Monday, June 1, 2009
Performance Based Navigation and RNP
As the national airspace system evolves, older navigation systems give way to safer, more reliable, and increasingly efficient technology. As students, we all studied the evolution of non-directional beacons and radio-ranges giving way to VORTAC’s and ILS’s. Today, these are now "legacy" navigation aids when you consider that the first ILS approach was flown into Pittsburgh on a snowy day in 1939.
How many of us have shared the excitement of a full procedure non-precision approach, flown in mountainous terrain in a non radar environment to an unfamiliar non-towered airport, at minimums, at night? And how many of us thought there’s got to be a better way! I promise you the transition to performance based navigation (PBN) and specifically required navigation performance (RNP) will revolutionize the way we fly.
PBN is a cornerstone within the FAA’s strategic plan for the national airspace system called NextGen. While a future article will be devoted to explaining NextGen, it would be difficult to explain PBN without understanding the significant role it plays within NextGen. "From flight decks and control towers to runways and radar stations, our national air transportation system is moving toward an unprecedented, paradigm-shifting change" according to the FAA’s NextGen Integration and Implementation Office. Regrettably, all of you hold-outs will be affected in the not too distant future, so the 8-track listening, dual sided-steel razor blade, "recently invested in a VCR" crowd, please pay attention!
RNP vs. RNAV
They are cousins and both members of the PBN family. While RNAV affords point-to-point navigation, it is limited to the service volume of referenced NAVAIDS or drift and update errors inherent to self-contained on-board systems like Doppler and IRS/INS. So while RNP is based on the RNAV principles, it is an advanced RNAV technology, providing a consistent lateral and vertical path for precision navigation. RNP features many additional benefits compared to RNAV such as vertical navigation, an on-board capability to monitor navigation performance, alerting the crew when the aircraft is exceeding its RNP and is at risk of transgression outside of the containment area. RNP also supports Radius-to Fix (RF) legs, which provide curved lateral and vertical path segments.
The precise path created by RNP is the result of fusion between global positioning system (GPS), the flight management computer and the binary coding specification of the procedure design. Unlike conventional "terrestrial-based" navigation aids, procedure designers are not limited to linear projections created by radio beams or arcs defined by the use of distance measuring equipment. Since RNP is significantly more precise than the technology it replaces, the accuracy or flight technical error (FTE) provides superior containment, presenting new opportunities for terminal and en route design. Terrain challenged or airspace limited airports and heliports alike can take advantage of VMC corridors tailored for operations in IMC conditions. Depending on individual aircraft capabilities, and obstacle penetration, RNP approaches can provide weather minimums equivalent to a Category I precision approach. While RNP procedures can be constructed to take advantage of non-linear paths, avoiding terrain and airspace boundaries, it also benefits the community by designing procedures that remain outside of noise sensitive areas, and with a steeper vertical path, reducing the environmental impact to the community. Kind of like a curved-path ILS to a heliport. Similarly, a curved path missed approach can be incorporated for the missed approach segment, facilitating a precision three-dimensional lateral and vertical path, avoiding terrain, obstructions and noise sensitive areas. When this same concept is applied to RNP departures, previously VFR-only heliports can enjoy all-weather departures or the ability to carry more payload by taking advantage of the terrain and obstacle avoidance.
RNP supports a precision approach at locations that are not served by an ILS for physical or economic reasons. They can also be designed as "overlay" approaches to existing ILS, or certain non-precision approaches. This provides a huge safety benefit through the elimination of the non-precision "dive and drive" decent profile and its associated risk of controlled flight into terrain. RNP provides a continuous decent profile to a DA or Decision Altitude similar to an ILS. At the DA, the pilot either transitions to a visual landing or a missed approach. You might question why overlay an ILS with an RNP procedure? Many ILS are plagued by signal interference for a variety of reasons, (obstructions, vehicles and taxiing aircraft, maintenance) and can cause undesired movement especially when coupled to the autopilot. Aside from a dependable and stable alternative to a co-located ILS, the RNP procedure can be used when the ILS is out of service. Aside from improved operational reliability, this capability can be provide lifesaving support during disaster relief, when terrestrial based navaids are no longer operational due to flooding, hurricane, typhoon and may remain out-of-service for days or weeks.
ANP vs. RNP
As previously discussed, RNP design features an on-board performance monitoring capability. So in a "NextGen navigation performance" world, each segment of navigation has a RNP value assigned to it. This RNP value is continuously compared to actual navigation performance (ANP). When ANP exceeds RNP, you no longer have adequate performance to continue the procedure. The lower the RNP value, the tighter the containment and FTE, providing greater accuracy and lower minimums. En route airspace would have a designation of RNP 5, Terminal RNP 1 and approaches can be as low RNP.11 (These are the ICAO standards the FAA is migrating to from RNAV 1 and RNAV 2).
Don’t RAIM on My Parade
Receiver autonomous integrity monitoring (RAIM) must be accomplished unless your aircraft has a certain type of receiver such as GBAS/SBAS enabled, or if your IRS/IRU meets certain drift standards. As the 24 satellite GPS constellation ages, depending on the time of day, aircraft position and mask angle, signal integrity issues exist with duration of 00:00:01 to 00:14:59. Hence, a RAIM prediction is created to ensure adequate signal coverage during the time forecast for approached planned in IMC conditions. These short duration signal outages are referred to as "RAIM holes". Signal outages scheduled for periods 15 minutes or longer will presented in the GPS NOTAMS. So the lower the RNP, the greater the requirement for signal accuracy and number of satellites that must be available. For this reason, many approaches are designed to an RNP.3 or with split minimums, providing crews with more operational flexibility, providing a balance between RAIM and actual weather.
You can find more information about PBN from ICAO at www.icao.org. Additional information about NextGen is available from the FAA at www.faa.gov/nextgen. RAIM prediction information is available free of charge at www.raimprediction.net/ac90-100/. Another Web site of interest is NOAA’s Space Weather Center, located at www.swpc.noaa.gov/today.html. Help is also available at www.hughesaerospace.com
RNP is a game changer with global reach for aviation, presenting huge potential for helicopters. After all, helicopters are the only true point-to-point source of transportation, and should have a navigation protocol that supports its unique capabilities. Globally, regulators and operators are working together to take advantage of RNP. They recognize its value as it provides immediate opportunities and access to more locations at a fraction of the costs associated with legacy navigation aids. Helicopters will be major beneficiaries of RNP, best able to exploit its potential with RNP airways, approaches and departures tailored to serve their needs. RNP can provide us with a safe and reliable stabilized approach with a reduced noise signature wherever they are needed, the latter of which neighbors on the ground will like.