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Patent No. 4999637
Creation of artificial ionization clouds above the earth (Bass, Mar 12, 1991)
Abstract
A method for forming a cloud of artificial ionization above the earth by initially heating the resident plasma at a desired altitude with electromagnetic radiation having a frequency approximately the same as that of the ambient plasma. As the plasma frequency increases due to heating, the radiation frequency is also increased until the final maintenance frequency is attained.
Notes:
DESCRIPTION
1. Technical Field
The present invention relates to a method for establishing a patch or cloud
of artificial ionization above the earth and more particularly, relates to a
method of forming a cloud of artificial ionization by heating naturally occurring
plasma with electromagnetic energy which is transmitted from the Earth's surface
at a variable, increasing frequency.
2. Background Act
Certain communication and radar systems operate by "bouncing" transmitted and/or
reflected signals off of naturally-occurring layers of ionization in the ionosphere.
One known system using this technique is "over-the-horizon" (OTH) radar. By
bouncing or reflecting the signals off an ionized layer, the signals can actually
travel "over-the-horizon", thereby substantially increasing the range of the
system.
However, while present OTH systems are capable of detecting objects at long
range (e.g. strategic threats), they are not well suited for detecting "close-in"
objects (e.g. missiles at 1000 kilometers or less). One problem lies in the
fact that as the beam angle of the radar is increased from the horizontal, the
frequency of the beam must be lowered in order to achieve refraction at a more
nearly normal incidence. As this frequency is lowered, the antenna system gain
is reduced and the radar cross section decreases for small close-in object.
These effects act to set a minimum range for the OTH system.
Another major problem with present OTH systems is related to the low radar cross
section of small targets at typical OTH operating frequencies. These objects
having small cross-sections produce a weak return signal even when the object
is within the range of the OTH radar since the OTH system is normally designed
for objects having much larger cross-sections, e.g. large aircraft.
A still further problem encountered by present OTH radar is directly related
to the unstable conditions in the ionosphere which widely vary depending on
seasonal, diurnal, and/or sunspot cycles. Accordingly, the operating frequency
of present OTH radar systems has to be constantly adjusted to allow for the
varying ionospheric conditions which may vary so much at times that the OTH
system is rendered inoperable.
Several techniques have been proposed to overcome some of the shortfalls of
present OTH radar systems. One known technique is disclosed in U.S. Pat. No.
3,445,844 wherein a cloud of artificial ionization is formed above the earth
to serve as a layer for redirecting communication signals. The cloud is formed
by "breakdown", i.e. creation of a high level flux of free electrons (i.e. plasma)
at a desired altitude by focusing electromagnetic energy thereon to heat a localized
region or area. The electromagnetic energy heats and accelerate the electrons
in the resident plasma to a degree such that their kinetic energy reaches the
level required for the occurrence of ionizing collisions. Scattering from a
cloud so formed takes place due to the discontinuity between this zone of enhanced
ionization and the surrounding medium.
A cloud formed in accordance with the method disclosed in U.S. Pat. No. 3,445,844
will provide a good reflection layer for OTH radar and like systems. However,
when a cloud is formed by breakdown as in the mentioned patent, the plasma frequency
of the cloud quickly adjusts to the hating frequency and breakdown is initiated
over the entire area of the cloud. By initiating and forming the cloud with
radiation having the same high frequency as that required for continued maintenance
of the cloud once formed, a substantial amount of power is required, much of
which is reflected or passes through the cloud while it is being formed and,
accordingly, is wasted.
BRIEF DESCRIPTION OF THE DRAWINGS
The actual construction, operation, and apparent advantages of this invention
will be better understood by referring to the drawings in which like numerals
identify like parts and in which:
FIG. 1 is a simplified schematical view of a system for forming a cloud of artificial
ionization above the earth for bouncing signals over-the-horizon in accordance
with the present invention; and
FIG. 2 is a schematic illustration of a cross-beam radiation transmission system
for forming a cloud of artificial ionization in accordance with the present
invention.
DISCLOSURE OF THE INVENTION
The present invention provides a method for forming a patch or cloud of artificial
ionization at an altitude above the earth wherein no substantial amounts of
power are wasted in forming and maintaining the cloud.
More specifically, the present invention provides a method wherein variable
frequency heating is used to form a cloud of artificial ionization. This is
accomplished by initially heating the resident plasma at the selected altitude
by transmitting electromagnetic radiation from the earth at an initial frequency
which is approximately the same frequency as the ambient plasma frequency. This
radiation, being of the same frequency as the plasma, will be efficiently absorbed
with relatively little being reflected from or passed through the ambient plasma.
The radiation heats the plasma and accelerates the free electrons in the plasma
thereby increasing the plasma frequency.
The frequency of the plasma is monitored by radar or the like and, as it increases,
the frequency of the radiation being transmitted also increases, preferably
in a manner where the radiation frequency continues to substantially match the
increasing plasma frequency. The radiation frequency is continuously increased
until the final maintenance frequency is attained, at which time, the transmission
of electromagnetic radiation is continued at the final frequency to maintain
the integrity of the cloud. The final plasma frequency (i.e. maintenance frequency)
is selected so that it is always greater than the frequency of any signals (e.g.
communications, radar) that are expected to be "bounced" off the cloud once
the cloud is used for its intended purpose.
To further conserve power and to reduce the power required for carrying out
this invention, the initial radiation is weakly focused so that only the center
area of the plasma within the cloud are will undergo initial heating. The focus
of the radiation is contracted as the radiation frequency is increased until
the entire cloud are is heated by the radiation. The radiation can be transmitted
by a single antenna system or by two spaced antenna systems positioned so that
their beams intersect at said altitude to thereby form the cloud.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring more particularly to the drawings, FIG. 1 is a illustration of how
the present invention is utilized with an over-the-horizon (OTH) radar system.
A cloud 10 of artificial ionization is formed at an altitude above the surface
of the earth 11 by transmitting electromagnetic energy 12 from an antenna system
13. An OTH radar system 14 transmits and receives signals 15 that are reflected
off cloud 10 to detect a target 16 that is "over-the-horizon" as will be understood
in the art.
In accordance with the present invention, cloud 10 is formed by variable frequency
heating. The degree of ionization in the ionosphere depends on the electron
temperature of the average energy of the free electrons; i.e. plasma, at a particularly
altitude. The electron energy can be increased by absorption of incident electromagnetic
radiation. This, in turn, increases the degree of ionization (i.e. number of
free electrons and ions per unit volume). The higher the electron density of
an ionized layer, the higher the frequency of radio or radar waves that can
be reflected from that layer in communications or radar applications.
To raise the electron temperature efficiently, it is necessary to irradiate
the ionosphere at or near the frequency of the plasma that is naturally present
at the altitude of interest. "Plasma frequency" is defined roughly as the highest
frequency that will be reflected from a particular altitude in the ionosphere
and this frequency will increase as the electron density increases. However,
if the frequency of the incident radiation used to heat the plasma is much higher
or lower than the plasma frequency a large portion of the radiation will not
be absorbed but will be reflected or passed through the heating zone and will
be wasted.
In the present method, a target altitude at which cloud 10 is to be formed is
selected and the frequency of the natural plasma is determined by tracking radar
or the like. Heating is begun by transmitting electromagnetic radiation 12 from
antenna system 13 at substantially the same frequency as that of the plasma
resident at the target altitude. Radiation 12 is absorbed efficiently by the
plasma at the target altitude which increase the electron density thereof which,
in turn, increases the frequency of the plasma frequency. As the plasma frequency
increases, the frequency of radiation 12 is increased to approximately match
the increasing plasma frequency. By constantly tracking the increasing plasma
frequency and adjusting the frequency of the heating radiation accordingly,
almost all, if not all, of the power being used to heat the ionospheric electrons
is absorbed efficiently into the plasma over the entire heating cycle. In this
way, the electron density (degree of ionization) can be raised to the desired
level without any substantial waste of power which otherwise would be considerable.
In the present method, the initial heating is carried out with the radiation
13 being broadly focused so that the central area within cloud 10 is being heated.
As the plasma frequency within the smaller area is increased, the focused area
of radiation 13 is contracted until the entire final area of cloud 10 is being
heated by the radiation 13. This minimizes the initial heating power requirements
and results in a further substantial reduction in the overall power requirements
for forming cloud 10.
The antenna system 13 required to transmit radiation 12 in the present invention
may be of any known construction having high direct inability capabilities;
for example, a phased array, beam spread angle (O) type see U.S. Pat. No. 3,445,844
and the "The MST RADAR at Poker Flat, Ak.," Radio Science, Vol. 15, No. 2, March-April,
1980; pps. 213-223, both of which are incorporated herein by reference. A phased
array antenna generating a steerable focused beam can be assembled at a single
site or two phased array antenna systems 20, 21 (FIG. 2) may be spaced from
each of other to generate two coherent beams 23, 24 of radiation that cross
each other at the selected altitude to form cloud 10a.
The key beam geometric parameters of a two antenna system are shown in FIG.
2. The beam of radiation from each antenna is assumed to diverge in the azimuthal
plane (b) and to be collimated in the elevational plane (a). This assumption
is appropriate for a backscatter cloud that must be inclined about 45 degrees
from the horizontal. For forward scattering, with cloud 10a either directly
overhead or downrange from an antenna, the beam will diverge in both planes
and "b" would be used for both antenna. The following relationships can be used
to calculate the various parameters of the system of FIG. 2: ##EQU1## wherein:
a=length of side of antenna array in elevational plane.
.lambda.p=Wavelength of Radiation Frequency
R=Actual distance from array to cloud 10a. ##EQU2## wherein:
b=length of side of antenna array in azimuthal plane
W=Width of cloud 10a ##EQU3##
S=distance between arrays.
D=Depth of cloud 10a.
While the size and characteristics of a particular cloud 10 will vary depending
on its application and actual conditions under which it is formed, the following
example will serve to better illustrate the present invention. A cloud 10 (FIG.
1) is to formed at an altitude of 90 kilometers (km) and is to have a final
area of 1 square km or larger with a thickness of from 3 to 10 meters. The resident
plasma density at 90 km is typically on the order of 10.sup.6 /cubic cm. A square,
phased array antenna system 13 of 100 meters on a side, beams electromagnetic
radiation 12 (i.e. power) at an initial or starting frequency to initiate heating
of the resident plasma. Antenna 13 is focused so that only the plasma at the
center of cloud 10 will initially be heated, thereby requiring substantially
less power than if the entire 1 square km area of the cloud was originally heated.
Since the plasma frequency of the ambient plasma is approximately 9 megahertz
(MH.sub.z), about 70 MW of power will be required. As the plasma heats, the
frequency thereof increased and is tracked by groundbased radar. As the frequency
of the plasma increases, the frequency of radiation 12 is increased accordingly
until a frequency of 15 MHz is reached. The power requirement at this point
will have dropped to approximately 27 MW.
At this point, the focus of the antenna is contracted to cover a greater, if
not all, of the area of cloud 10, and radiation 12 (i.e. heating) is now applied
over the entire area. Due to the increased heating area, the power requirement
is temporarily increased to approximately 33 MW but will quickly decrease as
the radiation frequency continues to increase until the final maintenance frequency
of 300 MH.sub.z is reached. The power requirement for the final maintenance
frequency is approximately 7 MW and final electron or plasma density will be
about 10.sup.9 /cubic cm.
The integrity of cloud 10 will be maintained as long as radiation 12 is transmitted
thereto at the final maintenance frequency. The final maintenance frequency
for any particular cloud is interrelated to the frequency of the radar or other
communication signal which is to be bounced off that cloud. That is, it is generally
preferably to have the plasma frequency of the cloud substantially higher than
the radar frequency. This will ensure a high degree of reflection for the radar.
Also, irregularities in the plasma density may form in the cloud. These will
commonly be spaced about one heater wavelength apart. This spacing should be
relatively small compared to a radar wavelength to avoid excessive scattering
of the radar signal in undesirable directions.