Look Without Being Seen
(Source: Swedish FMV; Issued March 21, 2018)
(Issued in Swedish; unofficial translation by Defense-Aerospace.com)
Three different kinds of passive sensor are placed in a star form with a central receiver station in the middle. Connected by microchip or optical cable, the devices can locate a target by triangulation and even identify it. (FMV image)
FMV conducts a demonstration of a so-called passive sensor system. On a screen two points are visible. By rapidly analyzing the combination of signals emitted from the aircraft, it appears that there are two JAS 39 that come low from the north along the coast at low altitudes.

On the screens that are set up in a lesson room at the air-defense regiment in Halmstad, every flight is seen across southern Sweden and Denmark. Even the ships around the coasts are visible on the screens.

"We are doing this demonstration to raise awareness of how this kind of so-called passive sensors for air and sea monitoring work," says Carl Fischerström, who coordinates the demonstration.

“We want to acquire more knowledge of passive sensor systems in order to make the right assessments on how to assemble the future sensor system for air monitoring,” says FMV’s Carl Fischerström.

The same position of air and sea “targets” is today obtained from the radar chains that the Armed Forces operate along the coasts, so why has FMV invited a Czech company to conduct a demonstration of its sensor system?

"The special thing about passive systems is that they are difficult to detect for the opponent. They only listen, while radar systems send out signals that are reflected against the object, making them easier to knock out. We and the Armed Forces need to know more how to assemble future sensor components to create an operational ability that can match a possible opponent,” says Carl Fischerström.

Triangulation gives the position

With three sensors positioned to form a star with a central antenna in the middle, it is possible to determine the direction, distance and height of the vehicle. This is done by the so-called TDOA technology which measures time differences in the received signals.

Lars Carlsson is a system engineer at FMV. He finds that a conventional radar is easier to find and knock out. While passive technology, a form of signal tension, can see without being seen. This provides improved opportunities for survival on the battlefield.

"You can measure as little time differences as down to 3 nanoseconds, and it allows the system to identify the position of the object in all three dimensions, even though the sensors are not so far apart," says Lars Carlsson, System Engineer at FMV.

Nosradarn, radio and Link 16 in a fighter aircraft are examples of emitters that provide signals that the passive sensor system may perceive. It is not enough to track the aircraft, but it is possible to identify what kind of aircraft it is.

“A database of transmission frequencies and pulse frequencies of emitters helps the operator identify the aircraft,” says Carl Fischerström. “You can also enter each individual emitter and draw conclusions about what mission the aircraft may have.”

Future road selection

In front of the screens, Miroslav Cerny has 30 years of experience in the continuous development of the passive sensor system. He points on the screen saying that there is an air tank in Denmark there over there.

"This is really no new technology, but modern signal processing, a library of information about emitters that the system itself creates, and fast IT processors have made it efficient and user-friendly," says Lars Carlsson.

FMV is now conducting a number of demonstrations of the system for invited guests in the defense sector as part of knowledge-building on passive sensor systems.

"It will give us better prerequisites for developing ground for choice of future sensor information systems," says Carl Fischerström.

-- VERA NG - Passive Sensor System
-- Passive monitoring, early warning
-- Detects electromagnetic energy
-- Tracking targets in two or three dimensions, in real time
-- Range of 400 km
-- Identification of 200 goals simultaneously
-- Initially, a Soviet system further developed by the Czech company ERA Omnipol.


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