Why RF receiver sensitivity is important for law enforcement

Listening devices used by terrorists and organized crime are discreet, highly intelligent, and incredibly difficult to locate. Even the smallest of bugs can pack a huge amount of technology and may be integrated into USB cables, light bulbs, and other electrical objects.

These micro-electronic devices are designed to lie dormant when not in use. Even when they have been activated and connected to a cellular or Wi-Fi network, they only need to “shake hands” momentarily every few hours to capture incriminating information. For the remainder of the time, they are undetectable to all but the smartest RF monitoring systems.

Technical Surveillance Counter Measures

For security services working in the field of Technical Surveillance Counter Measures (TSCM), finding and locating the RF sources of these devices is imperative because undetected RF surveillance and data transmissions will enable criminals to:

• Carry out intelligence, surveillance, and reconnaissance (ISR) operations against legitimate organizations
• Collect and disclose classified information
• Hijack critical command and control signals, causing financial disruption or worse
• Obtain vast sums of money and financial data
• Compromise law enforcement and intelligence agency operations

This is especially important for in-building deployments in embassies, critical infrastructure, and secure sights. The spectrum monitoring systems law enforcement deploy must be able to capture all RF transmissions in their purest form so key data can be extracted for analyses, decoding, and demodulation. Any irregularities can then be flagged, time-stamped, and swiftly dealt with. Signal purity is dependent on the sensitivity capabilities of the deployed RF receiver.

Video: The RFeye Guard, a TSCM solution

Factors impacting receiver sensitivity

Multiple factors can impact receiver sensitivity, including system design and tuning capabilities, installation, positioning of the different antennas, and third-party RF interference (RFI), which exists in natural and artificial formats.

When setting up spectrum analyzing equipment (portable or network-based) to monitor RF activities in an increasingly congested spectrum, the location of the equipment in relation to other wireless-enabled technologies in use must be considered.

This is because all electronic devices and components powered by wireless RF will emit RF interference, which will compromise receiver sensitivity capabilities and, ultimately, the fidelity of the captured data. System design and configuration will also impact sensitivity levels, so appropriate shielding and grounding products must be incorporated to mitigate this. The biggest barrier to capturing RF signals in their purest form, however, is a receiver’s ability to support the lowest signal-to-noise ratio.

The importance of minimizing the noise figure

Noise floor is the lowest point at which an RF signal can be detected before the signal-to-noise ratio is not strong enough for the captured data to have any value. To extract the high-fidelity data needed for accurate signal sampling, decoding, and demodulation, RF signal strength must always be higher than ambient noise levels (the noise floor).

If a receiver is unable to detect RF signals close to the noise floor, a wealth of potentially critical information could go undetected and, more crucially, untraceable. Failing to find and identify unrecognized RFI in an undercover mission could have far graver repercussions.  

A lower noise figure (NF) indicates better receiver performance, as the received signal has less additional noise. This is important for the following reasons: 

• It enables a receiver to detect weak signals in a noisy environment
• It improves the accuracy and reliability of the signal measurements, such as signal strength, frequency, and modulation— integral to accurate monitoring

However, achieving a low noise figure is difficult, especially in high-frequency applications such as law enforcement. Thermal noise, caused by the random movement of electrons in the receiver components, is a principal source of noise. Other noise sources include amplifier noise, flicker noise, and intermodulation distortion. Several techniques can be deployed to minimize NF and improve receiver performance in RF monitoring, such as using low-noise amplifiers, filtering out unwanted signals, using proper shielding and grounding, and optimizing the design and placement of the receiver components. 

The importance of wideband

Another challenge for the security services is not knowing what they are looking for. They need a signal-agnostic system that can search for unknown signals in a wide frequency range so new sources can be identified, demodulated, and decoded in real time to support insightful decision making.  

A 9 kHz-40 GHz RF receiver, for example, enables real-time monitoring of multiple frequency ranges, including handheld trunked radios and Radars. Similarly, a 100 MHz bandwidth capacity allows for a more comprehensive and detailed analysis of frequency hopping signals.

The consequence of substandard receiver sensitivity

During a covert operation, the RF environment can change very quickly, and adversaries will leverage RFI and noise floor to disrupt or interrupt legitimate communications channels. Special operational forces hunting down these adverse tactics require a spectrum monitoring system with configurable receiver sensitivity and dynamic range capabilities so they can detect, track, geolocate, and combine frequency ranges emitted just above the noise floor.

A congested RF spectrum makes accurate RF monitoring difficult

With dependency on wireless RF at an all-time high, it has become readily apparent that the open RF spectrum is an invaluable asset financially, politically, and economically. As such, it is tightly regulated by government agencies in many countries to ensure it is being used in a way that does not cause interference or disrupt other users.

The RF spectrum is also a finite resource that is becoming increasingly congested, with armed forces, law enforcement organizations, government bodies, and commercial organizations increasingly required to share frequency ranges. The congestion challenge has been amplified with the advent of 5G, IoT, and automation as they are further fuelling the RFI dilemma. Moreover, many RFI sources, particularly intentional RFI, emit close to the noise floor and are incredibly difficult to hunt down without a super-sensitive RF receiver.

Conclusion

Powerful spectrum analyzers with highly sensitive receiver capabilities are integral to detecting RF transmissions, monitoring illicit activities, and obtaining greater situational awareness for effective decision-making. Trying to locate the source of RF signals, illicit or otherwise, is an arduous task without the ability to measure RF signal strength and properties dynamically.

You need to quickly determine whether the source is an illegal transmitter, a frequency out of spec, an industrial controller interfering with an adjacent band, or just a poor connection. The RF receivers must, therefore, offer several key features, including a high tuning range, customizable data collection, communication interception as well as accurate signal decoding, demodulating, and decrypting capabilities. The financial costs of not having accurate monitoring capabilities are enormous; the human costs could be even worse.