ATM Modernization, Business & GA, Commercial, Military

What is RTSP?

By William Reynish | October 1, 2006
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We’ve all heard of RNP, or required navigation performance. Essentially it’s the concept that if an aircraft is appropriately equipped and its crew appropriately trained, it can be safely flown closer to obstacles than the normal rules allow. For example, under Alaska Airlines’ FAA RNP approvals, the operator’s Boeing 737s can descend through solid clouds to their approaches into the Palm Springs, Calif., airport when the ceiling is as low as 250 feet above the ground. Meanwhile, because of the surrounding mountainous terrain, non-RNP qualified aircraft may not descend below 1,850 feet unless they can clearly see the airport. Landing at Palm Springs, therefore, is an excellent illustration of what "performance-based" operations are all about: the ability to achieve varying levels of airspace access depending on the certified capabilities of the aircraft and its crew.

So RNP is a way of increasing system capacity by allowing flights to be completed under conditions that previously would have seemed impossible. But RNP alone isn’t enough to handle the doubling, perhaps even tripling, of air traffic projected by 2025. We must develop ways to further increase capacity. One way is to increase aircraft operating capabilities and another is to segregate aircraft of different capabilities. Air traffic managers already have been segregating aircraft in an elementary way for many years. Non-instrumented, general aviation airplanes, for example, are restricted in their access to controlled airspace. General aviation aircraft carrying the minimum approved level of avionics, on the other hand, can share controlled airspace equally with the most sophisticated airliners.

Tomorrow, that will not be the case. In the Next Generation Air Transportation System (NGATS), now in the planning stage, officials of the Joint Planning and Development Office (JPDO) have stated that air traffic control’s traditional "first come, first served" philosophy will be replaced in high-density and certain other areas by giving priority access and routings to the best equipped aircraft, with the less well equipped being directed to less busy and probably more circuitous flight paths. But even that step will not overcome the capacity challenge of the future, and the bar must be raised even higher, if the system is to avoid total gridlock in the skies.

It’s all a question of system capacity. During rush hours at what NGATS intends to designate as "super density" terminal areas, much tighter lateral and in-trail approach separations will be in force, requiring the highest levels of performance assurance, which in turn means admission will be restricted to those aircraft with the most advanced onboard systems. As the Palms Springs example shows, 5-mile lateral en-route separations are unnecessarily wasteful of airspace for new technology airplanes, and significant future reductions are possible. Environmental concerns, moreover, may one day force airports to build new runways between their existing ones, often turning two conventionally separated parallel runways into three much more closely spaced parallels, with simultaneous approaches on each demanding equally high performance-based standards to qualify for admission. And airspace planners are already looking at reducing en-route lateral separations from 5 miles to 3 miles for aircraft equipped with automatic dependent surveillance-broadcast (ADS-B), as well as using the system to approve a sort of "equivalent visual" operation while still in instrument conditions in certain terminal areas. Unquestionably, the bar must be raised significantly higher than today’s airspace entry criteria, if we are to avoid gridlock. One size will no longer fit all.

The required total system performance (RTSP) concept is seen as the means to accomplish this. Currently in its early definition phase, RTSP is seen as expanding on RNP by adding required communications performance (RCP) and required surveillance performance (RSP). RCP would ensure that an aircraft carried equipment appropriate to maintaining continuous communications with air traffic control over the entire route it planned to fly. For trans-Pacific flights that could mandate satcom, for example. RSP, meanwhile, would ensure that aircraft carried appropriate surveillance response avionics, such as ADS-B and perhaps Mode S transponders. And both RCP and RSP could translate to a requirement for dual systems in certain types of airspace, such as high-density terminal areas, where single system failures could disrupt the traffic flow.

Yet the airborne elements are only half of the equation. Equally important is the ground environment, which brings in required air traffic management performance (RATMP). The melding of the air and ground performance metrics will allow the development of target levels of safety (TLS) which can be applied to differing operations in differing flight regimes.

Some may regard this as total overkill, but it is, in fact, a logical extension of the advances in technology that have accompanied aviation’s progress as aircraft numbers, speeds and capabilities have increased over the years. In navigation, we have moved from the low frequency radio range to non-directional beacons, to VOR, to DME, to Omega and Loran, and on to inertial platforms and satellite navigators. In communications, high frequency radio transitioned to VHF, and we now use satellite communications and data links, as well. And in surveillance, "skin paint" primary radar has been augmented, and in many applications replaced, by secondary surveillance radar, along with the now increasing use of ADS-B and multilateration networks. Transponders, themselves, have been moving from Mode A to Mode C, and to Mode S and, more recently, to ADS-B-compatible universal access transceiver (UAT) and 1090ES units. Supplementing these advances has been the progressive introduction of weather radar, autoland, traffic and terrain avoidance systems, head-up displays, synthetic and enhanced vision systems, and a wide range of other flight deck technologies which have over time expanded the operational envelope. (Interestingly, the ILS, developed in the 1940s, remains as a "legacy" system and, while it has greatly matured since then, no one has yet forecast its replacement date with any certainty.)

RTSP is expected to be introduced gradually, commencing in the middle of the next decade. System experts are already busy defining the process under which all these avionics assets will be classified into the matrix yardsticks used to assess the performance capability of each aircraft. But the classification task is a demanding one, as RCP and RSP are much more difficult than RNP. For example, in the case of the RNP certification of Boeing’s 737 Next Generation (NG) aircraft, the aim was to confirm that when the aircraft was equipped with the factory RNP avionics and was flown by an RNP trained crew, it could remain within a specified ground track to an accuracy of one tenth of a nautical mile (608 feet) for 95 percent of the time, and would remain within two tenths for 99.9 percent of the time, while depending exclusively on the aircraft’s systems, with no assistance from ground guidance facilities. Demonstrating that earned the aircraft its RNP 0.1 certification. Aircraft with lesser equipage earn less demanding certification levels, such as RNP 0.3, for 0.3-mile track keeping accuracy, or RNP 2, for 2-mile en route accuracy.

RATMP Factor
Establishing RCP and RSP values will be much more challenging because the assessment in both cases must balance the combined capabilities of the aircraft and the RATMP ground environment to establish the target level of safety, which some feel could be set at 98 percent, or even higher. In communications, the key criteria will be availability, capacity, error rate and transmission delay, and where voice would be assessed as less efficient than data. In surveillance, availability and capacity will be matched with accuracy and data rate as the key parameters.

The issue then becomes the balance of the respective air and ground capabilities, with some avionics industry observers concerned that the performance onus could be placed more on the avionics systems, which in turn would call for increased operator costs in the retrofit of more sophisticated, and more expensive, units. This is particularly the case in surveillance, where one company official states that the accuracy and capacity of today’s secondary radars are essentially "unknown" and would be difficult to precisely measure. This could, he says, result in a requirement for more advanced, and therefore more costly, transponders to meet the required TLS.

Deriving an equitable balance between air and ground capabilities clearly will be a difficult task, and industry observers have already expressed some misgivings that the benefits of RTSP may accrue more to the air traffic management side than to aircraft operators. Interestingly, it has been suggested that, because ATM capabilities can be less advanced in less developed parts of the world, even the most modern aircraft could find themselves operating in a much lower TLS environment than in more regulated airspace. Call this a real headache for the planners.

However, it seems clear that eventually the airspace system will have at least two or more admission levels, determined by equipment eligibility. For many operators, the RTSP technology criteria could place costly avionics upgrade burdens on those, such as corporate aviation, who wish to enjoy complete flexibility in route and airport access. Here, however, it becomes the operator’s choice: in a performance-based environment, there are no mandatory avionics requirements. Airspace access will be determined by each aircraft’s capabilities, which can be achieved by a combination of systems. That is, two aircraft may each be allowed access to a given airspace even though they use quite different avionics suites to meet the performance requirements. For example, ADS-B may substitute for a conventional transponder, GPS/IRS may substitute for VOR/DME and, looking farther ahead, data link may substitute for voice radio.

More to Come
But more is yet to come. Already, planners are studying the likely future need for standards such as required environmental performance (REP), to reduce noise and emissions; required automation performance (RAP), for optimum ATM efficiency; required weather performance (RWP), to avoid inadvertent entry into turbulence and severe icing areas; required vision performance (RVP), which would call for synthetic, enhanced or other vision technologies at certain airports, and even required security performance (RSeP) to ensure that appropriate safeguards are in place when necessary. And there are also subgroups of requirements, described in a Boeing Phantom Works presentation to a recent Air Traffic Control Association conference. These would include required trajectory prediction performance (RTPP), conflict detection (RCDP) and conflict resolution (RCRP), among others.

Yet it is important to stress that these performance-based concepts do not simply reflect the desires of FAA bureaucrats in Washington. RTSP is also an important issue in Europe, which faces the same doubling or tripling traffic volumes by 2025 as does the United States. It is also high on the agenda of the International Civil Aviation Organization (ICAO), where a dedicated multinational panel of experts aims to have a Global ATM Performance Manual completed by late 2007, along with a companion RTSP roadmap document codeveloped by ICAO and the International Air Transportation Association (IATA).

Unquestionably, aviation operations will become more complex in the future. But it seems equally certain that they will become more efficient, more predictable and, above all, safer.

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