Perimeter camouflaged magnetometric detection devices.

Perimeter masked magnetometric detection devices..

Zvezhinsky Stanislav Sigismundovich, Candidate of Technical Sciences
Larin Alexander Ivanovich, Candidate of Technical Sciences

PERIMETER MASKED MAGNETOMETRIC DETECTION DEVICES

Perimeter detection devices (hereinafter referred to as detection devices, DD), designed to increase the effectiveness of protecting the boundaries of industrial, military and civilian facilities, occupy a special place in the special equipment industry.

The continuous growth of the global market for detection devices observed since the mid-60s is due to:

• the increasing threat of terrorism in relation to nuclear-hazardous, chemical, energy and other large-area objects; the high efficiency of using such means in local military conflicts (Middle East, Afghanistan);
• the increasing threat of the spread of religious and national extremism across state borders, illegal migration, smuggling of weapons and drugs. The greater the length of the perimeter, the higher the comparative efficiency of using technical means in relation to the human factor of security.

The SOs detect the intrusion of violators (people, vehicles, military equipment) into the protected area of ​​space — the detection zone (DZ) by characteristic disturbances of the physical field, which are recorded by the sensitive element (SE) of the device.

The type of SE determines the geometry of the detection zone, distributed along the protected boundary, in contrast to more compact point detection devices, in which the zone is distributed around and near their place (point) of installation.

In accordance with the physical principle of intrusion registration, there are various types of perimeter security systems: seismic, vibration, radio engineering and others [1,2,3], which can be divided into classes:

• masked or barrier (non-masked), depending on the secrecy (or visibility) of the components of the means, mainly the CE;
• passive or active.

Camouflaged COs, the sensitive element of which is placed in the ground at a depth of up to 50 cm, have a tactical advantage in that identification of the detection zone is difficult for an intruder. This makes it unlikely that it will be overcome by clever methods, which sharply reduce the detection ability of the barrier.

For masked SO, as a rule, the list of sources of significant interference is significantly smaller, the means do not require regular maintenance, and the range of temperatures affected is narrowed. On the other hand, barrier SO are generally cheaper and more practical, their installation and replacement in case of damage does not present any difficulties [ 3].

When using active SO, the intruder is detected based on the registration of his interaction with a specially created physical field (for example, a radio beam); in passive ones, he is detected by the disturbance introduced into the existing field (for example, the Earth's magnetic field).

The advantages of passive SS include, as a rule, significantly smaller weight and size characteristics and energy consumption, and compliance with radio masking requirements. The advantages of active SS include, in general, greater detection capability and greater opportunities for improving products.

Table 1 provides a classification and examples of known domestic and foreign perimeter SS.

In our country, the undisputed leader-developer is SNPO Eleron and its subsidiaries NIKIRET (Zarechny), Dedal (Dubna); Abroad – these are primarily the USA (Sandia, Honeywell, Sylvania, Southwest Microwave), Israel (Magal S.S., Galdor-Secotec, G.M. Advanced Security Technologies) and the UK (Geoquip, Remsdaq L.T.D.), to a lesser extent Switzerland (Alarmcom), Japan (Optex), Canada (Senstar-Stellar), France.

Table 1. Classification of perimeter detection equipment

A special place among the SO is occupied by magnetometric detection tools (MDS), based on the registration of useful signals (PS) of changes in the magnetic induction flux caused by moving intruder objects due to the presence of ferromagnetic objects [3,4].

The sensing element of the MDS is a cable line distributed along the detection zone and is a loop induction circuit with a differential structure formed by cable turns.

The cable is laid in the ground to a depth of 25…50 cm along the protected boundary in 1…3 trenches.

The attenuation of remote electromagnetic interference in the CE (by 30…60 dB) occurs practically without attenuation of useful signals.

The objects of detection — violators are vehicles and military equipment, as well as people carrying weapons (knife, pistol, machine gun), hand tools (pliers, nippers), various household items (keys, glasses), various ferromagnetic objects in shoes, clothes, luggage (nails, insoles, buttons, fasteners, etc.).

Magnetometric SI are less common than, for example, seismic ones, mainly for two reasons:

• due to the impossibility of detecting so-called “magnetically clean” violators (who have taken measures to remove ferromagnetic objects from their equipment);
• due to the restriction of use near sources of industrial electromagnetic interference.

The second restriction is significant, and the first, as practice shows, is insignificant:

• the proportion of “magnetically clean” intruders from the total number (unarmed, armed people, etc.) does not exceed a fraction of % (depending on the specifics of the facility), and their potential threat is minimal;
• it is extremely difficult for an intruder to determine the operating principle of a passive and masked sensitive element and the configuration of the detection zone.

The problem of a prepared intruder is inherent to all SI without exception. The solution to this problem lies in the ways of combining several means with different physical principles of detection, or using a combined SI with several principles of detection — for example, the seismomagnetometric means «Duplet» [ 4].

Magnetometric means, unlike seismic ones, are insensitive to the aggregate state of the soil and most natural and climatic factors (except lightning and magnetic storms), which determines their relatively high potential noise immunity. Tuning out from pulse electromagnetic interference is achieved by constructing a separate interference channel, as well as by algorithmic means [ 3]. There are MSOs capable of operating under concrete runway surfaces, under water, etc.

The MSO is “aimed” at objects of human activity (ferrous metal), which allows the most reliable way to distinguish between people and large animals, which, in some cases, are the main sources of interference for other types of SO. The unlikely and relatively weak effect of a source of mechanical interference, changing the configuration of the SE in the Earth's magnetic field, is secondary, unlike the barrier vibromagnetometric SO, where this effect is significantly greater and is decisive [ 6 ]. A distinctive feature of the MSO is also the ability to organize remote monitoring of operability with full depth, which is not available in other means.

The detection zone of the means is uniform along the entire boundary, without “dead” zones, typical, for example, for perimeter SO on the physical effect of the leaky wave line or for common radio beam SO.

As practice shows, the effectiveness of the MSO is very high in cases:

• when the construction of barriers at the security line is impractical or impossible (for example, on mountain slopes, flooded river banks, etc.), with intensive natural and seasonal migration of animals through the security line;
• for the tactical purpose of camouflaging the line, in the case of organizing a second security line and combining it with the barrier SO of the first line;
• in complex soil and geological conditions, when the effectiveness of other COs is sharply reduced (for example, marshy soil, quicksand — for seismic COs).

The MCO has an advantage over other means in that the mathematical modeling of the detection process corresponds to reality with greater reliability [ 6] :

• In the detection zone, vector summation of magnetic field inductions from the intruder and interference occurs, which can be considered quasi-static, instantaneous, without distortion and absorption. In other means, useful signals experience nonlinear distortions along their propagation path, and attenuation depends on a number of random factors.
• The environment for the formation and transmission of magnetometric signals — the magnetic field in the detection zone — is determined by the main (constant) magnetic field of the Earth. The fluctuation magnetic noise of the Earth's magnetic field is fairly stable, described by spectral functions, its spatial gradient is negligible and can be suppressed differentially. In other SOs, the influence of the physical environment is non-stationary, the noise depends on natural and climatic factors and the installation site, and it cannot be excluded differentially.
• The intruder in the MSO is actually detected directly, by the presence of an equivalent magnetic moment. In others CO is detected, as a rule, indirectly, by “traces” in the signal propagation environment, which introduces great uncertainty into the signal generation process. Therefore, when developing MSO and predicting their tactical and technical characteristics (TTC), mathematical modeling methods can be successfully used [ 6].

The main TTCs that determine the effectiveness of MSO are the probability P0 of detecting intruders and the average operating time Tls to false alarm. The value of P0 is determined not only by the sensitivity of the means – the detection threshold P0, but also by the useful characteristic of intruders, the magnetic moment M. The value of Tls is determined not only by the construction of the MSO, but also by the intensity and frequency of significant interference.

Methods for constructing the MSO

In any MSO, four sequentially connected functional units can be distinguished:

• CHE – distributed magnetometric converter of magnetic induction into an electrical signal of a differential type;
• amplifier-filter (AF);
• analog-to-digital converter (ADC) or its simplest analogue – threshold device (PD);
• a signal processing unit (SPU) that, in accordance with a specified processing algorithm, produces (or does not produce) a detection signal.

The series connection of a sensing element, an amplifier-filter and a threshold device is called a magnetometric detector (MD), which is the simplest magnetometric detection tool with (all other things being equal) the highest detectability and minimal noise immunity. A set of several MDs connected to a signal processing unit essentially forms a real MSO.

Methods for constructing magnetometric detection tools are divided into methods for constructing MDs and BOSs.

The former are divided into methods for constructing:

• ZO;
• CHE;
• a primary discrimination path for useful signals and interference.

The latter are divided into methods for increasing noise immunity and information content of signals.

The tracing of the sensitive element along the security boundary must be as precise as possible (in terms of the width and length of adjacent counter-connected sections) in order to ensure the maximum suppression coefficient of remote magnetic interference.

Double-line detection allows to significantly increase the noise immunity of the MSO, based on the time correlation of interference, simultaneously and with approximately equal intensity affecting both magnetometric detectors, spaced deep into the line, while the impact of the intruder on them is sequential in time. Compared with single-line (with the same detection ability), a gain in noise immunity of several times is achieved. Disadvantages of this solution: a double increase in weight, dimensions and cost, a loss in application tactics (expansion of the detection zone).

The two-flank construction of the perimeter MSO allows to increase the noise immunity to pulsed electromagnetic interference by 3 … 5 times.

The principle is based on the fact that the probability of simultaneous, within the uncertainty interval, crossing of two flanks of a hidden boundary by two intruders is negligible. Magnetic interference acts on both flanks simultaneously, and the uncertainty interval serves to eliminate the effects associated with the non-identity of the MO and the spatial gradient of interference.

The disadvantage of such a solution, compared to a single-flank one, is the increase in weight, dimensions, energy consumption, and cost.

However, if the detection zone length is limited (for example, by the magnetic noise level), then dividing it into two flanks is optimal.

There are three methods of primary discrimination of signals and interference, allowing their distinctive features to be realized:

• a separate (at the SE level) interference sensor;
• optimal filtering of useful signals against the background of interference and vice versa;
• selection of the optimal detection threshold P0.

The introduction of a separate interference sensor (distributed along the ZO has an advantage over a concentrated one) is extremely important in perimeter MSOs and allows increasing their interference immunity by more than 10 times. The interference sensor can also be used to remotely monitor the operability of the MSO.

The choice of the optimal detection threshold P0 is of the utmost importance.

For a distributed induction SE, there are three constantly acting limiting factors:

• magnetic noise of the Earth's magnetic field;
• thermal noise of the SE;
• the amplifier-filter's own noise.

Their combined effect, as studies show, limits the maximum achievable detection threshold (with a detection zone length of up to 500 m) to P0 = 1.5…2.0 nV/vit. If the MSO is located near sources of powerful industrial interference, for example in a city, P0 should be increased: its value should exceed the peak level of the total noise by at least 2.5 times.

Methods for increasing the noise immunity of the MSO are divided into signal processing by:

• discrimination by amplitude-time characteristics;
• accumulation;
• correlation processing from several MOs.

In essence, they are aimed at identifying and fixing in the BOS algorithm a set of features for distinguishing useful signals from interference, such as duration, intersignal (interpulse) pause, polarity of extremes, their alternation, etc. Analog (integration) or digital (pulse counting) accumulation of signals is an effective criterion for distinguishing, based on the fact that, as a rule, the duration and magnitude of PS are greater than those of interference.

Correlation processing of PS and interference involves their analysis by:

• magnitude;
• coincidence of time of appearance;
• sequence of pulses at the outputs of the corresponding MO.

The methods are reduced to identifying the interference situation when a ban on the detection signal is generated. As the ban time increases, the noise immunity increases, but the probability of missing an intruder increases, whose appearance in the ZO may coincide with the action of the interference.

The methods of increasing the information content of the PS are based on mathematical modeling of the magnetometric detection process and are a kind of “know-how” of the developers.

There are three types of MSO construction: single-line, dual-line and tri-line, depending on the number of cables along the security line, forming an induction SE [ 2].

In single-line devices, the CE is formed by a conductor wound section by section around a permalloy core (diameter ~ 8 mm), with a change in winding direction through the base A = 1.8…3 m so that the number of sections with the same direction is equal (Fig. 1).

A two-line FE is formed by a cable which, on its path along the detection zone (length L0), “crosses” from one parallel trenches to another through a distance A, forming an even number of “open circuits” of equal area, connected in opposite directions (Fig. 2).

A three-line FE is formed by three parallel cables running at an equal distance A from each other along the boundary using junction boxes; a turn of the FE covers two open, oppositely connected induction circuits lying next to each other at a distance A.

In single-line CE, the maximum coefficient of suppression of “long-range” electromagnetic interference Kp = 50…60 dB is ensured during manufacture. The cost of CE is relatively high (due to the use of permalloy or other magnetic filler with high m), the manufacturing technology is quite unique, the length L0 does not exceed 100…150 m.

All this determined its limited use, despite the tactical and technical advantages. The prototype created and tested at the turn of the 80s and 90s (“Dedal”) confirmed the high performance characteristics of the single-line MSO declared abroad. At present, the domestic technology for manufacturing such CE has been lost.

In two-line CE, the suppression coefficient reaches 35…40 dB, which is ensured by the accuracy of installation on the ground. The width of the CE “loop” (1.2…2.4 m) is optimized depending on the intruder model. The base size A = 2…50 m, which can vary along the ZO (up to 500…700 m long), depends on the electromagnetic environment and bends of the route.

This type is the most common, despite the fact that its main disadvantages are:

• rather strict requirements for the quality of installation (adjacent sections of the “loop” must be symmetrical);
• a slight decrease in the value of P0 and high seismic sensitivity of the MSO at the intersections;
• the impossibility of organizing the interference sensor (open circuit) in a single design with the SE.

In three-line CE (width, loop base” A = 1…1.2 m), despite the maximum number of cables and the shorter achievable length of the CE (no more than 300 m), due to the shorter base, the suppression coefficient of “far” interference reaches 45…50 dB. In this case, end switching boxes are required, with the help of which a differential structure of the CE and an open circuit of the distributed interference sensor are formed in a single cable structure.

The shielded cable used for two-, three-line CE (only such are considered below) (to prevent electrostatic interference) can be single-core or multi-core, and with an increase in the number of turns W, the detectability of the MSO generally increases and the requirements (in terms of noise) for the amplifier-filter are reduced.

At the same time, the reliability of the CE decreases, its own noise increases, the design becomes more complex (due to the appearance of soldering units), therefore, the choice of cable type seems alternative.

Fig. 1. Single-line magnetometric detection device (single-flank)
Fig. 2a. Two-line MSO (single-flank)
Fig. 2b. Two-line MSO (dual-flank)

Detection objects

The characteristic that adequately reflects the magnetic useful properties of an intruder at distances exceeding the linear dimensions of its inherent magnetized volume is the dipole magnetic moment M, the value of which is directly correlated with the mass of the ferromagnetic material [ 5 ]. The possibility of classifying intruders by the value of M to a certain extent reflects their potential threat to the protected object.

Table 2 shows the average values ​​of the magnetic moment M for different types of intruders.

The distribution of moments for similar intruders (the real range is about 20 dB) obeys a complex dependence, which can be extrapolated by the Rayleigh law.

Table 2. Average values ​​of magnetic moments for intruders

An intruder with a magnetic moment M, moving above the plane of a two- or three-line SE with a speed of V0 and a trajectory height of h0, in accordance with the law of electromagnetic induction, generates a useful signal, the average value of which (subject to condition A і h0) can be estimated by the formula:(1)

When substituting typical values ​​of the model of an unarmed intruder without luggage into (1) (M1 = 0.045 Am2, V0 = 1.0 m/s, h0 = 1.5 m), we obtain: 1/W @ 3 nV/vit; at the same time, for an intruder armed with a pistol or machine gun, we have 2/W @ 7 nV/vit; 3/W @ 30 nV/vit. As can be seen from (1) and in accordance with experimental data, the influence of nails or insoles in human shoes (M~ 0.003Am2, h0=0.3 m) is comparable to the influence of a knife or tools.

Foreign developments of the magnetic model of a person, compared to domestic ones, are characterized by a more pragmatic approach to assessing the magnetic model of a person [2.7]:

• A “magnetically clean” intruder is not an object of detection of the magnetic model of a person, since it does not pose a danger under the conditions of use of the device;
• an unarmed intruder moving into the ZO is wearing “heavy” boots and cold weather clothing, carries a folding knife and wire cutters, other ferromagnetic objects, i.e. has an equivalent moment of at least 0.1 Am2;
• the armed man model assumes the presence of a firearm of at least a machine gun, and the standard assessment is the magnetic moment M = 1.4 Am2 [8], which corresponds to an intruder with enhanced small arms (grenade launcher, machine gun), or having a full ammunition supply, radio station. Such an intruder, even at a minimum speed of movement of ~ 0.5 m/s, generates useful signals in the SE over 40 nV/vit.

Interference

Table 3 shows significant sources of industrial electromagnetic and seismic interference, average statistical estimates of their permissible proximity to the MSO with a detection zone of length L0 = 500 m, with a minimum known detection threshold П0 = 2.5 nV/turn (i.e. maximum sensitivity) and the efficiency of electromagnetic interference suppression of ~ 40 dB, which is quite achievable in practice with high-quality installation of cable SE using a tape measure.

Table 3. Significant sources of industrial interference for MSO

The degree of influence of magnetic interference on the MSO depends on:

• type of spatial distribution of the source (“homogeneous”, “single-wire” ~ 1/r, two-wire” ~ 1/r2, “dipole” ~ 1/r3, where r is the distance to the source);
• imbalance e – the value of the unbalanced area of ​​adjacent, differentially connected elements of the SE;
• the dimensions of the sensitive element;
• the power and orientation of the source relative to the sensitive element;
• interference spectrum relative to the band of registered frequencies of the PS. To ensure the greatest noise immunity of the MSO, before installation, it is possible to search for significant sources of interference using a scanner.

The influence of interference is proportional to the width of the sensitive area of ​​the SE, therefore, all other things being equal, it is necessary to strive to reduce it to the limit when the PS begins to decrease.

With increasing L0, the influence of all interference also increases proportionally, and the increase in the internal noise of the MP occurs ~ (L0)1/2 , which allows us to predict the achievable noise immunity of the MSO with any length of the detection zone. The influence of «homogeneous» interference (for example, a magnetic storm) is proportional to the accuracy e/A, with which the MP is installed on the ground, and does not depend on the number N of sections or the length of the base A.

The accuracy does not really change with increasing L0, therefore the influence of other types of interference in practice is determined by the value of the base A: with its decrease, the interference decreases approximately proportionally up to a certain limit Amin, which is defined as:

(2)

where k=1, 2, 3 corresponds to a single-wire”, “two-wire” and point” interference source.

Further reduction of A practically does not lead to a decrease in interference, therefore, knowing the type (k) and the approximate distance ri from all N significant sources to the SE, it is possible to calculate Ai according to (2) and determine minN{Ai}, which will be the optimal value.

Thunderstorms stand out among the interference factors of natural origin. Lightnings that occur during a nearby thunderstorm (at a distance of no more than 3…5 km) are sources of powerful pulsed electromagnetic fields — atmospherics, which are the main, constantly acting (with a seasonal interval) factor determining the interference immunity of the MSO.

Statistical data allow us to estimate the number of atmospherics per year, for example, for the central zone of Russia ~ 4400. Another natural factor is magnetic storms associated with solar activity, however, with precise editing, their influence is significantly less (they are “homogeneous”), in addition, their spectrum lies mainly below the band of recorded frequencies.

Roots of large trees, fences, unstable building structures in strong winds (over 10-15 m/s), swaying, produce low-frequency vibrations of the upper soil layer, which are a significant interference for masked MSO. The distance from the tree trunk, at which the interference does not exceed the level of fluctuation noise of the Earth's magnetic field, approximately corresponds to the average radius of the tree crown, which is associated with the correlation between the development of the root system and the crown of trees.

There are known methods for reducing the effect of seismomagnetic interference:

• limiting the area of ​​application by possible distance from sources;
• reducing the length of the detection zone: the dependences of the magnitude of magnetic and seismic interference are ~ L0 and ~ Ц L0, respectively;
• engineering preparation of the area: digging canals up to 50 cm deep along the EZ (which can then be filled with soft soil, turf) to drain water and weaken seismic surface waves from powerful sources of interference; compaction of the soil after laying the EZ (including with the help of motor vehicles), which also reduces the likelihood of the appearance of burrowing animals in the detection zone.

In the future, we hope to continue the discussion of magnetometric detection means for protecting perimeters and extended borders, and to dwell in more detail on the practical developments of the magnetometric detection method both in Russia and abroad.

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