Information processing in radio surveillance systems.
Vladimir Anatolyevich Silantyev,
Candidate of Technical Sciences
INFORMATION PROCESSING IN RADIO SURVEYING SYSTEMS
In a modern computer radio surveillance system, one has to deal with intensive information flows that are created by built-in controllers and digital signal processing devices of radio equipment. The task of the software of such a system is to extract from this flow, which characterizes a complex and constantly changing electromagnetic environment, the data necessary for reliable detection of the sought-after signals. The article examines the general principles of information processing in radio surveillance systems, which reflect the experience of developing and operating the RS1100 multichannel complex at a number of sites. This complex is designed to protect objects in the form of several spatially separated rooms from information leakage via a radio channel, power supply network and other wire lines.
The decision-making procedure for detecting a signal of interest in a radio surveillance system is generally a complex multi-stage process. At the first stage, during the panoramic analysis of the radio environment, occupied sections of the monitored frequency range are determined. Then, within these sections, individual signals are selected and detected. Finally, among all the detected signals, those created by specific sources of radio emissions are selected based on a number of characteristics. Modern radio intelligence equipment provides primary data on the radio environment at a very high speed: analysis of the 1-GHz range with a resolution of 12 kHz can be performed in less than 10 seconds with an information flow intensity of over 20 KByte/s. The software of automated radio surveillance systems must process this information in such a way as to provide the operator with the minimum data necessary to make informed decisions at the final stages of analysis. Otherwise, the speed qualities of the equipment will simply not be realized due to the limited capabilities of the operator.
Monitoring the radio environment in radio surveillance systems begins with constructing spectral panoramas that reflect the power distribution of radiation received by the antenna by frequency in the entire range under study. A spectrum analyzer or scanning radio receiver is used for this purpose, measuring the level of the received signal at each tuning step. The data array obtained in this way represents a spectral panorama in discrete form with a resolution equal to the receiver bandwidth. The control computer of the system can display spectral panoramas on the monitor and store them in memory for subsequent processing.
In multichannel systems, a separate spectral panorama is created for each of several spatially distributed antennas. In particular, in the RS1100 system of the Radioservice company, up to 26 channels can be used to detect and neutralize eavesdropping devices in several rooms of a building (see Automated RS1100 Complex: How to Build a Distributed Radio Monitoring System for an Object, Communication and Telecommunication Security Systems, No. 4 (16), 1997). Such systems operate continuously for a significant period of time (weeks, months), creating spectral panoramas in each scanning cycle that characterize the radio environment at a specific moment in time with different resolutions (overview and detailed). Separate spectral panoramas are convenient for visual monitoring; however, with a large number of them, searching for and processing the information of interest causes significant difficulties. To reduce the volume of stored information and simplify its analysis, the RS1100 system stores only the current spectral panoramas (i.e. those created during the last scanning cycle), while data on all previous ones is generalized as a result of processing, for example, accumulation or averaging (Fig. 1). Generalized spectral panoramas (they are called radio range loading diagrams or background panoramas) provide a statistical estimate of the radiation intensity over long time intervals and are successfully used to classify radiation sources. At the same time, it is obvious that during such processing some of the information related to previous scanning cycles is lost. In particular, it is impossible to determine the moments of appearance and disconnection of a certain signal, track the change in its parameters over time, etc.
Fig. 1. This is how spectral panoramas are displayed on the screen of the RS1100 system control computer: in the background is a generalized panorama reflecting the results of all previous scanning cycles, in the foreground is a current panorama.
To solve this problem, the new version of the RS1100 control program uses spectral panoramas as the source material for selecting the main information objects — detected signals. Each signal receives a unique identifier, and the program recognizes signals of the same name that are created by one radiation source in different antennas or are repeatedly detected in several scanning cycles. The selection and detection of the signal are performed together with the measurement of a number of its parameters: intensity, spectrum width, carrier frequency, etc. All information about the signal is placed in a database, which is filled automatically during the operation of the system without operator intervention. The database of detected signals (Fig. 2) not only ensures compact storage of all radio observation results, but also provides means for processing, classifying and displaying the necessary information, significantly increasing the efficiency of the operator's actions. By analyzing the information in the database using a standard query and report mechanism, it is possible to obtain comprehensive information on the evolution of each detected signal, perform statistical processing of its parameters (which is often necessary due to the non-stationarity of the spectra and the propagation conditions of radio waves), and also implement very important signal classification operations.
Fig. 2. Automatic creation and filling of databases of detected signals ensures completeness and compactness of information presentation in the radio surveillance complex.
The number of signals detected by the radio monitoring system in just one cycle of scanning the radio range in a large city reaches several hundred. The most important task of the software of the complex is to select from among the entire set of detected signals those that are of real interest to the operator. The program solves this problem by distributing signals from the database into groups based on a priori information about the radio environment, which is entered by the operator during setup or accumulated by the complex during operation (training). For example, the list of detected signals will be significantly reduced if the system ignores all emissions created by third-party «background» sources. Such signals are classified by the program based on data from loading diagrams — special spectral panoramas created in advance and characterizing the radio environment at the observation site.
To continue the classification, among all the radiation detected in each current scanning cycle, those are selected that have never been in the program's field of view. Generalized spectral panoramas that store all the observation results from the start of work on a specific task are used to select such «unknown» signals. Classification of «unknown» radiation allows focusing on current changes in the electromagnetic environment in relation to its already studied characteristics. In particular, if the complex operates in automatic mode and is periodically serviced by an operator, then only those radiations that have been detected since the last service will be of interest to him. Such signals are selected from the entire set of unknown signals by time/date of first detection and are called «new» in the RS1100 program.
Finally, many signals emitted by transmitters of radio broadcasting and communication stations exactly correspond to the legally established standards. If the database is filled with information about frequency assignments of such stations operating in the area of operation of the complex, the program will classify their signals as «standard».
Classification operations allow the number of detected signals of interest to the operator to be minimized and, in some cases, the desired object to be identified immediately. In other situations, the decision is made at the next stage after identification operations have been performed. During the identification process, the operator establishes that the detected signal was generated by a source of a certain type. Identification often comes down to spectrum analysis or signal demodulation. Special tests, such as harmonic analysis or acoustic probing, are also used in eavesdropping device search systems. Spatial identification methods have been developed for multichannel systems, which allow establishing that the detected signal is generated by an internal (located inside a building or premises) and not an external source. In the RS1100 system, the results of identification tests performed automatically are placed in the database as parameters of the detected signal. Sorting the lists by these parameters allows the detected signals to be arranged in order of their potential «danger».
The methods given do not exhaust the capabilities of software tools for automating information processing in radio surveillance systems. Depending on its purpose and composition, the software can successfully perform other operations, such as recording and automatic analysis of signal realizations at the demodulator outputs or other procedures for identifying radiation sources. And in these cases, the use of databases for storing, processing and analyzing information about the radio environment can significantly increase the efficiency of the radio monitoring system.