A multimode, multispectral antenna system (14) for detecting radiation from selected target regions in each of at least a pair of selected spectrum bandwidths through collimating lens (35), and rotatable, cooperative prisms (21) effective for collimating and scanning beams of radiation controllably with respect to said target regions.

CROSS REFERENCE TO RELATED APPLICATIONS 
The subject matter of this application is related to the subject matter of 
commonly owned U.S. patent application Ser. No. 800,937 filed on even date 
herewith and bearing the same title as the herein invention. 
TECHNICAL FIELD 
This invention is directed toward the technical field of electromagnetic 
antennas and particularly toward radar antenna for detecting energy in 
selected portions of the electromagnetic spectrum from a target region 
under observation. 
BACKGROUND ART 
Typical radar, electromagnetic detection and/or surveillance schemes of the 
past and present have employed only a single band or a single range of 
electromagnetic energy bands. 
Such systems are typically complex. This tends to discourage the 
development of schemes and systems operating in more than a single range 
or a single set of bands of electromagnetic energy. 
Prior art millimeter radar systems further typically operate with only one 
feed at the focal point of an antenna. For purposes herein, a feed is 
generally considered to be a source of electromagnetic radiation capable 
of receiving the same. Exceptions to this approach are known, (e.g., 
phased arrays, Luneberg lens antennas, multiple or extended feeds, etc.), 
but they are generally either very expensive or they result in degraded 
performance. 
On Feb. 27, 1984, however, the Applicant herein applied for a patent ("Wide 
Angle Multi-Mode Antenna," Ser. No. 584,273) on a radar antenna which did 
utilize two separate feeds in a common aperture arrangement operating at 
about 95 GHz. This system permits the operation of the antenna with each 
beam independently, or in concert. 
Using two kinds of beams operating in the same frequency range provides 
enhanced operational flexibility. However, such a system remains subject 
to diffraction, a fundamental resolution limitation. This diffraction in 
any such system remains directly proportional to the operating wavelength, 
thereby limiting the resolution of microwave and millimeter wave systems. 
Thus, the resolution attainable with millimeter radar, while better than 
that with lower frequency radar, still remains several orders of magnitude 
coarser than attainable with infrared systems operating in either the 3-5 
micrometer or the 8-12 micrometer wavelength region. These regions are 
often chosen for infrared systems because the Earth's atmosphere is 
relatively transparent. Furthermore, infrared systems can operate 
passively, i.e., they do not need to flood a target actively with 
radiation in order to observe the reflected energy, as do radar systems. 
Rather, passive infrared systems detect heat energy which is directly 
emitted by the target. This passive operation offers concealment during 
military operations, and is not susceptible to radar jamming techniques. 
On the other hand, infrared sensors cannot replace the function of radar; 
rather, radar and infrared systems complement each other, For example, 
infrared radiation can be attenuated to unusable levels by clouds, fog, 
rain, snow, etc. while radar can operate effectively in such weather. In 
addition, many target/background combinations appear significantly 
different when viewed in different regions of the electromagnetic 
spectrum. Some targets are therefore more easily detectable in one region 
than another. Furthermore, information received in two or more spectral 
regions can often aid in identification and recognition of a potential 
target, rather than simply in detection. Thus, the use of several types of 
sensors in conjunction with each other can yield a much higher probability 
of mission success under a greater variety of circumstances than can the 
use of one mode or kind of detector operating individually. 
Since space is always at a premium in packaging electromagnetic detection 
systems, particularly when the system is packaged in a missile, it is 
often impractical to consider the inclusion of separate sensors in a 
weapon delivery system. Separate optics, antennas and/or scanning systems 
would also undesirably result in high cost and weight. 
Accordingly, it is an object of this invention to develop a multimode 
antenna for millimeter radar including an infrared sensor, both detectors 
utilizing a common aperture system. 
It is further object of the invention to establish an electromagnetic 
scanning system which uses rotating prisms to direct the view of the 
detection system to selected target regions, said prisms being transparent 
to all modes of electromagnetic energy used in the antenna. 
SUMMARY OF THE INVENTION 
Accordingly, the invention herein is directed toward a multimode 
electromagnetic antenna arrangement operable and effective at several 
spectral bandwidths or frequencies, which employs the same collimating 
lens and rotatable prism scanning system for operation at all modes of 
operation. 
According to a preferred embodiment of the invention, one of the spectral 
bandwidths includes a passive mode of operation employing infrared 
radiation. 
According to a version of the invention, an additional beam focusing 
feature is interposed between the collimating lens and the passive or 
infrared detector in order to establish the position of said passive 
detector at a common focal region with the active radar system source. 
Other features and advantages will be apparent from the specification and 
claims and from the accompanying drawings which illustrate an embodiment 
of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 shows feeds respectively 12 and 13 for sending and receiving 
actively derived electromagnetic signals for transmission to and return 
from selected target regions (suggested, but not shown) generally to the 
right of the apparatus shown in the drawing. These feeds 12 and 13 operate 
in a multimode/multispectral detection system 14 according to the 
invention disclosed herein. The system 14 is considered multimode in that 
several different beams are received and/or transmitted by the system for 
detection and processing, and is multispectral in that at least two 
regions of the electromagnetic spectrum are utilized. 
The version of the invention set forth in detail herein deals primarily 
with the notion of active and passive beams of radiation. However, the 
embodiment disclosed additionally covers the employment of two modes of 
active radiation. 
Active feeds 12 and 13 of the multimode system 14 may each, for example, be 
horn type electromagnetic feeds or broad band signal antennas, 
respectively leading to waveguides 25' carrying the selected 
electromagnetic energy to the horn of feeds 12 and 13 from a suitable 
source such as radar transceiver 25. 
Feeds 12 and 13 are set transversely apart from another, preferably in a 
vertical manner in this instance. According to a preferred version of the 
invention, feed 12 is effective for producing a narrow, pencil beam 12' of 
radiation. This beam 12' expands until it reaches collimating lens 35 
which is a converging lens made of a single kind of material or several 
materials, as will be seen below. 
Feed 13, on the other hand, is effective for producing a broader beam of 
radiation, which can be and is frequently referred to as a fan beam 13'. 
The beam 13' "fans out" in response to the broadening action of lens 31, 
as discussed in greater detail below. The fan beam 13' is typically used 
for general surveillance, and the pencil beam is effective for tracking 
and homing purposes once a target has been detected or "acquired". 
The pencil beam 12' is focused by collimating lens 35 for direction through 
rotatable scanning prism assembly 21. The fan beam 13' is also focused by 
the collimating lens 19, and is then further shaped by a shaping lens 31 
which is generally cylindrical, before further direction through the 
scanning assembly 21. 
U.S. patent application Ser. No. 584,273, filed Feb. 27, 1984, shows 
preferred modes for carrying out portions of the structure of the overall 
system herein addressed insofar as it relates to the establishment of 
multiple active beams and the hardware related thereto. The inventors in 
that case are Peter E. Raber and John H. Cross. The title of the 
Application is "Wide Angle Multi-Mode Antenna". The contents of the 
Application are hereby expressly referred to and incorporated herein. 
The beams 12' and 13' generated are directed toward selected target regions 
by a rotating prism assembly 21 after being collimated by collimating lens 
35. This assembly 21 includes first and second cylindrical prisms 
respectively 21' and 21", which are rotatable about axis 99 coincident 
with the cylindrical axes of the prisms 21' and 21" extending toward the 
selected target region. Prisms 21' and 21" both rotate in the same 
direction at selected speeds, or in opposite directions, or one of them 
can be stationary. This effects the scanning or direction of beams of 
radiation in specific predetermined or preselected directions, without 
reliance upon cumbersome, complicated and expensive mechanical 
arrangements such as gimbal devices, for example, which are relatively 
unreliable and frequently prone to breakdown. Instead, a simple rotary 
drive mechanism 22, employing gears, belts, or friction means, for 
example, to rotate or counter-rotate prisms 21' and 21" can be employed. 
Such mechanisms 22 can conveniently be purchased commercially from any one 
of a number of vendors, or they can be custom designed according to 
well-known techniques from available parts and subsystems. 
In the passive mode of operation, the multimode system 14 includes, for 
example, an infrared or video detector 51. Interposed between the detector 
51 and the collimating lens 19 is a focusing system 54 which can, for 
example, include respectively an infrared lens 55 and an infrared beam 
expander 65. 
The infrared energy need not pass through a multiple element focusing 
system 54. The focusing system 54 may comprise a single component 
accomplishing both of the purposes of establishing a collimated beam from 
the converging return beam passing through collimating lens 35, and 
further focusing the beam to a desired focal point or region at which the 
IR detector is effective for detection. 
This focusing system permits the establishment of detection means for each 
mode of operation at the same general focal region. In other words, the IR 
detector 51 can be co-located in the same general area with feeds 12 and 
13, which of course act as detectors also, in conducting reception of 
radiation in their respective modes. 
Regarding the materials used, the collimating lens 35 can be made entirely 
of a single selected material, a cross-linked polystyrene material, such 
as Rexolite, for example, which is transmissive to both millimeter 
wavelength and visible or near infrared radiation. Rexolite, however, has 
mediocre resistance to abrasion, heat and weathering. Accordingly, other 
materials may be chosen for their transmission and structural 
characteristics in the frequency bands of interest. For example, zinc 
sulfide and zinc selenide are preferred materials at multimeter 
wavelengths and in both the 3-5 micrometer and the 8-12 micrometer 
infrared wavelength regions. 
The collimating lens 35 is made of a dielectric material, such as Rexolite 
according to one embodiment. In that instance, one side of the lens is 
preferably ellipsoidally convex and spherically concave. For Rexolite, the 
spherical concave surface is approximately flat--the sphere being very 
large in effect. 
Since the system 14 operates in multiple modes, in particular, modes 
involving substantially different frequency or wavelength bands or 
portions of the electromagnetic spectrum, it is frequently useful to use 
one kind of material for central portion 35' of the collimating lens 35, 
and another for the perimeter portion 35" of the lens 35, as suggested in 
both figures, but most effectively in FIG. 2. When this is done, the 
materials may each effectively be chosen to be opaque to the region to 
which the other is transparent, in order to avoid interference. 
By way of further detail, the scanning prism arrangement 21 shown in FIG. 1 
comprises two cooperative prisms, respectively 21' and 21", each of which 
is bounded by a cylindrical perimeter centered on the rotation axis 99. 
Each prism is shaped like a wedge having a base and apex when viewed from 
the side. As shown, the apex of one prism 21' points downward and the apex 
of the other 21" points upward. This wedge shape causes each prism to have 
a circular face and an elliptical face. 
The circular faces of the respective prisms are preferably maintained 
adjacent and parallel to one another, and rotate in a plane perpendicular 
to the axis 99 of the system. This changes the disposition of the 
elliptical faces (i.e., hypotenuse) of the prisms and modifies the 
direction of beams of electromagnetic energy passing through the 
arrangement. 
With the prisms rotated, as shown in the drawing, a beam of radiation 
passing through the scanning prisms would be passed without net angular 
redirection, albeit subject to some transverse displacement which, 
however, has no bearing upon system operation nor on the accuracy of 
detected signals. 
If either of the bases of the prisms 21 is rotated toward the viewer, 
however, the beam for each mode of energy received or transmitted is 
redirected somewhat toward the viewer as well. If only one of the bases is 
rotated, a net downward or upward redirection will also be effected. 
To produce exclusively sideward beam sweeping without any upward or 
downward redirection, the prisms are counter-rotated in coordination with 
each other, the maximum beam sweep being accomplished when both of the 
prism apexes are directed toward the viewer or away from the viewer. 
Exclusively upward or downward sweeping can be established by rotating both 
prisms 21 about the axis 90 degrees, and then equivalently counter 
rotating. 
By rotating both prisms 21 in the same direction at the same angular 
velocity, with any desired initial relative orientation, a conical beam 
sweep is established. 
Spiral, rosette, and other scan patterns can be established by rotating the 
prisms 21 at different angular velocities in the same or opposite 
direction even without angular acceleration. Materials such as zinc 
selenide and zinc sulfide are suitable for the collimating lens 35 and the 
prisms 21' and 21", since they are transmissive to millimeter wave 
infrared, and even visible radiation. Furthermore, they are much more 
resistant than Rexolite to temperature abrasion and weathering. 
Sapphire (i.e., crystalline alumina) is suitable for some applications of 
the system disclosed, but not for the 8-12 micrometer region, and not for 
applications in which the fan and pencil beams are polarized, because 
sapphire is by its nature birefringent and thus has a different effect 
upon each component of the polarized beam, creating undesired effects for 
which compensation is difficult to achieve. Sapphire is, however, 
particularly resistant to abrasion, weathering and adverse temperature 
conditions. 
Polycrystalline ceramic alumina material, which can be used for millimeter 
wavelength applications, is unfortunately not effective for multimode 
active and passive arrangements addressed herein, because the material is 
simply not infrared transmissive. However, in transparent "glassy" form, 
alumina would be as suitable as sapphire environmentally, without the 
birefringence problems of the latter. Such a form is provided by 
formulations based on alumina, such as aluminum oxynitride (ALON) and 
magnesium aluminate spinel (MgAl.sub.2 O.sub.4). 
It is thought and believed that the material of choice will be gallium 
arsenide, when it becomes available in large enough sizes, because it is 
infrared and millimeter wave transmissive and holds up well under adverse 
temperature conditions. 
Transmission of radiation herein is understood in two senses, depending 
upon context. In one sense, the system 14 actively transmits radiation in 
one or more spectral bands. Reflected portions of said radiation are 
transmitted back as well--even though this is not truly transmission, but 
reception. Similarly, when radiation passes through a prism or lens, it is 
said to be transmitted therethrough, even though system-wise the radiation 
may in fact actually be received radiation returning from a target. 
The information herein is likely to lead individuals skilled in the art of 
the invention to conceive of variations thereof which nonetheless lie 
within the scope thereof. Accordingly, attention to the claims which 
follow is invited, as these alone specify with authority and legal effect 
what the scope and impact of the invention actually is.