Radar Tech that Lets You See Without Being Seen

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Over the backdrop of the widening global use of stealthy aircraft, Germany has been exploring a new passive radar equipment. This technology area could challenge existing assumptions on stealth as a key organizing principle for air warfare.

Passive radar technology is essentially covert, meaning pilots entering a monitored area may be unaware they are being tracked. That could even be the case for pilots flying stealthy aircraft like the F-35, experts say, though there is no publicly available data pitting passive radar against low-observable aircraft designs and their radiation-absorbing coatings.

The German Air Force has created an FFF formal acquisition track for the technology, in order to check capability gap, derive requirements and design an investment program, according to defensenews.com.

Passive radar equipment computes an aerial picture by reading how civilian communications signals bounce off airborne objects. The technique works with any type of signal already present in a given airspace, including radio or television broadcasts as well as emissions from mobile phone stations.

Passive radar is set to become a supplement to conventional active radar within the next few years. It offers a decisive operational advantage: it cannot be located. Unlike active systems, passive radar does not emit any waves of its own, meaning that it cannot be jammed.

In November 2018, the German Air Force and the Defense Ministry’s defense-acquisition organization had tested a new technology in Southern Germany, aimed at visualizing the entire region’s air traffic through TwInvis, a passive radar system made by Hensoldt, but no details were given regarding the results of this demonstration.

The radar’s name is made up from “twin” + “invisible”, as neither TwInvis itself nor the targets to be detected emit any signals on their own, which means that they are “invisible”.  

The company’s passive radar system requires high computing power and extremely complex signal processing software. Their system uses VHF (very high frequency), digital radio and television frequencies as carrier waves rather than its own transmitter, according to hensoldt.net.

There are a number of additional applications for passive radar, including as an undetectable guidance system for missile defense or covert surveillance of borders and whatever lies beyond them, Hensoldt officials have said.