U.S. patent application number 10/246828 was filed with the patent office on 2004-09-02 for radome compensation using matched negative index or refraction materials.
Invention is credited to Barker, Delmar L., Schmitt, Harry A., Schultz, Stephen M..
Application Number | 20040169616 10/246828 |
Document ID | / |
Family ID | 32907492 |
Filed Date | 2004-09-02 |
United States Patent
Application |
20040169616 |
Kind Code |
A1 |
Schultz, Stephen M. ; et
al. |
September 2, 2004 |
RADOME COMPENSATION USING MATCHED NEGATIVE INDEX OR REFRACTION
MATERIALS
Abstract
A compensated radome is provided, comprising an inner layer of a
negative index of refraction material, often referred to as a
"metamaterial", and an outer layer of a positive index of
refraction material. The thickness of the two materials and their
respective refractive indices are adjusted so that a beam of light
passing through the radome is effectively not refracted. The
metamaterial-compensated radomes solve the bore sight angle problem
with a minimum of complexity.
Inventors: |
Schultz, Stephen M.;
(Spanish Fork, UT) ; Barker, Delmar L.; (Tucson,
AZ) ; Schmitt, Harry A.; (Tucson, AZ) |
Correspondence
Address: |
Patent Docket Administration
RAYTHEON COMPANY
Bldg. E1, M.S. E150
P.O. Box 902
El Segundo
CA
90245
US
|
Family ID: |
32907492 |
Appl. No.: |
10/246828 |
Filed: |
September 19, 2002 |
Current U.S.
Class: |
343/872 |
Current CPC
Class: |
H01Q 1/421 20130101;
G02B 1/007 20130101; B82Y 20/00 20130101; H01Q 1/422 20130101 |
Class at
Publication: |
343/872 |
International
Class: |
H01Q 001/42 |
Claims
What is claimed is:
1. A compensated radome, comprising an inner layer of a negative
index of refraction material and an outer layer of a positive index
of refraction material, wherein each layer has a thickness, with
said thickness of each layer and their respective refractive
indices adjusted so that a beam of electromagnetic radiation
passing through said radome is effectively not refracted.
2. The compensated radome of claim 1 wherein said inner layer has a
different thickness than that of said outer layer.
3. The compensated radome of claim 2 wherein said inner layer has a
different absolute value of index of refraction than that of said
outer layer.
4. The compensated radome of claim 1 wherein said inner layer and
said outer layer have an identical thickness.
5. The compensated radome of claim 4 wherein said inner layer and
said outer layer each have an identical absolute value of index of
refraction.
6. The compensated radome of claim 1 wherein said negative index of
refraction material comprises a photonic band gap material.
7. The compensated radome of claim 1 wherein said negative index of
refraction material comprises a left handed material.
8. The compensated radome of claim 1 wherein said electromagnetic
radiation is selected from the group consisting of UV, visible, and
IR wavelengths.
9. The compensated radome of claim 1 wherein said electromagnetic
radiation is of RF wavelength.
10. The compensated radome of claim 9 wherein said thickness of
said inner layer and said thickness of said outer layer have a
total thickness that is one-half wavelength of said RF
wavelength.
11. A missile including a radome, said radome compensated against
bore sight error, said radome comprising an inner layer of a
negative index of refraction material and an outer layer of a
positive index of refraction material, wherein each layer has a
thickness, with said thickness of each layer and their respective
refractive indices adjusted so that a beam of electromagnetic
radiation passing through said radome is effectively not
refracted.
12. The missile of claim 11 wherein said inner layer has a
different thickness than that of said outer layer.
13. The missile of claim 12 wherein said inner layer has a
different absolute value of index of refraction than that of said
outer layer.
14. The missile of claim 11 wherein said inner layer and said outer
layer have an identical thickness.
15. The missile of claim 14 wherein said inner layer and said outer
layer each have an identical absolute value of index of
refraction.
16. The missile of claim 11 wherein said negative index of
refraction material comprises a photonic band gap material.
17. The missile of claim 11 wherein said negative index of
refraction material comprises a left handed material.
18. The missile of claim 11 wherein said electromagnetic radiation
is selected from the group consisting of UV, visible, and IR
wavelengths.
19. The missile of claim 11 wherein said electromagnetic radiation
is of RF wavelength.
20. The missile of claim 19 wherein said thickness of said inner
layer and said thickness of said outer layer have a total thickness
that is one-half wavelength of said RF wavelength.
Description
TECHNICAL FIELD
[0001] The present invention relates generally to radomes, and,
more particularly, to compensated radomes for eliminating bore
sight errors produced by refraction of the radome.
BACKGROUND ART
[0002] Optically-transparent radomes suffer from refraction, due to
the bending of light through the radome material. It is desired to
compensate such radome effects that degrade performance of
present-day missile radar seekers. These radome-induced distortions
ultimately limit the missile's ability to detect stressing targets
in the presence of severe interference. RF adaptive array
processing places very stringent requirements on antenna
calibration and radome compensation. These requirements will only
increase in the future, as inexpensive countermeasures become
increasingly sophisticated and widespread.
[0003] Compensating bore sight shift is typically accomplished by
measuring the radome bore sight shift in a RF chamber and storing
compensation coefficients to be used in software algorithms.
Compensation coefficient tables are typically a function of RF and
gimbal angle, both azimuth and elevation. System requirements drive
compensation accuracy, which in turn drives the quantity and
quality of data that must be measured and stored for compensation.
Heating caused by aerodynamics, radome-to-radome variability, and
aging may also have to be considered when compensating. Such
compensating methods add to the cost and complexity of radome
systems
[0004] Thus, there is a need for compensated radomes that avoids
most, if not all, of the foregoing problems.
DISCLOSURE OF INVENTION
[0005] In accordance with the present invention, a compensated
radome is provided, comprising an inner layer of a negative index
of refraction material, often referred to as a "metamaterial", and
an outer layer of a positive index of refraction material. The
thickness of the two materials and their respective refractive
indices are adjusted so that a beam of light passing through the
radome is effectively not refracted.
[0006] The metamaterial-compensated radomes of the present
invention solve the bore sight angle problem with a minimum of
complexity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a schematic diagram, depicting the bore sight
error caused by refraction within a radome material;
[0008] FIG. 2 is a diagram depicting the refraction of an incident
ray for negative and positive refractive index material;
[0009] FIGS. 3a-3b depict a schematic diagram of a radome and its
refraction for (a) a conventional radome and (b) a compensated
radome of the present invention.
BEST MODES FOR CARRYING OUT THE INVENTION
[0010] FIG. 1 shows how the presence of a radome 10, forming the
forward portion of a missile 12 (shown in partial), introduces bore
sight errors. A sensor 14 is mounted on a gimbaled mount. The
nature of the sensor 14 is immaterial, and may comprise any of the
known sensors operating in the visible, IR, or UV portion of the
electromagnetic spectrum. Alternatively, the sensor 14 may instead
be an antenna or other receiving means for receiving a signal and
then conveying the signal to the actual sensor. While a missile
with a radome is specifically illustrated, it will be appreciated
that the present invention is not limited to missiles per se, but
rather is directed to improving radomes, regardless of the craft on
which they are used.
[0011] The function of the sensor 14 is to sense the location of a
target 16. However, the radome 10 comprises an optical material
having a finite thickness. Due to the nature of light passing
through different materials (the atmosphere 18 inside the radome
10, the transparent radome itself, and the atmosphere 20 outside
the radome), light is bent at each interface, as is well-known. As
a result, a target 16 that is actually at one location is perceived
by the sensor 14 to be at a different, apparent location 16'. The
angle .alpha. between the two positions 16, 16' relative to the
sensor 14 is called the bore sight error.
[0012] A variety of compensation mechanisms have been employed, as
described above. The present invention provides a compensation
approach that uses novel new metamaterials that possess a negative
index of refraction, whose magnitude can be matched to that of the
radome materials. These metamaterials can be either photonic band
gap (PBG) materials or the much more recently discovered left
handed materials (LHM). Both PBG materials, fashioned to have a
negative index of refraction, and LHMs can be used to compensate
for the effects of the radome 10; however, only the LHM are truly
left handed in the sense of having simultaneous -.mu. and
-.epsilon..
[0013] Examples of PBG materials and LHMs are increasingly
well-known. For example, PBG materials are described by D. F.
Sievenpiper et al, "3D Metallo-Dielectric Photonic Crystals with
Strong Capacitive Coupling between Metallic Islands", Physical
Review Letters, Vol. 80, No. 13, pp. 2829-2831 (30 Mar. 1998); D.
Sievenpiper et al, "High-Impedance Electromagnetic Surfaces with a
Forbidden Frequency Band", IEEE Transactions on Microwave Theory
and Techniques, Vol. 47, No. 11, pp. 2059-2074 (November 1999); and
M. Notomi, "Theory of light propagation in strongly modulated
photonic crystals: Refractionlike behavior in the vicinity of the
photonic band gap", Physical Review B, Vol. 62, No. 16, pp.
10,696-10,705 (15 Oct. 2000). The contents of the foregoing
references are expressly incorporated herein by reference.
[0014] Examples of LHMs are described by D. R. Smith et al, "A
composite medium with simultaneously negative permeability and
permittivity", Physical Review Letters, Vol. 84, No. 18, pp.
4184-4187 (1 May 2000), which discloses demonstration of a
composite medium, based on a periodic array of interspaced,
conducting, nonmagnetic split ring resonators (SRR) and continuous
wires, that exhibits a frequency region in the microwave regime
with simultaneously negative values of effective permeability
.mu..sub.eff(.omega.) and permittivity .epsilon..sub.eff(.omega.).
This structure forms a "left-handed" medium (that is, ExH lies
along the direction of -k for propagating plane waves), for which
it has been predicted that such phenomena as the Doppler effect,
Cerenkov radiation, and even Snell's Law are inverted. See also D.
R. Smith et al, "Negative Refractive Index in Left-Handed
Materials", Physical Review Letters, Vol. 85, No. 14, pp. 2933-2936
(2 Oct. 2000). The contents of the foregoing references are
expressly incorporated herein by reference.
[0015] The use of metamaterials should also have a number of other
benefits. Because the metamaterials solve the problem on the
"physical level", they are more likely to be able to provide
corrections over larger ranges of RF and gimbal angle, both azimuth
and elevation. Moreover, the PBG materials and the LHMs tend to be
inherently narrow band. This property means that it should be
possible to build into the radome 10 attractive properties, such as
cross-section reduction and improved protection against EMI
(electromagnetic interference) and EMP (electromagnetic pulse). If
the materials can be made tunable, and there is considerable
evidence that they can be, it should also be possible to
incorporate narrow band filtering/frequency agility into the radome
10 itself.
[0016] Specifically-designed PBG and LHM are composite materials
that exhibit the bulk property of a negative index of refraction.
The bulk index of refraction results in a sign reversal of Snell's
law as given by
n, sin(.theta..sub.1)=-n.sub.2 sin(.theta..sub.2),
[0017] where .theta..sub.1 and .theta..sub.2 are the propagation
angles of the electromagnetic waves in the two regions, where
region 1 is a conventional media with a positive index of
refraction and region 2 exhibits a negative index of refraction.
The sign reversal results in the flip of the refracted angle about
the surface normal. FIG. 2 shows a ray 22 incident on a slab 24 of
high index material. If the slab 24 of material has a positive
index of refraction (n>0), then the ray 22a is refracted toward
the normal 26 to the surface 24a, with the angle .theta. remaining
positive (measured counter-clockwise from the surface normal).
[0018] However, if the index of refraction has the same magnitude
but is negative (n<0), then the refracted ray 22b will still be
refracted towards the surface normal 26, with the refracted angle
becoming negative (measured clockwise). This results in a flip of
the refracted ray about the surface normal 26 as illustrated in
FIG. 2.
[0019] The result of propagation through a slab of material with a
negative index of refraction can be considered as a negative phase
advance rather than a positive phase advance exhibited by a slab of
material with a positive index of refraction. Both the approach
using Snell's law and the concept of phase advance result in an
equal but opposite response between positive and negative index
materials. This equal but opposite response of a material with a
negative index of refraction can be used to compensate for
refraction caused by traditional bulk material.
[0020] FIGS. 3a-3b illustrate how a slab 124 of negative index
material is used to compensate for a slab 24 of positive index
material.
[0021] Specifically, FIG. 3a shows a ray 22 transmitted through a
curved slab 24 of (conventional) positive index material. Examples
of such positive index materials commonly employed in radomes
include glass, transparent plastics, ceramics, and sapphire,
although any material that is heat-resistant, is aerodynamically
strong, and can still transmit radar signals may be employed in the
practice of the present invention.
[0022] After traveling through the slab 24 of material, the ray 22'
is traveling in a different direction than the ray 22 would travel
without the slab 24 of material present, and thus exhibits the
afore-mentioned bore sight error. FIG. 3b shows the same slab 24 of
material with an additional layer 124 of material with a negative
index of refraction. The material 124 with a negative index of
refraction exhibits an equal but opposite response to that of the
material 24 with a positive index of refraction. The resulting
response is a wave 122' traveling in the same direction as a ray 22
would without any material 24, 124 present. And thus, the material
is compensated so as to eliminate the bore sight error. Of course,
the material selected for the layer 124 must not only be of
opposite sign (negative), but also be substantially equal in
absolute value to that of the material comprising layer 24. While
slight deviations may be tolerated between the two values, best
results are obtained where both materials have essentially the same
absolute value of index of refraction.
[0023] On the other hand, if there is a mismatch in absolute value
of index of refraction, it is possible to alter the thickness of
the negative index material to compensate, using well-known
equations available in any optics text book. Further, it is
possible to alter the shape of either of the surfaces on the
negative index material 124, using spheric or aspheric surfaces to
form lensing surfaces, as is well-known in optical materials.
[0024] In addition to the bore sight error, another deficiency in
traditional radomes 10 is the reflection of RF energy off of the
radome. The radome reflection causes a substantial increase in side
lobes of the radar beam. This increase in the side lobe produces a
significant degradation in seeker performance. Requiring the radome
to be 1/2 wavelength thick minimizes the reflection of RF
radiation. When the radome is one half wavelengths thick, then the
wave that traverses the material undergoes a shift in phase of
.pi., thereby resulting in zero reflection. However, this can only
be maintained for one specific frequency and incident angle.
[0025] A material with a negative index of refraction produces a
negative phase advance rather than a positive one. A layer of
material with a negative index of refraction can be used to produce
a negative phase advance that compensates for the positive phase
advance produced by a traditional bulk material. Thus, the
compensated radome exhibits zero phase advance resulting in zero
reflection. Therefore, the compensated radome shown in FIG. 3b also
exhibits zero reflection, given that the thickness of the two
layers and the magnitude of the index of refraction are equal. This
zero phase advance is independent of incident angle. Furthermore,
the zero phase advance of the wave is independent of the thickness
of the material given that the two layers have an identical
thickness. This results in radomes with zero reflections that can
be fabricated to have any thickness.
[0026] The use of metamaterials to perform radome compensation is
novel for a number of reasons. Some of these have been cited
already. The use of negative index of refraction materials is
novel. In fact, LHMs have only been fabricated within the last year
and a definitive measurement of their left handed properties has
only been completed in the last few months. While PBG materials
have been around for a number of years, those with a negative index
of refraction have only recently been demonstrated. To summarize
again the novel properties of the present invention:
[0027] 1. Metamaterial-compensated radomes eliminate the bore sight
errors produced by refraction.
[0028] 2. Metamaterial-compensated radomes eliminate reflections at
all incident angles.
[0029] 3. Metamaterial-compensated radomes eliminate reflection
independent of the total thickness of the compensated radome.
[0030] 4. LHMs are narrow band, so it should be possible to reduce
the radar cross-section (RCS) and enhance electro-magnetic
interference/electro-magnetic pulse (EMI/EMP) protection.
INDUSTRIAL APPLICABILITY
[0031] The use of metamaterial-compensated radomes is expected to
find use in missiles and other optical sighting mechanisms.
* * * * *