Editor's Note

Europe’s Energized Mode S Program

By David Jensen | April 1, 2003
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Following a decade-long gestation period, Eurocontrol’s ambitious Mode S surveillance program is accelerating. On March 31, coordinated Mode S secondary surveillance radar (SSR) coverage was to commence over France, Germany, Switzerland, the UK and the Benelux states (Belgium, Luxembourg and the Netherlands), where air traffic in Europe is densest.

Mode S (S for selective) is a major European program that is expected to deliver a big return: cost/benefit analyses estimate that the about 1-billion-euro program will bring a net gain of 1.7 billion euros by 2017. The return would come largely from a reduction in flight delays, an increase in airspace capacity, and an improvement in air traffic controller productivity.

"ATS [air traffic service] providers will recover costs from user fees, and airlines will recover costs through improved operations," says Pascal Dias, Eurocontrol’s program manager for Mode S.

To initiate coordinated Mode S surveillance in Europe, France’s Service Technique de la Navigation Aerienne (STNA), Germany’s Deutsche Flugsicherung GmbH (DFS), and the UK’s National Air Traffic Services (NATS) are participating in a program to standardize the use of Mode S SSR by "following common regulatory principles and a common timeframe, and by accessing aircraft parameters in a similar way," says Dias.

This standardization is key to Europe’s intent of harmonizing the continent’s air traffic services–which is why Eurocontrol is delivering to the program participants common, validated tools for Mode S, such as procedures, safety assessments, hardware and software system components, regulatory guidelines, a common implementation plan, and a process to allocate interrogator identification (ID) codes. These tools derive from a nine-year-old Eurocontrol strategy for the Initial Implementation of Mode S Enhanced Surveillance (IIMSES).


A major part of this strategy was a program called POEMS, or Pre-Operational European Mode S, for which Eurocontrol established a coordinated approach with Europe’s ATS providers to develop two competitive Mode S ground stations. For the POEMS program, which was launched in 1995, Eurocontrol tested and approved Mode S radar systems from two selected manufacturers, Thales ATM and UK-based Raytheon Systems Ltd. (formerly Cossor).

ATS providers "can choose from the two suppliers, but the specs [for Mode S ground stations] have been made widely available," says Dias. "It is of paramount importance that the market remains open and that other companies that meet the specs can be suppliers, too."

The Mode S ground equipment, which comprises an interrogator and a transmitter, can be part of a new SSR or installed to upgrade a traditional Mode A/C secondary radar.

Test and evaluation against procurement specifications of Thales’ and Raytheon’s Mode S radars have been conducted since 2000 at three locations. From contracts signed in the 1996/97 timeframe, Thales established a POEMS ground station with rotating antennas at Paris’ Orly International airport, and Raytheon Systems installed its POEMS stations at Germany’s Dusseldorf airport and the UK’s London-Gatwick airport.

Two-Phase Approach

Eurocontrol divided its Mode S program into two phases. This is possible because Mode S permits various levels of performance and capabilities. This flexibility, in turn, allows ATS providers to tailor their Mode S surveillance to match their needs.

For example, basic surveillance Mode S is used in the United States primarily to support airborne aircraft/traffic alert and collision avoidance systems (ACAS/TCAS) and provides accurate altitude reporting. In addition, it provides simplified aircraft ID code assignments.

Mode S with enhanced surveillance delivers more data to controllers, such as aircraft attitude, heading and speed, as well as altitude, aircraft ID and ACAS/TCAS advisories. As part of enhanced surveillance, Eurocontrol wants Mode S to support a step towards controller-pilot data link communications (CPDLC), which also will be implemented using VHF data link Mode 2 (VDL-2) in Europe, to help reduce voice communication congestion.

"The principle is to set up a point-to-point link between aircraft and radar," Dias says. And the agency also wants Mode S to facilitate surface traffic management. "In this case, we would use Mode S for the ground surveillance multilateration process," he adds.

The Eurocontrol program’s first phase, called "elementary Mode S" surveillance, was to begin March 31, upon the commissioning of Mode S SSR software in the seven participating countries. The data the radars will receive includes the interrogated aircraft’s ID and three-dimensional position: distance, azimuth and altitude.

With elementary Mode S, Eurocontrol plans to achieve the following:

  • Resolve the problems associated with radio frequency interference;

  • Overcome Europe’s Mode A code shortage; and

  • Maintain current safety levels, as air traffic continues to increase.

By 2005, the European core-area states are expected to have about 40 Mode S SSRs in operation, and the second phase, "enhanced Mode S," is expected to deliver benefits. For this phase, Eurocontrol will not have to upgrade the SSR’s interrogator software (because the radars already are enhanced Mode S-capable) but will configure the stations into enhanced surveillance extraction mode, to acquire more aircraft-derived data, such as trajectories and intentions. This information will be displayed on controller screens and included with automated air traffic control (ATC) support tools and safety nets, such as short-term conflict alert and medium-term conflict detection.

"For enhanced [Mode S] surveillance, we see controller efficiency improved, and this would bring a 5 percent or so capacity increase, which means fewer flight delays," says Dias. Eurocontrol’s confidence in Mode S benefits derives, in part, from testing in the agency’s ATC simulator, conducted in conjunction with Germany’s DFS.

The enhanced Mode S phase also is to correspond with wider implementation and coordination of Mode S SSRs in Europe. Though participation in the Eurocontrol program is voluntary, the agency believes that Mode S coverage expansion beyond core Europe could ultimately include all European Civil Aviation Conference (ECAC) countries. It is confident that any new installation, upgrade or replacement of secondary surveillance radar will incorporate the Mode S function.

"Mode S will likely become standard for secondary radar," says Dias. "However, for the moment, we’re just trying to harmonize the deployment of Mode S in the high-density areas of Europe. We expect the decisions by other countries [to equip for Mode S] will come piecemeal."

A More Capable System

Mode S uses the same two frequencies employed by the SSR: 1030 MHz for interrogation and 1090 MHz for the reply. Mode S standards are defined in International Civil Aviation Organization (ICAO) Annex 10, volumes III and IV, amendment 77.

A primary advantage of Mode S over the commonly used Mode A/C is improved surveillance performance, with greater integrity, as a result of the selective interrogation and extraction of flight and positioning data from aircraft. Mode A/C transponders interrogate all aircraft within a beam width simultaneously, and this can compromise the system’s integrity. "When traffic gets dense, the [Mode A and C] signals are superimposed and we may lose the accuracy of the information," Dias explains. Mode S interrogates each aircraft individually, using a roll call.

Another principal advantage of Mode S over Mode A/C is its 24-bit address identifier, which allows each aircraft to have its own permanent code, or address. Mode A/C SSR alternatively interrogates airborne transponders; it requests flight altitude through Mode C and aircraft ID through Mode A. The Mode A reply is coded in just 12 bits, allowing for only 4,096 codes–and though they are reusable, these addresses can be depleted quickly, given the about 25,000 flights in ECAC countries each day. However, Mode S’ 24-bit capacity permits more than 16 million discrete addresses–more than enough to accommodate the individual registration of all aircraft in the world.

Additional Advantages

Eurocontrol plans to gain additional advantages from Mode S, including:

  • Enhanced integrity of the surveillance data–achieved through improved processing of a greater number of aircraft parameters, as well as more use of air traffic controller tools.

  • Enhanced aircraft position accuracy–from the Mode S monopulse method of measuring azimuth, and from smaller increments in altitude reports (25 feet instead of 100 feet) transmitted by Mode S transponders.

  • Reduced limits in flight data processing–accomplished from the capability to use direct flight identification and overcome the Mode A code shortage.

  • Improved controller efficiency–by complementing the current air situation picture with downlinked aircraft parameters.

  • And the option to implement future functions–such as automatic dependent surveillance-broadcast (ADS-B) by providing an air-ground and/or air-air data link.

Mode S transponders are required on most aircraft flying in North and South America, Europe and East Asia because they are integral to ACAS/TCAS. Europe, for example, requires ACAS II on all aircraft weighing 33,075 pounds (15,000 kg) and, beginning Jan.1, 2005, will require the system on aircraft weighing 12,570 pounds (5,700 kg).

However, the capabilities of airborne systems can vary as greatly as those of ground systems. For the Eurocontrol Mode S program, aircraft operators must upgrade their transponders.

"European aircraft have Mode S transponders suitable for ACAS, but they can accommodate the addresses from only 15 radars," says Dias. "They need to extend their capability for up to 78 radar address codes. And for enhanced Mode S, they must be able to transmit additional aircraft parameters." For the additional parameters, operators will have to link their transponders with such information sources as the flight management system (FMS).

A Demanding Program

"By 2008, everyone [all aircraft operators in Europe] will have Mode S," says Dias. The Eurocontrol program requires that by 2005, instrument flight rule (IFR) certified aircraft weighing 12,570 pounds (5,700 kg) or more must be equipped with transponders capable of enhanced Mode S. And by 2008, visual flight rule (VFR) aircraft must include elementary Mode S transponders.

Eurocontrol estimates that aircraft equipage will represent two-thirds of the Mode S program’s expense, and this has drawn detractors, especially in the aircraft operator community, which sees Mode S as primarily benefiting air traffic controllers. "The problem for the airlines is that the costs will come quickly, but the benefits will come slowly, over time," says Dias.

Aircraft operators worried in the past that ADS-B, using a data link other than Mode S (for example, VDL Mode 4), could succeed Mode S surveillance, and this would involve the needless expense of equipping for two systems. However, this has been overcome by the natural choice of using Mode S 1090 extended squitter as the primary means for fitting ADS-B in the short and medium term.

Europe’s national ATS providers will assume most of the Mode S program’s remaining, one-third expense when they acquire or upgrade their secondary radars. Eurocontrol expects to see about 100 Mode S-capable radars operational in Europe by 2010, according to Dias.

The United States already enjoys coast-to-coast Mode S radar coverage, with the capability fitted to 138 terminal control radars in operation (see sidebar). Indeed, Lincoln Lab made Mode S deployable in 1975 as part of a development program jointly supported by the United States and United Kingdom. The two countries wanted to prepare for the foreseen time when air traffic would surpass the capacities of Mode A/C radar surveillance.

But Federal Aviation Administration officials concede that, with its wide range of related activities and quest for greater systems performance, the European Mode S SSR program is much more rigorous. Mode S SSR, for example, is considered to be integral to the European Commission’s Single European Sky initiative. Eurocontrol plans to "cluster," or network the Mode S radars so data can be shared. This would reduce overlapping coverage and even allow countries to decommission some secondary radars. Eurocontrol has upgraded both Europe’s RADNET (radar network) and its ARTAS (ATC surveillance tracker and server–a multiradar processing system) to Mode S.

Mode S Lite in the United States

The U.S. Federal Aviation Administration (FAA) has incorporated Mode S capability in 144 of its secondary surveillance radars (SSRs). One hundred and thirty-eight of the radars are operational, and the remaining six serve in a support capacity. Most Mode S SSRs in the United States are terminal radars, providing coverage down to ground level. The remaining SSRs (about 20) are long-range, en route radars. Combined, FAA’s Mode S radars provide nationwide coverage down to 12,500 feet.

The FAA launched its Mode S program in 1984 and installed the first new radar at Baltimore-Washington International Airport (BWI). The program was completed in the 1997/98 timeframe.

FAA now plans to upgrade its en route radars, though it won’t opt for the data link capability, which is incorporated in the terminal radars. "There are no plans in the U.S. to provide the equivalent of enhanced Mode S services," says an FAA official. Also, he adds, "An audit was done, and it found that the data link function wasn’t used." The agency had thought of promoting Mode S for controller-pilot data link communications (CPDLC), but other data link bands (notably, VHF data link Mode 2) have been chosen for that mission.

Instead, FAA’s en route radars will be fitted with the air traffic control beacon interrogator Mode 6 (ATCBI6), which includes functions comparable to a Mode S interrogator. "It looks like Mode S and smells like Mode S, but it doesn’t have data link," says the official. Many in the industry, therefore, refer to the ATCBI6 beacon as "Mode S lite."

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