1.

INTRODUCTION

Nowadays, thanks to the dynamic technological development of the last few decades, in parallel with the development of radar technology, several different ways of combating and counteracting radio-electronic reconnaissance have been developed. For this purpose, there are used techniques like: physical destruction and disruption of proper operation (passive or active), modification of the electrical properties of the medium (ionization of the space, or the use of absorbing and / or scattering substances) and modification of the radio signature of the objects (changing the RCS, masking). [1]

In the overwhelming majority, in terms of effectiveness, the methods above can be considered comparable and usually complementary. However, in case of the effective conduct of warfare, the optimization of the cost-to-effect ratio has become increasingly important. In such future approach and in the context of restrictions on the availability of precision missile systems, the “superiority” of the idea of using active interfering devices is clearly noticeable. In particular, devices, which are characterized by a relatively low cost of implementation and, despite the low output power, can provide an effect comparable to that obtained using expensive high-power devices or “stealth” technology.

This paper describes the idea of creating interference characterized by the above-mentioned properties, obtained by application of a specific method of using miniature sources of active interference. This method concerns the creation of a spatially-distributed interference.

In order to maintain the operational capabilities on the battlefield the modern radars have many mechanisms increasing their resistance to interference, e.g. tuning in a wide frequency range, automatic selection of the least distorted frequency, high frequency selectivity, spatial selectivity, high dynamics of receiving systems with automatic gain control, elimination of non-synchronous and stationary interference, protection against information overload or interference bearing. Taking into consideration the issues presented above, in the authors’ opinion, the most adequate solution seems to be the use of synchronized sources of active interference, which generate signals in the specific frequency bands, corresponding to the operating frequency of the device being interrupted. Known and current solutions in operation, based on noise sources or digitally controlled with frequency synthesis, in order to provide the required efficiency, require wide frequency band (and higher output power level), or selective work (resulting in the need for additional mechanisms of their “online” control, depending on the signal of the interfering device). Therefore, it was proposed to create dedicated, active spatial interference, using miniature microwave oscillators, synchronized by injection signal from an interrupted device.

2.

THE BASICS OF DISTURBING THE RADARS

2.2

Determination of the range of the radar in the conditions of active interference

The range of the pulsed radar in a free space (R0), without interference, ignoring the influence of the environment on the radar antenna characteristics and the atmospheric attenuation, is determined by equation 1 [2, 8]

where:

• P - output power in pulse;

• τ - pulse duration;

• G - directional gain of radar antenna;

• λ - wavelength corresponding to the operating frequency;

• σ - effective surface reflection of an airborne object (RCS)

• F - noise figure of the radar receiver;

• V - visibility coefficient, defining the threshold ratio of reflected pulse power to the average noise power of the radar receiver;

• k - Boltzmann constant (k = 1,38 · 10-23 [W·s/K]);

• T₀ - receiver path temperature in degrees [K]);

• Δf - bandwidth of the radar receiver.

Product value: k·T₀·F·Δf determines, in the above equation, the noise power of the receiver P_szw.

Then, by excluding all elements, except those describing the parameter P_szw, from under the root in equation 1 and substituting a new variable C:

one can write the equation for the range of the radar in free space, as a function inversely proportional to the radar receiver’s noise power (Equation 4):

In the conditions of active external interference, due to the addition of the receiver’s noise power with interference noise power, the equation for the radar range takes form described by equation 5.

It should be mentioned, however, that in the case of active, non-noise interference, due to the fact that its power, in general, significantly exceeds the noise power of the radar receiver, the second element in the root can be omitted when assessing the effectiveness of interference [1].

Assuming, that the spectrum of the interference signal corresponds to the bandwidth of the receiver of the interrupted radar, the ratio of the radar range in the conditions of interference, to its range in free space (R_z / R₀) can be described using the spectral density of noise (equation 6):

where:

• S_szw - spectral density of the receiver’s noise, equal:

  

• S_szz - spectral density of noise interference at the receiver’s input, equal:

  

where:

• S_z - noise spectral density at the output of the interference transmitter;

• G_z - directional gain of the interference transmitter antenna;

• G_s - directional gain of the radar antenna in the direction of the source of interference;

• λ - wavelength corresponding to the frequency of interference;

• R_n - distance of the interference transmitter from the disturbed radar.

Describing the ratio of the spectral density of noise interference at the input of the receiver to the spectral density of the receiver’s noise (equation 9) as the K factor:

the ratio of the radar range in disturbed conditions to its range in free space, i.e. the radar range degradation factor, can be described by equation 10:

Similarly, the range of radar under noise conditions can be described by equation 11:

3.

THE IDEA OF CREATING ACTIVE SPATIAL INTERFERENCE

3.2

Simulation of spatial noise interference impact on a radar

Depending on the mutual configuration of the disturbed radar and the interference transmitters used, the observed impact of spatial interference on the radar detection range varies and strongly depends on the shape of the antenna characteristics of the radar. In addition, the situation changes dynamically during the movement of the antenna characteristics.

Assuming that the bandwidth of the noise signal is greater than the bandwidth of the disturbed radar receiver, interference at the receiver of the radar appears with the power _Pszz described by equation 12, which is the sum of the power of the interference signals coming from the i number of noise transmitters located on different distances and azimuths in the space around the radar:

where:

• G_s(βi) - antenna gain of the radar in the direction of the i^th source of interference;

• Gni - antenna gain of the i^th source of interference.

Examples of reducing the detection range of radar as a result of using various types of interference emitters are presented in Figures 1-6. The following sources of interference (jammers) have been used in the analysis:

• - high-power noise transmitters with an antenna with directional gain of 3 dB, placed on aircraft located at a distance of 100 km from the disturbed radar, generating noise interference with a spectral density of 10 W/MHz;

• - miniature low-power noise transmitters with antennae with directional gain of 0 dB, in the form of packages consisting of 20 pieces randomly scattered in an area of 2 km radius, generating noise interference with a spectral density of 0.001 W/MHz each.

Figure 1.

“Ideal” radar disturbed by 2 low-power interference transmitters’ packages.

Figure 2.

“Ideal” radar disturbed by 1 high-power interference transmitter on the aircraft.

Figure 3.

“Ideal” radar disturbed by 4 low-power interference transmitters’ packages.

Figure 4.

“Ideal” radar disturbed by 4 high-power interference transmitters on the aircraft.

Figure 5.

Typical radar disturbed by 4 high-power interference transmitters on the aircraft.

Figure 6.

Typical radar disturbed by 4 low-power interference transmitters’ packages.

4.

INJECTION-LOCKED SYNCHRONIZED MICROWAVE OSCILLATORS, AS A SOURCE OF ELECTROMAGNETIC INTERFERENCE

Due to the fact that modern radars have many mechanisms to prevent interference (the most important ones are listed in section 2.1), in the authors’ opinion, the greatest effectiveness of interference on a radar can be obtained only as a result of using active sources of interference. This thesis is clearly confirmed by the analysis carried out in the previous chapters. Therefore, the concept was originally considered to build the proposed system of spatial interference, based on miniature transmitters of the noise interference. The examples of simulation tests presented in section 3.2 show that, in principle, this is the correct assumption. However, such a solution forces the use of broadband transmitters, or selective with the possibility of re-tuning. This, unfortunately, results in the need of use higher levels of output power in the first case, or in the second case, the need of extending (complicating) the transmitter design with elements that enable control of the interference signal in accordance with the radar signal.

As a result of the conducted tests and subsequent analyzes, it has been found, that the mentioned limitations, associated with the use of noise sources, can be eliminated by using synchronized sources of active interference, e.g. in the form of digitally controlled transmitters with frequency synthesis.

Therefore, it is proposed, in order to ensure the most simplified design (and thus better resistance to external factors and cheapest implementation) to use the analog microwave oscillators with injection-locking. Such transmitters, in the form of miniature, microwave oscillators dedicated to specific radar band and synchronized with its signal, are devoid of the disadvantages of noise sources. Additionally, with a relatively simple design, they are able to generate interference in the radar frequency band, depending on the probe signal [5]. As a result, an oscillator of this type can be a kind of “intelligent” source of the interference signal, which may adapt in an analogue way, the generated signal to the parameters of the radar signal. A typical block diagram of the injection-locked oscillator [5] is shown in Figure 7.

Figure 7.

General block diagram of the injection synchronized oscillator.

As numerous publications show, the phenomenon of injection-lockingis quite widely used in practically designed microwave oscillators, used in, among others, radio communication systems [5]. However, due to the fact, that the base source for all solutions based on this phenomenon is the publication of R. Adler [6], the application of injection-lockingfor the purpose of disturbing the microwave radars, requires certain modifications, in order to “adjust” the mechanism proposed by Adler.

The main problem here is the fact, that the theory presented by Adler and the calculations presented do not take into account the specificity of microwave systems, associated with the use of high frequencies. In addition, many authors assume that the resonant circuit (passive part) of the oscillator is characterized by high Q parameter, which strongly narrows the oscillator’s synchronization band. As a result, it is possible to synchronize it by external signals only with a very small frequency range. This is very undesirable fact when the disturbance of the radar with variable frequency of the transmitted signal is considered. Then, it is necessary to design the oscillator, which will ensure a sufficiently wide synchronization band, enabling locking the oscillator at any frequency injected from the disturbed device, i.e. radar. This, however, corresponds with reduction of the Q parameter during the design of the oscillator.

Another problem is the use of reflective-type models describing the phenomenon of injection and admittance representation of individual circuit elements [7]. This approach is very difficult to implement in practice, especially in the case of microwave technology, where it would be more adequate and natural to use a transmission model using scattering coefficients. Partly, these problems were eliminated by D. Sommer and N. J. Gomes in [5], who presented a S-matrix-based reflection model, and demonstrated the possibility of designing oscillators with a wide synchronization band.

The above-mentioned issues, along with other quite numerous simplifications and understatements, presented in the available publications mean, that the use of injection-locked oscillators as the sources of the interference, requires the development of new, dedicated mathematical model, which will then be used to construct a real oscillator. This is the subject of ongoing research work of the authors of this paper.

5.

CONCLUSIONS

On the basis of the research results obtained so far, it can be clearly stated, that the idea of disturbing the radars with the use of miniature packages of synchronized transmitters creating spatial interference in the area around the radar, is the correct conception. As demonstrated in subsection 3.2, by the use of very low power transmitters, one can interact with a radar very effectively, even more effectively compared to the use of higher-power transmitters. In the case of modern radars, the creation of the effective interference, limiting or preventing detection, by the use of single transmitters (even with high output power), is only possible in the direction of the source of interference (carrier of the transmitter). The distance position of the carrier and the objects located in a narrow sector are then masked. The interference carrier, as a signal source, also exposes its presence. As a result, even single radar can determine the location of the maximum disturbance signal (bearing) and, acting in a system with other radars, can indicate the location of the disturbance carrier to active combat agents. In addition, one should not forget that the use of aircrafts equipped with high-power jammers, especially above their own positions, masks their location for enemy radars, but also effectively interferes with their own devices. There will be no such problem, when low-power interference transmitters are used, that are located in the distant, from our own, position, in enemy radar dislocation zone.

An analysis of the available literature gives the conclusion, that there is a possibility to design an extremely interesting and, as the content of the previous chapter shows, very practical solution of interference transmitters, “tailored” to specific radars. The examples of the use of injection-locked oscillators presented by different authors are often not adapted for use in the design of microwave devices, or they present idealized models, based on assumptions impeding their practical implementation in the real designs. However, the analysis carried out by the authors of this paper and preliminary simulation tests have shown, that it is possible to design and apply a real dedicated microwave oscillator that will be adapted to be synchronized with a typical radar signal. This will be possible in the future works, mainly thanks to the constant development of new dedicated mathematical model, which takes into account all the aspects related to real microwave design, that are missing in the analyzed publications.