The Evolution from Area Navigation Required Navigation Performance, to RNP RNAV

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       ICAO GNSSP IP11

 

GLOBAL NAVIGATION SATELLITE SYSTEM PANEL
Meeting

Rio de Janeiro, Brazil

(22 October – 1 November, 2001)

Information Paper

 

The Evolution from Area Navigation (RNAV), Required Navigation Performance (RNP), to RNP RNAV

(Presented By: Jeff Williams)

 

(Prepared By: Thomas Meyer and Jerry Bradley)

 

This Paper identifies differences between RNAV – a method, RNP – a concept, and RNP RNAV as defined in RTCA DO-235A/EUROCAE ED-75.  This paper is presented to define differences and to clarify and eliminate the current confusion that exists between the use of the term RNP in multiple applications, specifically, those that exists between RNP for the en route domain and RNP for approach, departure and landing operations.
1.0  Introduction

The continuing growth of aviation places increasing demands on airspace capacity and emphasizes the need for the best use of the available airspace.  These factors, along with the accuracy of modern aviation navigation systems and the requirement for increased operational efficiency in terms of direct routings and track-keeping accuracy, have resulted in the concept of required navigation performance or RNP.

Rapid changes in technology in the area of navigation performance, including the change from source-referenced area navigation systems, provide the foundation for aviation’s global evolution.  This progress will be marked by combining all elements of communication, navigation, and surveillance (CNS), and air traffic management (ATM) into tomorrow’s CNS/ATM based systems.  Concepts in CNS/ATM such as RNP provide the path for this transition.

RNAV is a method of navigation that permits aircraft operation on any desired flight path: e.g., “user preferred routes.”  The future CNS/ATM operating environment will be based on navigation defined by geographic waypoints (expressed by latitude and longitude).  Instrument procedures and flight routes will not require aircraft to overfly ground-based navigation aids (source-referenced systems) defining specific points.

The application of RNAV techniques has been shown to provide a number of advantages over conventional (source-referenced) forms of navigation, including:

1.  User preferred routes – tracks that consider pressure altitude and wind
2.  Establishment of more direct routes, resulting in shorter flight distances;
3.  Establishment of dual or parallel routes to accommodate a greater flow of en route traffic;
4.  Establishment of bypass routes for aircraft overflying high-density terminal areas;
5.  Establishment of alternatives or contingency routes, either planned or unplanned (e.g., severe weather avoidance); and
6.  Establishment of the best locations for holding patterns.
2.0  The RNP Concept

Required Navigation Performance (RNP) is a statement of the navigation performance accuracy necessary for operation within a defined airspace.  ICAO Document 9613, Manual on Required Navigation Performance defines the RNP concept.  RNP can include both performance and functional requirements, and is indicated by the RNP type. These standards are intended for designers, manufacturers, and installers of avionics equipment, as well as service providers and users of these systems for global operations.  The MASPS provides guidance for the development of airspace and operational procedures needed to obtain the benefits of improved navigation capability.

The term RNP is also applied as a descriptor for airspace, routes, and procedures (including departures, arrivals, and instrument approach procedures (IAP)).  The descriptor can apply to a unique approach procedure or to a large region of airspace.  RNP applies to navigation performance within an airspace, and therefore includes the capability of both the available infrastructure (navigation aids) and the aircraft.  RNP type is used to specify navigation requirements for the airspace.  ICAO has standardized the following RNP Types, RNP-1, RNP-2, RNP-12.6 and RNP-20.  The required performance is obtained through a combination of aircraft capability and the level of service provided by the corresponding navigation infrastructure.  From a broad perspective:

aircraft capability + level of service = access.

In this context, aircraft capability refers to the airworthiness certification and operational approval elements (including avionics, maintenance, database, human factors, pilot procedures, training, and other issues). The level of service element refers to the national airspace system infrastructure (including published routes, signal-in-space performance and availability; and, air traffic management).  When considered collectively, these elements result in providing access.  Access provides the desired benefit (airspace, procedures, routes of flight, etc.).

While it has been a goal for RNP to be sensor-generic, this goal is unachievable as long as the aircraft capability is in any way dependent on external signals.  The aircraft navigation system always consists of specific sensors or sensor combinations and the navigation infrastructure consists of specific systems.  In addition, flight inspection and maintenance personnel ensure the quality of specific navigation systems.  Sensor independent operations (where pilot does not have to be concerned about what sensors are being used) could be established when the airborne equipment provides the protections and system alerting based upon specific navigation systems.

The sensor-specific options available for any given route, procedure or airspace depend on the available infrastructure (level of service) and the requirements (aircraft capabilities) that promote access to a given airspace or procedure.

RNP types specify the minimum navigation performance accuracy required in an airspace.  Normally, aircraft not meeting the RNP type are excluded from RNP airspace.  If appropriately equipped, an aircraft with a level of navigation performance more accurate than that specified can fly in the airspace concerned (e.g. RNP 1 certified aircraft in RNP 4 airspace). 

Note that when an aircraft’s capability meets the requirements of a more stringent RNP airspace, based on upon specific infrastructure, this capability might not meet the requirements of a less stringent RNP airspace (due to the lack of supporting infrastructure appropriate to its navigation equipment fit), e.g. RNP 1 (DME/DME only) certified aircraft is not capable of operation in RNP 10 (oceanic) airspace.

 

 

 

FIGURE 1.  Evolutionary Path for Implementation

Figure 1 shows the U.S. plan for how the elements of certification, operational approval, and level of service relate to access.  The first column represents the airworthiness certification.  The second column addresses the operational approval (this includes operational approval, continuing airworthiness, training, etc.).  Combining the first and second column collectively represent the aircraft capability component of the equation.  The third column represents the level of service (requirement for navigation performance accuracy) independent of the navigation source and the fourth column represents access.
Access in this case denotes the ability to operate within specific airspace domains and or to conduct an operation.  Access relates directly to the benefits addressed in the introduction.

Where airspace is designated as RNP-x, the required performance is an accuracy value met by using an appropriate RNAV system. The accuracy requirement is that the total system error (TSE) does not exceed the specified RNP value (‘x’) for 95% of the flight time in either cross-track or along track.

Note:  Precision approaches are based upon specific sensors and not upon navigation performance for an operations/airspace.

3.0  The RNP RNAV Concept (see RTCA DO-236A).

3.1  RTCA DO-236A/EUROCAE ED-75A, Minimum Aviation System Performance Standards (MASPS):  Required Navigation Performance for Area Navigation (RNP RNAV) contains system requirements for operations in an RNP RNAV environment.

3.2  It is important to distinguish between RNP and RNP RNAV operations.
There is no requirement for airborne monitoring of the achieve accuracy; instead, accuracy is ensured operationally by qualifying specific sensors or through ATM.  These parameters (accuracy, integrity, and continuity) are defined and quantified by RNP RNAV.

Where airspace is designated as RNP-x RNAV, performance requirements include containment (see Figures 2 and 3).  Containment is a set of interrelated parameters used to define the performance of an RNP RNAV navigation system.  These parameters are containment integrity, containment continuity, and containment region. The accuracy requirement is the 95th percentile of TSE (same as RNP).  Integrity and continuity are specified relative to a containment region, whose limit is equal to twice the RNP value (e.g., for RNP-0.3 RNAV the containment region is 0.6 NM).  RNP RNAV has additional functional requirements beyond those of RNP, i.e., alerting of the loss of RNP capability must occur in the flight crew’s primary field of view. RNP RNAV avionics assume the ATS service provider ensures their navigation infrastructure meets desired performance requirements.  These assumptions should be listed in the AFM.  However, since all ATS service providers may not provide identical performance from their navigation infrastructure, the operator must ensure the service provider’s existing infrastructure supports the desired RNP RNAV operation.

The containment region (see Figure 3 below) quantifies the navigation performance where the probability of an unannunciated deviation greater than 2 x RNP is less than 1 x 10-5.  This means that the crew will be alerted when the Total System Error (TSE) can be greater than the containment limit. This performance assurance is intended to facilitate the assessment of operational risk and safety for applications where ATC intervention is not feasible or timely (e.g., instrument procedure). The RNP RNAV containment region (the area defined by 2 times the RNP value) could help with safety assessments for separation and obstacle clearance in the development of routes, areas, and procedures.

        
           FIGURE 2.  RNP    FIGURE 3.  RNP RNAV
Figure 2 depicts the lateral region defined by an RNP.  Figure 3 depicts the lateral containment region for an RNP-x RNAV.  
Containment in this case is not equal to required obstacle clearance surfaces or aircraft-to-aircraft separation.  It is a tool to support obstacle clearance and separation although the methods to use the tool have not been defined.

Containment integrity for RNP RNAV is the basis for user confidence in the correctness and reliability of the navigation capability including the monitoring and alerting for the RNP.        The 2 x RNP containment limit represents a 99.999% percent or better probability per flight hour level of assurance that an undetected fault or condition leading to misleading RNP status has not occurred (e.g., aircraft is outside of 2 x RNP when indications are that it is within 2 x RNP, and no alert is issued).

Containment continuity is the capability of the total system to satisfy the containment integrity requirement without unscheduled interruption during the intended operation.  The 2 x RNP containment continuity limit is a 99.99 percent or better probability that the navigation performance is satisfied without unscheduled interruptions during the intended operation.  Containment continuity helps prevent unnecessary interruptions and false warnings of a loss of navigation capability.

Normally, the loss of a single aircraft radio navigation/self-contained navigation in the national airspace system is classified as a minor hazard with a continuity requirement of 99.99 percent probability and misleading navigation is classified as a major hazard with a containment integrity of 99.999 percent probability.  This is what the RNP RNAV 2 x RNP continuity values is based on.  However, there are many operations where continuity and integrity requirements are more stringent that those specified for RNP RNAV.  In the conduct of RNAV operations, specifically terminal and approach operations predicated upon an RNP RNAV capability, a continuity probability of 99.99 percent may also be inadequate in an obstacle rich environment (ORE).   In such an ORE, this issue may be resolved by increased equipment redundancy/availability, operational performance requirements/limitations, or some other mitigation of risk.  If no solution exists, these operations will be prohibited. 

Based upon their mean time between outage, many single system/sensor installations may not qualify for RNP RNAV operations when there is not an underlying navigation infrastructure available or when an ORE requires increased RNP RNAV containment continuity.
 
4.0  Standard RNP RNAV Types

Standard RNP RNAV types are generally associated with a type of operation. RNP RNAV types for U.S. public procedures are shown in Table 1.  The operations identified in column 2 of Table 1 are not intended to limit other RNP RNAV types for that operation (e.g., RNP-1 RNAV may be used for en route operations). In the United States, the latest version of AC 25.1309-1, System Design Analysis; AC 23.1309-1, Equipment, Systems, and Installations in Part 23 Airplanes; AC 27-1, Certification of Normal Category Rotorcraft; and AC 29-2, Certification of Transport Category Rotorcraft, provide a description of hazard classifications.
 
 

TABLE 1.  U.S. STANDARD RNP RNAV TYPES

RNP RNAV
Type Applicability/ Typical
Operation
 Normal Performance
95% Accuracy Airborne Containment Region Hazard Class
(Misleading Information/Loss of Navigation)2,3
RNP – 2 RNAV En route 2 NM +/- 4 NM Major / Minor
RNP – 1 RNAV1 Terminal Area  1 NM +/- 2 NM Major / Minor
RNP - 0.3 RNAV Approach
 0.3 NM +/- 0.6 NM Major / Minor

 
Note 1:  RNP-1 RNAV applies when conducting a published Standard Instrument Departure Procedure (SID), Standard Terminal Arrival (STAR), Missed Approach Procedure, and for segments of an IAP prior to the final approach segment.

Note 2:  Determination of the hazard classification is dependent upon the obstacle clearance surface and assumes use of a Collision Risk Model to achieve the Target Level of Safety.  These failure classifications with their associated design assurance levels may not be adequate for an Obstacle Rich Environment (ORE).  An ORE would require other risk mitigation techniques, special equipment, and special authorization. “An environment is obstacle rich when is it not possible to construct an unguided, discontinued approach using procedural means.” Approach operations in an ORE require supplemental guidance to assure an appropriate routing for a climb to minimum vectoring altitude or minimum IFR altitude, whichever is lower.  Special procedures may require additional risk mitigation techniques that are not applicable to public procedures.  The missed approach/aircraft extraction must be evaluated for operational approval.

Note 3: Typically, loss of RNP has a minor hazard effect.  However, a loss of all navigation capability creates a major hazard effect.  Analysis of the supporting infrastructure and/or availability of specific navigation systems are critical to this assessment.

Given these RNP RNAV types, certain types of RNP RNAV equipment will utilize default RNP RNAV types dependent upon phase of flight.  For example, Wide Area Augmentation System (WAAS) stand-alone navigation units (TSO-C146) support operations for standard RNP RNAV types of 2, 1, and 0.3.  This allows WAAS to satisfy the RNP RNAV requirements without having to impose unique operational issues and/or procedures on the equipment and users of that equipment.

5.0  Application of RNP RNAV – Airspace Design

RNP RNAV is a navigation requirement and is only one factor to be used in the determination of required separation minima.  RNP RNAV alone cannot and should not imply or express any separation standard or minima.  When establishing air traffic route spacing and aircraft separation minima, in addition to the navigation element, one must also consider the supporting airspace infrastructure, including surveillance and communications.  Additionally, the parameters of Air Traffic Management, such as intervention capability, capacity, airspace structure, and occupancy or passing frequency (exposure) must be evaluated.  A general methodology for determining separation minima has been developed by the ICAO (see Document 9689).  The general methodology is to conduct an operational safety assessment that considers all CNS/ATM elements.

Near-term implementation of RNP RNAV includes RNAV instrument approach procedures for GPS or DME/DME RNP-0.3 RNAV certified aircraft. 

6.0  Conclusion

RNP RNAV satisfies the navigation element of the CNS/ATM equation.  Through the implementation of RNP RNAV, including the use of leg types, etc.; predictable and repeatable flight paths allow navigation to contribute to meeting the long term concept of “Free Flight” and in shaping air navigation as an integral part of the Global CNS/ATM Plan.

 

thunderland

Thanks for sharing!