Parametric location is a new method for detecting hidden objects.

Parametric location is a new method for detecting hidden objects.

SHCHERBAKOV Grigory Nikolaevich,
Doctor of Technical Sciences, Professor

Known methods for detecting stationary objects in concealing environments are based on recording various LF, HF, microwave anomalies (electromagnetic, thermal, etc.) at the locations of these objects.

In this case, active location methods — radar, induction, acoustic, etc. — use the existing contrasts between the search object and the natural background (soil, vegetation, water).

The name of the method is usually determined by the type of probing field [1].

In earlier works, the author demonstrated the possibility of using a fundamentally new method for searching for small-sized objects, based on recording artificially induced contrasts between the search object and the background due to additional irradiation of the investigated space, along with the main probing, with various physical fields.

The emergence of these contrasts is due to the different reaction of the search object of artificial origin and elements of the natural background to the exciting field.

Physical foundations of parametric location

It is known that electrical circuits in which at least one of the parameters changes according to some given law are called parametric.

An electromagnetic field scattered, for example, by a search object may differ from the incident field in its parameters: amplitude, phase, frequency and polarization.

Under the influence of an additional exciting field (acoustic, laser, etc.), these parameters may change in time and space.

The law of change of these parameters will be determined primarily by the characteristics of the exciting field (power flux density, frequency, etc.).

From this it follows that the proposed type of location can be called parametric.

Figure 1 shows possible combinations of probing and exciting physical fields.

Fig. 1. Physical fields used in parametric location

In addition to electromagnetic, acoustic and seismic fields, radioactive radiation can also be used as exciting fields when searching for various objects.

The effect of these radiations (neutron and gamma) on the electronic devices of the search objects changes their parameters (base resistance, barrier and diffusion capacitance of junctions, etc.) [2] and, accordingly, the reflective characteristics of these objects.

This can be recorded by using probing electromagnetic fields.

It should be noted that many of the proposed parametric location options are based on physical effects known in technology as “harmful”.

For example, mechanical vibrations of a target (aircraft, tanks, etc.) cause “target noise”, which worsens the search characteristics of radars, especially coherent ones [3, 4].

The cross-modulation effect is considered “harmful”, creating mutual interference between adjacent communication channels [5, 6].

Excitation of electron-optical observation and target designation devices by laser radiation often leads to their “blinding” [7].

Irradiation, even short-term, of electronic equipment with ionizing radiation (neutron, gamma) causes reversible and irreversible changes in its element base, primarily semiconductor components [2, 8].

However, the management of all these effects, primarily due to the selection of optimal parameters (energy, time, frequency, etc.) of the exciting field, allows them to be transformed from “harmful” into “useful” and used for the purpose of locating objects of artificial origin.

Table 1: Characteristic parameters of exciting fields.

In parametric location, the excitation signal “colors” the probing signal when it is reflected from the search object of artificial origin.

“Coloring consists of giving it characteristic amplitude, frequency-time and polarization features, which can then be detected in the receiver of the search system.

The parametric process of forming a secondary signal can be either linear or nonlinear.

The first case occurs, for example, when a microwave electromagnetic field is scattered by a vibrating search object — due to its additional irradiation by a powerful acoustic field.

The second — when a microwave field is scattered by an excited nonlinear object on harmonics.

Excitation of nonlinear elements of the search object (transistor and diode junctions, etc.) can be carried out by an electromagnetic field of the LW, MW, and HF ranges, which leads to a corresponding change in the NEPR of the entire object.

This makes it possible, for example, to determine the operating frequency of the sought radio receiver of the explosion control line, radio station. It is typical that the search object (radio receiver) can be switched off.

It is necessary to note the presence of a significant number of possible combinations of probing and exciting physical fields.

The choice of a particular combination should be made taking into account many factors: the availability of a priori information about the design features of the search objects, the characteristics of the surrounding background, the required detection range, etc.

With regard to the problem under consideration, the most promising should be considered combinations of various electromagnetic fields of the LF, HF, and microwave ranges. This is mainly due to their ability to penetrate through covering semiconducting media.

It is possible to use various combinations of probing and exciting fields in one search system — in order to increase the reliability of detecting various small-sized objects.

Based on the above, the author proposes the following definition of parametric location: this is an active method of detecting objects, in which changes in the parameters of the probing field are recorded due to the irradiation of these objects with an additional exciting field (acoustic, microwave electromagnetic, laser, etc.).

Using microwave nonlinear parametric interactions to detect small arms and explosive devices

Previously conducted studies in our country have shown that nonlinear radar can be used for remote detection of small metal objects. This is due to the presence of nonlinear electrical properties in metal contacts [9, 10].

However, the use of the well-known METRRA (USA) and similar NRS for detecting small arms has proven to be ineffective.

This is explained by its insignificant nonlinear properties at the third harmonic — due to the absence of point pressure contacts.

The numerous flat metal contacts present in the design of small arms have insignificant nonlinear properties at microwave frequencies, which is explained by the shunting effect of their large intrinsic capacity.

Some increase in the NEPR of point metal objects (by 18-20 dB) can be achieved with a dual-frequency NRS mode with registration of third-order combination frequencies.

However, even in this case, detection of small metal objects (pistols, etc.) is difficult.

A qualitative leap in detection of such objects can be achieved by using a nonlinear parametric method.

The proposed version of the method is based on enhancing the nonlinear properties of flat metal contacts by additionally exposing them to powerful microwave short radio pulses. This effect causes, under certain conditions, an electrical plasma breakdown of the dielectric oxide films covering the contacting metal surfaces.

Breakdown consists of several elementary fast-acting nonlinear electronic processes: emission of electrons from the cathode into the dielectric (oxide film), electron multiplication due to impact ionization, formation and destruction of a negative space charge.

At the moment of breakdown, the nonlinear properties of the contacts increase sharply, which can be recorded by the NRS receiver. The increase in nonlinear properties can be explained by the formation of a nonlinear plasma layer.

The effect of breakdown of thin dielectric films is considered in electronics [2, 11] and electrical engineering [12] as “harmful”.

However, in our case, its special creation allows us to enhance the unmasking properties of small-sized metal search objects, that is, it makes the effect “useful.”

It is known that clean surfaces of almost all metals, when in contact with an atmosphere containing oxygen, are immediately covered with films of their oxides.

In a relatively short period of time, an initial oxide layer several atomic cells thick is formed on the metal surface, which in turn is covered with an absorbed gas film.

The laws of oxide film growth for different metals are different.

For metals with dense oxides, the law of oxide film growth at low temperatures is closer to logarithmic.

In particular, this is observed for iron at an oxidation temperature of up to 3750C, for copper — up to 1000C, for aluminum — up to 2250C.

The rapid decrease in the growth rate of these films over time is due to their protective effect.

Such films are called passivating.

All of the above allows us to conclude that there is rapid oxidation of flat metal contacts (various metal objects) contained in small arms and explosive devices after damage to the oxide films.

The latter occurs, for example, due to dry friction between elements of contacting surfaces (oxide films are torn off) — when pulling the bolt of a pistol, installing explosive devices on the ground surface.

The electric field strength required to break through the oxide film depends on many factors: the roughness of the contacting surfaces, the thickness of the film, the chemical composition of the film, the frequency of the applied voltage, etc.

An analysis of the literature devoted to electrical processes in the contacting system “metal-oxide-metal” shows that in most cases the breakdown electric field strength is 105-107 V/cm.

It is very important that the duration of the applied voltage can be very short — units-tens of nanoseconds.

Accordingly, the duration of exposure to the electromagnetic field irradiating the search object can be just as short.

With an oxide film thickness of units-tens of microns, its breakdown occurs at a voltage of 10 V or more.

The required pulse power of the exciting microwave transmitter of the decimeter range irradiating a small-sized object (lob » 0.1-0.2 m) is determined by the formula:

(1)

where Upr is the minimum required breakdown voltage of the oxide film of a flat contact.

Hence, for example, at Upr = 10 V, r = 5 m, l in = 0.3 m, l ob = 0.15 m, Gu = 10, the generator power (in a pulse) should be no less than 913.9 W. To increase the reliability of electrical breakdown of contacts (Upr > 20-30 V), this power should be several kW.

With a probing radio pulse duration of 100 ns and a repetition rate of 1000 Hz, the average power in the transmitting antenna will be an insignificant value — tenths of a watt.

The output power amplifier can use pulse bipolar microwave transistors connected according to the power addition scheme.

It is necessary to use a non-reciprocal element (a ferrite isolator or circulator) between the output of the microwave power amplifier and the antenna — since microwave transistors cannot withstand high SWR. The latter can happen, for example, when bringing the transmitting antenna close to the operator's body or the ground surface.

A search system that records the excited» nonlinearity of the metal contacts of the search volume can be technically implemented in several ways.

For example, using a NRS with dual-frequency microwave irradiation with registration of second-order combination frequencies or by implementing the cross-modulation effect.

In this case, it is necessary that one of the two microwave signals irradiating the object be significantly stronger than the other.

The expression that allows us to estimate the range of a dual-frequency NVR in free space is as follows:

(2)

where s nk(1) is the normalized nonlinear EPR of a metal search object at the combination frequency.

Calculations show that at Рu1 = 103 W; Рu2 = 102 W, Gu1 = Gu2 = 10, s nk(1) = 10-11 m6/W2, Ap = 10-2 m2, Рpr(min) = 10-12 W, the detection range of a small metal object (pistol, mine) will be about 5 m.

With a probing pulse duration of 10-100 ns, a repetition rate of 1000 Hz, the total average power in the transmitting microwave antennas will be only tens of mW.

The main difficulty that arises when implementing the cross-modulation effect is obtaining a second signal (less powerful) with a “clean” spectrum, i.e. without its own amplitude modulation.

This interfering noise modulation appears both in the microwave generator itself and when the probing signal is reflected from the inhomogeneities of the natural background [13].

To increase the detection range of small metal objects, resonance phenomena can also be used.

It is also possible to place a more powerful microwave transmitter near the area being investigated (units of meters), and a second microwave transmitter and receiver at a significant distance (tens of meters).

It has been experimentally established that for metal objects with point pressure contacts (models of explosive devices), the disappearance of nonlinear properties is observed at a primary microwave field flux density of more than 10-30 W/m2.

This can be explained by the “burning” of point contacts — due to the high concentration of microwave power in them. For metal objects with planar contacts (PM pistols), the “burning” effect was not observed.

In most cases, the reflected signal at the combination frequency increased with the growth of the exciting microwave field.

The working hypothesis about the excitation of plasma breakdown in planar contacts is confirmed by the following observed phenomena:

• the presence of a threshold value Ppad, below which the nonlinear properties of the pistols disappeared;
• noise “coloring” of the reflected signal, which is typical for microwave diagnostics of plasma.

Thus, experimental studies in the decimeter wave range confirmed the correctness of the conclusions about the feasibility of using the effect of plasma breakdown of the oxide film of flat metal-to-metal contacts to detect small-sized “man-made” metal objects.

During the research, it was also revealed that the signal reflected from the pistol can be comparable to the signal reflected from a bunch of keys.

In this regard, it is advisable to conduct additional studies in the future aimed at identifying characteristic signatures (spectral, polarization, resonance, etc.) inherent in individual weapons and other small-sized objects, which would facilitate their selection against the background of other objects.

The expected detection ranges of small-sized objects, implemented using a portable version of parametric locators, lie within the range from tens of centimeters to tens of meters.

The first digits are typical when using quasi-stationary exciting LF electromagnetic fields — for locating objects in highly absorbing environments (wet clay soil, building structures, etc.).

The second digits are for using directed microwave and laser excitation fields in free space – for location, for example, of camouflaged passive optical-electronic devices (target sensors of explosive devices, radio-controlled “video bugs”, etc.).

The idea of ​​parametric location is a logical continuation of the nonlinear radar method.

Both methods record spectral differences in signals received from stationary objects.

However, unlike nonlinear radar, the range of detectable objects in parametric location is much wider.

The main disadvantage of parametric location is the increased energy consumption for creating the exciting field.

Possible areas of application of parametric location: information security, counter-terrorism, forensics, construction, archeology.

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