Radio fuses.
Recently, when carrying out terrorist acts in populated areas using explosive devices, devices equipped with radio fuses are becoming increasingly widespread.
This trend is due to the fact that the detonation of explosive charges by electrical means using a wire line in a populated area causes a whole series of technical difficulties associated with the installation, masking and protection of the wire line from mechanical damage and from the influence of so-called induced currents in wire lines (the sources of the latter are current-carrying rail tracks, leakage currents from electrical networks, sources of electromagnetic radiation, electrostatic charges, the inductive influence of power networks and lightning discharges).
The use of delayed-action fuses based on clock (timer) mechanisms and chemical retarders in terrorist attacks does not guarantee the destruction of a moving object due to the difficulty of ensuring that the moment of the target's entry into the destruction zone coincides with the moment of the explosion.
The operation of radio fuses is based on the transmission of a coded radio control signal by a command-transmitting device (command-transmitting device, command device, transmitter) and its subsequent capture by a receiving and executing device (executive device, receiver), to which an electric detonator is connected.
A typical command and transmission device includes an encoder unit that generates a coded signal and a transmitter unit that transmits a control command over the air.
A typical receiving and executing device includes:
- a receiver unit that receives a control command;
- a decoder unit that decodes the received command;
- an actuator unit that, after receiving the appropriate control command, issues a current pulse to the electric detonator, which is necessary for its operation.
The transmission range of the radio control signal is determined by the specific technical parameters of the radio detonator (primarily the transmitter power and the receiver sensitivity at a specific operating frequency) and external conditions, which include: the state of the atmosphere and the underlying surface, terrain, geomagnetic conditions, underground and above-ground utilities, buildings and other stationary and moving objects, the level of unintentional and intentional interference.
In reality, the transmission range of the radio control signal can range from several tens of meters to several tens of kilometers.
When carrying out terrorist acts, the maximum transmission range of the radio control signal is determined by the technical capabilities of the radio detonator and, to a greater extent, the ability to visually monitor the situation in the area where the explosive device is installed and to promptly give the command to detonate it.
The minimum range is determined based on the safety of the perpetrator of the terrorist act from the point of view of the possibility of exposure to the damaging factors of the explosion and, to a greater extent, the possibility of a covert approach, waiting for the target and subsequent withdrawal, taking into account possible counteraction by the target’s personal security.
It is natural to assume that the perpetrator of a terrorist act will strive to transmit a radio control signal from the maximum possible distance.
It should be noted that due to the high rates of “fee” in case of success and no less high responsibility otherwise, it is unlikely that the perpetrator will entrust “pressing the button” to an outsider.
It is also unlikely that the perpetrator would knowingly risk himself, having at his disposal such a sophisticated system for remote detonation of an explosive device as a radio detonator (kamikaze terrorists, as a rule, use the simplest contact electrical or mechanical switches). Statistics of planned and carried out terrorist acts show that in most cases in urban conditions the range of control commands was on average 100 — 120 m, in open areas — 150 — 500 m.
The most frequently used radio detonators in improvised explosive devices are radio control equipment for models, radio stations with an individual tone call function, and elements of security alarms for cars and stationary objects.
The conditions for using a radio detonator dictate the need to make the command and transmitting device small-sized and portable, although it can also be installed on cars.
In this case, the receiving device, on the one hand, must have a high level of noise immunity in the presence of a large number of radio transmitting devices and various industrial sources of interference (trolleybuses, trams, welding equipment, etc.) and, on the other hand, — high reliability of operation at a given moment in time with a minimum delay from the moment of “pressing the button” to the moment of explosion of the explosive charge, including in the presence of the external sources of interference discussed above.
To increase the noise immunity (reduce the probability of false alarms) of radio fuses, encryption systems with a large code volume and systems that combine the ability to repeat commands multiple times with subsequent correction of a possible error can be used. Quartz filters and resonators in receivers and transmitters are also used for these purposes.
In general, the creation of a radio fuse that meets the requirements for reliable operation at a given time and noise immunity is a complex technical task, errors in the solution of which are very costly for the manufacturer of the radio fuse and the perpetrator of a terrorist act using such a device.
In addition, the specific nature of this “activity” does not allow for the involvement of a wide range of relevant specialists to create higher-quality radio detonators and conduct their full-scale testing in laboratory and field conditions.
Possible consequences of errors in the development and manufacture of a radio detonator may be:
- unauthorized activation of a fuse, which, in the absence of an appropriate safety-actuating mechanism (long-range arming mechanism), in most cases leads to the death of the manufacturer and/or the perpetrator of the terrorist act;
- failure to activate when the command to detonate is given;
- delayed activation after the command to detonate is given, in which case the target of the attack leaves the affected area and remains unharmed or receives minor damage.
In the last two cases, the manufacturer of the radio detonator and the perpetrator of the terrorist act may not only end up without a fee or part of it, but also become victims of an assassination attempt by both the target of the assassination attempt and the customer.
In addition, if the device fails to trigger, the law enforcement agencies get their hands on an explosive device that contains a significant amount of information about its creator.
It is natural to assume that if the manufacturer of a homemade radio fuse has enough knowledge to develop and produce it, then the level of qualification of the manufacturer allows him to correctly assess the occupancy of frequency ranges and correctly select the operating frequency for his product.
In this regard, the most likely production of homemade radio fuses is in the frequency ranges of 26 — 29 MHz, 140 — 170 MHz.
With a significantly lower probability, the frequency ranges of 20 — 26, 29 — 48 MHz, 110 — 140 MHz, 170 — 260 MHz, 300 — 700 MHz can be used, which is quite logical and explainable.
The use of a frequency range below 20 MHz seems extremely unlikely due to the significant complexity of creating a portable command and transmitting device capable of operating effectively with short antennas.
Moreover, this would require appropriate full-scale development and testing with the corresponding expenditure of time and money without a guarantee of success.
It is most likely that the manufacturer will go the way of using ready-made proven circuit solutions from the composition of radio-controlled models (frequency ranges of 26 — 28 MHz and 144 — 146 MHz, transmitter power of 0.5 — 1.5 W) as the most accessible, cheap and reliable; radio stations of domestic and foreign production in the frequency range of 26 — 28 MHz, 144 — 174 MHz, 390 — 470 MHz (for example, “VEDA”, “ALAN”, “DRAGON”), as well as various alarm systems for the protection of cars and stationary objects operating in the same frequency ranges (for example, equipment such as CAR ALARM SYSTEM with an operating frequency of 27.145 MHz and “Sova” with an operating frequency of 26.945 MHz).
This is confirmed by domestic and foreign statistical data on terrorist acts carried out and planned using explosive devices equipped with radio fuses.
It should be noted that the intervals between the specified frequency ranges are densely occupied by radio broadcasting and television stations, as well as powerful radio transmission facilities of various services.
For example, television broadcasting channels occupy the frequency bands of 48 — 66 MHz, 76 — 100 MHz, 174 — 230 MHz, 470 — 622 MHz.
Radio broadcasting in the VHF range uses the frequency ranges of 66 — 73 MHz and 88 — 108 MHz. The sections of the broadcast frequency range and those allocated for service radio communications are characterized by continuous radiation from powerful radio transmitting equipment, which practically excludes the operation of radio fuses with an operating frequency within these ranges in their area of action.
It is also important that for the production of individual samples of radio fuses, the cheapest and simplest, on the one hand, and reliable, on the other hand, is the equipment of the frequency range of 25 — 30 MHz. The equipment of the frequency range of 144 — 146 MHz and higher is more complex, and a radio fuse based on it can be created by specialists of a sufficiently high class.
Most likely, in this case, ready-made units and blocks of existing receiving and transmitting devices will be used with their subsequent modification.
Theoretically, it is possible to use portable radio stations of the 420 — 470 MHz frequency range from Motorola, Kenwood and others to create a radio fuse.
The main advantage of this range is the small length of the antenna of the receiving and transmitting devices — 5 — 15 cm. In practice, a number of circumstances prevent the creation of radio fuses in this range.
These radio stations are usually made in the form of a monoblock, and to upgrade them to the level of a radio fuse, it is necessary to carry out the corresponding engineering calculations by a highly qualified specialist, as well as check and adjustment using the appropriate specialized equipment.
The short length of radio waves in this range helps to reduce their diffraction ability, that is, the ability to bend around obstacles.
As a result, objects made of metal (cars, trade kiosks and pavilions, etc.) or fine-cell reinforced concrete with a cell size commensurate with the wavelength (0.7 — 0.75 m) located in the path of radio wave propagation are an obstacle for these radio waves, causing their weakening (radio shadow effect) or strong re-reflection. In particular, a receiving device installed under the bottom of a car is largely shielded by its body.
However, when conducting radio communication between two subscribers located inside a reinforced concrete building or in neighboring buildings, at frequencies of 400 — 450 MHz, due to the effect of the formation of peculiar random waveguides in metal structures such as a central heating system, local improvements in the quality of communication are possible.
In this case, the specific result is determined not only by the distance between the subscribers, but also by the relative distance (in relation to the wavelength) between the subscribers and the metal structures. In addition, the radio waves emitted by the transmitters, reflected from obstacles, can be added or subtracted at the receiving point, forming a complex interference pattern.
In this case, a whole series of points are formed in space, where the signal is weakened as a result of interference. The distance between these points, naturally, depends on the wavelength.
The longer the wavelength, the greater the distance between these points, and vice versa, the shorter the wavelength, the better the radio waves are reflected from obstacles and the smaller the distance between these points and the more often they are found in space.
The influence of the interference pattern on the quality of radio communication becomes especially noticeable in the frequency ranges of 700 — 900 MHz. This phenomenon does not cause much harm to mobile radio communication, since the subscriber, having detected a weakening of the signal, can take a few steps to the side and leave the point where the adverse effects of the interference effect occur. If we are talking about a radio fuse, then difficulties of a certain order arise with optimizing the location and movement in space of the receiving and executing device, especially if an electric detonator and explosive charge are connected to the latter.
Thus, the use of the frequency range of 400 — 470 MHz and higher in radio fuses is theoretically possible, but in practice it is an unlikely event.
A separate point is the question of the possibility of using paging communication to create radio fuses based on it. Theoretically, the solution to such a problem is not difficult, especially when using digital pagers. A number of factors prevent the practical implementation of the idea of such radio fuses.
Paging systems, as a rule, are a set of stationary command and transmitting equipment and a large number of individual receiving devices — pagers. Existing paging systems use frequency ranges of 136-175 MHz and 440-490 MHz.
Recently, there has been a growing trend to switch to frequencies of 440 — 490 MHz. This is explained by the desire to reduce the length of the antenna and the dimensions of the pager itself and to increase the number of subscribers simultaneously placed in one frequency range.
It is difficult to imagine that someone, when creating a radio fuse, will try to create their own paging complex.
The most likely attempt is to transmit a command to the pager through the operator of the paging company.
Taking into account the quality and reliability of the telephone network (especially domestic), the time it takes to dial the operator's number, talk to him, the time it takes to dial and transmit the message from the operator to the subscriber, the total time from the decision to detonate the explosive device to the activation of the electric detonator can range from several tens of seconds to several minutes (in some cases — up to 10 minutes or more).
Under these conditions, the range of movement of a moving object (in a car or on foot) relative to an explosive device can be from several tens to several hundred meters (in a car — up to several kilometers), which in principle excludes the possibility of hitting such an object.
In addition, at frequencies above 400 MHz, problems arise with the reliability of transmitting a command to a pager associated with diffraction and interference of waves.
All this, as well as possible errors by paging company operators when dialing a subscriber's number, make the use of paging communication for remote detonation of an explosive charge unlikely.
Similar problems arise when attempting to use cellular communications operating at frequencies of 450-470 MHz, 800-900 MHz and 1800 MHz to create radio detonators.
In this case, several more problems are added, related to possible errors by numerous subscribers of the cellular and landline telephone network when dialing phone numbers and related to such a service of telephone companies as automatic dialing of cellular subscribers and informing them of various information (for example, about the balance of funds on the subscriber's account and payment terms).
Thus, the most likely creation and use in terrorist acts of radio detonators whose operating frequencies lie in the ranges of 26-29 MHz and 140-170 MHz with an average command transmission range of 100-170 MHz. 120 m in populated areas and 150 — 500 m in open areas.
The frequency ranges 20 — 26 MHz, 29 — 48 MHz, 110 — 140 MHz, 170 — 260 MHz and 300 — 700 MHz can be used with a significantly lower probability.
This is confirmed by domestic and foreign statistics.