Laser microphones — a universal reconnaissance tool or another fashion fad?.
Laser microphones — a universal reconnaissance tool or another fashion fad?
A. V. LYSOV
“PPSh LABORATORY”
In recent years, information has emerged that intelligence agencies in various countries and unfairly competing firms are increasingly using remote portable acoustic reconnaissance equipment to obtain unauthorized speech information. These reports naturally raise serious concerns among heads of security services at enterprises and organizations. The most modern and effective are considered to be laser acoustic reconnaissance systems (LSAR), which allow reproducing speech, any other sounds, and acoustic noise during laser-location probing of window panes and other reflective surfaces.
According to the press (including specialized publications), in the USA, for example, in the mid-80s, sellers of special equipment noted a surge of interest among buyers specifically in laser microphones. No less interest is currently being shown in these products in Russia. In this regard, it is appropriate to analyze the current level of development of LSAR, touch upon the physical features of information retrieval and consider a number of factors influencing the results of using this tool.
The history of the creation of the first LSAR goes back to the 30s, when they tried to design similar devices using a lamp and a light filter. At the same time, laboratory tests could be considered successful. C with the development of laser technology already in the 60s, it was possible to create and put into service the first specialized information retrieval systems for the CIA.
Today, a whole family of laser acoustic reconnaissance devices has been created. An example is the SIPE LASER 3-DA SUPER system. This model consists of a radiation source (helium-neon laser), a receiver of this radiation with a noise filtering unit, two pairs of headphones, a power battery and a tripod. Aiming. laser radiation at the window glass of the desired room is carried out using a telescopic sight. The optical attachment allows changing the divergence angle of the output beam, high stability of the parameters is achieved due to the use of an automatic control system. The model provides for the removal of speech information from window frames with double glazing with good quality at a distance of up to 250 m.
Advances in laser technology have significantly improved the technical characteristics and reliability of these reconnaissance systems. Thus, the Hewlett-Packard HPO150 laser device has a rated reconnaissance range of up to 1000 m. In addition, there are reports of the potential for operation at a distance of up to 10 km.
Let us consider in more detail the physical processes that occur when intercepting speech using LSAR. The probed object — usually a window pane — is a kind of membrane that vibrates at an audio frequency, creating a soundtrack of the conversation. The radiation generated by the laser transmitter, spreading in the atmosphere, is reflected from the surface of the window pane and modulated by an acoustic signal, and then is received by a photodetector, which restores the reconnaissance signal.
In this technology, the modulation process is of fundamental importance, which can be described as follows.
The sound wave generated by the acoustic signal source falls on the air-glass interface and creates a kind of vibration, i.e. deviations of the glass surface from its original position. These deviations cause diffraction of light reflected from the interface. If the dimensions of the incident optical beam are small compared to the length of the “surface” wave, then the superposition of the various components of the reflected light will be dominated by the zero-order diffraction beam. In this case, firstly, the phase of the light wave turns out to be modulated in time with the sound frequency and uniform over the beam cross-section, and secondly, the beam “sways” with the sound frequency around the direction of specular reflection.
It is necessary to take into account that the following factors affect the quality of the received information:
parameters of the laser used (wavelength, power, coherence, etc.);
parameters of the photodetector (sensitivity and selectivity of the photodetector, type of processing of the received signal, etc.);
parameters of the atmosphere (scattering, absorption, turbulence, background illumination level, etc.);
quality of processing of the probed surface (roughness and unevenness caused by both technological reasons and environmental influences — dirt, scratches, etc.);
level of background acoustic noise;
level of intercepted speech signal; specific local conditions.
All these circumstances leave their mark on the quality of the recorded speech, so you cannot take data on reception from a distance of hundreds of meters on faith — these figures were obtained under proving ground conditions, or even by calculation.
In particular, representatives of the company RK ELECTRONIC quite correctly call the interception range of their product PK1035-SS from several meters to 500 meters. In addition, many Western users in open publications claim that in urban conditions there is no point in talking about hundreds of meters. The same result was obtained by a few of our compatriots who own LSAR (by the way, mainly of Western manufacture at a price of 15,000 DM). As a result, the interest of specialists in laser information collection systems and, accordingly, in organizing information protection from leakage through this channel is somewhat weakening. To a large extent, disappointment befell those who are eager to buy a black box with a red button, by pressing which, without effort, knowledge, skills and without risk, you can get a brilliant result. In fact, the use of such a complex, one might say, capricious system as LSAR is unthinkable without long, painstaking preparation and significant costs both for the collection system and for the equipment for processing the results.
A mandatory condition for using LSAR is also the study of the tactics of its use in various conditions.
From all of the above, we can draw the following conclusions: laser pickup systems exist and, if used correctly, are a very effective means of obtaining information; LSAR is not a universal means, since much depends on the conditions of use; not everything that is called a laser reconnaissance system by the seller or manufacturer is a laser reconnaissance system; without qualified personnel, thousands and even tens of thousands of dollars spent on purchasing a LSAR will be wasted; security services must reasonably assess the need to protect information from LSAR. If there is a real threat, protection should be organized taking into account the specifics of the location and operation of the facilities, taking into account the technical and financial capabilities of the opposing side, and also in compliance with environmental, ergonomic and aesthetic requirements.
Literature
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