4999637
Creation of artificial ionization clouds
above the earth
Inventor: Bass
Date Issued: March 12, 1991
Application: 07/049,881
Filed: May 14, 1987
Inventors: Bass; Ronald M. (Houston, TX)
Assignee: APTI, Inc. (Washington, DC)
Primary Examiner: Cangialosi; Salvatore
Assistant Examiner: Attorney Or Agent: Faulconer; Drude
U.S. Class:
Field Of Search:361/231; 342/352; 342/5; 342/367; 376/123; 376/124
International Class: U.S Patent Documents:3133250; 3174150; 3189901; 3300721; 3445844; 3518670; 3882393; 4035726; 4686605; 4712155
Foreign Patent Documents: Other References: Radio Science, vol. 15, No. 2, pp. 213-223, (4/80), "MST Radar at Poker Flat, Alaska", Balsley et al..
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.
Claim: What is claimed is:
1. A method of forming a cloud of artificial
ionization at an altitude above the earth, said method comprising:
initiating heating of the resident plasma at said altitude by transmitting
electromagnetic radiation from the earth to said altitude at an initial
frequency which is approximately the same as the original frequency of said
resident plasma; and increasing said frequency of said electromagnetic
radiation as said frequency of said resident plasma increases, until a final
maintenance frequency is attained, said maintenance frequency being t or above
the plasma frequency necessary to provide a plasma having an electron density
capable of reflecting communication or like signals which come into contact with
said plasma.
2. The method of claim 1 including:
defocusing said
electromagnetic radiation so only the center area of said cloud is initially
heated; and contracting the focus of said electromagnetic radiation as
the frequency of said radiation is adjusted until the entire area of said cloud
is heated.
3. The method of claim 1 wherein said electromagnetic
radiation is transmitted by a single antenna system.
4. The method of
claim 1 wherein said electromagnetic radiation is transmitted by two antenna
systems, each spaced from the other, and inclined whereby the beams of said
electromagnetic radiation transmitted from said systems will intersect each other
at said altitude.
5. A variable frequency heating method for forming a
cloud of artificial ionization at an altitude above the earth, said method
comprising:
transmitting electromagnetic radiation form the earth to said
altitude at an initial frequency which is approximately the same as the original
frequency of the plasma naturally present at said altitude; focusing said
electromagnetic radiation to heat said plasma to thereby accelerate the free
electrons therein thereby increasing the frequency of said plasma;
monitoring the frequency of said plasma as it increases; increasing the
frequency of said electromagnetic radiation as said frequency of said plasma
increases; continuing to increase said frequency of said electromagnetic
radiation until a final desired maintenance frequency is attained; said final
desired frequency being at or above the plasma frequency necessary to provide a
plasma having an electron density capable of reflecting communication signals or
the like which come into contact with said plasma; and continuing to
transmit said electromagnetic radiation at said final frequency to maintain the
integrity of said cloud.
6. The method of claim 5 wherein said final
frequency is greater than the frequency of any communication and/or radar
signals expected to be reflected by said cloud.
7. The method of claim 6
wherein said electromagnetic radiation is initially focused whereby only the
center area of said plasma is initially heated.
8. The method of claim 7
including:
contracting the focus of said electromagnetic radiation as the
said frequency of said radiation is increased whereby the entire area of said
cloud is heated.
9. The method of claim 8 wherein said frequency of said
electromagnetic radiation is increased to approximately match said increasing
frequency of said plasma.
10. The method of claim 9 wherein said
electromagnetic radiation is transmitted by a single antenna system.
11.
The method of claim 9 wherein said electromagnetic radiation is transmitted by
two antenna systems, each spaced from the other, and inclined whereby the beams
of said electromagnetic radiation transmitted from said systems will
intersect each other at said altitude.
Description:
1. Technical Fieldd
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 thatas 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 ofthe 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 ground based
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.
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