Federal Aviation Administration Twentieth Edition
1.4.2.1 Track to Fix. A Track to Fix (TF) leg is 1.4.2.2 Direct to Fix. A Direct to Fix (DF) leg is a intercepted and acquired as the flight track to the path described by an aircraft’s track from an initial following waypoint. Track to a Fix legs are area direct to the next waypoint. Narrative: “left sometimes called point.to.point legs for this reason. turn direct BARGN WP.”  See FIG ENR 1.19.3. Narrative: “on track 087 to CHEZZ WP.” NOTE. See FIG ENR 1.19.2. FIG ENR 1.19.2, FIG ENR 1.19.3 and FIG ENR 1.19.4
illustrate TF, DF, CF and RF leg types.

FIG ENR 1.19.2
Track to Fix Leg Type

FIG ENR 1.19.3
Direct to Fix Leg Type

1.4.2.3 Course to Fix. A Course to Fix (CF) leg is a path that terminates at a fix with a specified course at that fix. Narrative: “on course 078 to PRIMY WP.” See FIG ENR 1.19.4.
FIG ENR 1.19.4
Course to Fix Leg Type

1.4.2.5 Heading. A Heading leg may be defined as, but not limited to, a Heading to Altitude (VA), Heading to DME range (VD), and Heading to Manual Termination, i.e., Vector (VM). Narrative: “climb heading 350 to 1500”, “heading 265, at 9 DME west of PXR VORTAC, right turn heading 360”, “fly heading 090, expect radar vectors to DRYHT INT.”
1.4.3 Navigation Issues. Pilots should be aware of their navigation system inputs, alerts, and annunci-ations in order to make better.informed decisions. In addition, the availability and suitability of particular sensors/systems should be considered.
1.4.3.1 GPS. Operators using TSO.C129 systems should ensure departure and arrival airports are entered to ensure proper RAIM availability and CDI sensitivity.
1.4.3.2 DME/DME. Operators should be aware that DME/DME position updating is dependent on FMS logic and DME facility proximity, availability, geometry, and signal masking.
1.4.3.3 VOR/DME. Unique VOR characteristics may result in less accurate values from VOR/DME position updating than from GPS or DME/DME position updating.
1.4.3.4 Inertial Navigation. Inertial reference units and inertial navigation systems are often coupled with other types of navigation inputs, e.g., DME/DME or GPS, to improve overall navigation system performance.
NOTE.
Specific inertial position updating requirements may apply.
1.4.4 Flight Management System (FMS). An FMS is an integrated suite of sensors, receivers, and computers, coupled with a navigation database. These systems generally provide performance and RNAV guidance to displays and automatic flight control systems.
1.4.4.1 Inputs can be accepted from multiple sources such as GPS, DME, VOR, LOC and IRU. These inputs may be applied to a navigation solution one at a time or in combination. Some FMSs provide for the detection and isolation of faulty navigation information.

Federal Aviation Administration Twentieth Edition
1.4.4.2 When appropriate navigation signals are available, FMSs will normally rely on GPS and/or DME/DME (that is, the use of distance information from two or more DME stations) for position updates. Other inputs may also be incorporated based on FMS system architecture and navigation source geometry.
NOTE.
DME/DME inputs coupled with one or more IRU(s) are often abbreviated as DME/DME/IRU or D/D/I.
2. Required Navigation Performance (RNP)
2.1 General. RNP is RNAV with on.board naviga-tion monitoring and alerting, RNP is also a statement of navigation performance necessary for operation within a defined airspace. A critical component of RNP is the ability of the aircraft navigation system to monitor its achieved navigation performance, and to identify for the pilot whether the operational requirement is, or is not being met during an operation. This on.board performance monitoring and alerting capability therefore allows a lessened reliance on air traffic control intervention (via radar monitoring, automatic dependent surveillance (ADS), multilateration, communications), and/or route separation to achieve the overall safety of the operation. RNP capability of the aircraft is a major component in determining the separation criteria to ensure that the overall containment of the operation is met.
The RNP capability of an aircraft will vary depending upon the aircraft equipment and the navigation infrastructure. For example, an aircraft may be equipped and certified for RNP 1.0, but may not be capable of RNP 1.0 operations due to limited navaid coverage.
2.2 RNP Operations
2.2.1 RNP Levels. An RNP “level” or “type” is applicable to a selected airspace, route, or procedure. As defined in the Pilot/Controller Glossary, the RNP Level or Type is a value typically expressed as a distance in nautical miles from the intended centerline of a procedure, route, or path. RNP applications also account for potential errors at some multiple of RNP level (e.g., twice the RNP level).
　
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