PHASE-CONJUGATING RETRODIRECTIVE CROSS-EYE RADAR JAMMING

20210382139 · 2021-12-09

Inventors

Warren Paul du Plessis (Pretoria, ZA)

Cpc classification

International classification

Abstract

An antenna array is configured for signal jamming and comprises at least two antennas each configured to receive an incoming signal and transmit a retrodirective retransmitted signal based on the incoming signal and at least two repeater components. Each one of the antennas has one of the repeater components coupled thereto. Each one of the repeater components is configured to utilize a reference signal which is common to all of the repeater components. Each one of the repeater components is configured to negate a phase of the retransmitted signal relative to the incoming signal of its coupled antenna as a function of at least the reference signal, at least one of the repeater components is further configured to introduce at least a phase adjustment to the retransmitted signal. In this way, an angular error may be induced in a threat radar.

Claims

1. An antenna array configured for signal jamming, the antenna array comprising: at least two antennas each configured to receive an incoming signal and transmit a signal based on the incoming signal; and at least two repeater components, wherein: each one of the antennas has one of the repeater components coupled thereto; each one of the repeater components is configured to utilize a reference signal which is common to all of the repeater components; at least all but one of the repeater components is configured to modify a phase of the retransmitted signal relative to the incoming signal of its coupled antenna as a function of at least the reference signal; at least one of the repeater components is further configured to introduce at least a phase adjustment to the retransmitted signal; and the combined effect of the components is to cause the signals retransmitted from the antennas to phase differences of 170° to 190° and amplitude differences of less than 3 dB at the source of the incoming signal.

2. The antenna array as claimed in claim 1, which includes only two antennas and two repeater components, wherein only one of the repeater components is configured to introduce the phase adjustment.

3. The antenna array as claimed in claim 2, wherein the phase adjustment is 170° to 190°.

4. The antenna array as claimed in claim 1, wherein all of the repeater components introduce the phase adjustment.

5. The antenna array as claimed in claim 4, wherein the phase adjustment is such that the retransmitted signal has a phase difference of 170° to 190° at the source of the incoming signal.

6. The antenna array as claimed in claim 1, wherein at least one of the repeater components is further configured to introduce an amplitude adjustment to the retransmitted signal.

7. The antenna array as claimed in claim 6, wherein the amplitude adjustment is such that the retransmitted signals from each antenna have amplitude differences of up to 3 dB at the source of the incoming signal.

8. The antenna array as claimed in claim 1, wherein the phase adjustment is configured to change the operation of a repeater beacon to make it a repeater cross-eye jammer by ensuring that the signals have a phase difference of 170° to 180° and an amplitude difference of less than 3 dB at the source of the incoming signal.

9. The antenna array as claimed in claim 1, wherein respective repeater components are coupled to one of the respective antennas only, in a one-to-one relationship.

10. The antenna array as claimed in claim 1, wherein the repeater components are configured to negate the phase of the retransmitted signal also as a function of an offset or phase shift.

11. The antenna array as claimed in claim 10, wherein the offset or phase shift is relative to the reference signal.

12. The antenna array as claimed in claim 10, wherein the offset or phase shift of one of the repeater components is relative to that of one of the other repeater components as all of the repeater components utilize the reference signal.

13. The antenna array as claimed in claim 1, wherein the reference signal is provided by a signal network interconnecting the repeater components.

14. The antenna array as claimed in claim 13, in which the signal network is phase-matched.

15. The antenna array as claimed in claim 14, in which the signal network is symmetrical in that a length of the network is equal between a source of the reference signal and the respective repeater components.

16. The antenna array as claimed in claim 1, in which either: the reference signal is generated by one source for all of the repeater components; or the reference signal is generated by multiple synchronized sources.

17. The antenna array as claimed in claim 1, in which each of the antennas is in the form of a sub-array of antenna components.

18. A method of operating the antenna array as claimed in claim 1, the method comprising: negating, by each one of the repeater components, a phase of the retransmitted signal relative to the incoming signal of its coupled antenna as a function of at least the reference signal; and introducing, by at least one of the repeater components, at least a phase adjustment to the retransmitted signal.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0044] The invention will now be further described, by way of example, with reference to the accompanying diagrammatic drawings.

[0045] In the drawings:

[0046] FIG. 1 illustrates a schematic view of a PRIOR ART retrodirective array transmitting signals in the direction of an incoming signal;

[0047] FIG. 2 illustrates a schematic front view of a PRIOR ART Van-Atta array showing a common centre of all antenna pairs;

[0048] FIG. 3 illustrates a schematic rear view of the Van-Atta array of FIG. 2 showing the repeater components as cylinders and the equal-length feed networks;

[0049] FIG. 4 illustrates a schematic view of a PRIOR ART rotation of a pair of antennas around its centre which does not change the total distance a retransmitted wave propagates;

[0050] FIG. 5 illustrates a schematic view of a PRIOR ART delay of a Van-Atta retrodirective array which places an apparent target behind the position of the retrodirective array (Δr), especially when antenna elements are widely spaced;

[0051] FIG. 6 illustrates a schematic front view of a PRIOR ART phase-conjugating array showing an arbitrary antenna layout;

[0052] FIG. 7 illustrates a schematic rear view of the phase-conjugating array of FIG. 6 showing the repeater components as boxes and a common reference;

[0053] FIG. 8 illustrates a schematic view of PRIOR ART operation of a phase-conjugating array showing how transmitted signals add in phase at a reference plane;

[0054] FIG. 9 illustrates a schematic view of a PRIOR ART Van-Atta retrodirective cross-eye jamming implementation;

[0055] FIG. 10 illustrates a schematic view of an antenna array, in accordance with the invention;

[0056] FIG. 11 illustrates a schematic view of geometry of the antenna array of FIG. 10 in a near field of the array;

[0057] FIG. 12 illustrates a schematic view of geometry of the antenna array of FIG. 10 interacting with monopulse radar antenna elements;

[0058] FIG. 13 illustrates a graphical representation of a simulation of a PRIOR ART Van-Atta cross-eye jammer when the radar is rotated with the jammer at a 30° angle (θ.sub.c=30°);

[0059] FIG. 14 illustrates a graphical representation of the simulation of a PRIOR ART Van-Atta cross-eye jammer of FIG. 13 when the jammer is rotated with the radar pointing towards the jammer (θ.sub.c=0°);

[0060] FIG. 15 illustrates a graphical representation of a simulation of the antenna array of FIG. 10 when the radar is rotated with the array at a 30° angle (θ.sub.c=30°); and

[0061] FIG. 16 illustrates a graphical representation of the simulation of the antenna array of FIG. 10 when the jammer is rotated with the radar pointing towards the jammer (θ.sub.c=0°).

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT

[0062] The following description of an example embodiment of the invention is provided as an enabling teaching of the invention. Those skilled in the relevant art will recognise that changes can be made to the example embodiment described, while still attaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be attained by selecting some of the features of the example embodiment without utilising other features. Accordingly, those skilled in the art will recognise that modifications and adaptations to the example embodiment are possible and can even be desirable in certain circumstances and are a part of the present invention. Thus, the following description of the example embodiment is provided as illustrative of the principles of the present invention and not a limitation thereof.

[0063] FIGS. 1-9 generally relate to prior art antenna implementations and have been described in the BACKGROUND OF INVENTION.

[0064] FIG. 10 illustrates an antenna array 100, in accordance with the invention. The antenna array 100 may be considered a phase-conjugating retrodirective cross-eye radar jamming assembly, as will become apparent from the description below.

[0065] The antenna array 100 has a pair of antennas 110, 111 which may be the simplest implementation. An antenna array with more than two antennas may also be practicable using the teachings of this invention; a version with more than two antennas may even be advantageous or desirable compared to the simplest two-antenna implementation. The antennas 110, 111 are each configured to receive an incoming signal and transmit a retrodirective retransmitted signal based, at least partially, on the incoming signal.

[0066] Each antenna 110, 111 has connected thereto a repeater circuit component 120, 121. Respective repeater components 120, 121 are coupled to respective antennas 110, 111 in a one-to-one relationship. In other words, each antenna 110, 111 has only one repeater component 120, 121 associated therewith. Further, that repeater component 120, 121 is uniquely associated with that antenna 110, 111. The repeater components 120, 121 are configured to conjugate a phase of the retransmitted signal relative to the incoming signal of its coupled antenna 110, 111.

[0067] The antenna array 100 has a signal network 132 configured to provide a reference signal 130 to the repeater components 120, 121. Each of the repeater components 120, 121 receives the same reference signal 130 and it is therefore a common reference signal 130. Sections of the signal network 132 between a signal source (e.g., an oscillator) and the repeater components 120, 121 are phase-matched so that the same reference signal 130 is provided to each of the repeater components 120, 121.

[0068] In this example embodiment, the repeater components 120, 121 are configured to generate the retransmitted signal based on the incoming signal and two additional factors: the reference signal 130 and internal phase criteria specific to each repeater component 120, 121. The phase criteria may specify, for example, that the phases of the retransmitted signals produced by the repeater components 120, 121 are different, e.g., out of phase, e.g., by 180°. An amplitude may also be varied.

[0069] As a result of the fact that the antenna array 100, being a repeater retrodirective cross-eye jammer, is new, no mathematical analysis of this case exists. Accordingly, the Inventor has conducted an extensive mathematical analysis of such an antenna array 100. While it may be assumed that the far-field analysis for retrodirective arrays provided above can be reused, this is actually not the case, because the antenna array 100 must operate in its near field to be effective [14], so the far-field analysis cannot be reused. A mathematical analysis of the case where a phase-conjugating array is not operating in its far-field region is thus provided below.

[0070] Starting with an analysis of a retrodirective phase-conjugating array, and then moving to a cross-eye jammer, an electric field E.sub.t relative to a phase reference Ø.sub.z at a radar from a number of repeaters in the far field of the radar is given by

[00001] $\begin{matrix} E_{t} \propto {.Math.}_{n = 1}^{N} {[P_{r} (θ_{rn}) P_{c} (θ_{cn})]}^{2} E_{rn} & (4) \\ \propto {.Math.}_{n = 1}^{N} {[P_{r} (θ_{rn}) P_{c} (θ_{cn})]}^{2} \frac{a_{n} e^{j (2 β r_{n} + Ø_{n} - Ø_{z})}}{r_{n}^{4}} & (5) \end{matrix}$

where β is the free-space propagation constant, N is the number of repeaters, and a subscript n denotes repeater n. The parameters for each repeater are the radar and repeater antenna gains P.sub.r(θ.sub.rn) and P.sub.c(θ.sub.cn) at angles θ.sub.m and θ.sub.cn shown in FIG. 11, the electric field E.sub.rn, the gain and phase shift a.sub.n and Ø.sub.n, and the range r.sub.n shown in FIG. 11.

[0071] Ensuring that the signals from a number of repeaters add in phase at the radar (the goal of a retrodirective array) requires that Ø.sub.n=−2βr.sub.n to ensure that all signals are received in phase. However, achieving this objective may be impractical as the range to each repeater is not precisely known.

[0072] Explicitly accounting for the 2π ambiguity of phase gives

[00002] $\begin{matrix} E_{t} \propto {.Math.}_{n = 1}^{N} \frac{a_{n} e^{j (2 β r_{n} + Ø_{n})}}{r_{n}^{4}} e^{- j Ø_{z}} e^{j 2 π 2 m_{n}} & (6) \\ \propto {.Math.}_{n = 1}^{N} \frac{a_{n}}{r_{n}^{4}} e^{j (β r_{n} + Ø_{n} - Ø_{z} + 2 π 2 m_{n})} & (7) \\ \propto {.Math.}_{n = 1}^{N} \frac{e^{j (β r_{n} + 2 π m_{n} - Ø_{z})}}{r_{n}^{2}} a_{n} e^{j Ø_{n}} \frac{e^{j (β r_{n} + 2 π m_{n})}}{r_{n}^{2}} & (8) \end{matrix}$

where m.sub.n are integers. The three factors in the summation in (8) correspond to the influence of the path from the radar to repeater n relative to a phase reference, the effect of the repeater, and the influence of the path from the repeater back to the radar, respectively.

[0073] The conjugation of the signal at the repeater means the signal transmitted by each repeater, E.sub.sn must be

[00003] $\begin{matrix} E_{sn} \propto [\frac{e^{j (β r_{n} + 2 π m_{n} - Ø_{z})}}{r_{n}^{2}}] & (9) \\ \propto \frac{e^{- j (β r_{n} + 2 π m_{n} - Ø_{z})}}{r_{n}^{2}} & (10) \\ \propto \frac{e^{j (β r_{n} + 2 π m_{n} - Ø_{z})}}{r_{n}^{2}} e^{- j 2 (β r_{n} + 2 π m_{n} - Ø_{z})} . & (10) \end{matrix}$

[0074] This result corresponds to setting the repeater phase shift in (8) to

Ø.sub.n=−2(βr.sub.n+2πm.sub.n−Ø.sub.z) (12)

which leads to

[00004] $\begin{matrix} E_{t} \propto {.Math.}_{n = 1}^{N} \frac{e^{j (β r_{n} + 2 π m_{n} - Ø_{z})}}{r_{n}^{2}} a_{n} e^{- j 2 (β r_{n} + 2 π m_{n} - Ø_{z})} \times \frac{e^{j (β r_{n} + 2 π m_{n})}}{r_{n}^{2}} & (13) \\ \propto {.Math.}_{n = 1}^{N} \frac{e^{- j (β r_{n} + 2 π m_{n} - Ø_{z})}}{r_{n}^{2}} a_{n} \frac{e^{j (β r_{n} + 2 π m_{n})}}{r_{n}^{2}} & (14) \\ \propto {.Math.}_{n = 1}^{N} e^{j Ø_{z}} \frac{a_{n}}{r_{n}^{4}} & (15) \\ \propto e^{j Ø_{z}} {.Math.}_{n = 1}^{N} \frac{a_{n}}{r_{n}^{4}} & (16) \end{matrix}$

which has all the repeater phases equal, as required for a retrodirective array.

[0075] An important observation from an implementation perspective is that the phase reference Ø.sub.z is arbitrary, so the only requirement is that it be the same for all repeaters. The implementation in FIG. 10 achieves a common phase by sharing the reference signal 130 from a common oscillator to all jammer channels via phase-matched cables of the signal network 132. However, synchronisation by other means or the use of miniaturised atomic clocks is also possible and opens the possibility of using multiple platforms to implement cross-eye jamming.

[0076] Considering two repeaters and modifying (12) to

Ø.sub.n=−2(βr.sub.n+2πm.sub.n−Ø.sub.z)+Ø.sub.on (17)

where Ø.sub.on is a phase offset, changes (16) to

[00005] $\begin{matrix} E_{t} \propto e^{j Ø_{z}} {.Math.}_{n = 1}^{2} \frac{a_{n}}{r_{n}^{4}} e^{j Ø_{on}} & (18) \\ \propto e^{j Ø_{z}} [\frac{a_{1}}{r_{1}^{4}} e^{j Ø_{o 1}} + \frac{a_{2}}{r_{2}^{4}} e^{j Ø_{o 2}}] & (19) \\ \propto \frac{a_{1}}{r_{1}^{4}} e^{j (Ø_{z} - Ø_{o 1})} [1 + \frac{a_{2} r_{1}^{4}}{a_{1} r_{2}^{4}} e^{j (Ø_{o 2} - Ø_{o 1})}] & (20) \\ E_{t} \frac{r_{1}^{4}}{a_{1}} e^{j (Ø_{o 1} - Ø_{z})} \propto 1 + \frac{a_{2} r_{1}^{4}}{a_{1} r_{2}^{4}} e^{j (Ø_{o 2} - Ø_{o 1})} & (21) \\ E_{t} \frac{r_{1}^{4}}{a_{1}} e^{j (Ø_{o 1} - Ø_{z})} \propto 1 + {ae}^{j Ø} & (22) \end{matrix}$

where a=a.sub.2 r.sub.1.sup.4/a.sub.1r.sub.2.sup.4 and Ø=Ø.sub.o2−Ø.sub.o1. Letting a.fwdarw.1 and Ø.fwdarw.180° gives two signals of approximately equal amplitudes and a phase difference of 180°, thereby achieving the conditions required for cross-eye jamming.

Simulations

[0077] The following parameters of a cross-eye jamming engagement are used to simulate a representative missile threat against an aircraft or ship [15], [16], [22], [26]: [0078] 10 GHz frequency, [0079] 1 km engagement range (r=1 000 m), [0080] 10 m jammer baseline (d.sub.c=10 m), [0081] 2.54 wavelength separation of the radar antenna elements (d.sub.r=2.542) to give a radar sum-channel beamwidth of approximately 10°, [0082] 30° jammer-rotation difference (θ.sub.c=30°), [0083] jammer-amplitude match of 0.5 dB (a.sub.n=0.9441), and [0084] jammer phase difference of 175° (ϕ.sub.n=175°).

[0085] The relevant geometry is shown in FIG. 12, with monopulse radar antenna elements on the left denoted by circles and the antennas 110, 111 on the right denoted by crosses.

[0086] The simulations were performed with the nec2c version 1.3 implementation of the Numerical Electromagnetics Code (NEC) [27] using horizontal dipoles of length 0.4860 wavelengths with 21 segments to model the phase-comparison monopulse antenna elements and the jammer antennas. Horizontal dipoles were used as to minimise the coupling as end-on dipoles have low coupling [28], and the length was chosen to minimise the imaginary component of the input impedance. A first simulation was performed with the two radar dipoles were excited with 1 V sources at their centre segments, and the voltages at centre segments of the jammer dipoles were determined. A second simulation was then performed with voltages at the centre segments of the jammer antennas set to values corresponding to the jammer transmission signals, and the voltages at the centre segments of the radar dipoles were determined. These voltages at the two radar dipoles were then combined to form the sum- and difference-pattern returns, after which monopulse processing was performed.

[0087] The first set of simulations serve to validate this approach by considering a Van-Atta cross-eye jammer. The simulated monopulse indicated angles are compared to theoretical results which have been experimentally validated [15], [16], [29] in FIGS. 13-14. FIGS. 13-14 illustrate an indicated angle induced in a radar by a Van-Atta cross-eye jammer, where letters “N” on the top and right axes indicate positions of the first sum-channel nulls. The agreement between the simulated and validated theoretical results is almost perfect, thereby confirming that the above approach to using NEC-2 to simulate cross-eye jammers is valid.

[0088] The second set of results considers a phase-conjugating cross-eye jammer, and the results are shown in FIGS. 15-16. FIGS. 15-16 illustrate an indicated angle induced in a radar by the antenna array 100, where letters “N” on the top and right axes indicate positions of the first sum-channel nulls. FIG. 15 shows that a large indicated angle error is obtained for all radar rotations, thereby demonstrating the ability of phase-conjugating cross-eye jammer to cause large angular errors. More importantly, FIG. 16 shows that the indicated angle, which should be zero, remains high over the wide range of jammer rotations considered. FIG. 16 thus demonstrates that a phase-conjugating has retrodirective properties because the jammer rotation does not have a significant influence on the resulting angular error.

[0089] The asymmetry and non-monotonic variations in FIGS. 15-16 for the phase-conjugating cross-eye jammer are believed to be due to the fact that the signals to each of the jammer antennas travel different distances and thus experience different path losses. This effect cannot occur in a Van-Atta cross-eye jammer as both signals travel along identical paths, just in different directions, so the path losses are equal. Accordingly, the Applicant believes that the disclosure provides an effective cross-eye jammer based on phase-conjugating retrodirective arrays.

REFERENCES

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