may last for a few seconds. They fade as the electrons spread out from the main trail and the level of ionisation decreases. Under-dense trails are far more common than the over-dense ones and they are found as a result of random meteor trails as well as during meteor showers. These trails are used more for commercial radio communications applications, although they can still be used for ham radio.
The maximum distance which can be achieved using meteor scatter is about 2000 km. Performance also falls off at short distances, generally below about 400 km. Here very high angles of radiation have to be used and this results in requiring the signal to be reflected that a large angle and this limits the effectiveness of the system. Generally the optimum distance is around about 1000 km or so.
When using meteor scatter radio communication, it is found that the optimum beam heading is not exactly the line between the two stations who are communicating. Instead most of the useable reflections occur slightly off to the side or the other. Usually this is between ten and fifteen degrees away from the direct path.
The antenna gain is an important feature for stations using meteor scatter communications. Obviously the main advantage of the gain is that signal strengths can be improved. This can be of great importance for amateur radio systems where transmitter power can be a limiting factor. However this gain can bring some disadvantages with it. By increasing the aerial gain the beam width is reduced. This in turn this means that the amount of the sky which is illuminated is reduced. By doing this the number of meteor trails which can be used is also reduced. If the antenna gain is increased too far then this can actually reduce the number of meteor trails that can be used and degrade the performance of the system.
Meteor scatter operating and bands
Meteor scatter or meteor burst radio communications can be used on a wide variety of frequencies. For ham radio most of the communications take place on the two metre band, although there are some contacts made on the 70 centimetres ham radio band, but this is very close to the absolute top limit for this form of radio propagation.
Some meteor scatter operation takes place on the 50MHz ham radio band. The lower frequencies here mean that the reflections are more effective. However on occasions, around the peak of the sunspot cycle, propagation is more likely to take place as a result of ordinary ionospheric propagation. This is one of the factors that determines the lower limit for meteor scatter radio communications.
Using meteor scatter
Meteor scatter communications can be very different to what may be termed the more normal or traditional forms of radio communications used by ham radio operators. The bursts of short-lived signal paths between the two ham radio stations means that special techniques are required. To achieve this specialised protocols or ways of worked were developed to enable the communication to be established and the information passed effectively between the two stations. A single meteor trail may only support a number of the steps needed to exchange information, and a complete contact may require the use of several meteor trails over a period of time.
A variety of transmission modes can be used with meteor scatter. For ham radio users in Europe the use of high speed Morse was popular. Using Morse transmissions speeds up to 800 words a minute were used. Originally the Morse was pre-prepared and speeded up using tape recorders, the reverse process being used to enable it to be deciphered later. For ham radio operators in North America Single Sideband was widely used.
The widespread availability of computers now means that they can be used to provide much greater levels of flexibility. Not only can they be used for the generation and reception of high speed Morse, but they have also enabled the creation of specialised transmission modes developed especially for meteor scatter operation.
One popular form of transmission for use in ham radio with its associated computer programme is known as WSJT. Developed for ham radio use by K1JT it was written explicitly for meteor scatter communication. It only requires the use of a computer sound card, and possibly an interface box to ensure the right levels are presented to each input. This makes it ideal for use within ham radio as little new equipment is needed.
WSJT includes several modes that can be used. The first mode, and the one that is most widely used is known as FSK441. It employs multi-frequency shift keying with four tones and a data rate of 441 baud. The system is also self-synchronising, as a result of the character codes used in the protocol and this has the advantage that it does not require an explicit synchronization tone. FSK441 is generally used on the 2 metre and 70 centimetre amateur radio bands.
WSJT in the FSK441 mode can utilise very short pings, and this means that communication does not rely on the longer pings normally only found during meteor showers. Accordingly it can be used at any time.
Meteor scatter overview
Meteor scatter communications, or as it is sometimes called, meteor burst communications is a particularly interesting mode of radio communications open to ham radio operators, and it can be used to very good effect at VHF. It offers a form of propagation that can be used to provide contacts over distances up to a maximum of around 2000 km, and as such it is unique at these frequencies.