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German Air Force explores passive aggressive surveillance

1st November 2019 - 12:00 GMT | by Thomas Withington in Toulouse

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The Luftwaffe (German Air Force) is blazing a trail with its interest in passive radar, with the technology promising to enhance its ground-based air surveillance capabilities.

Whereas conventional radar transmits RF (Radio Frequency) energy which collides with an object and is then reflected back, allowing the radar’s determination of the target’s velocity, bearing and altitude, passive radar exploits RF energy already in the environment. 

Passive radar can work in two ways: Firstly, it can detect disturbances made by aircraft in existing RF transmissions. For example, a passive radar can determine disturbances in local television or cell phone signals, geo-locate where these disturbances are taking place and thus determine an aircraft’s position. 

Conversely, a passive radar will detect RF emissions made by an aircraft’s radar, satellite communications, transponder, radio altimeter, radio navigation and/or voice communications, and locate the sources of these transmissions to determine the aircraft’s location. 

The key attraction of passive radar is that it does not emit RF. As such, it is difficult to determine where such a radar maybe located and thus difficult to jam. 

A spokesperson for the Luftwaffe told Shephard that the service is interested in such technology to enhance low-altitude air surveillance within Germany. Some passive radar systems are deployable meaning that they can be set up in areas where conventional ground-based air surveillance coverage maybe lacking, or difficult due to undulating terrain obscuring a conventional radar’s line-of-sight. 

It was revealed in October 2018 that the Luftwaffe and Germany’s MoD had performed trials of Hensoldt’s Twinvis passive radar system in southern Germany. 

Twinvis monitors transmissions across a very high frequency to ultra-high frequency waveband of 30MHz to 3GHz principally detecting disturbances in FM (Frequency Modulation) broadcast radio transmissions on a waveband of 65.8MHz to 108MHz; DAB (Digital Audio Broadcasting: 174MHz to 240MHz/1.452GHz to 1.492GHz) radio and DVT-B (Digital Video Broadcasting-Terrestrial) television transmissions using frequencies of 474MHz to 738MHz. 

The system can monitor FM, DAB and DVT-B simultaneously. Detection ranges of up to 134nm (250km) and an accuracy of 164ft (50m) are achievable, while the system provides 360° surveillance. 

A spokesperson for the company stated that such accuracy could be adequate for ground-based air defence fire control: ‘Twinvis’ accuracy is sufficient to guide a missile to the target position, where the final target approach would be guided by [the missile’s] on-board sensors.’

For now, Hensoldt and the Luftwaffe are keeping quiet on the scope of the tests. That said, the company did disclose that beyond the German Air Force it has ‘conducted numerous measurement campaigns with interested parties including armed forces, police and air traffic control authorities.’

While the Luftwaffe is plainly interested in the technology, their spokesperson added that there is still some way to go before any operational systems are deployed: ‘The number, configuration and technology of the passive radar systems is not yet determined. The German air force is now in the process [of defining] the requirements for their operational needs.’

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