7 March 2024

Good things come in small packages

By Mathew Willmot

7 min read

If your perception of advanced passive sensors involves inflexible fixed installations, think again. A new generation of highly portable Saab sensors is enhancing situational awareness on the land, water and in the sky.

Picture this. Sometime in the not-too-distant future, an enemy jet aircraft has just quietly slipped over an isolated part of your national border. Its task: destroying a critical piece of infrastructure. As the jet speeds deeper into your airspace, the pilot uses radar and weapons systems to search for threats such as other aircraft and Ground Based Air Defence (GBAD) batteries. But what they can’t see are your passive sensors on the ground. Many kilometres away on a ridge, a small network of man-portable passive sensors has detected the jet’s approach and through triangulation determined its position, bearing and speed. By analysing its distinctive electronic signature, the sensor system quickly identifies it as a threat and alerts a nearby GBAD unit. The unit switches on its radar and quickly fires a series of missiles to neutralise the aircraft.

A breakthrough in passive sensing

Passive sensors have long been a vital part of most nations’ defence, surveillance and intelligence gathering capabilities. Unlike radar systems which emit electromagnetic radiation to locate and track targets, passive systems work instead by detecting and analysing those same type of electromagnetic emissions emitted by other sources, for instance the radar on the aforementioned jet fighter. With no emissions of their own, these sensors allow for continuous monitoring and detection of foes with an extremely small likelihood of being detected themselves. Traditionally, passive sensors for military use have often been relatively large and in fixed emplacements. Their size has dictated that they be either mounted on ground-based masts or attached to suitably sizeable vehicles, warships, or Airborne Early Warning & Control (AEW&C) platforms.

Passive sensors have long been a vital part of most nations’ defence, surveillance and intelligence gathering capabilities.

But a key breakthrough came in 2022 when Saab released the Sirius Compact, a lightweight and affordable reimagining of the passive sensor. Measuring just 460 by 150 millimetres, each sensor unit weighs less than five kilograms, enabling it to be easily carried by a soldier, mounted to a drone or to be carried on a small vehicle or vessel. The system’s ease of operation and excellent transportability enables it to be quickly relocated to new locations as the threat scenario evolves. And its affordability compared to traditional passive sensors allows for a much wider roll-out, greatly enhancing coverage and situational awareness.

Sirius Compact can easily carried by a soldier, mounted to a drone or to be carried on a small vehicle or vessel.

Extremely accurate Direction Finding (DF)

At the heart of each Sirius Compact unit is a digital channelised receiver providing 360 degree instantaneous coverage between either 2-18 GHz or 1-18 GHz, depending on the variant. An integrated inertial measurement unit and global navigation system enables the sensor unit to calculate its exact position and orientation on (or above) the earth’s surface. When the electromagnetic pulses of a radar signal sweep over the sensor, it is able to use the amplitude, energy and phase difference between pulses to very accurately provide a direction finding to the source of the emission.

In a typical use case scenario, three Sirius Compact sensors might be deployed in the field several kilometres apart and connected via a standard IP network. A ruggedised laptop at one location hosts the Tasking and Reporting System (TRS) for remotely controlling the array of sensors, providing a digital map display of emitters and access to a threat library containing information on previously identified enemy radar signatures. When a suspect signature is detected by one sensor, the other sensors can automatically be tasked to search for it. By working together, the direction of the signal from each sensor can be determined to within two degrees – when these results are merged, the emitter can be geolocated by using triangulation.

Supporting GBAD

One key application for Sirius Compact is complementing the operation of GBAD units, as described above. Current conflicts are demonstrating that when GBAD units keep their radar constantly switched on, they tend to be quickly located by the enemy – and targeted with radar-seeking missiles. But with an array of Sirius Compact sensors providing an early warning and detection capability, GBAD units need only activate their radar when a threat draws near. This reduces their risk of detection and increases their chance of mission success. In this way, Sirius Compact serves as a force multiplier, enhancing and strengthening the effect of other assets. As such, it plays a vital role in the kill chain.

Another application is gathering intelligence and identifying enemy positions. Sirius Compact sensors carried by ground troops or attached to light vehicles can be sent out in the field to listen for radar-emitting enemy installations, pinpointing their location and collecting details for later targeting. Importantly, Sirius Compact sensors themselves do not contain information from the threat library and only access this via the TRS. This means that should a sensor fall into enemy hands; it won’t reveal sensitive intelligence on which enemy signatures have been identified.

In the air, on the water

Sirius Compact also has a myriad of other applications, whether it be peace or war. Its ease of deployment and agility means it can be used for surveillance of remote border locations, helping to maintain national integrity and keep the peace. In this role, the system typically becomes part of a multi-sensor solution complementing fixed passive sensors, radar installations and other infrastructure.

The light weight of the Sirius Compact also makes it ideal for mounting to drones for aerial duties, both in peacetime and during conflicts.

The light weight of the Sirius Compact also makes it ideal for mounting to drones for aerial duties, both in peacetime and during conflicts. A drone-agnostic design allows it to be mounted on a variety of aerial craft, in a variety of orientations. To reduce the chance of own platform detection, the sensor can be disconnected from the IP network during missions and the drone quietly flown on a pre-programmed route, any gathered emitter data can then be downloaded upon the drones return. The variant for use in drones weighs in at just three kilograms; even less than the man-portable system.

Yet another application during both peacetime and conflict is littoral monitoring. Sirius Compact sensors can be attached to light vessels, such as small Combat Boats and Unmanned Surface Vessels (USV), to search for signs of enemy radar use. In a hybrid conflict scenario, Sirius Compact can play a useful role in tracking vessels owned by persons of interest to security services. Owners of luxury yachts, for example, may seek to avoid detection while travelling by turning off the transponders that transmit their vessel’s identity. However, they still rely on commercial navigation radar with a distinctive signature that can be picked up by the Sirius Compact’s sensors.

40 years of passive sensor experience

While there may be a handful of other man-portable passive sensors available, none come with the pedigree of the Sirius Compact. It is part of the Saab Sirius family of products, which include advanced systems for gathering communications intelligence (COMINT) and electronic intelligence (ELINT) across land, sea and air domains. Saab has more than 40 years of experience developing passive systems for use in assets including submarines, warships and AEW&C aircraft.

This depth of experience is important. The electromagnetic spectrum is becoming increasingly crowded, with the traditional lines between communication and radar becoming blurred. The advent of civilian 5G networks has brought a never-before-seen complexity to signals intelligence and advanced, flexible systems that use Artificial Intelligence (AI) are needed to maintain robust situational awareness.

With electronic signals management become increasingly important as Western forces prepare for more peer-on-peer conflicts, Sirius Compact provides a reliable, simple and affordable way for defence forces to expand their sensor coverage. Saab refer to this as “Silent Power”