Introduction

Meteor burst communication (MBC) can provide inexpensive,* very low data rate connectivity on links up to about 900 sm (1440 km) long. In this discussion, very low data rates are in the range of 10 to over 100 bps average throughput.

MBC utilizes the phenomenon of scattering of a radio signal from the ionization trails caused by meteors entering the atmosphere. A meteor trail must have some form of common geometry between one end of a link and the other. The usable life of a trail is short: from tens of milliseconds to several seconds. Thus a particular link can sustain useful communication with that trail for a very short period of time and then the users will have to wait for another trail entering the atmosphere with similar geometry characteristics. On a particular MBC link a transmitter bursts data when a common trail is discovered, waits for another trail, and bursts data again. The time between useful trails is called waiting time.

The useful radio-frequency range for meteor burst operation is between about 20 and 120 MHz. The lower frequencies are ideal and provide the best performance. As we have seen in Chapter 12, receivers in these "lower" frequencies are externally noise limited. Galactic and man-made noise are of such a magnitude in comparison with the expected signal levels of MBC systems that these lower frequencies become virtually unusable for meteor burst communications. As shown in Figure 12.21, this external noise drops off as the square of the frequency. Unfortunately, MBC performance also drops

* Inexpensive in the context of LOS microwave, troposcatter, and possibly satellite communications over equivalent distances.

Radio System Design for Telecommunications, Third Edition By Roger L. Freeman Copyright © 2007 John Wiley & Sons, Inc.

Diagram Meteor Burst Communication
Figure 13.1. The concept of operation of a meteor burst communication link.

off as a function of frequency. It has been found by experience that the range 40-55 MHz is a compromise operational frequency band for MBCs.

MBC transmitters have output powers from under 100 W to 5 kW or more. Antennas for fixed-frequency operation are usually Yagis, and horizontally polarized log periodics (LPs) if we wish to cover a bandwidth as wide as 5 MHz. Figure 13.1 illustrates the concept of a MBC system.

The implementation of MBC systems is attractive, especially from the standpoint of economy. The low data rate and the waiting times are disadvantages. One common application is remote sensing of meteorological conditions and/or reporting of seismic data. One large MBC system is installed in the Rocky Mountains in the United States to provide data on snowfall and accumulated snow. The system is aptly called SnoTel. MBC also has application in high latitudes because such systems are generally unaffected by aurora, polar cap events, or magnetic storms where HF systems often become virtually unusable.

Another application for MBC is for a "reconstitution orderwire.'' During natural disasters and general war, large portions of the PSTN or other networks may appear to be wiped out. Well-planned MBC systems can serve to coordinate efforts to reconstitute the PSTN or what is left of it.

13.2 METEOR TRAILS 13.2.1 General

Billions of meteors enter the earth's atmosphere every day. One source (Ref. 1) states that each day the earth sweeps up some 1012 objects that, upon entering the atmosphere, produce sufficient ionization to be potentially useful for reflecting/scattering radio signals.

Meteors enter the atmosphere and cause trails at altitudes of 70-140 km. The trails are long and thin, generating heat that causes the ionization. They sometimes emit visible light. The forward scatter of radio waves from these trails can support communication. The trails quickly dissipate by diffusion into the background ionization of the earth's atmosphere.

Meteor trails are classified into two categories, underdense and overdense, depending on the line density of free electrons. The dividing line is 2 X 1014 electrons per meter. Trails with a line density less than the value are underdense; those with a line density greater than 2 X 1014 electrons per meter are termed overdense. The dividing line of about 2 X 1014 electrons per meter corresponds to the ionization produced by a meteor whose weight is about 1 X 10 3 g. When averaged over 24 h, the number of meteors is almost inversely proportional to weight. As a result, we would expect that the number of underdense trails would far exceed the number of overdense trails. However, the signals reflected from underdense trails fall off roughly in proportion to the square of the weight, whereas signals from overdense trails increase only a little with weight. In practice, though, we find perhaps only 70% of received MBC signals are from underdense trails. Even so, the mainstay of a MBC system is the underdense trail.

Another interesting and useful fact about sporadic meteors is their mass distribution. This distribution is such that the total masses of each size of particle are approximately equal (i.e., there are ten times as many particles of mass 10 4 g as there are particles of mass 10 3 g). Table 13.1 lists the

TABLE 13.1 Estimate of Properties of Sporadic Meteors
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