Communications: directive radio wave systems and devices (e.g. – Directive – Utilizing correlation techniques
Reexamination Certificate
2002-05-07
2004-02-24
Blum, Theodore M. (Department: 2862)
Communications: directive radio wave systems and devices (e.g.,
Directive
Utilizing correlation techniques
Reexamination Certificate
active
06697017
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to the field of bearing measurement of wideband electromagnetic emitters, used in communication radar and electronic warfare systems
Systems involved in reception of electromagnetic signals, such as electronic warfare systems, radar systems and communication systems, often are required to locate the bearing of an electromagnetic source with high accuracy and high probability of intercept. Many systems are known in the art for performing this task, using different methods. One of the most advanced methods is known as “multimodal interferometry” or “circular interferometry”. The basic idea started with the “Adcock bearing measurement”, described in UK Patent No. 130490. This is basically a four element form of the circular interferometer. The principle of the Adcock bearing measurement has been expanded to any number of elements, with practical system implementations described, for example, in IL Patent No. 57910; and also by P. M. Eyring, “Compact DF antenna delivers high AOA accuracy”,
Microwave
&
RF,
June 1997; and by Murthy et al., “Models simulate Butler-matirix-based DBDs”,
Microwave
&
RF,
June 1996. Some expansions of the theory of phase modes have been published. See for example: B. Shellig, “A matrix-fed circular array for continuous scanning”,
Proceedings of the IEEE,
November 1968, pp. 2016-2027; D. E. N. Davis et al., “An adaptive circular array for HF direction finding and null steering”,
IEEE LCAP
1985 (Conf. Proc. 248); J. R. F. Guy and D. E. N. Davies, “Studies of the Adcock direction finder in terms of phase-mode excitations around circular arrays”,
The Radio and Electronic Engineer,
vol. 53 no. 1, January 1983; and R. Eiges and H. D. Griffiths, “Mode-space spatial spectral estimation for circular arrays”,
IEEE Proc.—Radar, Sonar Navig.,
vol. 141 no. 6, December 1994. All of these expansions suffer from severe practical implementation difficulties.
The concept of using phase modes in a bearing measurement system now will be described briefly, referring to FIG.
1
. An aggregate of receiving antenna elements
1
are equispaced around a circle. This aggregate feeds a RF Butler matrix
2
which is a modal-beam-forming netwvork that includes RF hybrid combiners and phase shifters, interconnected by RF transmission lines, as described, for example, in T. Macnamara, “Simplified design procedures for Butler matrices incorporating 90° hybrids or 180° hybrids”,
IEEE Proc.,
vol. 134 part H no. 1, February 1987. The function of RF Butler matrix
2
is to phase-shift and sum the signals from all antenna elements
1
into several outputs, which are called modal beams. For modal beam numbered M in an array consisting of N elements
1
, the weighted sum is:
S
(
M
)
=
-
1
N
∑
i
=
0
N
-
1
s
(
i
)
exp
(
2
π
i
M
N
)
where s(i) is the signal out of element i and S(M) is the signal out of modal beam M. This operation, which is equivalent to a Discrete Fourier Transform, is performed in the RF domain.
The modal beams have the following two major properties:
1. Each beam has a quasi-omnidirectional amplitude pattern, which means that the signal strength does not depend on the azimuth angle of the source.
2. The phase of modal beam M, &thgr;(M), is quasi-proportional to the azimuth angle &phgr; of the source, with M being the constant of proportioinality:
&thgr;(
M
)≡
M&phgr;
Thus, by measuring the modal phases, it is straightforward to extract an estimate of the azimuth angle of the source. The higher the mode, the higher the accuracy; but measurements of lower order modes is required in order to resolve ambiguities occurring in the higher order modes. The inherent circular symmetry of the phase modes concept lends itself to most compact, minimal channel count designs, with minimal frequency, polarization and elevation dependence.
The modal beams from RF Butler matrix
2
are input to a set of multichannel receivers
3
. Phase measurements of the modal beams have been implemented using several types of multichannel receivers
3
, all tackling the task of amplification, dynamic range handling and correlating (phase measurement). Finally, the outputs of multichannel receivers
3
are input to a digital processing unit
4
that carries out digital signal processing to extract the bearing of the received signal. The types of receivers
3
that have been used in practical systems include:
1. Direct broadband receivers, with broadband limiting amplifiers and broadband correlators.
2. Homodyne receivers of several implementations, for example the implementation taught by Manuel in U.S. Pat. No. 5,661,485. These receivers downconvert all the channels to a single tone Intermediate Frequency (IF) signal, using one of the modal beams as a reference local Oscillator (LO), after the proper frequency shift. The dynamic ranging and the correlation between modal beams is performed in IF.
3. Super heterodyne receivers, which downconvert a predetermined portion of the spectrum into IF signals and perform the correlation in IF.
All of the prior art implementations use RF Butler matrix
2
as the spatial processor that produces the modal beams in the RF domain, the shortcomings of this approach stem from the limitations imposed by RF Butler matrix
2
:
1. RF Butler matrix
2
has a considerable insertion loss (5 to 8 dB at 18 Ghz). This reduces the system's sensitivity. Using a low noise preamplifier on each antenna element
1
alleviates this problem, at the expense of complexity, cost and also reduced accuracy, because of the transmission mismatch between amplifiers, which is hard to calibrate.
2. RF Butler matrix
2
has inherent inaccuracies which are hard to control, especially at the broadband and high frequencies often encountered in practical systems. These inaccuracies transform into deviations from the ideal phase-azimuth relationship, generating large bearing estimation errors. While sonic of these errors may be linearized, not all of these errors can be corrected; and the overall system performance is very sensitive to measurement errors, multipath, multisignals and reflections.
3. RF Butler matrix
2
is a very complicated RF supercomponent, imposing realization difficulties, high cost and mechanical constraints.
4. Advanced bearing algorithms for implementing null steering, multisource discrimination, etc., can not be used in practice because of the limited accuracy and the practically available phase modes from RF Butler matrix
2
.
There is thus a widely recognized need for, and it would be highly advantageous to have, a device and method for broadband reception and bearing measurement of RF sources that does not employ a very difficult, highly specialized RF beamforming supercomponent such as RF Butler matrix
2
.
SUMMARY OF THE INVENTION
The present invention is an innovative device and method for circular interferometry. The present invention eliminates the drawbacks and limitations of the RF processing unit used in the prior art by replacing the RF processing performed by this unit with digital or IF processing.
According to the present invention there is provided a device for determining a bearing of an incoming RF signal, including: (a) a plurality of primary antenna elements for receiving the RF signal; (b) for each primary antenna, a receiving channel for downconverting the received RF signal to a respective IF signal, each IF signal having a respective amplitude and a respective phase; and (c) a processing mechanism for inferring the bearing from the amplitudes and the phases.
Preferably, the primary antenna elements are equally spaced around at least a portion of a circle. Preferably, each primary antenna element is provided with an amplifier for amplifying the received RF signal.
Preferably, the receiving channels are homodyne receivers or heterodyne receivers.
Preferably, the device further includes a source of a local oscillator signal. The oscillator signal is introduced to each receiving ch
Blum Theodore M.
Friedman Mark M.
Rafael-Armament Development Authority Ltd.
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