Problems of detection and identification of radio signals of covert information control means.

Problems of detection and identification of radio signals of covert information control means.

Problems of detection and identification of radio signals of covert information control means

Part 1. Basic requirements for radio signal detection systems

Kargashin Viktor Leonidovich
Candidate of Technical Sciences

Special equipment, No. 3, 2000

Radio frequency signal detection and identification systems are currently among the most common devices used to counter covert information control. Such devices are implemented in the form of broadband electromagnetic field indicators, scanners and automated radio monitoring systems based on digital receivers. All of them are based on the presence of an obvious unmasking sign of the operation of special radio equipment (SRTS), in the form of radio emission that exists during the period of switching on the SRTS in the transmission mode.

When detecting radio signals, the main requirement is guaranteed identification of the received radio emission as dangerous, which makes the task of optimizing the structure and parameters of radio signal detection devices extremely urgent. Guaranteed detection and identification of SRTS signals are possible if a number of conditions are met that determine the following aspects of signal reception:

• information;
• energy;
• temporal;
• spatial.

All these aspects are, as a first approximation, independent of each other, which allows us to consider separately their influence on the structure and technical characteristics of radio signal detection devices.

InformationThe aspect is used to the greatest extent for identifying SRTS signals against the background of numerous interfering signals from external and extraneous stations, including for determining broadcasts. The essence of the information aspect lies in the possibility of determining the received signal for belonging to the SRTS by the presence of the corresponding information content in the received message. As a possible class of information message, as a rule, a speech signal is considered, which exists in one or several controlled premises. For this purpose, most receiving complexes and means allow demodulating the signal according to standard procedures provided by the receiver (AM, FM, PM, etc.), as well as using additional identification capabilities in the form of the presence of a correlation between the demodulated signal and the acoustic signal of the room. Moreover, such procedures allow not only automated complexes to solve problems, but also scanners and field indicators, implementing the phenomenon of «acoustic tie». Of the existing equipment, it is worth noting the field indicators «RICH-2», «RICH-3», «D-006», «D-008», «IPSh-012», scanners «Oscor-5000», «Xplorer», «Cub», «Scorpion», automated complexes of the «ARK», «RCC-2000», «Rodeya» series.

Without denying the usefulness of this procedure for most practical tasks, it should be noted that the solution is limited for SRTS variants that do not provide for analog types of modulation, are not intended for intercepting speech information, and use complex types of modulation, coding, and cover. Taking into account the prospects for the development of information transmission technology, including for SRTS, and theoretical methods for using noise-like signal systems, the possibility of identifying signals by information features will be reduced to a minimum.

At present, the signal emitted by the SRTS should be considered as noise, which excludes the possibility of its demodulation without knowledge of the method of converting the original information and, moreover, the presence of demodulation options is not necessary for the effective detection of SRTS signals.

In general, a signal from a SRTS can be represented as a signal with a random base in the frequency-time domain without taking into account the method of its formation. Currently, signals with a random base are to a certain extent implemented using signals with pseudo-random frequency hopping (PRFH) and by compressing signals in time. From the standpoint of counteraction, limiting the types of bases of received and identified signals leads to uncertainty in assessing the capabilities of search devices with respect to emerging and implemented types of signals. Fig. 1 shows some types of signal bases demonstrating significant differences between signals with deterministic bases and signals with random bases.

Existing search devices and systems are designed to detect and identify continuous signals or short-term signals in the presence of a remote control in the SRTS. The ability of search devices to detect signals with frequency hopping is determined by specific signal parameters, i.e. the channel switching speed, the number of these channels and the bandwidth of each channel. The problem of detecting and identifying signals with frequency hopping is independent and must be solved using specialized radio receivers with a high frequency tuning speed.

The ability to detect short-term signals when using tunable receivers depends on the speed of its tuning in comparison with the speed of change of the signal frequency, but with a long-term analysis of the frequency range, the probability of detecting such a signal tends to 1.

Fig. 1. Types of SRTS signal databases

A signal with a random base is characterized by the appearance of radio emission at random moments in arbitrary parts of the frequency range. It is obvious that the correct identification of such a signal by traditional methods is extremely difficult. Considering the SRTS signal as a noise process allows us to build detection tools and complexes in an optimal way. For such a situation, energy receivers such as radiometers, known in radio engineering, are optimal, the use of which for the problem being solved has a number of specific points that will be analyzed in the next article.

Energyaspect in solving the problem of detecting signals from the SRTS means determining the requirements for the components of radio monitoring devices taking into account the sensitivity of the receiver, the radiation power and the conditions of the SRTS location. It is obvious that in the presence of artificially introduced restrictions on the maximum values ​​of the SRTS parameters, the solution to the detection problem will have a threshold nature, and, therefore, significantly reduce the potential efficiency of detection systems. This problem can be solved without using energy characteristics, for which we will consider the generalized scheme of covert information control, presented in Fig. 2.

As a model of covert information control, it is advisable to consider the situation of optimal reception of the SRTS signal at the control point, that is, the organization of coherent reception taking into account the distance and the attenuation of the signal introduced by the building structures.

Fig. 2. Radio monitoring scheme of signals from the SRTS

Reception of the SRTS signal by the detection receiver is obviously incoherent, since the parameters of this signal are unknown, and the possibilities of effective detection are determined by the following prerequisites:

• the distance is greater than the distance at which the antenna of the detection device is located;
• the SRTS signal is additionally weakened by the building structures along the propagation path.

If the SRTS is calculated optimally, then the signal power at the inputs of both receivers can be linked through the transmitter range and the transmitter radiation power. Using known expressions for the radio communication channel, a simple relationship can be obtained between the distances  and in the following form:

,

where the symbol denotes the quantile function of the normal distribution, — power attenuation introduced into the signal by building structures, and — the probability of a false alarm and correct detection on the SRTS receiver, and — the probabilities of false alarm and correct detection for the detection receiver.

The formula is obtained for the following conditions, assuming identical propagation of radio waves along the paths to both radio receivers:

• the receiver at the checkpoint performs coherent analysis;
• the detection receiver performs incoherent analysis;
• the attenuation of the radio signal by the building structures is determined by the factor , which is frequency-dependent for structures of different types ().

Thus, the requirements for the location of the SRTS signal detection devices can be determined only from certain regulatory considerations and specific conditions at the monitoring facility without taking into account the energy indicators of the SRTS radio transmitter. The choice of specific values ​​of the probabilities of false alarm and correct detection for both receivers is determined by the requirements for the quality of signal reception and can be made from the following assumptions:

• probability of false alarm  for the SRTS receiver, the order of 10-3…10-2 can be selected, since the situation of erroneous reception for covert control is not significantly dangerous;
• the probability of a false alarm for the detection receiver should be significantly lower, on the order of 10-8…10-10, since erroneous reception in this case is critical and leads to additional costs for decoding false responses;
• the probability of correct detection of the signal for the SRTS receiver should be high enough, about 0.9…0.99, since guaranteed signal reception is the main requirement for covert information control systems;
• the probability of correct detection  can be selected of the same order of 0.9…0.99, since the requirement of guaranteed detection of an unknown signal should also be considered as the main one.

For variations of the specified values ​​of the standard parameters (,,,) the ratio between the distances and is reduced to the following form:

.

In an extreme situation, when the attenuation values ​​of the signal introduced by the building structures are unknown, it is advisable to choose and determine the distance based on a simple ratio . For the frequencies of the traditional VHF range used to transmit signals from the SRTS, the amount of attenuation by building structures is insignificant, which makes the choice of the coefficient justified., and, in addition, the case of signal attenuation by structures and in the path is not excluded, which removes the problem of assessing the effect of signal attenuation on the choice of detection device parameters. Reducing the distance compared to the distance is the price to pay for the unknown signal parameters. For the most stringent requirements, the distance between the antenna of the detection device and the antenna of the supposed SRTS transmitter should be no more than 0.075 of the distance between the transmitter and the SRTS receiver. For example, if the distance to the possible location of the SRTS checkpoint is 100 meters, the placement of the antennas of the detection device should be organized in such a way as to ensure the distance from the antenna of the sought radio transmitter to the antenna of the detector is no more than 7.5 meters, i.e. the distance between the antennas (or antenna positions) should be no more than 15 meters. For smaller distances to the checkpoint, the problem of placing the antennas of the detection device becomes practically difficult to resolve.

From the energy standpoint, the operating principle of detection devices such as electromagnetic field indicators have certain advantages, since when moving across the survey space, the required or smaller distance between the antennas of the radio transmitter and the indicator is achieved (however, one should take into account the limitations of the SRTS model for field indicators in terms of the need for the SRTS to operate in radiation mode during survey, as well as the reduced sensitivity of the indicator given the differences in its operating frequency band compared to the width of the signal spectrum).

Stationary automated complexes are currently equipped with a system of receiving antennas (for example, the ARK series complexes), which allows for signal detection in several rooms. Taking into account the developed requirements, the number of receiving antennas of the complex can be determined depending on the expected distance to a possible checkpoint.

For typical situations, this number is not so significant, since a distance of 15 meters corresponds to their installation approximately through a room 5-6 meters wide. At shorter distances to the control point (15-20 meters), the optimal use of stationary radio monitoring systems becomes problematic, since the distance between the receiving antennas in this case should be no more than 3 meters, which increases the required number of antennas by almost 30 times.

SpatialThe aspect is the assumed presence of directional properties of the SRTS transmitter antenna, taking into account the location of the control point. When assessing the influence of the energy aspect, it was assumed that the receiving antennas of control and detection operate under identical conditions. In the traditional VHF frequency range, creating directional properties for small antennas is problematic and the antenna is usually a dipole with a circular radiation pattern. With an increase in the frequency range of the SRTS, such a possibility becomes more real, for example, due to the use of not dipole, but vibrator antennas, which allow for some directionality. The most significant is the non-zero probability of unsuccessful placement of the receiving antennas of the detector in corner zones with minimal radiation in the desired direction. Taking into account directional properties will significantly worsen the requirements for the minimum distance of the detector antenna to the transmitter antenna, and in the microwave range it can generally lead to a difficult situation of the need to place receiving antennas on the outer surfaces of buildings on the side of the control point.

Temporaryaspect is the necessity of matching the operating time of the SRTS transmitter in the radiation mode and the detection receiver setting to the corresponding frequency range. From Fig. 1 it is evident that for signals with a random base or short-term messages, as well as for SRTS with external control, such a match is of a probabilistic nature. Search devices can be divided into two groups according to this factor:

• electromagnetic field indicators and scanners, which assume the continuity of the radiation of the sought signal during surveys;
• stationary detection systems designed for long-term observation of signals in the frequency range.

The first group of search devices is designed to detect radiation from continuous-action SRTS, which currently represent a fairly large class of covert information control devices. By moving such search devices, the problems of energy and spatial aspects are also solved to a certain extent. For the adopted signal model, such search devices are practically optimal in terms of their operating principle.

The second group of search tools is designed to detect signals of a wider class of SRTS, and the main problem is the compliance of the requirements for the tuning time of detection receivers with possible models of SRTS time parameters. A possible approach to developing requirements for the tuning speed of receivers will be considered in the following articles. However, based on general considerations, it can be stated that with an infinitely long time of signal observation in the monitored frequency range, SRTS signals with arbitrary radiation characteristics in time will be guaranteed to be detected, including short-term transmissions or signals with frequency hopping.

Thus, the use of radio emission from SRTS transmitters as a telltale sign can be optimized both by the structure of search devices depending on the model of the detected signal and on the conditions of the checkpoint location. In this case, the main factors that allow optimization of search devices are:

• focus on the SRTS signal as a signal with a random base, that is, a noise process, the demodulation of which does not make sense in the context of solving the problem of counteracting covert information control;
• when detecting signals from optimally designed SRTS, the antenna(s) of the detection devices must be located at a certain minimum distance from the antenna of the desired SRTS transmitter;
• search equipment such as broadband electromagnetic field indicators and scanners can be used to detect signals from SRTS operating in continuous emission mode, provided that they are moved in space to achieve minimum distances between the antennas of the search device and the transmitter;
• automated stationary radio monitoring systems make it possible to detect a radio signal from a wider class of SRTS with arbitrary types of control over the SRTS operating mode, but require justification of the requirements for the frequency tuning speed of the base receivers used.

When practically using the search and detection equipment and systems available on the market for specialized equipment in specific situations, a clear understanding of the models of SRTS signals to be detected is required, or consultation with a specialist in optimizing the design of signal detection systems is required in relation to to the conditions of the location of the protected object.