Fast switching polarization diverse radar antenna system

A fast switching, variably polarized lens is placed in front of a radar antenna which radiates an electromagnetic wave having any fixed polarization. The radiated electromagnetic wave is coupled to the lens and the switching elements within the lens transform the polarization of the radiated electromagnetic wave as commanded. Each transmit pulse may take on any polarization which the lens is capable of providing independent of the polarization of the previous transmit pulse. Similarly, during receive mode the lens can be set up to allow the radar system to receive a return signal of any polarization independent of the polarization of the return signal's associated transmit signal.

TECHNICAL FIELD 
This invention relates to a polarization diverse radar antenna system and 
more particularly to a system capable of switching the polarization of the 
radiated and received electromagnetic pulses on a pulse to pulse basis. 
BACKGROUND ART 
In the field of radar design there is an ever existing emphasis on 
increasing both the detection and identification capability of the radar. 
This has lead to designs utilizing narrower and narrower pulses of 
transmitted electromagnetic energy to break out more features (i.e., 
increase the resolution) associated with a target. However, narrow pulse 
systems are prohibitively expensive, especially baseband pulse systems. 
One solution which increases the radar system's detection capability 
utilizes a polarization diverse antenna systems capable of transmitting 
and receiving pulses of various polarizations. In practice however, these 
systems are often very complex, unacceptably heavy and physically too 
large for use in airborne applications. As an example, U.S. Pat. No. 
3,720,947 assigned to the United States government discloses a mechanical 
switching system for changing the polarization of a radar antenna to 
either a linear, circular right or circular left polarization. This 
mechanical switching system is not compatible with recent requirements for 
airborne radar systems, especially a fighter aircraft radar system. In 
addition, the mechanical switching system is too slow to allow a radar 
system to switch polarization on a pulse to pulse basis. 
One way to provide a polarization diverse radar system is to provide a lens 
in front of a radar antenna to switch the polarization of the radiated and 
received electromagnetic waveform to the desired polarization. U.S. Pat. 
No. 4,901,086 assigned to the Raytheon Company discloses a radar system 
incorporating a lens which transforms a linearly polarized waveform to a 
circularly polarized waveform on transmit, and transforms a circularly 
polarized waveform to a linearly polarized waveform on receive. However, 
this system does not have the flexibility to vary the polarization of the 
transmit and receive waveforms on demand, or pulse to pulse. 
DISCLOSURE OF THE INVENTION 
An object of the present invention is to provide a polarization diverse 
radar system. 
Another object of the present invention is to provide a radar system which 
can rapidly switch the polarization of the radiated electromagnetic wave. 
Yet another object of the present invention is to provide a system which 
can switch the polarization of the radiated electromagnetic wave on a 
pulse to pulse basis. 
A further object of the present invention is to provide a lens which can be 
placed in front of an existing radar antenna aperture and switch the 
polarization of the radiated electromagnetic wave on a pulse to pulse 
basis. 
Another object of the present invention is to provide a lens which can be 
placed in front of an existing radar antenna aperture and switch the 
polarization of the received electromagnetic wave on a pulse to pulse 
basis. 
Still another object of the present invention is provide high isolation 
between the orthogonal components of the radiated and received pulses. 
According to the present invention a fast switching, variably polarized 
lens is placed in front of a radar antenna which radiates and receives an 
electromagnetic wave having any fixed polarization, the radiated 
electromagnetic wave is coupled to the lens and switching elements within 
the lens transform the polarization of the radiated electromagnetic wave 
as commanded while providing high isolation between the orthogonal 
components of the radiated electromagnetic wave. 
Each transmit pulse may take on any polarization which the lens is capable 
of providing independent of the polarization of the previous transmit 
pulse. Similarly, during receive mode the lens can be set up to allow the 
radar system to receive a return signal of any polarization independent of 
the polarization of the return signal's associated transmit signal. 
An advantage of the present system is that a polarization diverse antenna 
system provides improved target definition data due to the enhanced return 
information provided by the diverse polarization return signals. 
These and other objects, features and advantages of the present invention 
will become more apparent in light of the following detailed description 
of a preferred embodiment thereof, as illustrated in the accompanying 
drawings.

BEST MODE FOR CARRYING OUT THE PRESENT INVENTION 
Referring to FIG. 1, a polarization diverse radar system 10 includes a 
source antenna 12 and a lens aperture 14 along with a receiver 16, a 
transmitter 18 and a transmit/receive switch 20 (e.g., a circulator). The 
receiver 16 and transmitter 18 operate in a well known manner and transmit 
and receive signals on lines 22,24 respectfully which are routed to and 
from the source antenna 12 via the switch 20. An RF transmission line 26 
(e.g., a waveguide) carries the signals between the switch and the source 
antenna. 
The source antenna 12 includes a plurality of radiators 27 through which 
electromagnetic waves in the radar frequency range (e.g., 16.5 GHz at Ku 
band) propagate. As an example, the source antenna may consist of 820 
resonant slot radiators (a few of which are shown) with a radial 
distribution for radiating pattern sidelobe control of the electromagnetic 
wave. The source antenna radiates a vertically polarized electromagnetic 
wave which is coupled to the lens aperture. The radar system 10 is 
controlled by a system computer 28 which communicates with the receiver 16 
and transmitter 18 via a digital data bus 29 (e.g., MIL-STD-1553 or ARINC 
629). The system computer 28 also communicates with the lens 14 via a 
signal on a line 25 to control the polarization switching of the lens 14 
which will discussed in detail hereinafter. 
FIG. 2 illustrates a side view of the source antenna 12 and the lens 14. 
The source antenna 12 and the lens 14 may be spaced a distance L 29 of 
0.1.lambda. to several .lambda. (e.g., 5.lambda.). The preferred spacing 
is on the order of 0.25.lambda. to 1.0.lambda.. Test data indicates that 
the best antenna patterns achieved from the present system occur when the 
spacing is 0.38.lambda. or 0.63.lambda.. At Ku band .lambda. would be in 
the order of 0.72 inches. 
The vertical polarized electromagnetic wave radiated from the source 
antenna 12 is coupled to the lens 14 via a plurality of coupling elements 
30. The electromagnetic wave received at each coupling element 30 (e.g., a 
simple microstrip dipole element) is input to an associated switching 
element 32 having an electronic driver circuit 33 and a diode switch 34 
and which resides in a lens housing 31 such as aluminum. Each diode switch 
34 should be compact, light weight and offer high isolation and low loss. 
As an example each diode switch may consist of two shunt diodes in each 
output leg to achieve the isolation necessary (e.g., 30 dB) for cross 
polarization suppression. Each switching element 32 switches the coupled 
wave to a horizontal radiating element 35 or a vertical radiating element 
36 on a interpulse basis. The switching speed of each switching element 32 
is preferably on the order of one to five microseconds at a switching rate 
of approximately 15 KHz or greater. In an airborne multimode radar system 
each element should be capable of handling high power, such as, four watts 
average and sixty watts peak. Attention is drawn to the fact that 
radiating elements 35,36 may be placed in any orthogonal relationship to 
one another and as a result the present invention is clearly not limited 
to only horizontal and vertical alignments on the lens 14. If the 
electromagnetic wave is radiated from the horizontal radiating element 35 
the radiated waveform will have a horizontal polarization, whereas if the 
waveform is radiated from the vertical radiating element 36 it will have a 
vertical polarization. During receive mode, the lens 14 can be configured 
to receive a waveform having either horizontal or vertical polarization. 
The vertical and horizontal radiating elements 35,36 may be cross wired or 
printed dipoles, crossed stripline notches, slots or preferably due to 
simplicity, dual polarized patches. Other candidates include any known 
radiating elements capable of operating as a high isolation orthogonal 
radiating element pair. 
FIG. 3 is a breakaway front view of the lens array 14 partially 
illustrating the arrangement of horizontal radiating elements 35 and 
vertical radiating elements 36. Note, in the interest of clarity in FIG. 2 
the horizontal and vertical radiating elements 35,36 associated with the 
switching diodes are illustrated as the horizontal radiating element being 
above the vertical radiating element. While in FIG. 3 the horizontal and 
vertical radiating elements are illustrated in the preferred relationship 
of being adjacent. However, one of ordinary skill in the art will 
certainly appreciate that the invention is clearly not so limited, and 
that in fact there are many different arrangements for the radiating 
elements which will operate properly. 
The present invention is clearly not limited to switching between only 
horizontally and vertical polarizations. In fact, it is anticipated that 
the lens can be configured to switch between many different polarizations 
including: horizontal, vertical, circular and elliptical. FIG. 4 
illustrates one such alternative embodiment 49 which incorporates well 
known phase shift circuitry to provide either a vertical, horizontal, 
circularly left, circularly right, or an elliptically left or right 
polarized electromagnetic wave. 
Referring to FIG. 4, the source antenna 12 provides a vertically polarized 
waveform to the coupling elements 30 which provide the coupled waveform to 
a plurality of phase shifter elements 50. Each phase shifter element 50 
includes a power divider 52 (e.g., a 3 dB hybrid) and a variable value 
multi-bit phase shifter element 54. The dividers 52 split and route the 
power to both a non-phase shift path 55 and to the phase shifter 54 which 
provides a variable amount (e.g., 0,22,45,90,135 etc. degrees) of phase 
shift. These waveforms are then radiated by orthogonally disposed 
radiating elements 56,58 and combined in space to provide a radiated 
waveform 60 of the desired polarization. 
FIG. 5 illustrates how the orthogonally disposed linear radiating elements 
56,58 may be oriented on the face of the lens. Each radiating element 
56,58 is inclined 45 degrees to the vertical in order to facilitate the 
various desired polarizations such as horizontal, vertical, circular or 
elliptical. An example of the polarization diversity of the present 
invention is now in order. 
Table 1 lists how the phase shifter 54 (FIG. 4) would be commanded to 
achieve various polarizations assuming the polarization from the source 
antenna 12 is vertical. 
TABLE l 
______________________________________ 
RADIATED WAVEFORM 60 
POLARIZATION PHASE SHIFTER #1 
______________________________________ 
Vertical 180 deg 
Horizontal 0 deg 
Circular left 90 deg 
Circular right 270 deg 
Elliptical left 22 deg 
Elliptical right 338 deg 
______________________________________ 
As an example, if the desired polarization of the radiated electromagnetic 
waveform 60 (FIG. 5) is vertical, the phase shifters 54 will be commanded 
to 180 degrees of phase shift to provide the vertically polarized radiated 
waveform. Another example shown in Table 1 discloses that if circular left 
polarization is desired, the phase shifter is commanded to ninety degrees 
of phase shift. The remaining phase shifter commands necessary to achieve 
the other polarizations presented in Table 1 are self explanatory. 
In situations where a balanced structure and higher isolation is required a 
phase shifter may be required in both paths from the power splitter 52 in 
which case the commands to the phase shifters to achieve the various 
polarizations may be as illustrated in Table 2. 
TABLE 2 
______________________________________ 
RADIATED WAVEFORM 
60 POLARIZATION SHIFTER #1 SHIFTER #2 
______________________________________ 
Vertical -90 deg 90 deg 
Horizontal 0 deg 0 deg 
Circular left +45 deg -45 deg 
Circular right -45 deg +45 deg 
Elliptical left +11 deg -11 deg 
Elliptical right -11 deg +11 deg 
______________________________________ 
An example illustrating the polarization diversity switching on a pulse to 
pulse basis of the present invention is now in order. 
FIG. 6 illustrates a series of electromagnetic pulses 70 which are radiated 
from and received by the lens 14. Power is plotted along the vertical axis 
and time along the horizontal axis. The series of pulses 70 includes a 
plurality of transmit pulse 72,74,76,78 and a plurality of receive pulses 
80,82,84,86 which represent the reflected return from the target 
associated with transmit pulses 72,74,76,78 respectfully. As an example of 
the polarization diversity of the present invention, the first transmit 
pulse 72 may have a vertical polarization and the second transmit pulse 74 
may have circular left polarization. The third transmit pulse 76 may have 
a circular right polarization followed by a fourth transmit pulse 78 
having an elliptical left polarization. Any combination of transmit pulse 
polarizations is possible. 
It should also be understood that the present invention is not limited to 
receiving only a pulse of the same polarization as the last radiated 
transmit pulse. As an example, even though the first transmit pulse 72 has 
a vertical polarization, the radar system 10 is not constrained to receive 
only a pulse having a vertical polarization at time T 88. One of the 
features of the system 10 is that is can change the polarization of the 
signal it receives return pulse to return pulse. That is, the lens 14 can 
be configured to receive any polarization during receive mode independent 
of the polarization of the last transmitted pulse. 
To achieve complete polarization diversity for both transmit and receive 
pulses, each switching element must be capable of switching at a rate at 
least two times greater than the pulse repetition frequency (PRF) of the 
transmit signal. 
The present invention is clearly not limited to antennas having only a 
monopulse aperture. FIG. 7 illustrates an alternative embodiment 
polarization diverse radar antenna system 90 of the present invention 
including a flat plate antenna 92 having several apertures such as a 
monopulse aperture 94, a tri-aperture interferometer 96, and a guard 
aperture 98. A polarization switching lens 100 according to the present 
invention may be placed over only the monopulse aperture 94 as shown in a 
spaced relationship as discussed hereinbefore. 
The present invention is also applicable to an electronically steerable 
phased array antenna. Switching elements can be placed in front of each of 
the source antenna's elements to control the polarization of the radiated 
and received waveforms. The antenna can be entirely electronically scanned 
or scanned electronically in one dimension and mechanically scanned in the 
other dimension. It should be noted with a phased array antenna system 
that the spacing of the radiating elements on the lens should be about 
one-half a wavelength (i.e., 0.5.lambda.) as shown by distances 110,112 in 
FIG. 2. An example of a phased array antenna that may incorporate the 
present invention is the Joint Surveillance Tactical Radar System (JSTARS) 
manufactured by Norden Systems Incorporated of Norwalk, Conn., a 
subsidiary of United Technologies Corporation the assignee of the present 
invention. The polarization diversity switching provided by the present 
invention can benefit any radar systems working in synthetic aperture 
radar (SAR) mode, moving target indicator (MTI) mode, or other well known 
radar operating modes such as a foliage penetration mode. 
The present invention is not limited to the embodiments illustrated herein. 
As an example referring to FIG. 2, clearly the drivers 33 do not have to 
be collocated with each switching diode 32. Instead, several driver blocks 
each capable of switching many switching diodes (e.g., forty) may be used 
depending on factors such as space available in the lens, cooling etc. In 
addition the present invention is certainly not limited to systems using 
switching diodes, ferrite switches are a well known alternative amongst 
others. 
The coupling elements generically discussed herein can be septum polarizer, 
orthomode transducers, patches, notches, probe fed slots, open end 
waveguide or dipoles. Coupling elements are well known in the art. 
An advantage of the present invention is that each transmit pulse may take 
on any polarization which the lens is capable of providing independent of 
the polarization of the previous transmit pulse. Similarly, during receive 
mode the lens can be set up to allow the radar system to receive a return 
signal of any polarization independent of the polarization of its 
associated transmit signal. 
Although the present invention has been shown and described with respect to 
a preferred embodiment thereof, it should be understood by those skilled 
in the art that various other changes, omissions, and additions may be 
made to the embodiments disclosed herein, without departing from the 
spirit and scope of the present invention.