Page 201 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 12: Space-based Wireless WANs

12.2.2 Basic Characteristics Notes

Telecommunications systems developed to use meteor trails transmit data in bursts as intermittent
meteor trails arrive for use; however, not all meteors create trails usable for communications.
Though meteors of all sizes continuously are bombarding or being swept up by the Earth’s
atmosphere, their numbers, or frequency of occurrence, vary relative to their size. Those that
create usable trails for propagating radio waves for effective beyond line of sight (BLOS)
telecommunications are near a milligram in size. They collide with our atmosphere traveling
thousands of miles per hour. At that velocity, friction causes them to heat up and vaporize. The
vaporized atoms escaping from the surface of the meteor collide with those of the atmosphere,
ionizing them and leaving trails of free electrons. This occurs at between 50 and 75 miles altitude.
The trails range from 10 to 20 miles in length with a radius of close to a meter at the head,
when created. However, they dissipate rapidly, generally within one half of a second, though
some trails may last up to several seconds. The latter type is called an overdense trail, because
the density of the free electrons is more than 1014 electrons per meter. Trails with less electron
density are termed underdense. Overdense trails result from larger meteors and thereby are
fewer in number and occur less frequently. When planning an MBC, system, communicators
generally ignore overdense trails due to their lower frequency of occurrence.

Did u know? MBC, systems are designed around predicted availability of the larger
numbers of underdense trails.
The trails are useful for telecommunications because they reflect radio waves back to the
Earth’s surface. The reflections occur in two ways, depending on whether the trail is overdense
or underdense. Overdense trails are packed tightly enough to reflect radio waves in much the
same manner as do layers of the ionosphere; however, underdense trails are less tightly packed,
allowing penetration by the waves, which excites the free electrons and causes them to act as
individual antennas reradiating the wave back to Earth in a scattering fashion. The reflection
properties of meteor trails vary by frequency. Attenuation of waves radiating at frequencies
above 50 MHz increases with the cube of the frequency. Thus, frequencies above 30 MHz but
below 50 MHz, in the lower very high frequency (VHF) spectrum, propagate most efficiently
using meteor trails. Also, radio waves of these frequencies travel in line of sight (LOS) paths.
Thus, to be useful for communications between two points, the trail must be connected to both
points through specular angles of reflection. That is, the signaling wave will be reflected at an
angle equal to the angle of incidence of the transmitted RF wave and will travel in a straight LOS
path.

Another MBC feature is the variation in numbers of meteor trails available to reflect radio waves.
Two principal variations based on movements of the planet must be considered when planning
an MBC system. As the Earth rotates toward the sun in the morning (meteors are moving away
from the sun), it sweeps-up or overtakes meteors, thereby causing large numbers of trails. But
in the evening hours, near sunset, when meteors must overtake the rotation of the planet, the
overtaking trails are fewer. This phenomenon is called the diurnal variation, and its order of
magnitude is approximately 4:1. Similarly, the nominal orbits of meteors are such that most
intersect the Earth’s orbit in the Northern Hemisphere in August and fewest in February (Figure
2). This seasonal variation is in the order of 3:1. Other variations, such as showers of meteors from
specific points in the heavens, occur but are so infrequent that they are not useful for continuous
communications.
MBC system planning should rely on meteors arriving sporadically and occurring at normal
(Gaussian) frequency. This leads to reliance on statistical predictions and the use of models
in identifying link parameters in MBC system designs. Accurate forecasting of meteor trail
availability is critical to successful system design.

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