Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Frequency of cyclic current or voltage
Reexamination Certificate
2001-08-08
2003-04-29
Le, N. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Frequency of cyclic current or voltage
C356S367000
Reexamination Certificate
active
06556002
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to apparatus and methods for determining the polarization of electromagnetic signals.
Electromagnetic signals such as radio waves and light have a property referred to as polarization. Radar operates by transmitting an electromagnetic signal to a target and comparing the signal reflected from the target with the transmitted signal. In modern electronic warfare, targets avoid detection from enemy radar by using various countermeasures such as, jamming an enemy radar signal impinging on the target with a signal denying range information to the enemy and creating false reflected signals to deceive the enemy radar system. To be effective, the signals created by the countermeasure system should have characteristics such as polarization corresponding to the signal characteristics expected by the enemy system as, for example, characteristics of the return signals expected by an enemy radar system. In some cases, the enemy radar may change its signal polarization rapidly. Such a radar system is referred to as “polarization agile.” If the enemy radar is polarization agile, the countermeasure system must be capable of determining the polarization of the transmitted signal rapidly, so that the countermeasure system can change the signals which it emits. For example, a jamming system carried on an aircraft and intended to defeat a polarization agile enemy radar system should determine the polarization of the incoming radar signal and alter the polarization of the jamming signal accordingly. If the jamming system does not do this, the jamming signal will not match the polarization of the return signals from the aircraft. The enemy radar receiver can reject the jamming signals and acquire meaningful return signals. Delay in measuring the incoming signal polarization can allow the enemy system to acquire meaningful return signals for a sufficient time to find the position of the aircraft. Conversely, where a radar or communications system must overcome enemy jamming, it is desirable to measure the polarization of the jamming signal and transmit the radar or communications signal with a different polarization.
However, traditional polarization measuring techniques do not provide polarization measurements rapidly enough to counteract a polarization agile enemy system. Just as the receiving system becomes accustomed to one polarization, the enemy system changes polarization.
At a given point in space along the path of an electromagnetic wave and at a given instant in time, an electric field points in a particular direction, denoted by a vector, {right arrow over (E)}. This vector is perpendicular to the direction of travel of the signal or “propagation vector.” The polarization of an electromagnetic wave is described by the orientation of the electric field vector and the manner in which this vector varies with time.
The polarization vector can be split into components E
x
and E
y
along orthogonal x and y axes perpendicular to the direction of travel of the electromagnetic wave. The component along the x axis commonly is referred to as the “horizontal” component, whereas the component along the y axis is referred to as the “vertical” component. Although these terms are used herein, it should be appreciated that these directions may be arbitrary directions unrelated to the normal gravitational frame of reference. At any given point in space, E
x
and E
y
vary with time. For example, for a sinusoidal wave having frequency &ohgr;, E
x
=Asin(&ohgr;t) and E
y
=Bsin((&ohgr;t)+&agr;), where t is time, &agr; is a phase difference and A and B are the magnitudes of the E
x
and E
y
components. When the E
x
and E
y
components are in phase (&agr;=0), the electric field is linearly polarized. In this condition, the electric field vector at a given point always lies on the same plane. When the E
x
and E
y
components are out of phase (&agr;≠0), elliptical polarization results. When the E
x
and E
y
components of an elliptically polarized electromagnetic signal are of equal magnitude (A=B) and are 90° or 270° out of phase, the signal is said to be circularly polarized.
To measure signal polarization, a dual-aperture (polarized) antenna and a device known as a polarimeter are required. The dual-aperture antenna provides one electrical signal V
h
representing the E
x
or horizontal component of the electric field of a signal impinging on the antenna, and another electrical signal V
v
representing the E
y
or vertical component of the electric field of the same signal. These signals typically are amplified and filtered separately in a dual-channel receiver before passing to the polarimeter. The polarimeter compares these signals to determine their relative magnitudes and the phase difference between them.
A prior art analog polarimeter is shown in FIG.
1
. The horizontal signal V
h
is supplied to one input of a four port directional coupler
200
of a type referred to as a “hybrid.” The vertical signal V
v
is supplied to the input of a phase shifter
202
which applies a known phase shift &ggr; to that signal. The phase-shifted signal is supplied to another input of the directional coupler
200
. The coupler
200
couples the signals supplied to its input and provides a signal at a first output
204
representing the coupled power output or sum of the input signals supplied to the circuit, and also provides another signal at a second output
206
representing another combination of the input signals with a specific phase shift, 180° in this example, between the input signals. The first or sum output
204
of circuit
200
is supplied to the input of a further phase shifter
208
which applies a known phase shift &phgr;. The output of this phase shifter is connected to one input of another directional coupler
210
, which is similar to the first
200
. The second or difference output
206
of coupler
200
is connected directly to the other input of coupler
210
. Thus, when time-varying V
v
and V
h
signals are applied to the polarimeter, one time-varying output signal, referred to as the &Sgr; signal appears at the difference output
212
of coupler
210
. Another time-varying output signal referred to as the &Dgr; signal, appears at the sum output
214
of coupler
210
. The output signals are supplied to a dual-channel receiver and logarithmic amplifier
216
which monitors the amplitudes of these signals and provides a signal representing a ratio between their amplitudes. This ratio signal is supplied to a null adaptive tracker
218
, which adjusts the phase differences &phgr; and &ggr; applied by the phase shifters to achieve a null condition as discussed below.
The relationships between the &Sgr; and &Dgr; output signals appearing at the outputs
212
and
214
of the second 180° coupler
210
and the input signals V
v
and V
h
are referred to as the “transfer functions” of the polarimeter. These transfer functions depend on the phase shift values &ggr; and &phgr; applied by the phase shifters
202
and
208
. Conversely, there is a relationship between the transfer functions which yield output signals with particular characteristics and the phases and amplitudes of V
v
and V
h
. Stated another way, there is a relationship between the phase shifts &phgr; and &ggr; which yield particular output signal characteristics and the phases and amplitudes of the input signals V
v
and V
h
.
In particular, for the components illustrated in
FIG. 1
, the ratio
&LeftBracketingBar;
Δ
&RightBracketingBar;
&LeftBracketingBar;
Σ
&RightBracketingBar;
between the amplitude |&Dgr;| of the &Dgr; output signal and the amplitude |&Sgr;| of the &Sgr; output signal will be at a minimum or null condition when:
γ
=
2
tan
-
1
(
b
a
)
,
and
(
1
)
φ
=
3
π
2
-
α
.
(
2
)
Where:
a is the amplitude of the horizontal component V
h
; b is the amplitude of the vertical component V
v
; and &agr; is the phase difference between these com
Greenbaum Myron H.
Schineller Eugene R.
He Amy
ITT Manufacturing Enterprises Inc.
Le N.
Lerner David Littenberg Krumholz & Mentlik LLP
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