Navigation Terminology (Advanced)

This page is the developer reference behind Navigation Terminology. It is aimed at contributors who add or change uORB fields, parameters, MAVLink handling, log analysis tools or documentation.

PX4 flies aircraft, drives rovers, and can drive boats, so its code and documentation use the vocabulary of aviation, marine navigation and ground vehicle dynamics. These traditions use the same words (heading, course, track, bearing, drift, sideslip), and they do not always mean the same thing.

This page collects the official definitions, the reference frames they are measured in, and the rule PX4 applies when they conflict. PX4 uses these definitions, with one deliberate exception: it uses course for the actual direction of motion, and course setpoint for the intended one (see Course Is the Actual Direction). Where PX4 represents or computes something differently from the official definitions, this is described separately in Where PX4 Differs.

Sources and Precedence

The definitions are taken from these authorities (see References):

• Aviation: ICAO (Annex 2, Doc 9713), FAA Pilot's Handbook of Aeronautical Knowledge (PHAK) and FAA Pilot/Controller Glossary (P/CG).
• Marine: NGA The American Practical Navigator (Bowditch, Pub. No. 9), IHO S-32 Hydrographic Dictionary, NMEA 0183.
• Ground vehicles: ISO 8855 (road vehicle dynamics vocabulary) and SAE J670 (vehicle dynamics terminology).
• Frames: WGS 84 (NGA, adopted by ICAO), ISO 1151 (flight dynamics), SNAME (marine vehicle motion) and ISO 8855 (road vehicles).

Where definitions of the same word conflict between these traditions, PX4 applies this rule:

1. Aviation definitions take precedence (ICAO first, then FAA). They are international standards, and most existing PX4 interfaces already follow them. The one exception is course (see Course Is the Actual Direction).
2. Marine terms are used for marine concepts that aviation does not define, such as speed through the water, leeway, and set and drift (see Terms Not Yet Used in PX4).
3. Ground vehicle terms are used for vehicle dynamics concepts that navigation does not define, such as tyre slip angle (see Terms Not Yet Used in PX4). For directions (heading, course, drift angle), the navigation terms apply to rovers too.
4. Where a word is ambiguous between traditions, use the official qualified term: for example desired track (FAA P/CG) instead of bare track for an intended path, and drift angle (FAA PHAK) instead of bare drift.

The conflicts and the resulting choices are listed in Terminology Conflicts.

Frames and Reference Directions

Every position and direction is relative to a reference frame. This section describes the frames PX4 uses, and the official convention each one is based on.

Global Frame (ECEF, WGS 84)

The global frame is the Earth-centred, Earth-fixed (ECEF) frame of the World Geodetic System 1984 (WGS 84), the reference system that ICAO requires for aeronautical data and that GNSS receivers use. Its origin is the Earth's centre of mass, and it rotates with the Earth. Global positions are given in it as geodetic latitude, longitude and altitude on the WGS 84 ellipsoid.

• Latitude and longitude are in degrees, positive north and east.
• Altitude has two different references, which must never be mixed up:
  • Height above the WGS 84 ellipsoid, as output by GNSS receivers.
  • Altitude above mean sea level (AMSL), which differs from the ellipsoid height by the local geoid height (up to about 100 m).

In PX4, vehicle_global_position carries lat, lon, alt (AMSL) and alt_ellipsoid. On MAVLink and in mission items, the reference of each altitude is given by its MAV_FRAME.

Local Tangent-Plane Frame (NED)

ISO 1151 defines the normal earth-fixed axis system: an earth-fixed frame whose z-axis points vertically down. Aviation and marine engineering conventionally align its horizontal axes with north and east, which gives the north-east-down (NED) frame:

• x points to true north, y points to east, z points down.
• It is a plane tangent to the Earth at a chosen origin.
• It is right-handed, so positive rotation about z is clockwise when viewed from above. This matches the clockwise sense of headings, courses and bearings.

In PX4, the positions of vehicle_local_position (fields x, y, z) are in this frame. Its origin is the position where the estimator was initialised, or where it was set manually (ref_lat, ref_lon, ref_alt). This is the same frame as MAVLink MAV_FRAME_LOCAL_NED.

See Local Frame Is a Map Projection for how PX4 relates the local tangent-plane frame to the global frame.

Local Navigation Frame (NED)

The local navigation frame has the same north, east and down axes, but moves with the vehicle:

• Its origin is the body frame origin.
• Its axes point to true north, east and down at the vehicle's current position.

In PX4, the velocities of vehicle_local_position (fields vx, vy, vz), the attitude and the heading are relative to this frame. Close to the local origin, its axes and those of the local tangent-plane frame are practically the same.

Body Frame (FRD)

ISO 1151 (aircraft) and SNAME (ships and submarines) define the body-fixed axes the same way:

• x points forward (nose, bow or front).
• y points to the right.
• z points down, completing a right-handed frame.

PX4 calls this frame FRD (forward, right, down), which is the same frame as MAVLink MAV_FRAME_BODY_FRD. Its origin is the vehicle's centre of mass. The vehicle's attitude is the rotation from the local navigation frame to FRD, and its heading is the direction of the body x-axis in the horizontal plane.

INFO

Some ground vehicle and robotics conventions use z-up frames instead:

• ISO 8855 defines the road vehicle axis system with x forward, y to the left and z up (FLU), and an earth-fixed frame with z up. Angles about z are then positive counterclockwise when viewed from above, the opposite of navigation practice. SAE J670 allows both the ISO z-up and the traditional SAE z-down axis systems, so check which one a source uses.
• ROS uses ENU for the local frame and FLU for the body frame. See ROS 2 & PX4 Frame Conventions.

Track Frame (Along-Track and Cross-Track)

To describe how well a vehicle follows a path, area navigation (RNAV, ICAO Doc 9613) splits the horizontal position error into two components relative to the desired track:

• Along-track: along the desired track, in the direction of travel.
• Cross-track: perpendicular to the desired track (see Cross-Track Error).

PX4 path-following controllers (NPFG, pure pursuit) work in this frame.

North References

ICAO defines directions as measured "from North (true, magnetic, compass or grid)", and Bowditch uses the same four references:

• True north: The direction of the geographic North Pole along the local meridian.
• Magnetic north: The direction of the horizontal component of the Earth's magnetic field. The angle between true and magnetic north is called magnetic variation in aviation (FAA PHAK) and variation in marine navigation (Bowditch). Geomagnetic models such as the World Magnetic Model call the same angle magnetic declination.
• Compass north: The direction an installed compass indicates. It differs from magnetic north by the deviation caused by the vehicle's own magnetic fields.
• Grid north: The direction of the north axis of a map projection grid. It differs from true north away from the projection's central meridian.

PX4 uses true north internally. The magnetometer measures compass north, so PX4 applies the magnetometer calibration, which removes the deviation. The estimator then applies the declination (from the World Magnetic Model, or from EKF2_MAG_DECL depending on EKF2_DECL_TYPE).

Angle Conventions

The official aviation and marine definitions express directions in degrees, measured clockwise from north from 000° to 360°. East is 090°, south is 180°, west is 270°. MAVLink follows this convention for navigation angles (for example GPS_RAW_INT.cog in centidegrees from 0 to 35999).

PX4 uses the same zero and the same sense of rotation, but a different unit and range internally: see Angles in Radians.

Definitions by Tradition

The Navigation Terminology page gives one definition per term. This section lists the definitions, where they differ, and the choice PX4 makes.

Heading

ICAO: "The direction in which the longitudinal axis of an aircraft is pointed, usually expressed in degrees from North (true, magnetic, compass or grid)." — ICAO Doc 9713, International Civil Aviation Vocabulary, and Annex 2, Rules of the Air

FAA: "Heading—direction in which the nose of the aircraft points during flight." — FAA PHAK, Chapter 16

Bowditch: "Heading (Hdg., SH) is the direction in which a vessel is pointed at any given moment, expressed as angular distance from 000° clockwise through 360°. It is easy to confuse heading and course." — Bowditch (2019), Art. 107

Aviation and marine definitions agree, and ground vehicle dynamics (ISO 8855, SAE J670) calls the same angle the yaw angle (see Ground Vehicles).

Course

FAA: "Course—intended path of an aircraft over the ground or the direction of a line drawn on a chart representing the intended aircraft path, expressed as the angle measured from a specific reference datum clockwise from 0° through 360° to the line." — FAA PHAK, Chapter 16

FAA: "COURSE - The intended direction of flight in the horizontal plane measured in degrees from north." — FAA P/CG

Bowditch: "Course (C, Cn) is the horizontal direction in which a vessel is intended to be steered, expressed as angular distance from north clockwise through 360°. Strictly used, the term applies to direction through the water, not the direction intended to be made good over the ground." — Bowditch (2019), Art. 107

In aviation and marine navigation, the course is an intended direction. In aviation it is intended over the ground, while in strict marine usage it is the direction steered through the water.

PX4 deviates from both: it calls the intended direction of motion over the ground the course setpoint, and uses course for the actual direction, the course over ground (see Course Is the Actual Direction).

Course setpoints in PX4: fixed_wing_lateral_setpoint.course and position_setpoint.course.

Course Over Ground

IHO: "course over ground: The direction of the path over the ground actually followed by a vessel. It is normally a somewhat irregular line." — IHO S-32

Course over ground (COG) is the actual direction of motion over the ground: the direction of the horizontal ground velocity vector. With $v_N$ and $v_E$ the north and east components of the ground velocity:

$$\chi =\mathrm{atan}2(v_E,v_N)$$

Ground vehicle dynamics (ISO 8855, SAE J670) calls the same angle the course angle (see Ground Vehicles). PX4 calls it the course, and uses course over ground as a synonym (see Course Is the Actual Direction).

It is the direction of the track at the current point. This is why avionics display it as TRK (track angle), and why NMEA 0183 VTG labels it "track made good". COG is undefined when the vehicle is stationary, and noisy at low speed, so code that uses it must check a minimum ground speed or the course accuracy.

In PX4: sensor_gnss.course, MAVLink GPS_RAW_INT.cog, NMEA RMC and VTG.

Track

ICAO: "TRACK [ICAO] - The projection on the earth's surface of the path of an aircraft, the direction of which path at any point is usually expressed in degrees from North (True, Magnetic, or Grid)." — FAA P/CG, quoting ICAO

FAA: "Track—actual path made over the ground in flight. (If proper correction has been made for the wind, track and course are identical.)" — FAA PHAK, Chapter 16

FAA: "DESIRED TRACK - The planned or intended track between two waypoints. It is measured in degrees from either magnetic or true north. The instantaneous angle may change from point to point along the great circle track between waypoints." — FAA P/CG

Bowditch: "Track (TR) is the intended horizontal direction of travel with respect to the Earth. The terms intended track and trackline are used to indicate the path of intended travel." — Bowditch (2019), Art. 107

In aviation the track is the actual path over the ground, and the desired track is the intended path between two waypoints. In marine navigation the track is the intended path.

Following the precedence rule, PX4 uses the aviation terms:

• Track: The actual path over the ground. Its direction at any point is the course.
• Desired track: The intended path over the ground, from which the cross-track error is measured. This is usually the line between two waypoints, but it can also be a line along a course setpoint the vehicle holds, which the FAA definition does not cover.

Marine readers understand bare "track" as the intended path, so always write desired track for the intended path.

Bearing

FAA: "BEARING - The horizontal direction to or from any point, usually measured clockwise from true north, magnetic north, or some other reference point through 360 degrees." — FAA P/CG

Bowditch: "Bearing (B, Brg.) is the direction of one terrestrial point from another, expressed as angular distance from 000° (North) clockwise through 360°." — Bowditch (2019), Art. 107

Aviation and marine definitions agree.

WARNING

A bearing is not a commanded direction. The direction a guidance law commands is a course (or heading) setpoint. See Field Names That Deviate.

Cross-Track Error

Cross-track error (XTE) is a standard output of RNAV systems and marine navigation receivers. Bowditch lists it alongside course over ground, speed over ground and bearing to the next waypoint (Art. 2501). NMEA 0183 reports it in the XTE, APB and RMB sentences, as a distance together with the direction to steer (L or R) to return to the desired track.

The cross-track error is the perpendicular distance of the vehicle from the desired track, together with the side of the track the vehicle is on. The official definitions do not define a sign: see Signed Cross-Track Error for how PX4 represents it.

Drift Angle

FAA: "Drift angle—angle between heading and track." — FAA PHAK, Chapter 16

The drift angle is caused by wind for aircraft, and by wind and current for vessels. For ground vehicles, it is caused by wheel slip, and ISO 8855 calls it the vehicle sideslip angle (see Ground Vehicles).

WARNING

Marine drift is the speed of a current, while the aviation drift angle is an angle between heading and track. Always write drift angle for the angle, and never bare drift.

INFO

For aircraft, the drift angle is not the sideslip angle ($\beta$, see Terminology/Notation). In flight dynamics (ISO 1151), sideslip is measured against the airflow, so a plane flying through a crosswind with its nose into the wind has a large drift angle and zero sideslip. Ground vehicle dynamics uses the same name for a ground-referenced angle: see Ground Vehicles.

In PX4, the aerodynamic sideslip is used by the EKF2 synthetic sideslip fusion (estimator_aid_src_sideslip, estimator_innovations.beta).

Ground Vehicles: Yaw, Course and Sideslip Angle

ISO 8855 and SAE J670 describe the horizontal motion of a ground vehicle with three angles, measured in the ground plane:

• Yaw angle ($\psi$): The angle from the earth-fixed x-axis to the vehicle x-axis. This is the heading.
• Course angle ($\nu$): The angle from the earth-fixed x-axis to the horizontal velocity of the vehicle over the ground. This is the actual direction of motion, which is the PX4 course, not the intended course of the FAA definition.
• Sideslip angle ($\beta =\nu -\psi$): The angle from the vehicle x-axis to the horizontal velocity over the ground. This is the ground vehicle counterpart of the drift angle, and not the air-relative sideslip angle of flight dynamics.

ISO 8855 measures these angles counterclockwise about a z-up axis, so its sideslip angle has the opposite sign of the PX4 drift angle.

Following the precedence rule, use these terms for rovers:

• Heading for the yaw angle, course for the course angle and drift angle for the vehicle sideslip angle.
• When code or documentation follows vehicle dynamics literature, the ISO 8855 terms may be used, but write vehicle sideslip angle and cite ISO 8855, so that it is not confused with aerodynamic sideslip.

For a rover that drives without wheel slip (Ackermann or differential drive), the course equals the heading when driving forwards, and the heading plus 180° when reversing. A drift angle appears when the wheels slip, for example on loose ground or in fast turns, and by design on mecanum rovers.

Airspeed, Ground Speed and Speed Over Ground

FAA: "Airspeed—rate of the aircraft's progress through the air." "GS—rate of the aircraft's inflight progress over the ground." — FAA PHAK, Chapter 16

Bowditch: "Speed over the ground (SOG) is the actual speed of the vessel over the surface of the Earth at any given time." — Bowditch (2019), Art. 106

Ground speed (aviation, FAA PHAK) and speed over ground (marine) are the same quantity: the magnitude of the horizontal ground velocity, whose direction is the course over ground. For vessels, the water-relative counterpart of airspeed is the speed through the water, which a log (paddle wheel) measures and a GNSS receiver does not.

Wind and Current Triangle

The wind triangle has a marine counterpart for vessels:

$${\mathbf{v}}_{ground}={\mathbf{v}}_{water}+{\mathbf{v}}_{current}$$

The current is described by its set and drift, and ${\mathbf{v}}_{water}$ is offset from the heading by leeway.

Terminology Conflicts

These are the terms where the traditions disagree, and the official term PX4 uses as a result of the precedence rule. Heading (yaw angle in ISO 8855), bearing, course over ground and cross-track error mean the same in all of them.

Term	Aviation (ICAO/FAA, ISO 1151)	Marine (Bowditch)	Ground vehicles (ISO 8855/SAE J670)	Term to use in PX4
Track	Actual path over the ground; the intended path is desired track	Intended path over the ground	—	Track (actual) and desired track (intended)
Course	Intended direction over the ground	Direction steered through the water (strict sense)	Course angle: actual direction of motion over the ground	Course (actual, also course over ground) and course setpoint (intended); differs from aviation, see Course Is the Actual Direction
Drift	Drift angle: angle between heading and track	Speed of the current, with set as its direction	—	Drift angle (angle); never bare drift
Sideslip	Angle between the body x-axis and the airflow	—	Angle between the vehicle x-axis and the velocity over the ground	Sideslip angle (air-relative) and drift angle or vehicle sideslip angle (ground-relative)
True to magnetic north	Magnetic variation	Variation	—	Declination (World Magnetic Model term, used by EKF2)

Where PX4 Differs

PX4 follows the official definitions above, except for the meaning of course. The sections below describe where PX4 represents, computes or names a quantity differently from the official definitions, so that you can convert correctly.

Course Is the Actual Direction

The official definitions use bare course for an intended direction (FAA, Bowditch), and course over ground (IHO) or track angle (avionics) for the actual direction. PX4 deliberately uses:

• Course: The actual (measured or estimated) direction of motion over the ground. Course over ground (COG) is a synonym.
• Course setpoint: The intended direction of motion over the ground, which aviation calls the course.

The reasons are:

• PX4 names a quantity by its actual value and adds setpoint for the commanded value, as for heading and heading setpoint, or velocity and velocity setpoint. With the aviation meaning, the course setpoint would be the setpoint of an intended value, and the actual value would need a different word.
• It matches the course angle of ground vehicle dynamics (ISO 8855) and what GNSS receivers report as course (sensor_gnss.course, NMEA RMC and VTG).
• Existing fields already fit: sensor_gnss.course is measured, and the course fields of setpoint topics (fixed_wing_lateral_setpoint.course, position_setpoint.course) are setpoints.

When you read aviation or marine sources, translate their course to the PX4 course setpoint. For example, the FAA wind correction angle is applied to the course setpoint in PX4 terms.

Angles in Radians

The official definitions express directions in degrees from 000° to 360°. In uORB, PX4 expresses them in radians, usually wrapped to $[-\pi ,\pi ]$, with the same zero (true north) and the same sense (clockwise). East is $+\pi /2$ and west is $-\pi /2$. Some fields use another range, for example transponder_report.heading ($[0,2\pi ]$) and unwrapped yaw setpoints, so check the field's [@range].

Convert with matrix::wrap_2pi() and math::degrees() when you output to MAVLink or to users, as the MAVLink streams do.

Local Frame Is a Map Projection

The estimator keeps the vehicle's global position (latitude and longitude), and derives the local position from it with an azimuthal equidistant projection centred on the local origin (MapProjection in src/lib/geo). As a result:

• The x-axis of vehicle_local_position points to true north at the origin. At the vehicle's position it points to the grid north of the projection, which differs from true north by the meridian convergence. The difference grows with distance from the origin, mainly east or west of it. It is about 0.1° at 10 km east of an origin at 47° latitude.
• Heading and velocity are relative to true north at the vehicle (local navigation frame), not to the projection's grid north.
• A straight line between two waypoints in the local frame is a straight line in the projection. The FAA desired track follows the great circle between the waypoints. The two are identical for lines through the origin, and practically identical over typical mission distances.

For vehicles that operate tens of kilometres from the origin, take this into account when you compare local-frame directions with true directions.

Signed Cross-Track Error

The official definitions express cross-track error as a distance and a side (NMEA: the direction to steer). PX4 represents it as a single signed distance:

• It is the shortest horizontal distance from the vehicle to the desired track.
• It is positive when the vehicle is to the right of the desired track and negative when it is to the left, looking in the direction of travel.

With $\mathbf{t}$ the unit vector along the desired track and $\mathbf{d}$ the vector from a point on the track to the vehicle (north and east components):

$$e_{xt}=t_N{d}_E-t_E{d}_N$$

This is the convention used by NPFG (fixed_wing_lateral_guidance_status.signed_track_error) and by pure pursuit (pure_pursuit_status.crosstrack_error). A vehicle with a positive cross-track error gets NMEA "steer L".

Drift Angle Sign

The FAA defines the drift angle as the angle between heading and track, without a sign. Where PX4 needs a signed value, it is wrap_pi(course - heading): positive when the vehicle moves to the right of where its nose points.

Wind as a Velocity Vector

The official definitions describe wind by the direction it blows from. PX4 wind estimates (wind, airspeed_wind) are velocity vectors in NED (windspeed_north, windspeed_east), which point in the direction the air moves towards. A wind reported as "from 360°" has windspeed_north < 0 in PX4.

Field Names That Deviate

Some existing names predate these pages or come from external protocols, and do not match the official terms. Read them with the meaning given here, and use the official term in new code:

Field	Actual meaning	Official term
position_setpoint.course (comment: "desired course (bearing)")	Intended direction of motion over the ground.	Course setpoint
position_controller_status.nav_bearing (MAVLink NAV_CONTROLLER_OUTPUT.nav_bearing)	Intended for the direction commanded by the guidance law. Not populated by current controllers (always 0).	Course or heading setpoint
position_controller_status.xtrack_error (MAVLink NAV_CONTROLLER_OUTPUT.xtrack_error)	Intended for the signed cross-track error. Not populated by current controllers (always 0).	Cross-track error
fixed_wing_lateral_guidance_status.bearing_feas	Feasibility of the course commanded by NPFG (NPFG calls it "bearing").	Course setpoint
transponder_report.heading (MAVLink ADSB_VEHICLE.heading)	Course over ground of the other aircraft, as reported by ADS-B.	Course
bearing_setpoint in the Ackermann and mecanum rover position controllers (AckermannPosControl, MecanumPosControl)	Course setpoint, taken from the bearing to the pure pursuit lookahead point (PurePursuit::calcTargetBearing(), which is a bearing). Ackermann rovers use it directly as the heading (yaw) setpoint, assuming no slip.	Course setpoint (mecanum), heading setpoint (Ackermann)
"Direction (bearing)" of the velocity setpoint in Rover Velocity Tuning and "bearing setpoints" in Rover Position Tuning	Intended direction of motion over the ground.	Course setpoint

Guidelines for Contributors

When you add or change uORB fields, parameters, MAVLink handling, log analysis tools or documentation:

• Use the official terms on these pages. Do not introduce new navigation terms. If a concept is missing, look up its official name in the references and add it here.
• Use heading only for the direction the vehicle points, course for the actual direction of motion over the ground, course setpoint for the intended direction over the ground, and bearing only for the direction from one point to another. Do not use bare course for an intended direction, even where an aviation or marine source does (see Course Is the Actual Direction).
• Write desired track for an intended path, and never use bare drift.
• Use sideslip angle only for the angle against the airflow. For the ground-relative angle, use drift angle, or vehicle sideslip angle with a reference to ISO 8855.
• Use the PX4 signed cross-track error convention.
• State the reference (true north), unit and range in the field comment, and add the [@frame] and [@range] tags (see uORB Documentation Standard).
• When you convert to or from an external protocol, check that protocol's definition of each field: names alone are not reliable.

Terms Not Yet Used in PX4

These terms have official definitions, but PX4 does not use them yet. They are kept here for reference, so that new code uses the official term when it needs one of these concepts.

Term	Official meaning	Source
Track made good	Net direction from the point of departure to the point of arrival.	Bowditch
Relative bearing	Bearing measured from the vehicle's heading.	Bowditch
Wind correction angle	Correction applied to the course setpoint to obtain a heading that keeps the track on it.	FAA
Tyre slip angle	Angle between the wheel plane and the direction of travel of the tyre contact patch.	ISO 8855, SAE J670
Leeway	Leeward motion of a vessel due to the wind.	Bowditch
Set and drift	Direction (towards) and speed of a current.	Bowditch

Expand the block below for the official definition of each term.

Official definitions

Track Made Good

Bowditch: "Track made good (TMG) is the single resultant direction from the point of departure to point of arrival at any given time. The use of this term is preferred to the use of the misnomer 'course made good.'" — Bowditch (2019), Art. 107

Track made good (also called course made good, CMG) is the net direction between two positions, not an instantaneous value. The matching speed is the speed made good (SMG): the distance between the two positions divided by the elapsed time. PX4 controllers do not use it, but it is useful in log analysis.

Relative Bearing

Bowditch: "A relative bearing is measured relative to the ship's heading from 000° (dead ahead) clockwise through 360°." — Bowditch (2019), Art. 107

• Relative bearing: The bearing measured from the vehicle's heading. Bowditch: "True Bearing = Relative Bearing + True Heading."
• Reciprocal bearing: The bearing in the opposite direction, which differs by 180°.

Wind Correction Angle

FAA: "WCA—correction applied to the course to establish a heading so that track coincides with course." — FAA PHAK, Chapter 16

The wind correction angle (WCA, also called crab angle) is the angle by which the heading is offset from the course setpoint into the wind, so that the track follows the course setpoint. The FAA definition calls this intended direction the course (see Course Is the Actual Direction). In the wind triangle, it is the angle between the heading and the course setpoint.

Tyre Slip Angle

ISO 8855 and SAE J670 define the slip angle of each tyre: the angle between the wheel plane and the direction of travel of the wheel's contact patch. It describes how a tyre generates lateral force, and is a vehicle dynamics term with no navigation counterpart. Write tyre slip angle for it, to distinguish it from the vehicle sideslip angle.

Leeway

Bowditch: "Leeway is the leeward motion of a vessel due to that component of the wind vector perpendicular to the vessel's track." — Bowditch (2019), Art. 1006

Leeway is the sideways push of the wind on a vessel's hull and superstructure, relative to the water. It is the marine counterpart of an aircraft being displaced by the wind, and contributes to the drift angle. In the current triangle, ${\mathbf{v}}_{water}$ is offset from the heading by leeway.

Set and Drift

Bowditch: "Set refers to the current's direction, and drift refers to the current's speed." — Bowditch (2019), Art. 1006

The set is the direction towards which the current flows, and the drift is its speed. When a dead reckoning (DR) position is compared with a fix, the same terms describe the total unexplained offset: "The set is the direction from the DR to the fix" (Bowditch, Art. 1008).

This is the opposite of wind direction, which is the direction the wind blows from. A current with set 000° and a wind from 180° push a vehicle the same way. PX4 wind estimates are velocity vectors, so they point the same way as current set.

WARNING

Marine drift is the speed of a current, while the aviation drift angle is an angle between heading and track. Write drift angle for the angle, and set and drift (of the current) for the current.

References

Aviation:

• ICAO Doc 9713, International Civil Aviation Vocabulary, and ICAO Annex 2, Rules of the Air (heading, track).
• ICAO Doc 9613, Performance-based Navigation (PBN) Manual (along-track and cross-track components).
• ICAO Doc 9674, World Geodetic System — 1984 (WGS-84) Manual.
• FAA, Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), Chapter 16: Navigation.
• FAA, Pilot/Controller Glossary (BEARING, COURSE, DESIRED TRACK, TRACK, TRACK [ICAO]).
• SKYbrary, Heading, Track and Radial (summary of the ICAO definitions).

Marine:

• National Geospatial-Intelligence Agency, The American Practical Navigator (Bowditch), Pub. No. 9, 2019 edition, Volume I: Art. 106 and 107 (speed and direction), Art. 1006 and 1008 (leeway, set and drift), Art. 2501 (receiver outputs, including cross-track error).
• International Hydrographic Organization, S-32 Hydrographic Dictionary (course over ground, course made good).
• National Marine Electronics Association, NMEA 0183 (RMC, VTG, HDT, XTE, APB, RMB).
• SNAME, Nomenclature for Treating the Motion of a Submerged Body Through a Fluid (body axes for marine vehicles).

Ground vehicles:

• ISO 8855:2011, Road vehicles — Vehicle dynamics and road-holding ability — Vocabulary (axis systems, yaw, course and sideslip angle, tyre slip angle).
• SAE J670 (2022), Vehicle Dynamics Terminology.

Frames:

• ISO 1151, Flight dynamics — Concepts, quantities and symbols (earth-fixed and body axis systems, aerodynamic sideslip).
• National Geospatial-Intelligence Agency, World Geodetic System 1984 (NGA.STND.0036).

PX4:

• Navigation Terminology: the commonly used terms.
• Terminology/Notation: symbols, frames and acronyms.
• uORB Documentation Standard: how to document frames, units and ranges of message fields.