![1 Kopie The RC-135V/W Rivet Joint is perhaps the most recognisable Western SIGINT aircraft. The aircraft’s comprehensive array of COMINT DF antenna is clearly visible beneath the fuselage, while the distinctive ‘chipmunk cheeks’ at the forward fuselage contain its ‘Automatic Electronic Emitter Locator System’ (AEELS). [Luca Chadwick]](https://euro-sd.com/wp-content/uploads/2025/06/1-Kopie.jpg)
Skybound scanners: Rebuilding Europe’s airborne SIGINT fleet
Luca Chadwick
With just 11 SIGINT aircraft currently operational among European NATO members, and growing demand for the capabilities they provide, Europe faces a critical shortage in one of its most vital intelligence disciplines. New German, French, and Italian programmes aim to partially fill this gap over the next few years, but a more comprehensive solution would be welcome over the longer term.
Today, by exploiting the most militarily-relevant Radio Frequency (RF) and Microwave portions of the electromagnetic spectrum (spanning roughly 3 MHz to 300 GHz), SIGINT supports military operations through two primary disciplines:
- Communications Intelligence (COMINT): Intercepts voice and text communications signals to reveal adversary intentions and movements.
- Electronic intelligence (ELINT): Captures non-communications signals, primarily from adversary radar systems. ELINT further subdivides into:
- Technical ELINT (TechELINT) – Analyses emitter parameters, such as operating frequencies and pulse repetition rates, to create threat libraries and mission data, helping develop ECM and identify, jam and deceive adversary systems.
- Operational ELINT (OpELINT) – Tracks emitters positions, behaviours, and activation cycles across the battlefield.
Together, these sources contribute to an Electronic Order of Battle (EORBAT), documenting adversary emitter types, locations, capabilities, operating frequencies, and activation cycles within a given area of responsibility. This links specific units to certain emissions, facilitating the monitoring of their movements and capabilities.
In conflict, SIGINT might enable safer ingress and egress routes for strike and close-air-support aircraft or identify targets for Suppression of Enemy Air Defences (SEAD) operations. For example, SIGINT might detect unfamiliar radar emissions near a contested border, pinpointing a new surface-to-air missile battery. By monitoring the battery’s activation cycles, the EORBAT identifies vulnerability windows for low-risk SEAD strikes.
In peacetime, SIGINT provides threat warning and situational awareness by revealing deviations in electromagnetic activity, potentially revealing hostile intent. Furthermore, monitoring adversary training exercises provides valuable insights into tactics, capabilities, and techniques, allowing decisionmakers to plan accordingly.
Today, the increasing sophistication of adversary threat systems, and changing operational behaviour, all demand equally advanced SIGINT capabilities. For instance, in 2023, US F-35s gathering OpELINT on NATO’s Eastern Flank failed to detect the Russian S-300PMU series systems, which were using unfamiliar ‘war-reserve’ frequencies, emphasising the spectrum’s evolving, deceptive nature.
To address these challenges, there is a growing demand for advanced ‘deep sensing’ capabilities: high-altitude SIGINT platforms designed to “see and sense more, farther, and more persistently,” as described by US Army Secretary Christine Wormuth. In essence, SIGINT platforms must monitor wider areas from further away, beyond the reach of adversary air defences.
SIGINT from the skies
Airborne SIGINT platforms offer key advantages over land- and maritime-based systems, particularly in complex terrains and signal-dense environments, where physical obstructions hinder signal interception. Signals above 30 MHz primarily follow Line-of-Sight (LoS) propagation, meaning they travel in straight paths that can be blocked or weakened by terrain, buildings or the curvature of the Earth. As a result, ground-based SIGINT is limited by these barriers, as ground-level obstructions absorb or reflect signals, reducing detection range and accuracy.
In contrast, aircraft operating at altitude can ‘look down’ into obstructed areas, significantly increasing their field of view and intercept probability. This advantage is reinforced by the radio horizon rule, wherein the higher a RF receiver antenna is, the greater the distance at which it can detect signals before the Earth’s curvature obstructs its LoS. Altitude is hence critical for pursuing RF signals, providing an optimal vantage point to capture an array of signals at range.
ELINT focusses predominantly on radar emissions, which span a range of bands, but typically the higher-frequency bands are where most fire-control radars used in ground-based air defences operate. For example, common systems such as Russia’s S-300PM series and later S-400 surface-to-air missile systems (NATO reporting names: SA-10A ‘Grumble’ and SA-21 ‘Growler’ respectively) rely on 30N6 series and 92N6 series fire control radars that operate in the X-Band (8-12 GHz).
However, detection of hostile radar signatures needs to be paired with an understanding of where these emitters are located geographically. Emitter geolocation, or ‘Direction Finding’ (DF), is then central to ELINT. Two of the techniques commonly used include:
- Angle of Arrival (AoA) DF: This uses at least two RF receivers arranged at a known distance from one another. These measure the angle at which the signal arrives, and use trigonometric principles to plot the signal paths, and so determine the approximate location of the emitter at the intersection of the plotted paths.
- Time Difference of Arrival (TDoA) DF: This relies on multiple, fixed-position receivers to measure the signal’s arrival time at each receiver. Since the distance between each receiver is known, and electromagnetic waves travel at a constant speed (the speed of light), the time differences can be used to calculate the difference in distance from each receiver to the emitter. The average distance defines a ‘hyperbolic curve,’ along which the emitter must be located.
Both methods require multiple, fixed RF receivers in known positions, which can be mounted along an aircraft’s fuselage or wings of an aircraft for dynamic, mobile coverage. In contrast, ground-based DF systems rely on at least two fixed antenna stations, making them slow to deploy and manoeuvre, thus vulnerable to targeting, while they are restricted by LoS obstructions.
These challenges underscore the requirement for ‘deep sensing’ SIGINT aircraft that can:
- Operate at higher altitudes, extending their coverage range.
- Offer long endurance and high cruising speeds to reach areas of interest quickly and remain on-station for extended periods.
- Carry and power sensor payloads that can quickly process vast quantities of data for persistent, wide-area surveillance.
An air of dependence
Despite these advantages, the early 2010s marked a substantial regression in European airborne SIGINT capabilities, as shown in Table 1. While these retirements were justified by increasing maintenance burdens and increasingly complex supply chains, most operations did not acquire immediate successors, leaving European NATO to face decade-long capability shortfalls, underpinning a dependency on US assets.
Table 1: NATO SIGINT aircraft retirements (2010 onwards) | ||||
Country | Aircraft | Fleet size | Retirement year | Notes |
France | C-160G Gabriel | 2 | 2022 | Withdrawn three years ahead of schedule.
Acquired a single leased SAAB 340 until replacement aircraft arrive from 2026. |
Germany | BR1150M Atlantique | 5 | 2010 | |
Italy | G.222VS (Versione Speciale) | 1 | 2011 (est.) | Operated a single leased Gulfstream III from Lockheed Martin between 2012-2013.
Currently operating a single leased King Air 350 in L3Harris’ SPYDR configuration with SIGINT capability. |
Spain | Boeing 707 ELINT | 1 | 2014 | No replacement. |
UK | Nimrod R.1 | 3 | 2011 | Replaced by 3x RC-135W Rivet Joints from 2013. |
Most of these retirements followed NATO’s 2011 intervention in Libya, which emphasised the value of SIGINT, particularly in supporting SEAD operations. SEAD’s success relies on effective ELINT data, which made platforms including the C-160G, G.222VS and Nimrod R.1 instrumental in locating Libyan ground-based air defence radars. This enabled the coalition’s intended tempo of approximately 10 SEAD sorties a day to enforce the no-fly zone. On average, SEAD assets have comprised around 5% of the air component during major air campaigns since 1991.
However, as noted in a 2011 Defence Today article by Dr Carlo Kopp, Libyan air defences were “relics,” and “in a state of disrepair.” Currently, Libya remains NATO’s most recent direct confrontation with a state adversary; this is an important distinction, as peer-conflicts will undoubtedly present greater challenges. Elsewhere, Thomasz Smura, writing in the Pulaski Policy Papers and Lt Col Andreas Schmidt in the Journal of the JAPCC have both highlighted the increasing sophistication of threat systems.
Near-peer/peer opponents such as Russia and China both leverage multi-layered, mobile networks of IADS. These networks incorporate a selection of sensors and weapon systems to create overlapping coverage at multiple range bands, seeking to complicate opponents’ ability to detect, target and engage them, with the intention of eliminating many threats before they reach the front-line. As such, the 5% contribution from SEAD assets will surely only increase in higher-threat environments, making high-fidelity SIGINT even more critical in potential future conflicts.
While US support in SIGINT collection efforts is extremely valuable, this has largely been driven by necessity, given European NATO members’ more modest capabilities. As such, amid rising peer threats and evolving transatlantic dynamics around European security, Europe’s shortfall in sovereign SIGINT capabilities has eroded its overall credibility, threatening to become a strategic vulnerability, particularly given the need for scalability in support of high-intensity operations, such as SEAD, to counter sophisticated IADS.
European developments
Encouragingly, as Luca Peruzzi outlined (ESD 3-2023), NATO’s European members are steadily expanding airborne SIGINT capabilities, signalling progress toward building a robust architecture that reduces their dependence on the US. Table 2 outlines the current, and soon-to-be operational SIGINT aircraft across European NATO fleets, with Germany, France, and Italy procuring modern ‘deep sensing’ platforms, reflecting a step-change compared to legacy systems.
Table 2: European NATO SIGINT aircraft as of May 2025 | |||||
Country | Aircraft | Operator | Fleet size | Status | Notes |
Finland | C295M | TukiLLv / 3 Flight, Tampere-Pirkkala Air Base, Finland | 1 | In service. | Single aircraft configured using Lockheed Martin’s ‘Dragon Shield’ roll-on-roll-off ELINT suite. |
France | Falcon 800X Archange | Escadron électronique aéroporté 1/54 ‘Dunkerque,’ Évreux-Fauville Air Base, France | 3 | On order. | Deliveries expected to commence from 2026. |
France | SAAB 340 | Escadron électronique aéroporté 1/54 ‘Dunkerque,’ Évreux-Fauville Air Base, France | 1 | In service. | Temporary lease until Archange is delivered. |
Germany | Global 6500 ‘Pegasus’ | Taktisches Luftwaffengeschwarder 51 ‘Immelman’, Shelswig Air Base, Germany | 3 | On order. | First aircraft undergoing testing, expected delivery 2026. Deliveries projected to be completed by 2028. |
Italy | Beechcraft King Air 350ER (SPYDR) | 71° Gruppo, Pratica Di Mare Air Base, Italy | 1 | In service. | Temporary lease until AISREW is delivered. Uses L3Harris SPYDR configuration. |
Italy | Gulfstream 550 ‘AISREW’ | 71° Gruppo, Pratica Di Mare Air Base, Italy | 2 | On order. | Both conversions are expected to be completed by L3Harris by June 2026. |
Sweden | S102B ‘Korpen’ (Gulfstream IV) | 74th Special Aviation Squadron, Malmen Airport, Sweden | 2 | In service. | Replacement discussions projected to commence after 2025. |
Türkiye | CASA CN235EW | Genelkurmay Elektronik Sistemler, Etimesgut Air Base, Türkiye | 3 | In service. | Unclear operational status. |
UK | RC-135W Rivet Joint | No. 51 Squadron, RAF Waddington, UK | 3 | In service. | US-UK sustainment currently agreed until 2035; US fleet projected to remain in service until ~ 2050 |
Table 3 summarises comparisons of these platforms’ core characteristics, including estimated detection ranges, and minimal detectable signal strengths, based on data provided by Dr Thomas Withington, Associate Fellow at the Royal United Services Institute (RUSI). These estimates are derived using the principle of ‘free space path loss,’ which Withington explained is “how much strength a signal loses as it travels through space”. The principle being that the farther a signal travels from an emitter source, the weaker it becomes, therefore limiting a receiver’s detection range.
While legacy and modern platforms share similar signal sensitivities, modern systems benefit from higher altitude ceilings, expanding their geographic coverage and enhancing their ability to detect farther away emissions at standoff ranges.
Table 3: Comparing estimated capabilities of legacy to newer European SIGINT aircraft | ||||||||
Country | Aircraft | Status | Cruise speed (km/h) | Flight range (km) | Service ceiling (m) | LoS range to sea level emitter (km) | Minimum detectable signal for handheld radio1 (dB) | Minimum detectable signal for generic ATC radar2 (dB) |
France | C-160G Gabriel | Legacy | 482 | 4,630 | 8,230 | 373 | -104 | -71 |
France | Falcon-8X Archange | Incoming | 945 | 8,334 | 15,545 | 513 | -106 | -74 |
Germany | BR1150M Atlantique | Legacy | 509 | 4,074 | 9,144 | 393 | -104 | -71 |
Germany | G6500 PEGASUS | Incoming | 1,087 | 11,112 | 15,545 | 513 | -106 | -73 |
Italy | G.222VS (Versione Speciale) | Legacy | 444 | 4,630 | 7,620 | 359 | -103 | -71 |
Italy | G550 AISREW | Incoming | 833 | 8,056 | 13,716 | 482 | -106 | -73 |
Sweden | S102B Korpen (Gulfstream IV) | Current | 850 | 7,815 | 13,716 | 482 | -106 | -73 |
UK | RC-135W Rivet Joint | Current | 815 | 6,276 (without aerial refuelling) | 15,240 | 508 | -106 | -74 |
Notes:
1) Assuming radio at 341 MHz, 31 dBm ERP at full LoS range. 2) Assuming air traffic control radar at 2.5 GHz, 81 dBm ERP at full LoS range. |
Evidently leading this transition are ultra-long-range business jets, which include the Bombardier Global 6500 (G6500) and Gulfstream 550 (G550), offering far better altitude, range, electrical power, and reliability compared to predecessor platforms. The aforementioned platforms outperform older types across most core characteristics, offering mission durations of up to 18 and 13 hours respectively. Furthermore, payload capacity is a critical factor for a SIGINT aircraft to accommodate fuel, mission systems and crew. In these cases, the G6500 supports approximately 2,617 kg, and the G550 up to 2,812 kg. Each aircraft also accommodates up to 10 operator workstations to enable real-time signal processing.
Operational availability is similarly vital. In commercial use, the Global series’ dispatch rate stands at 99.83% while the G550’s boasts 99.9%, while their maintenance intervals are equally optimal, equating to ~750 flight hours for the Global Series and ~500 for the G550. Both types also benefit from established, global networks of certified maintenance facilities.
Business jet-based platforms also feature ready-integrated power solutions to supply the energy-intensive detection and on-board processing systems. Bombardier’s Global series includes four variable frequency generators and an APU, while L3Harris’ G550-based ‘Strategic Airborne ISR System’ delivers 120 kVa of power via its APU, alongside 60 kW from engine-driven systems.
The new systems in detail
Germany is revitalising its airborne SIGINT capabilities through the ‘Persistent German Airborne Surveillance System’ (PEGASUS), based on the G6500, filling a 15-year long capability gap.
At its core is Hensoldt’s Kalætron Integral SIGINT system, which provides wideband coverage from <30-40 GHz. The lower end of this range helps capture COMINT signals, such as VHF and UHF communications traffic, as well as long-range surveillance radars using HF signals. The upper end, meanwhile, covers the X-, Ku- and Ka Bands, where synthetic aperture radar (SAR), air-to-air radar, and fire control radars typically operate.
In April 2025, PEGASUS completed cold-weather certification trials in Canada, establishing its readiness for operations Europe’s increasingly contested High North. NATO considers this region a “critical focal point in safeguarding collective defence,” with significant monitoring by American and British RC-135s, and Sweden’s S102Bs already underway, particularly over the Barents Sea and at the Russo-Finnish border.
The Arctic Institute notes increasing regional competition driven by rich gas and oil reserves, with Russia rapidly expanding its regional military footprint. In April 2025, Russia demonstrated its ‘Monolit-B’ lower X-Band coastal radar at Nagurskoye, which is compatible with the Uran and Kalibr cruise missile families, and reportedly capable of classifying and tracking up to 30 targets, up to 100 km away (or 250 km under super-refraction conditions) in active mode. This underscores the region’s growing militarisation, and the complex challenges that demand sustained SIGINT gathering across Europe.
Finally, from mid-2026, Italy will field two “Airborne Intelligence, Surveillance, Reconnaissance and Electronic Warfare” (AISREW) G550s based on L3Harris’ Joint Airborne Multi-Mission Sensor System (JAMMS) package. Open sources confirm that AISREW will use L3Harris’ Rio family of COMINT systems. The Rio Grande variant, understood to be part of AISREW’s suite, is capable of continuous scanning from 2 MHz to 8 GHz, alongside TDoA geolocation. It also includes continuous recording capabilities, enabling operators to conduct retrospective analysis, enhancing detection of fleeting, intermittent or low-power transmissions. Rio Grande also reportedly features digital adaptive beamforming with up to 30 dB of gain. This technique adjusts the phase and amplitude of incoming signals to suppress interference, enhance clarity, and track weak or moving emitters in dense signal environments.
Italy’s renewed focus on strategic SIGINT will strengthen NATO’s surveillance posture in the Mediterranean Sea, an area of increasing concern for the Alliance due to its dense network of critical undersea infrastructure (CUI), including gas and oil pipelines and underwater power and internet cables. Unlike many other regions within NATO’s area of responsibility, this region is further complicated by a wide spectrum of threats, ranging from state-sponsored sabotage, to smuggling and terrorism. Persistent, multi-domain ISR is therefore important to detect such threats.
Strategic implications
A key challenge with higher-altitude platforms is that while they can ‘see further,’ signal strength weakens over distance, demanding acute signal sensitivity to detect faint signals. To overcome this, newer platforms, including Archange and PEGASUS, are leveraging Artificial Intelligence (AI) to enhance signal detection. In a signal-dense environments, such as the Baltics, civilian or benign radars may conceal a distant radar of interest. In these case, AI-enabled systems can learn to identify subtle cues, such as unique waveforms or radar pulse patterns, to enable target recognition against complex electromagnetic clutter.
This is a vital development that helps expedite the intelligence cycle faster and more precisely. For example, in during one incident in July 2024, Russian drones operating in Ukraine’s Donbas region evaded counter-UAV systems by shifting their operating frequencies from 700 MhZ to 1 GHz, to the unorthodox 400-500MHz range. Such shifts have not been uncommon among both sides, and are likely to remain a challenge for SIGINT operators to deal with. AI-enabled systems could detect environmental changes, vulnerabilities and deviations from known signal patterns that may elude human operators.
Finally, amid growing uncertainty surrounding transatlantic support, indigenously-developed systems such as PEGASUS and Archange support the shift toward an self-sufficient SIGINT ecosystem, leveraging non-US industrial partners: Dassault and Thales (Archange), and Bombardier, Lufthansa Technik and Hensoldt (PEGASUS). By integrating European designed architectures into widely available airframes, these systems challenge US dominance in the special mission aircraft market, reducing overseas dependencies and reinforcing Europe’s industrial base.
Current systems
The UK’s RC-135W fleet remains European NATO’s most advanced airborne SIGINT asset, operating alongside Sweden’s S102Bs to monitor key centres of gravity including the Black Sea, the Russo-Finnish border and the Kaliningrad exclave. The RC-135s form part of a US-UK cooperative fleet, supported by rolling baseline upgrades with L3Harris that see each aircraft undergoing modernisation every four years. The current agreement is in place until at least 2035.
Compared to many platforms in the same role, the RC-135 offers a greater sensor payload and crew capacity, and is uniquely capable of air-to-air refuelling, enabling enhanced range and mission endurance. Its sensor suite includes both low- and high-band subsystems, supported by the Multiple Position COMINT Emitter Location System for COMINT DF, alongside the Automatic Electronic Emitter Locator System (AEELS) for radar emitter geolocation and classification. Ongoing Baseline 14 upgrades are integrating continuous recording and High Probability of Intercept capabilities; both are imperative to detecting Actively Electronically Scanned Array (AESA) emitters, which can rapidly hop frequencies, transmitting for brief durations, and in multiple directions, making them challenging to detect.
Finally, France and Italy maintain interim capabilities using a SAAB 340 and Beechcraft King Air 350, respectively. Though limited in range and ceiling, these aircraft ensure continued operator proficiency and readiness ahead of full-capability replacements.
Closing thoughts
Expanding Europe’s airborne SIGINT capacity is essential, however, as Table 1 demonstrates, the overall density of this capability remains low and concentrated between a select number of operators. As of May 2025, only 11 aircraft are confirmed to be operational, increasing to just 17 by 2028 (due to removal of currently-leased aircraft). Availability will be further constrained by maintenance cycles and attrition as for example, the UK’s RC-135W fleet often comprises just two serviceable aircraft, due to its rigorous sustainment program.
This low-density, high demand context presents two challenges:
- Intelligence gaps: Limited European SIGINT capacity threatens vulnerabilities in electromagnetic situational awareness, especially amid uncertainty in transatlantic political alignment. The brief suspension of US intelligence support to Ukraine in February 2025 illustrates this fragility. Additionally, given NATO’s current dependence on American SIGINT provisions, any future disruptions risk degrading allied intelligence output.
- Post-conflict monitoring: Any potential resolution to the war in Ukraine will increase demand for persistent electromagnetic monitoring to detect non-compliance with agreements or renewed hostilities.
To address these challenges, European NATO must continue upscaling its airborne SIGINT capacity. As Justin Bronk noted in a 2025 RUSI paper, some member states are “simply too small” to significantly bolster non-US EW capacity. However, RORO systems such as Lockheed Martin’s Dragon Sheild ELINT suite offer a practical, cost-effective solution to fill gaps, as demonstrated by Finland. Compatible with common airlift platforms, including the C-130 Hercules and C-295M, Dragon Sheild uses modular, palletised compartments, which can be installed and removed as required.
13 NATO members currently operate the C-130, including five with the latest C-130J variant, while a further five operate the C-295M. This strong supply of compatible platforms means equipping just one aircraft per compatible fleet could significantly boost capacity with minimal impact on airlift capabilities. This would reduce pressure on high-end platforms and enable high-resolution, localised SIGINT collection, deepening NATO’s operational resilience.
Leveraging a European-developed platform as the backbone of such a force would signal Europe’s commitment to strategic autonomy, while strengthening resilience amid an evolving electromagnetic threat environment.
Author: Luca Chadwick holds a Master’s Degree in Terrorism and Insurgency from the University of Leeds. Throughout his studies, Luca specialised in security studies, focussing on air power and coercion while also attending Yorkshire Universities Air Squadron.