Artificial ionospheric mirror composed of a plasma layer which can be tilted
United States Patent 5,041,834
Koert August 20, 1991

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United States Patent 5,041,834
Koert August 20, 1991
Artificial ionospheric mirror composed of a plasma layer which can be tilted

Abstract

This invention relates to generation of a Artificial Ionospheric Mirror
(AIM), or a plasma layer in the atmosphere. The AIM is used like the
ionosphere to reflect RF energy over great distances. A tiltable AIM is
created by a heater antenna controlled in phase and frequency. The heater
antenna phase shift scans a beam to paint a plasma layer. Frequency is
changed to refocus at continually higher altitudes to tilt the plasma layer.

Inventors: Koert; Peter (Washington, DC)
Assignee: APTI, Inc. (Washington, DC)
Appl. No.: 524435
Filed: May 17, 1990

Current U.S. Class: 342/367; 342/372
Intern'l Class: H04B 007/00; H01Q 003/22
Field of Search: 342/367,353,371,372 455/64

References Cited [Referenced By]

U.S. Patent Documents

3445844 May., 1969 Grossi et al. 342/367.
4253190 Feb., 1981 Csonka 455/12.
4686605 Aug., 1987 Eastlund 361/231.
4712155 Dec., 1987 Eastlund et al. 361/231.
4817495 Apr., 1989 Drobot 89/1.
Primary Examiner: Issing; Gregory C.
Attorney, Agent or Firm: Foley & Lardner

Claims

What is claimed is:

1. A method for generating an AIM, comprising the steps of:

(a) creating avalanche ionization in the atmosphere using a heater antenna;

(b) refocusing said heater antenna to alter the altitude of said avalanche
ionization by frequency shifting said heater antenna; and

(c) scanning said heater antenna to paint an avalanche ionization layer.

2. A method for generating an AIM as claimed in claim 1 wherein said heater
antenna is focused in the near field.

3. An apparatus for generating an AIM comprising:

(a) a phased array heater antenna which is focused at an altitude to cause
an avalanche ionization area to be created in the atmosphere;

(b) means for controlling frequency of individual radiators of said phased
array heater antenna to refocus said altitude of said avalanche ionization
area; and

(c) means for controlling phase of the individual radiators to scan said
phased array heater antenna.

4. An apparatus for generating an AIM as claimed in claim 3 wherein said
phased array heater antenna is focused to cause said avalanche ionization
area to be substantially a line.

5. An apparatus for generating an AIM as claimed in claim 4 wherein said
means for controlling phase moves said line substantially at a constant
altitude and said means for controlling frequency moves said line to
different altitudes.

6. An apparatus for generating an AIM as claimed in claim 4 wherein said
phased array heater antenna is a rectangular array and said line is formed
parallel to a long dimension of said rectangular array.

7. An apparatus for generating an AIM as claimed in claim 3 wherein said
phased array heater antenna is focused to cause said avalanche ionization
area to be substantially a point.

8. An apparatus for generating an AIM as claimed in claim 7 wherein said
means for controlling the phase moves said point substantially at the same
altitude and said means for controlling frequency moves said point to
different altitudes.

9. An apparatus for generating an AIM as claimed in claim 3 wherein said
phased array heater antenna is focused in the near field.

10. A method of generating an AIM comprising the steps of:

(a) focusing a phased array heater antenna at an altitude to cause an
avalanche ionization area to be created in the atmosphere;

(b) controlling the frequency of individual radiators of said phased array
heater antenna to refocus said altitude of said avalanche ionization area;

(c) controlling phase of the individual radiators to scan said phased array
heater antenna.

11. A method of generating an AIM as claimed in claim 10 wherein said step
of focusing causes said avalanche ionization are to be substantially a line.

12. A method of generating an AIM as claimed in claim 11 wherein said step
of controlling phase moves said line substantially at a constant altitude
and said step of controlling frequency moves said line to different
altitudes.

13. A method of generating an AIM as claimed in claim 11 wherein said phased
array heater antenna is a rectangular array and said line is formed parallel
to a long dimension of said rectangular array.

14. A method of generating an AIM as claimed in claim 10 wherein said step
of focusing causes said avalanche ionization area to be substantially a
point.

15. A method of generating an AIM as claimed in claim 14 wherein said step
of controlling phase moves said point substantially at the same altitude and
said step of controlling frequency moves said point to different altitudes.

16. A method of generating an AIM as claimed in claim 10 wherein said step
of focusing is performed in the near field.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to generation of a Artificial Ionospheric Mirror
(AIM), or a plasma layer in the atmosphere. The AIM is used like the
ionosphere to reflect RF energy over great distances.

2. Description of the Related Art

In the past, the technique of using the ionosphere as a mirror to reflect
radio waves, or RF energy, has given Ham Radio operators the ability to send
transmissions over long distances. This technique has also provided radar
systems the ability to look "over the horizon." Variations and fluctuations
in the ionosphere, however, can render the effectiveness of such
communications uncertain. Thus, the desirability of creating controllable
plasma layers in the atmosphere for communications purposes has been
recognized. See, for example, U.S. Pat. No. 4,686,605 issued to Eastlund and
U.S. Pat. No. 4,712,155 issued to Eastlund et al.

Previous experiments directed toward creating plasma layers for
communications have suffered from the inability to control the inclination
of the plasma layer so that signals could be transmitted and received from
various ranges. In other words, while one could create a plasma layer in the
atmosphere at a lower altitude than the ionosphere, point to point
communications would be limited in range based on the reflection angles of
the transmitted and reflected signals.

SUMMARY AND OBJECTS OF THE INVENTION

In view of the limitations of the related art it is an object of this
invention to generate a plasma layer that could be angled or tilted with
respect to the horizon in order to affect signal transmission range.

The present invention provides a system and method for generating a plasma
layer at controlled altitudes and inclinations that acts as an artificial
ionospheric mirror (AIM) to reflect RF signals. The AIM increases the range
and predictability with which RF energy may be reflected off the AIM for
communications purposes. More specifically, a tiltable AIM is created by a
heater antenna controlled in phase and frequency. The heater antenna phase
shift scans a beam to paint a plasma layer. The heater antenna continually
refocuses at a higher altitudes by frequency shifting to tilt the plasma
layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details of the present invention are explained with the help of the
attached drawings in which:

FIG. 1 shows creation of an AIM by a heater antenna and use of the AIM for
tracking aircraft and reflecting radio waves.

FIG. 2 shows a typical heater array.

FIG. 3 shows the spacial relationship for a heater array used in defining
heater array focusing.

FIG. 4 is a graph showing that power is at its upper bound at the antenna
focal point.

FIG. 5 shows generation of plasma by a heater array.

FIG. 6 illustrates generation of a plasma layer by scanning a heater
antenna.

FIG. 7 illustrates generation of a tilted plasma layer by scanning and
refocusing a heater antenna.

FIG. 8 shows generation of a plasma layer using a heater antenna to scan
with a line rather than a point.

FIG. 9 shows the phase corrections to move the antenna focal point from 60
Km to 61 Km.

FIG. 10 shows the frequency corrections to move the antenna focal point from
60 Km to 61 Km.

FIG. 11 is a plot of altitude v. distance location of plasma without
frequency chirping.

FIG. 12 is a plot of altitude v. distance location of plasma with frequency
chirping.

FIG. 13 shows the power density change after refocusing using frequency
chirping .

FIG. 14 graphs the free electron density v. altitude for an unfocused array.

FIG. 15 shows an antenna power pattern without grating lobes.

FIG. 16 shows an antenna power pattern with grating lobes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the creation and use of An Artificial Ionospheric Mirror
(AIM) for tracking aircraft and reflecting radio waves. A heater antenna 1
radiates power causing avalanche ionization or breakdown releasing free
electrons in the atmosphere to generate the AIM 2. The heater antenna 1 is
an array which can be used to focus energy at varying altitudes and
elevations to tilt the AIM 2 using phase and frequency control. The AIM 2
simulates the ionosphere 3 which is also used to detect "over the horizon
targets" 5. In addition, the AIM 2 can reflect radio signals transmitted
from a transmitter 6 to a receiver 7 over long distances.

A typical heater antenna is shown in FIG. 2. It consists of an array of
multiple active radiating elements 10 having their individual phase and
frequency controlled from a control module 12. The radiating element 10 is
used here to represent all possible antennas, including, but not limited to,
dipoles, slots, small or large horns, log-periodic antennas, large parabolic
reflectors, etc.

FIG. 3 shows the spatial relationship for a focused heater array. To have
the electric fields from all of the array elements focus, or arrive in phase
at a distance R.sub.o in the near field of the array, it is necessary to
correct the phase of each element to compensate for the phase delay
difference from the center element due to the additional phase path
W.sub.ij. If R.sub.o is much larger than the maximum D.sub.ij in FIG. 3,
then the phase delay can be approximated in wavelength to be:

W.sub.ij =(D.sub.ij).sup.2 /(4*R*g) (1)

where g is the wavelength of the heater frequency. Equation 1 is referred to
as the quadratic phase error. If this error is less than g/8 when the
element (i,j) is on the outer edge of the array, then the distance R.sub.o
is said to be in the far field of the array.

In order to focus the array at R.sub.o, it is necessary to have several
wavelengths of phase error from the outer elements of the array. That is,
the term "focus" is used in this context to mean that the electric field
from the array is concentrated in a desired spatial region.

FIG. 4 shows the degree of focusing that can be accomplished. This is a
vertical pattern of an array whose elements have been phase shifted to focus
at 60 Km. The array has 400 elements with a total width and length of 2000
g. The peak of the pattern is determined by the 1/R.sub.o.sup.2 dependence.

The AIM ionization layer is created by using this focused power to ionize an
area in the atmosphere, as shown in FIG. 5. The microwave breakdown of air
occurs where free electrons gain enough energy from an electric field to
generate additional free electrons until no more can be generated, thereby
resulting in avalanche ionization, or breakdown. This causes the generation
of a plasma layer 21. For example, a pulse of power from the heater begins
to propagate in the z direction shown in FIG. 5. As the field propagates,
more free electrons are generated. A breakdown point descends vertically
from the focal point of the propagating field giving thickness to the
ionized layer, or plasma layer, until all ionization stabilizes. This
"clamping" creates a thin vertical plasma layer.

Simulation results show that when an array 20 is focused at a point 22,
electric field power peaks at the focal point. Simulation results shows that
given a focused microwave source avalanche ionization, or breakdown will
occur at a power level 3-10 dB below the focal point power level.

To create an AIM, the heater array is focused at a desired altitude to
maximize power at a point and thereby generate plasma. The heater antenna
then "scans" the phase of each array element to move the focal point.

FIG. 6 illustrates creation of a noninclined AIM layer. The heater array 30
is first focused at point 31. The heater array scans horizontally by phase
shifting to a point 32 creating an avalanche ionization line 33. Next, the
heater array scans from a point 34 to a point 35 creating another avalanche
ionization line 36. The heater array continues this process to create an
ionization plane or AIM layer.

In order to form an inclined AIM cloud, each new ionization line must occur
at a slightly higher altitude. By altering the phase or frequency of the
array elements, the focal point can be moved up in altitude, as described
below.

FIG. 7 illustrates creation of an inclined AIM. The heater array 40 is first
focused at point 41. The heater array scans along the x direction to point
42 to generate avalanche ionization along line 43. Next, as in creation of a
non-inclined AIM, the heater array scans along the x and y directions
directly below point 44. The heater array 40 alters either phase or
frequency to refocus to a higher altitude in the z direction to the point
44. The heater array then scans along the x axis to point 45 to create the
avalanche ionization line 46. The heater array continues this process to
create a tilted ionization plane or tilted AIM layer.

FIG. 8 shows that the preferred method of generating a plasma layer uses a
heater antenna to scan with a line rather than a point. Scanning using a
line is preferred since an AIM can be created in the atmosphere in less
time. To create lines of ionization rather than points, a rectangular array
50 is used. In the array 50, radiating elements are focused only along the
plane of the long dimension of the rectangular array, creating a line of
ionization 53. The array is then scanned along the x-y axis and in altitude
along the z axis to create another ionization line 55. More ionization lines
are similarly generated to form a tilted AIM layer.

In order to create a tilted AIM it is necessary to refocus the heater array
at successively higher altitudes. Moving the focal point by changing the
phase of each element of the heater in a very precise manner is not
practical. Moving the focal point away from the initial location requires
changing the phase on each element. The phase change required is near the
rms tolerance level, typically 1 degree. FIG. 9 shows the required phase
corrections to move the focal point from 60 Km to 61 km. Elements 5, 10, 15,
and 20 have distances 5d, 10d, 15d, and 20d, respectively from the center of
the antenna, where d=25 meters It is clear from FIG. 9 that it is
impractical to alter numerous antenna element phases to move the focal point
to create tilted patches for AIM applications. 2000 elements may be required
here to generate enough power to ionize the atmosphere.

The second method of refocusing is accomplished by first setting the phases
of all elements for the initial focal point and then moving the focal point
by changing the frequency rather than the phase. This frequency chirping
method is less precise, but easier for hardware implementation because
precise phases for 2000 elements need not be changed. FIG. 9 shows the
required phase corrections to move the focal point from 60 Km to 61 Km. FIG.
10 shows that the focal point can be moved 100 meters by increasing the
frequency approximately 1 Mhz. The resulting focal point power levels are
not completely optimized, but simulation shows that there is less than a 0.1
db difference between the frequency shifted peaks and those obtained by
phasing.

Tilting the AIM using frequency chirping is practical to achieve in a real
system. FIG. 11 shows the plasma layer location with no frequency chirping.
FIG. 12 shows the plasma location of the same heater creating a tilted AIM
by increasing frequency from 550 MHz to 559.375 MHz while scanning
horizontally. The result is a smooth patch with a 45 degree inclination.

While it is true that frequency chirping does not achieve the same power as
phase focusing at the higher altitude, the difference for small frequency
chirps is negligible. FIG. 13 shows actual power density data generated by a
300 MHz heater focused at 70 km with the frequency chirped to 308 MHz.

In the far field region, power meets its upper bound without focusing. For a
far field or unfocused array, there is no way to raise the ionization
altitude or create a tilted AIM. Ionization takes place at a point where
there is enough power to initiate breakdown and where there is low enough
neutral density (i.e. pressure). This usually occurs between 40 and 50 km
altitude as shown in FIG. 14. Consequently, a near field focused antenna is
required to create a controlled AIM.

The focused pattern is a picture of constructive and destructive
interference of the fields from the elements of the array. Other
interference positions, or grating lobes, outside the focal point occur when
some of the array elements add up in phase. The power of grating lobes can
be kept below that of the main lobe, or focal point, by having a large
number of elements in the array and spreading them out over the array
aperture. This is called thinning the array. For square arrays having 400
elements or more grating lobes can be kept down by 20 db or more from the
focal point.

The degree of focusing depends on the ratio of focal range to aperture size.
The half power width from peak "V" can be approximated as:

V=2*g*(R.sub.o /L).sup.2 (2)

where L is the length of the array which is assumed square for equation 2.
The power gradient at the half power point "grad(P)" can be approximated as:

grad(P)=10/V(db/meter) (3)

For an AIM it is desirable that the power gradient be high because this
directly determines the gradient of the electron density of the generated
ionized cloud. The electron density must be high to avoid RF losses caused
by absorption. Hence V be small, preferably less than 2 Km. A heater
frequency of 300 MHz and a focal distance of 70 Km would project an aperture
size greater than 2 Km. Note in equation 2 that array size scales with the
square root of frequency.

Since a near field antenna is required, the near field of the heater antenna
may be required to extend to reach distant points. This is accomplished by
increasing the array size. It may not be economically feasible to fill this
entire aperture with elements, hence a thinned array is utilized.

If a thinned array had its elements uniformly distributed, there would be
many grating lobes in the radiation pattern of the array. These grating
lobes can be eliminated by randomly spacing elements. However, random
spacing puts power from the grating lobes into the average side lobe level.
If no new elements are introduced when the aperture is increased, then the
peak power of the main lobe remains constant and the main lobe receives less
of the total power as its beamwidth decreases. In order to preserve the
efficiency of the heater array, grating lobes must be utilized in creating
the AIM cloud or the array can not be heavily thinned. FIG. 15 shows an
array with uniform spacing having grating lobes. FIG. 16 shows an array with
randomized spacing which eliminates the grating lobes.

Although the invention has been described above with particular reference to
certain preferred embodiments thereof, it will be understood that
modifications and variations are possible within the spirit and scope of the
appended claims.

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