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74 Focus | Transponders European Commission, and put simply, they will allow pilots to select their own flight path from take-off to landing without having to use established airways (air corridors), as they do now. The rationale is to help save fuel and thus reduce the cost of air travel by allowing aircraft to fly the most direct route to their destination. However, this departure from the established airways system makes it essential that aircraft can accurately signal their position in the sky to traffic controllers on the ground and to other aircraft. Satellite navigation, specifically the GPS satellite constellation, will form the cornerstone of NextGen as it will be used by the ADS-B (Automatic Dependent Surveillance-Broadcast) radio protocol to transmit the position of the aircraft, based on its GPS coordinates. The transmission of ADS-B information will be performed by using modified so- called Mode-S transponders. Mode-S is a transponder protocol that will broadcast a unique 24-bit address assigned by the International Civil Aviation Organisation; it will be part of the aircraft’s Certificate of Registration and will remain with the aircraft throughout its operational life, along with the ADS-B information. Mode-S protocol information will be transmitted to ATM SSRs, which gather data on an aircraft’s situation. Whereas an ATM primary surveillance radar (PSR) provides information regarding an aircraft’s range and bearing, an SSR collects information on the aircraft’s altitude and its identity (in the form of the 24-bit address). ADS-B will thus provide additional information regarding the aircraft’s GPS position. Both SSRs and PSRs are often co-located, with the SSR antenna mounted on top of the PSR to ensure that the SSR and PSR information are displayed together to the ATC. ADS-B is increasingly being mandated for aircraft. For example, transponders compatible with ADS-B are now compulsory for all aircraft in Australia flying above 30,000 ft, while in the US, aircraft will be required to carry them by 2020, and in Europe from 2017. UAV operators will therefore need to ensure they have transponders that meet both Mode-S and ADS-B requirements if they are to fly in unsegregated airspace. Military UAV operators have a further consideration to manage. As well as ensuring that their aircraft meet Mode-S and ADS-B requirements in order to fly in the same airspace as civilian aircraft, their transponders will also have to meet the requirements of Mode-5. Like ATM Summer 2015 | Unmanned Systems Technology This UAV transponder is compatible with the Mode-5 IFF protocol (Courtesy of Sagetech) This reduced size transponder is aimed at platforms with SWaP constraints, making it suitable for UAVs (Courtesy of BAE Systems)