Although security threat can emerge from many sources, the one which is really brutal and catastrophic is terrorism. Terrorism has far reaching consequences like loss of human life and property, and change in behavior pattern of individuals, many of them being irreversible.

Spread of knowledge and education, coupled with strict vigilance, can help contain terrorism and other security threats. However, for robustness in security, electronic threat detection mechanisms are recommended. The main advantage is the passive monitoring by these electronic solutions, which does not impact routine life of any individual and still ensures safety and security. Generally, terrorist attacks are made in densely-populated or sensitive or tourist areas like bus stations, airports, railway stations, hotels, places of worship and other crowded places.

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The frequency spectrum is allotted in India by WPC. This means any frequencies used by anyone who is not legally allotted these raise suspicion, and also any illegal activities carried out in the licensed band are suspect. If the illegal and suspicious frequency and its location are determined by any monitoring system, it can help to locate and isolate the suspect without much manual intervention. This technique of detecting and locating the illegal and suspicious RF source is called RF surveillance. Here we discuss the RF surveillance technique which can be used both in preventive and tactical ways to effectively counter terrorist threats.

The need
It is not easy to determine illegal activities in communication and mobile services due to their use en-mass. However, communications between terrorists and anti-social elements can be stopped either by shutting down the network or by jamming all the operating frequencies. This would stop all forms of communications, including security agencies’, and is therefore counter-productive. That is why RF surveillance is required and is gaining popularity, as it enhances security perimeter beyond just physical and visual security.

Fig. 1: Basic block diagram of an RF surveillance system
Fig. 1: Basic block diagram of an RF surveillance system
Fig. 2: Multiple RF sensors to determine emitting devices in Hotel Taj Mahal building
Fig. 2: Multiple RF sensors to determine emitting devices in Hotel Taj Mahal building

How it works
Radio frequency (RF) surveillance systems must be capable of measuring unknown or unfriendly transmissions and possibly extract the target information content. Direction finding (DF) and geo-location are usually part of RF surveillance signal recovery; also knowledge of the emitter location is offered as part of such systems. The basic block diagram of a generalised RF surveillance system is as shown in Fig. 1.

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The three steps that describe it are as below:

Search. Performs the high-speed signal search operation in the selected frequency band and identifies active signals.

Classify. Performs signal classification operation to sort out the signals of interest based on the selected threshold level, alarm conditions and automatic modulation recognition algorithm.

Monitor. On the signal of interest, performs the demodulation and identifies the intelligence available in it. If required, records the signals and the intelligence for further analysis. Once intercepted RF signal is monitored and recorded, the system investigates RF spectrum to identify the target message content.

Who needs it
Surveillance of wireless signals is generally required by spectrum regulators, public safety agencies, border and coastal security, and military intelligence. In the government and military areas, these transmissions are often characterised as signals intelligence (SIGINT). These systems are useful to intercept and locate the different wireless transmitters such as global mobile personal communications by satellite (GMPCS), cellular handsets and VHF/UHF frequency range walkie-talkies. Thus, an RF surveillance system plays a vital role and ensures success in security mission.

Fig. 3: Use case of proactive indoor monitoring of sensitive areas
Fig. 3: Use case of proactive indoor monitoring of sensitive areas

Various intelligence and security department personnel like Homeland security department, Border and Coastal Surveillance department, critical infrastructure and perimeter protection forces, signal intelligence groups of Army, Navy and Air Force, technical and aviation research organisations, defence agencies, government end-users, paramilitary forces, regulatory agencies, commercial spectrum monitoring agencies, railways, hospitals, transportation security department, including aviation and maritime transportation, can use these systems.

Overview of use case
Let us define various use cases for RF surveillance systems in different operating environments. Three use cases are stated below:
1. Tactical use case
2. Preventive use case
3. Costal surveillance use case

Note that these are just indicative use patterns, and the RF surveillance system is not limited to these applications.

Tactical use case for critical infrastructure protection
RF sensors can be installed in the suspected area around the building as shown in Fig. 2, or RF sensors can be mounted in the sensitive buildings as shown in Fig. 3. All RF sensors can be connected via LAN backhaul either with a wired or wireless system. The sensors can then be time-synchronised and, with its core detection algorithm, the software can detect illegal or threat signal. The system is very easy to install within a few minutes.

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Fig. 4: Activity sequence for commission of RF surveillance system
Fig. 4: Activity sequence for commission of RF surveillance system

This method is best explained with the example of Hotel Taj Mahal in Mumbai where terrorists attacked. Assuming that the terrorists were still using RF communication for their coordination, it was possible to detect their signals and their precise origination, if multiple RF sensors (Fig. 2) were put in all corners of hotel building. Using cross-correlation technique the exact RF origination or emitting source, and hence terrorists’ locations, could be detected and they could be isolated and neutralised.

Though this technique is reactive and tactical, it can be really quick to spot the emitter and corrective actions can be taken to avoid larger damage. A better way, however, is to proactively monitor indoor areas of the sensitive buildings.

Fig. 3 shows a large hotel building with multiple floors and rooms. Each RF sensor scans its designated area and sends information to a common control location where all the RF-emitting sources (legal or illegal) in the hotel can be identified. Since sensors can time-synchronise their sweeps, the energy detection too gets synchronised. Power-level comparisons can then determine whether a transmitter is inside vs outside, and if inside, in which room.

This approach works well even if attenuation through the walls, ceilings and floors of buildings is substantial. The series of actions performed by this system in current example is summarised in Fig. 4.

Preventive use case for critical infrastructure protection
In preventive application of an infrastructure protection, a distributed network of RF surveillance systems can be used to perform the operation in remote mode, probably from network operations centre (NOC). The surveillance systems can be installed at various locations by connecting them on backhaul connectivity such as the Internet, 3G or a captive LAN network. These systems stay synchronised in time with GPS and therefore have the capability to geo-map suspicious emitters.

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Fig. 5: Preventive technique for emitter detection using RF surveillance
Fig. 5: Preventive technique for emitter detection using RF surveillance
Fig. 6: Distributed network of RF sensors for coastal surveillance
Fig. 6: Distributed network of RF sensors for coastal surveillance

Let us take the example of Mumbai city, which has a large number of commercial buildings and other important establishments and tourist centres like BSE building, Hotel Taj Mahal and Chhatrapati Shivaji Terminus. A typical deployment and control scheme for Mumbai city is shown in Fig. 5, where RF sensors are located at strategic locations and their time-stamped data is pulled at NOC for network view of RF operations in the city. This is a true proactive monitoring because one would know of any incremental RF emission as soon as it happens.

The process of installation and commission of sensor-based systems remains the same as explained in Fig. 4. For better insight of the entire city, drive test with data recording facility can supplement areas in which the RF sensors cannot be easily deployed. The drive data and the stationary sensor data can be correlated at NOC and emitters can be determined.

Coastal/border surveillance
The third and final example is of very large- scale monitoring, which can be coastal or border surveillance systems. The system works on the same philosophy as described in previous section, with a difference that the RF sensors are located along the coast or border, and that the granularity of deployment is not as dense as in previous case. The deployment locations can be hill-top or resorts where uninterrupted data connectivity to NOC is available.

These systems also work 24×7 for continuous signal monitoring and analysis. The operational activities of this coastal/border application are the same as in Fig. 5, where the surveillance systems along the coastal or border area, with suitable backhaul connectivity, can cover the entire sensitive border area. Fig. 6 is an indicative example of such deployment.


The author is application engineer in Electronic and Measurement group of Agilent Technologies at Hyderabad. Along with Master of Engineering degree and specialisation in systems and signal processing, he has over 12-year experience in RF surveillance and communication, EW systems development, integration and testing

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