A typical radar works by emitting – often quite powerful – electromagnetic (EM) signals in a certain frequency band, and then ‘listening out’ for their return. These signals tend to consist of a series of pulses, and once these pulses reach a solid object, such as an aircraft, some of their energy is reflected back toward the radar, and it is these echoes that radars are listening out for. However, in the process of emitting these signals, a radar also makes itself vulnerable to detection by hostile electronic intelligence (ELINT).  

Unlike a typical (active) radar, passive radar operates in receive-only mode, listening out for reflected energy from various other kinds of EM emission sources. This quality offers incredible potential – the ability for a radar system to detect and track aerial targets without emitting anything itself, and therefore without giving away its position to hostile forces. Passive radar development has been receiving serious attention from industry in recent years, and this has been made possible due to the confluence of two main factors.

Firstly, the world at large today is increasingly saturated with EM signals of all kinds, from AM and FM radio, to TV signals, to cellular networks, and now satellite internet services such as Starlink. All of these serve as persistent emission sources which can generate returns for a passive radar to listen out for. While many are aware that when they board an aircraft, it is typically being tracked by networks of ground-based radars throughout its flight. However, less attention is paid to the many other layers of other EM noise the aircraft passes through on its journey. It is these latter layers that passive radars take advantage of.

Secondly, emitting on the battlefield has become increasingly risky, as illustrated by the War in Ukraine. For both sides, electronic intelligence (ELINT) and communications intelligence (COMINT) capabilities have expanded, making many minds of common emission sources – from radios, to radars, to mobile phones, at risk of detection and destruction by the other side. The lesson of observing emissions control (EMCON) has often been learned the hard way by both sides. At present, the same lesson is gradually working its way through NATO allied militaries, albeit at varied speeds.  

Finnish company Patria launched the MUSCL (Multi-Static Coherent Locator) in 2018 as their entry into the passive radar domain, and have continued to expand its capabilities since. The system is available in several configurations, including a standard 20 ft ISO container with internal operator positions, and 25 m mast, as well as a towed variant without a container, and a 24 m or 21 m transportable field mast version. All feature two passive receiver antenna arrays tuned to different bands. Patria has stated the system is already in service with an undisclosed user.

At the AOC Europe 2026 conference, Warsight was taken into a MUSCL operator post, part of the containerised configuration. Here, Marko Tikkinen, Product Director at Patria explained that the post we were speaking in was being fed real-time air track data from a small network of four MUSCLs based at different locations around Finland. One workstation displayed a map overlaid with the air situation picture of this MUSCL network, where Warsight was able to observe various aerial tracks gradually crawling their way across the map. One track stood out in particular, as it was above central Estonia, over 150 km away from the nearest MUSCL, and around 275 km from the furtherst. Radar cross section (RCS), speed, and heading indicators were all available for each of the contacts, despite the fact that none of the radars in this network were emitting. Everything we could see was simply derived from listening in to reflected returns from ambient FM radio or digital TV (DVB-T/T2) signals.

While their lack of emissions allows them to remain fairly hidden from hostile receivers, a common concern with passive radars is the reliability and resilience of coverage. Since the radar operates as a receive-only system, it is therefore entirely reliant upon preexisting sources of emissions to generate tracks. This can lead to a kind of anxiety about whether passive radars would work outside of major populated areas.

Addressing these concerns, Tikkinen said areas with no coverage tend to be somewhat sparser than many people consider: “take a look at Europe, or Asia, or America, there is quite good coverage almost everywhere. Of course, there is like Alaska…or Siberia in Russia where there are no such good things, but in areas where a lot of people are around, then there are always digital television, FM radio.”

Furthermore, with passive radars, the emitters do not necessarily have to be friendly ones. To prove his point, Tikkinen turned back to the live air picture. Working the controls, he changed a setting and various new markers lit up on the map, revealing the positions of various radio and digital TV emitters close enough for the passive radar network to use. He drew attention to several new markers that cropped up in Sweden and Russia. “So totally we are not dependent on the in-country transmitters, because we can also…let’s say borrow from the neighbour,” he said with a slight smile.

Tikkinen added that MUSCL had already been successfully tested in Lapland – which with around 1.9 people per km², is among the least-populated parts of Europe. The idea seemed that if it could work there, it could work nearly anywhere.

According to the manufacturer, MUSCL is also capable of detecting low-observable (stealth) aircraft. Tikkinen explained, “there are two reasons why our system can detect also those. The first one is there’s the lower operating frequencies we are using, so the stealth coating does not have any effect on those frequencies. And then also the multi-static geometry, which means that we are illuminating the target from different directions, and there are also, in let’s say, some high-amplitude side view radar echoes from the target.” He concluded, “stealth target is a normal target for us.”

To unpack this somewhat, stealth aircraft tend to rely upon a couple of primary means to decrease the range at which radars detect them.

• The use of radar-absorbing materials, to absorb some of the radar energy and result in a weaker reflection returning. However, these need to be optimised against a specific range of frequencies the aircraft is attempting to hide from.
• Structural features such as S-shaped engine ducts, sawtooth edges, and angular frontal profile. All of these are designed to minimise the return in the direction of the emitter, which in a typical monostatic system is co-located with the receiver.

The radar-absorbing material benefits are largely nullified by the fact that no stealth aircraft are known to be optimised against very low band emissions, such as the 87-108 MHz or 470-700 MHz bands MUSCL operates in.

Structural features such as angling radar returns away from the receiver are largely nullified by two factors. For starters, the emitter and receiver are not co-located, so it is functionally unrealistic to expect to angle a stealth aircraft successfully against a receiver that isn’t emitting anything. Secondly, as their full name suggests, MUSCLs are designed to work together as part of a multi-static radar system. This means even in cases where returns toward one particular MUSCL might be decreased, its neighbours in the network will not face the same problem.

Low-frequency signals have often been considered insufficient to achieve a weapons-quality track, since the margin of error in target position increases with wavelength. However, operating a multi-static configuration can offset the inaccuracy of any one individual sensor. As such, Warsight asked if it is possible to attain a weapons-quality track from MUSCL track data, sufficient to launch a seeker-equipped missile to the approximate position of the target, whereupon its seeker would be able to pick up the target independently and conduct a successful engagement.

Tikkinen stated that “Yes, yes, it is. And if you are only using DVB-T, this position uncertainty is even less, so it can be even some tens of metres.”

While passive radars do not emit to fulfil their primary function of detection, the same does not necessarily apply to networking. When tying multiple radars together, data invariably need to be shared between them. At short distances wires or fibre optic cables could be used to carry said data, but the wires datalinks are entirely impractical when multiple receivers in an array are spaced hundreds of kilometres apart. As such, Warsight then asked how it was possible to network these radars while staying relatively quiet in the EM spectrum.

Here, Tikkinen explained that it was possible to network several MUSCLs via an encrypted 4G cellular connection, in what can be likened to security through obscurity. The 4G transmitter can be mounted away from the receiver if needed, however Tikkinen stressed that even if mounting the networking connection on the MUSCL mast, if a hostile ELINT operator were to observe the signal, it would simply appear as if “somebody’s watching or streaming some video,” from a mobile phone.

Tikkinen added that “from the MUSCL system point-of-view, any communications network from wireless mobile, satellite or backbone network providing IP connectivity can be used.”

Finally, if additional capability is required, MUSCL is also optionally offered with Patria’s CATCHR electronic support measures (ESM) system, to provide signal intelligence gathering capability, in support of various missions such as emitter detection, threat classification, threat analysis, and mission planning.

The faint WISPR of artillery

Continuing on a theme, at Eurosatory 2026, Patria displayed a new addition to their passive radar lineup in the form of WISPR (Weapon Indication and Sensing Passive Radar), a new passive radar aimed specifically at detecting and tracking very small targets such as artillery rockets, artillery shells, and mortar bombs. As Tikkinen noted, it is the first passive radar to be developed for this role.

Unlike MUSCL, which operates in both FM radio and DVB-T/T2 bands, WISPR’s two antenna arrays only operate in the latter of these two bands. But going all-in on DVB-T/T2 has some advantages. These signals are higher-frequency, making it easier to detect small targets with them, and WISPR has much larger and more sensitive arrays optimised for the sector surveillance role, with new, specialised signal processing capabilities, allowing for the detection of smaller targets than possible with MUSCL. WISPR’s total field of regard when including both antenna arrays sits at over 100°, which is seen as suited for its use case.  

Having said that, the two systems are in many ways complementary, as their roles are quite different. MUSCL is intended to search for larger aircraft, or drones with wingspans of around 1.5 m or more, while WISPR is much more optimised for the counter-battery role, it can also be used to detect smaller aerial threats, such as NATO Class 1 Mini drones.

In terms of WISPR’s readiness for production, Tikkinen noted that the company had already secured a European launch customer, with deliveries slated for 2027. Patria’s images showed two possible configurations of WISPR on offer – one comprised a 20 ft ISO container with internal space for operators, and a collapsible 25 m high mast; and another simpler, towed variant which simply consisted of a towed mast mounting the antenna array. The latter variant was understood to require remote operation from a separate operator post.

In a noisy spectrum, it pays to listen

The level of capability able to be derived from modern passive radars is quite impressive, while at the same time, the EM spectrum is getting increasingly contested, and emitting can often be risky.

There are some downsides which look set to remain – for instance, identification friend-or-foe (IFF) necessarily requires emitting, and is therefore incompatible with the idea of a silent radar. As such, passive radars would still benefit from operating within an ecosystem where active emitters are able to fill this role. Likewise, some kinds of deployments to very austere or uninhabited areas may not have enough ambient emission sources, and so may not always be compatible with some forms of passive radar.

Having said that, the technology is progressing, and current areas under examination include using Starlink satellites to develop passive radar sensing capability, thereby solving the austere location problem. It has been eye-opening seeing what is possible with current passive radar technology, and how much progress has been made in the last few years, with the number of use cases expanding. All told, the case against passive radar is getting increasingly difficult to make.