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Meteor Burst Communications The Gap Filler
Daniel E. Warren
Rome Laboratory
Robert Desourdis
Science Applications International Corporation
Introduction
Each day billions of meteors enter the upper atmosphere with enough energy
to ionize gas molecules sufficiently to reflect radio waves and enable
communications beyond line of site. Since the typical height of an ionized
meteor trail is about 100 kilometers, this mechanism will support beyond
line of site communication typically to a distance of 1800 kilometers.
Previously, there was no way to detect and take advantage of a usable trail
before it dissipated; thus meteor burst communications was a curiosity,
occasionally used by radio amateurs and had very little practical
application. The advent of modern, low cost digital technology and high
speed numerical processing equipment has brought about a rapid evolution of
meteor burst technology that is commercially available and will provide
reliable beyond line of site communications when other media fail. The
purpose of this paper is to introduce the reader to the principles of
meteor burst communications and to show where it fills a need for reliable
communications. The present state of the art will be discussed followed by
a discussion of current efforts to enhance the performance of this form of
communications.
Principles of Operation
A typical meteor burst network is shown in figure 1. It consists of one or
more master stations and a number of remote stations. The master stations
can communicate with a remote station or another master station. The remote
station can only communicate directly with a master station. Communications
from one remote station to another remote station must be relayed through a
master station. Once a usable trail is detected and its quality determined,
a quantity of digitized data is transmitted in a high speed burst. The
existence of a usable trail is determined by the reception of a probe
signal, transmitted by a master station, by another station in the network.
When another station receives the probe signal it transmits back an
acknowledgement to the master station indicating that a usable trail exists
and it is ready to exchange data. This handshaking uses a considerable
portion of a trail's useful lifetime and takes place each time a burst of
data is transmitted, often several times during the life of a meteor trail
and it . This trade off sacrifices some of the data rate for improved
systems reliability. Typical transmission data rates vary from a few
kilobits per second to over 100 kilobits per second.
Characteries of Meteor Trails
In order to understand the limits of a meteor burst system it is necessary
to discuss the characteristics of the trails in more detail. The meteors
that create these trails are, for the most part, bits of metallic and
nonmetallic material in orbit around the sun. This matter enters the upper
layers of the atmosphere at a velocity between 11 km per second, the escape
velocity from the earth, and 72 km per second, the vector sum of the escape
velocity from the Earth, the escape velocity of the solar system and the
orbital velocity of the earth around the sun. When these particles strike
the molecules of gas in the upper atmosphere the released kinetic energy
raises the temperature of the gas molecules to a temperature at which they
ionize and form a plasma of electrons and positive ions, not much different
than that in the ionosphere, except that the trail is more in the form of a
line charge. Only a small number of meteors that produce an ionized trail
will be usable for a given link. In order to be usable they must meet the
following three criteria. First they must produce an ionized trail with an
acceptable reflection loss, typically about 70 db; this takes an electron
density of about 10e+14 electrons per meter requiring a meteor with a
typical mass of .001 grams and a diameter of 1mm. Larger meteors are even
more desirable but their abundance is inversely proportional to their mass;
smaller ones are more plentiful but not as useful. The next criteria is
that they must be within a common volume that is within line of site of
both stations and at an altitude between 80 and 120 kilometers. Last, they
need to be tangent to the surface of an ellipse of which the two stations
are the foci, as shown in figure 2. This last criteria can be relaxed
somewhat for the more dense trails which will be discussed later. A meteor
trail does not dissipate by recombination, as does the ionosphere, but by
dispersion. Due to the repulsive nature of the electrons for each other,
the trail begins to spread in a radial direction once it is formed. As the
trail spreads, the radio waves scatter off different portions of the trail
causing a multipath situation which degrades the quality of the signal.
This becomes more severe at the higher frequencies where the wavelengths
are shorter.
If the electron density of the trail is less than about 10e+14 electrons
per meter most of the energy passes through the trail and only a small
amount of energy is scattered back to the receiving station. This is
referred to as an underdense trail. Trails with higher electron densities
reflect more completely, as if they were an electrical conductor and are
referred to as overdense; they require meteors of larger mass which are not
as common. As the overdense trail spreads, the reflection is less specular
and the reflection spreads out over a larger footprint on the surface of
the earth relaxing the orientation requirement of the meteor trail with an
elliptical surface. The trail is then referred to as nonspecular overdense.
These characteristics can be identified by observing the received signal
strength over a period of time. Figure 3 shows the variation of the
amplitude of the received signal with time for three types of trails.
Figure 3a shows the sharp spike associated with three underdense trails.
The rounded top of the response shown in figure 3b is indicative of an
overdense trail which is usually associated with a larger magnitude of
received signal. Figure 3c is the response of a nonspecular overdense
trail; the signal strength grows as the trail expands and the footprint of
the return signal widens. Multipath is evidenced by the slow temporal
amplitude variation seen in this figure. The signal return from an
overdense trail is intense and may be capable of transmitting a high rate
of data for several seconds, but as the trail spreads, the multipath
corrupts the modulation of the signal which reduces maximum rate of data
transmission.
The duration of a useful signal depends upon a number of factors such as
the altitude at which they form, the initial density of the trail, the
length of the communications link and the presence of winds in the upper
atmosphere. The signal return from higher density trails will persist
longer but multipath may degrade the signal quality and reduce the maximum
rate of data flow. Trails that form at higher altitudes, usually due to the
faster meteors, will dissipate more rapidly in the thinner atmosphere.
Degradation due to multipath is not as severe in the longer communication
links because the difference in path lengths, as the signal scatters from
different portions of the trail, is not as extreme. Ionospheric winds can
prematurely disrupt an underdense trail or may distort an overdense trail
to where a nonspecular link is closed. Increasing the transmitted power and
antenna gain will raise the received signal higher above the received noise
giving the trail a longer useful time as well a higher channel capacity.
The over all rate at which data can be transmitted over a meteor burst link
is also a function of the rate of incident meteors; this has a diurnal and
a seasonal variation. Meteors occur in two categories sporadic meteors and
those that are associated with periodically occurring meteor showers. Since
the shower type are of small significance, meteor burst links are designed
to utilize the sporadic meteors. They are more plentiful at sunrise as the
earth sweeps them up as it orbits the sun and they are less plentiful at
sunset as the earth runs away from them. A typical ratio of the number of
meteors at sunrise to those at sunset is about 4 to 1, as shown in figure
4, but this does not mean that four times as much data can be transmitted
over the link in the morning as in the evening. The average velocity of the
morning meteors is higher than the evening meteors, so they tend to ionize
at a higher altitude and their trails dissipate more rapidly. There is
approximately a 4 to 1 seasonal variation in meteor flux with the peak in
August and the minimum in February as shown in figure 5. A link is designed
to work for the worst case conditions and everything above that is a bonus.
Although meteor burst links are designed to utilize sporadic meteor trails
for the reflecting medium there are other phenomena which can also be used
to advantage. Intermittently, the E layer of the ionosphere, which is at
about the same altitude as the meteor trails, will also reflect VHF signals
and provide a continuous link at a high data rate. This is referred to as
sporadic E and one can not depend upon it in the link planning. For the
shorter links, occasionally an aircraft will provide sufficient reflection
to close the link. If two stations are close enough there may be sufficient
diffraction over the terrain or even a line of sight condition which will
close the link.
Frequencies
The frequencies most commonly used for meteor burst are between 40 and 60
Mhz. While these are not hard limits, frequencies outside of these bounds
have limited applications. The lower frequency is set by atmospheric and
galactic noise, physical antenna size and attenuation due to the D layer of
the ionosphere. At frequencies higher than 60 Mhz, phase dispersion from
multipath limits the useful duration of a given trail. The propagation loss
also increases proportionally as frequency squared.
Antenna Considerations
When considering an antenna to be used in a meteor burst network, there are
two contradicting parameters that must be optimized. The first
consideration should be the coverage of the common volume by the antenna
pattern so as not to miss any usable meteor trails. For all but the extreme
ranges, this requires a considerably wider beamwidth than one would use for
systems that propagate along the great circle route between stations. This
wide beamwidth implies a low gain antenna which adversely impacts the link
power budget and many available meteor trails will be rendered unusable.
The compromise that is often used is to cover about half of the common
volume. Most of the usable meteor trails are concentrated in a region
either side of the great circle route. If one wishes to enhance system
performance by increasing antenna gain, provisions must be made to detect
the usable trail and focus the antenna on it quickly. This is a capability
that will be addressed in a later section of this paper.
Noise Environment
The sensitivity of any receiver is limited by its noise floor, wether the
noise is generated by the receiver itself or comes from an outside source.
Because the scattering losses from most meteor trails are very high, it is
important to obtain the lowest noise floor possible. The limit to the noise
floor in a meteor burst network should be set by the received galactic
noise. This is noise that, for the most part, is radiated by the galaxy. At
meteor burst frequencies, it is typically 10 to 15 db above that which
would be received if the antenna were pointed towards at an infinite
surface that was at a temperature of 290 degrees Kelvin. This level varies
a few db during course of the day as the antenna scans different portions
of the galaxy due to the rotation of the Earth. This low noise condition
will only be met if the receiver is located in an environment away from
major sources of manmade noise such as cities, highways and power lines.
Thunderstorms that are within line of site of the receiver can also raise
the noise level to unacceptable levels.
The Utility of Meteor Burst
What good is a modern communications system with a throughput that is only
measured in kilobits per second? It is often better than one that is too
big, too expensive, too unreliable or that requires too much power, and it
is a lot better than no communications at all. Even with modern technology,
there are not many ways to achieve beyond line of site radio
communications. Satellites are usually the first choice, but due to the
limited number of channels available, authorization for their use does not
come easy, and they are expensive. Satellites are vulnerable to hostile
threats and in the polar regions none of the geosynchronous satellites are
visible. Techniques that rely upon ground wave and tropospheric scatter
offer ranges that are on the order of a few hundred miles and they require
large amounts of power and have large antennas. The HF spectrum, 3-30 Mhz,
can provide communications up to a few thousand miles sometimes, but has
diurnal variation and is easily degraded by solar activity and nuclear
detonations. Atmospheric noise is also quite high at these frequencies.
Meteors, even though they are sporadic, can provide a dependable media for
communications even thought the data rate may be less than we desire. Other
media boast of high data throughput, but that is a luxury that modern
technology has enticed us to become dependent upon. Most routine
information can wait a few seconds, and ten thousand bits per second, about
1200 words per minute, is faster than several people reading simultaneously
can manage. Meteor burst can fill a need for routine communications when
other means are not feasible. As testimony to the success of meteor burst
communications, Meteor Burst Communications Cooperation manufactures
terminals for commercial use. One of the largest groups of users are the
utility companies who need to collect data from remotely located sensors.
The interrogation rate for each sensor is in the order of a few times a day
but they have a large number of sensors in the net. Because they are
transmitting data at a low data rate, they can use low power transmitters,
smaller antennas and redundancy to increase the reliability of
transmission.
Another advantage of meteor burst communications is the low probability of
intercept by an unauthorized station due to the required geometry of the
meteor trail as explained before. This means that the footprint of the
scattered signal on the ground only covers a small area in the immediate
vicinity of the receiver.
HEMBLE
In an effort to increase the throughput rate of a meteor burst link, a
joint effort sponsored by ARPA is aimed at accomplishing this by increasing
the effective radiated power. The High ERP Meteor Burst Link Experiment
(HEMBLE), designed by Science Applications International Cooperation
(SAIC), has been installed to operate between the Air Force Rome Laboratory
research facility at Verona, New York and the Navy NAVELEX facility at
Charleston, South Carolina. The goal of this facility is to demonstrate a
time average data throughput rate of up to 10 kbs.
Each of the High Effective Radiated Power (HERP) units uses an array of 8
yagi antennas mounted on four poles as shown in figure 6 for the
transmitter and another eight yagis for the receiver. Each yagi has a
nominal gain of 13 dbi. If one assumes a perfect ground reflection, this
yields a gain of 28 dbi for each array. The two yagis on each pole are
connected in phase and each pole is a port of an array with four degrees of
freedom to scan in azimuth. The arrays are capable of being scanned to
about 25 degrees either side of boresite which is the beamwidth of the
individual yagis. Since the array beamwidth is typically 18 degrees it is
necessary to scan the beam in order to focus on the meteor trail. Each
array is steered by an individual computer at present. The transmit array
is fed with a four channel power amplifier with 1200 watts of power per
channel for a total output of 4800 watts. The power amplifier is excited
with the output of a MCC 6560B master station. The B series is a unit that
has been modified to modulate at a rate of 128 kbs using QPSK modulation.
Because of the sporadic availability of useable meteor trails, another MCC
6560B unit, using a single stack of two yagis each on transmit and receive,
is used as a reference link to compare against the HERP. Data collected
data on Hemble is very preliminary as of this writing, but the HERP link
does show a significant improvement over the reference link as indicated in
figure 7.
Hemble also functions as a research facility to explore advanced meteor
burst concepts such as improved beam forming techniques and transmitting
digitized voice and imagery. These two sites are also being used
experimentally to communicate with a disadvantaged vehicle, that is one
with a lower transmit power and a lower gain antenna. Modern data
compression techniques have enabled the demonstration of voice
communication at this facility; image transmission is planned to be
demonstrated soon. Another current study is the measurement of the phase of
the received signal between antennas in the array in order to determine the
direction to each meteor trail. This data is used to form a data base to be
used to predict the optimum steering of the array beam for a given time of
the day.
Conclusion
Meteor burst communications has been demonstrated to be a reliable form of
beyond line-of -sight communications even though its throughput rate varies
diurnally and seasonally and has a lower capacity than most other media.
For low data rates meteor burst may be economically advantageous over other
media and will usually work when they fail. The data rate can be raised
considerably by increasing the effective radiated power but this may
require the ability to focus an antenna array on the meteor trail. Raising
the effective radiated power of the master station will also offset the
handicap of a disadvantaged remote terminal. A modern HERP link has been
established that can demonstrate these advantages and is available to
evaluate other techniques that may increase the performance and the utility
of meteor burst communications.