Drafted — not technically validated
Record revision 1 · Radials, bearings, variation and deviation, station passage, waypoints, RAIM and the military grid.
Definitions are quoted or closely paraphrased from the source named with each word. Meanings change between editions and theatres; the brief and the current publications take precedence.
The words of navigation
These are the words of finding the way: bearings and radials, the compass’s errors, flying to and from a station, waypoints and satellite and inertial systems, and the military grid. The Navy’s air traffic controller manual and its T-45 instrument flight instruction come first, with the Army’s map-reading manual for the grid that every service uses. [1] [2] [3]
The FAA’s manual and handbook are cited for area navigation, GPS integrity and the difference between heading, course and track, which military and civil pilots share. [4] [5] TACAN and NOTAM are in the airfield and ATC terms.
15 terms
- Deviation
The compass's own error from the aircraft's magnetic fields. Metal, electrical current and equipment turn with the aircraft, so deviation changes with heading; a deviation card near the compass gives the correction.
Source: [1] NAVEDTRA 14342A, pp. 2-6–2-7
- Direct to fix
An area navigation leg type (DF) that runs from wherever the aircraft is straight to a waypoint.
Source: [4] Aeronautical Information Manual, ¶1-2-1
- Drift
The decay of an inertial navigation system's accuracy with time after it is aligned with a known position.
Drift is also the angle between heading and track; see drift angle. [5]
Source: [4] Aeronautical Information Manual, ¶1-1-15
- Drift angle
The angle between the aircraft's heading, where the nose points, and its track over the ground.
Source: [5] FAA-H-8083-25C, pp. 16-9–16-10
- Fly-by waypoint
A waypoint at which the aircraft begins the turn to the next course before reaching the point (turn anticipation). A fly-over waypoint must be crossed before the turn begins.
Source: [4] Aeronautical Information Manual, ¶1-2-1
- Grid zone designation
The zone number and latitude band letter that begin a Military Grid Reference System (MGRS) reference, such as “16S”. Between 80°S and 84°N the earth is divided into 60 zones 6° of longitude wide, numbered west to east from the 180° meridian, and 20 bands lettered C to X from south to north.
Source: [3] FM 3-25.26, pp. 4-12–4-13
- Homing
Flying to a station by keeping the bearing pointer at the top of the instrument without correcting for wind. In a crosswind it flies a curved path.
Source: [2] CNATRA P-1204, p. 64
- Lead point
Where a turn starts so that the aircraft rolls out on the course or arc. For a 90° intercept the lead is 1% of ground speed, and a proportion of that for smaller angles.
Source: [2] CNATRA P-1204, pp. 65–66
- Radial
A magnetic bearing from a VOR or TACAN station. Whatever its heading, an aircraft on the 090 radial is due east of the station, and flying to the station on that radial its magnetic course is 270.
On the horizontal situation indicator, the tail of the bearing pointer sits on the radial the aircraft is on. [2]
Source: [1] NAVEDTRA 14342A, p. 2-28; [2] CNATRA P-1204, pp. 12–13
- RAIM
Receiver autonomous integrity monitoring. A GPS receiver's check of its own solution, using a satellite beyond those needed for a position. Without it the pilot has no assurance that the position is right.
Source: [4] Aeronautical Information Manual, ¶1-1-17
- Relative bearing
The angle measured clockwise from the aircraft's nose to an object, so an object directly behind is at 180 on any heading. Heading plus relative bearing gives the true bearing.
Source: [1] NAVEDTRA 14342A, pp. 2-11–2-13
- Slant range
The straight-line distance from the aircraft to a station, including height, which is what DME measures, rather than horizontal distance.
Source: [4] Aeronautical Information Manual, ¶1-1-7
- Station passage
Crossing over the station. With VOR, it is when the bearing pointer falls past the 90° benchmark; TACAN has a larger cone of confusion over the station, so TACAN station passage is when DME stops decreasing.
Source: [2] CNATRA P-1204, p. 65
- Track
The actual path of the aircraft over the ground. Course is the intended path; heading is where the nose points.
Source: [5] FAA-H-8083-25C, pp. 16-9–16-10
- Variation
The angle between true north and magnetic north. It is east when magnetic north lies east of true north and west when it lies west, and it differs from place to place.
Source: [1] NAVEDTRA 14342A, p. 2-6
Sources
- NAVEDTRA 14342A, Air Traffic Controller, chapter 2: Air Navigation and Aids to Air Navigation. US Navy, Naval Education and Training Professional Development and Technology Center (copy hosted by Integrated Publishing). https://navyaviation.tpub.com/14342A/Tactical-Air-Navigation-Tacan-System-105.html (accessed 2026-10-25). Printed pages 2-6 to 2-7 (variation and deviation), 2-11 to 2-13 (bearings) and 2-28 (VOR and TACAN).
- CNATRA P-1204, Flight Training Instruction, Instrument Flight, T-45C (TS, ADV and IUT), (4-01) Original. Naval Air Training Command (copy hosted by Integrated Publishing). https://navyflightmanuals.tpub.com/P-1204/ (accessed 2026-10-25). Pages 12 to 13 (the HSI) and 64 to 66 (proceeding direct, intercepts and station passage). An older revision of the T-45C instrument instruction.
- FM 3-25.26, Map Reading and Land Navigation. Headquarters, Department of the Army, 20 July 2001. https://archive.org/download/MManuals/Fm3-25.26MapReadingAndLandNavigation.pdf (accessed 2026-10-25). Pages 4-12 to 4-13, the Military Grid Reference System, which all US services share.
- Aeronautical Information Manual, chapter 1: Air Navigation. Federal Aviation Administration. https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap1_section_1.html (accessed 2026-10-25). Section 1, Air Navigation Aids (¶1-1-7 DME, ¶1-1-15 inertial reference units, ¶1-1-17 GPS) and section 2, Performance-Based Navigation and Area Navigation (¶1-2-1). Cited by paragraph.
- FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge, chapter 16: Navigation. Federal Aviation Administration. https://www.faa.gov/sites/faa.gov/files/18_phak_ch16.pdf (accessed 2026-10-25).