A naval frigate is tasked with tracking a small vessel suspected of trafficking arms. But despite its surface-search radar system, the frigate cannot compete with the curvature of the Earth, which limits radar’s line-of-site and makes locating the criminal ship impossible from sea level. Acutely aware of this limitation, the small vessel stays undetected beyond the radar horizon—every minute that passes increases the chances that the vessel will slip into safe waters.
Recognizing this traditional limitation, the navy deploys a high-altitude ISR drone, which ascends rapidly, rising above the limitations of the Earth’s shape. RF sensors begin to detect and geolocate signals from maritime VHF radio transmissions that were previously beyond the radar horizon. The frigate can now change course and intercept the suspicious vessel.
To see further, you need to go higher.
Traditionally warships have deployed helicopters to search; however, this is an expensive approach. The following sections describe the benefits of integrating real-time RF sensors into different types of lower-cost unmanned airborne platforms and detail several practical applications.
Tethered drones
For certain (land-based) applications, tethered drones offer significant advantages over other types of ISR drones. The first advantage is their endurance, as tethered drones are powered directly through a cable connected to a ground station. This setup allows them to maintain flight for as long as necessary (50+ hours), providing uninterrupted surveillance.
The tether also provides a continuous, stable, and reliable power supply to its RF sensor payload. The physical connection of the tether allows for fast, reliable data transfer, which is essential for ISR operations that require immediate analysis and response. Additionally, this wired communication method is less susceptible to jamming or interception as the backhaul does not have to be sent over a wireless data link, providing a secure link that is vital for sensitive missions. The tether also improves the mission’s security by decreasing RF emissions from the drone.
In terms of performance, tethered drones offer improved stability, particularly in challenging weather conditions. The tether acts as a stabilizing force, reducing the impact of wind and turbulence, which can affect the accuracy of data collection. This increased stability ensures that the drone can continue to gather reliable intelligence even in less-than-ideal conditions.
Finally, tethered drones are cost-effective solutions for ISR operations. Their ability to operate continuously without the need for frequent maintenance makes them a suitable choice for missions that require sustained surveillance.
Practical application: Electronic support measures (ESM)
Tethered drones can be easily transported and launched from the back of 4x4 trucks, offering a highly flexible solution for conducting ESM in remote battlefield or border locations. Their quick deployment and retraction capabilities allow them to be operational within approximately ten minutes of arrival. Once airborne, these drones provide operators with an immediate altitude advantage, enabling effective monitoring of enemy communications and electronic signals. The intelligence gathered can be critical for either jamming enemy communications or protecting friendly channels from interference.
Practical application: protecting strategic positions
In a conflict scenario, there will be pressure to establish strategic positions such as forward operating bases (FOBs). Protecting these areas with robust spectrum monitoring technology is paramount; however, creating the necessary infrastructure, such as masts and wired connections for backhaul, is unlikely to be achievable at first.
Easily deployable and maneuverable tethered drones with RF sensors as payloads could allow tactical spectrum managers and CAEMA operators to ensure the electromagnetic spectrum around the FOB is clear, with any anomalies being detected and potential threats being located in real-time to keep personnel safe.
Traditional rotary drones
While comparatively restricted in terms of endurance and range, traditional rotary drones offer flexibility and versatility—the ability to carry payloads up to approximately 25kg (heavier payloads reduce endurance), a maximum flight range of 40km, and a flight duration of up to 150 minutes.
With vertical takeoff and landing capabilities, rotary drones can operate in spaces that may be restricted for other air platforms. They can also be transported easily and deployed quickly, making them efficient for multiple missions. The most valuable feature is arguably their excellent maneuverability and ability to quickly change altitude, direction, and speed, allowing them to adapt to dynamic environments.
Traditional rotary drone’s size, quiet operation, and ability to fly at low altitudes mean they have stealth capabilities when conducting sensitive missions with passive RF sensors in missions requiring they remain undetected. They can transmit data via secure wireless backhaul to a ground station or command centers, enabling real-time decision-making during ISR missions.
Practical application: Gathering tactical battlefield intelligence
Rotary drones equipped with RF sensors can quickly and easily help improve situational awareness on the battlefield.
Not requiring large runways and operating from forward operating and refueling areas (FARPs), they can be easily manoeuvred to a tactically advantageous position on the frontline to soak up enemy communications for real-time decision making as well as post-processing and decoding. When they use detector-based recognition of known signals of interest, for example, the sensors integrated into these drones run geolocation workflows that can precisely reveal the positions of enemy forces.
Rotary drones can detect acts of enemy electronic warfare, such as jamming or spoofing, protecting friendly communication systems and allowing allied forces to deploy effective countermeasures. Constantly deploying a network of these ISR drones can allow operators to establish baselines and patterns of life—any sudden changes in the battlefield environment could have repercussions for tactical decision-making.
Practical application: Monitoring illegal border activities
Rotary drones with integrated RF sensors are an effective tool for detecting and geolocating signals from smugglers or human traffickers along inaccessible border areas. As these criminal activities require communication to be successful, deploying ISR drones that can intercept signals from communication devices such as push-to-talk radios and cellular phones allows border security forces to help identify and geolocate the criminals behind these operations.
The real-time intelligence gathered by rotary drones can be shared directly with ground patrols to coordinate law enforcement efforts. Moreover, data collected by rotary drones can be used as evidence to build a case and prosecute the individuals involved in trafficking and smuggling.
Fixed wing drones
MAME (medium altitude medium endurance) MALE (medium altitude long endurance) ISR drones can take off from a rough airstrip, cruise at approximately 100 km/h, have an endurance of 20 hours, and carry a payload of up to 50 kg. If an RF sensor is integrated as a payload, operators have a tool to flexibility carry out long-range spectrum monitoring tasks. They can also run advanced geolocations of transmitters of interest by networking with ground-based sensors. Flying up to 3000 meters above sea level the drones have an enhanced coverage area as they are not affected by the limitations of RF signal propagation; thus, they have a wider detection cone. This makes them useful in a variety of applications.
Practical application: maritime domain awareness
The maritime domain is fraught with problems. In national waters, authorities face issues such as protecting the areas where undersea cables converge and detecting dark ship transfers of illegal goods. Policing a nation’s exclusive economic zone involves actively searching for illegal fishing, migrants, and human trafficking. In international waters, challenges relate to addressing piracy, pollution, and vessels in distress.
Coastguards and navies can only solve these challenges by gathering multiple sources of intelligence. No one method is a panacea; each one has strengths and weaknesses. The key is to layering as many methods as possible to ensure that, for example, if it is foggy and the camera does not work, the authorities can switch to a different sensor.
Fixed-wing drones with space for multiple sensors can provide this capacity. ISR drones with wideband RF sensors as a payload can detect and intercept signals of interest (from maritime radio navigation in the VHF band to IoT devices in the SHF). Once the signals are identified, operators can then geolocate them—providing intelligence that could help detect criminal behavior.
Practical application: ISTAR in remote areas
Carrying out ISTAR (Intelligence, Surveillance, Target Acquisition, and Reconnaissance) operations in remote areas is a challenge. If at all possible, physically moving troops into an area to conduct operations is likely to be resource-intensive and physically difficult due to logistical constraints.
In this scenario, militaries could employ manned aircraft, such as the Boeing P-8 Poseidon: a highly effective, but expensive method of conducting ISTAR. However, using ISR military drones is an efficient and cost-effective way of gathering intelligence in remote areas. A fleet of drones conducting wide-area surveillance can connected to a maritime domain awareness C2 system to ensure rapid response. By monitoring the spectrum for specific signals of interest and then running detector-based geolocations of these signals, commanders can gain a comprehensive understanding of the operational environment and can make informed decisions regarding which resources to use.
Aerostats
While ISR drones are often used due to their flexibility and discretion, aerostats (or high-altitude balloons) are chosen for the opposite reasons. For certain applications, there is an advantage of putting a huge, highly visible balloon in the sky equipped with a multitude of surveillance equipment. However, despite their size, they can operate with minimal RF emissions, which is advantageous for carrying out covert operations while reducing the chances of being detected.
Aerostats can stay airborne for extended periods, making them well-suited to long-term ISR missions. Their ability to remain in flexed positions allows them to focus on a specific geographic area. Operating at 4,600 m (15,000 feet) gives the integrated RF sensors a vastly extended line-of-site compared to smaller airborne platforms.
Practical application: Border security monitoring
In remote border locations, aerostats can maintain a fixed position for extended periods of time. They are an optimal platform for RF sensors to collect signals intelligence over vast areas. Their conspicuousness means they can act as a visual deterrent, while positioning a network of aerostats across a border allows them to function as a virtual sensor wall, passively collecting data on any transmissions. This is more practical than creating a fixed network, more convenient than deploying border forces, and cheaper than deploying a manned surveillance aircraft.
A versatile RF sensor built for ISR drone integration
CRFS has engineered the lightweight Node 100-18-LW (LW Node) to meet the growing demand for a versatile and lightweight RF sensor solution that can seamlessly integrate into unmanned aerial vehicles (UAVs). The design of the LW Node prioritizes high RF performance while adhering to the stringent weight constraints essential for UAV operations. Weighing in at under 2kg, this sensor is well within the Maximum Take-off Weight (MTOW) limitations typically imposed on UAVs. Additionally, it is built to withstand harsh environmental conditions, with its connectors rated at IP55 or higher for robust protection. The LW Node also features an enhanced GNSS chipset, capable of supporting the L1, L2, and L5 frequency bands, ensuring precise geolocation capabilities.
The integration of the LW Node into UAVs enables a significant extension of RF spectrum monitoring and Time Difference of Arrival (TDoA) mission ranges. By deploying RF sensors at elevated altitudes, UAVs can effectively increase the detection range of monitoring networks. This aerial advantage also enhances TDoA geolocation accuracy, as the increased altitude of the sensors provides a broader and more precise geolocation coverage area.
When operating in conjunction with ground-based sensors, UAVs equipped with the LW Node form a comprehensive, integrated network. This network is managed by a Command and Control (C2) system that receives data from both airborne and ground sensors. The result is a clear line of sight across vast areas, maximizing the effectiveness of RF detection and geolocation capabilities. This combination of high-altitude UAVs and ground-based systems delivers a powerful, adaptable ISR solution.
