Current Mode C secondary radarsystems in Europe's most congested airspace sectors - terminal manoeuvring areas (TMA) around Frankfurt, London and Paris - are beginning to be replaced by Mode S radars.
Gradually, the new radar system will be installed across other sectors where traffic is heavier, if less complex, than in TMAs. Mode C radar is a limited system, but designed for surveillance, not as a controller-pilot datalink communications (CPDLC) medium.
Mode S radar will increase the amount of data the controller can access about any aircraft on the display without sacrificing any of the inherent surveillance integrity of a radar system - which is vital in congested or complex airspace.
Automatic dependent surveillance systems, where the aircraft reports its position automatically for surveillance, is judged suitable for uncongested airspace or sectors where most traffic is en route, but it does not have the integrity essential for busy TMAs.
Rather than just indicating aircraft identification, type, height and destination, Mode S will carry a range of data. For controllers it will be the first provider of predictive aircraft trajectory information, which UK National Air Traffic Systems head of air traffic operations, policy and performance George Ennis says is a more urgent priority for his organisation than CPDLC.
It would be easy to overload controllers and clutter the display with additional data, but in the London TMA, controllers will probably be shown the height selected by the crew on the autopilot when a level change is authorised, so that if the wrong height is selected a "level bust" can be foreseen and prevented. Similarly, heading selected can be indicated. This information may go into data-blocks on screen, but to avoid clutter could go on to electronic flight progress strips.
Mode S has more datalinking capacity than will be used upon first installation, according to Ennis, but its two basic characteristics make it a better tool for dense traffic: Mode C radar can only handle 4,096 "squawk" identifier codes, Mode S has no limit; and while, at its busiest, today's system can overload and "lose" a radar return momentarily - a phenomenon known as radio frequency pollution - Mode S has no such problem. It also checks height to an accuracy of 25ft (7.6m) instead of the present 100ft.
Ennis says Mode S will soon be used for "conformance" checks. This means it will automatically interrogate an aircraft's flight management system (FMS) to see if its programmed trajectory conforms with its cleared trajectory, enabling it to issue an alert to the controller if it does not.
It can also provide, in a window on the controller's display, a vertical profile of aircraft in a holding pattern.
Controllers will also be able to use Mode S to interrogate the aircraft FMS for actual wind direction and speed, which can help approach controllers manage optimum approach spacing.
Source: Flight International
