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version: "1.0.0" name: matlab-convert-aerospace-coordinates description: > Perform aerospace unit conversions, time conversions, coordinate frame transformations, and rotation representations using Aerospace Toolbox. Use when converting units (length, velocity, angle, acceleration, angular velocity, force, mass, pressure, temperature, density), computing Julian dates or decimal years, transforming between coordinate frames (ECEF, ECI, LLA, flat Earth, geodetic/geocentric, NED, body, wind, stability), or working with rotation representations (Euler angles, DCM, quaternion, Rodrigues vector). Also use when the user asks about aerospace coordinate systems, reference frames, or rotation conventions. license: MathWorks BSD-3-Clause metadata: author: MathWorks version: "1.1"

Aerospace Fundamentals

Core Aerospace Toolbox functions for unit conversions, time conversions, coordinate transformations, and rotation representations.

When to Use

Converting between aerospace unit systems (SI, imperial, nautical)
Computing Julian dates, modified Julian dates, decimal years, or TDB time
Transforming positions between ECEF, ECI, LLA, or flat Earth frames
Converting between geodetic and geocentric latitude
Building DCMs for frame transformations (ECI↔ECEF, ECEF↔NED, body↔wind, body↔stability)
Converting between Euler angles, DCMs, quaternions, and Rodrigues vectors
Using the quaternion object for rotation math, interpolation, or composition
Identifying which coordinate frame data is expressed in

When NOT to Use

Atmosphere models (atmosisa, atmoscoesa) — use matlab-compute-aerospace-environment
Airspeed corrections (correctairspeed) — not covered here
Orbit propagation or satellite maneuvers — use Aerospace Toolbox, Aerospace Blockset or Satellite Communications Toolbox
Navigation-specific transforms (lla2enu, lla2ned) — use Navigation Toolbox
Simulink blocks — use /model-based-design-core:building-simulink-models with Aerospace Blockset

Workflow

Identify the coordinate frame — Determine what frame your data is in and what frame you need. See references/coordinate-systems.md for frame definitions and decision guide.
Convert units first — Ensure inputs match the function's expected units before calling transforms.
Apply the transformation — Use the appropriate function with correct argument ordering.
Verify — Round-trip the result back to the original frame; error should be < 1e-10.

Key Functions

Unit Conversions

Function	Converts	Units
`convlength`	Length	`'ft'`, `'m'`, `'km'`, `'in'`, `'mi'`, `'naut mi'`
`convvel`	Velocity	`'ft/s'`, `'m/s'`, `'km/s'`, `'in/s'`, `'km/h'`, `'mph'`, `'kts'`, `'ft/min'`
`convang`	Angle	`'deg'`, `'rad'`, `'rev'`
`convacc`	Acceleration	`'ft/s^2'`, `'m/s^2'`, `'km/s^2'`, `'in/s^2'`, `'km/h-s'`, `'mph/s'`, `'G''s'`
`convangacc`	Angular acceleration	`'deg/s^2'`, `'rad/s^2'`, `'rpm/s'`
`convangvel`	Angular velocity	`'deg/s'`, `'rad/s'`, `'rpm'`
`convforce`	Force	`'N'`, `'lbf'`
`convmass`	Mass	`'kg'`, `'lbm'`, `'slug'`
`convpres`	Pressure	`'Pa'`, `'psi'`, `'psf'`, `'atm'`
`convtemp`	Temperature	`'K'`, `'R'`, `'F'`, `'C'`
`convdensity`	Density	`'kg/m^3'`, `'slug/ft^3'`, `'lbm/ft^3'`, `'lbm/in^3'`

All conversion functions use the same signature: output = convXXX(value, fromUnit, toUnit)

Time Conversions

Function	Purpose	Since
`juliandate`	Calendar → Julian Date	R2006b
`mjuliandate`	Calendar → Modified Julian Date (JD − 2400000.5)	R2006b
`decyear`	Calendar → decimal year	R2006b
`leapyear`	Test if year is leap year	R2006b
`tdbjuliandate`	Terrestrial Time → TDB Julian Date	R2015a

Coordinate Transformations

Function	From	To	Since
`lla2ecef`	LLA (geodetic)	ECEF	R2006b
`ecef2lla`	ECEF	LLA (geodetic)	R2006b
`lla2eci`	LLA	ECI	R2014a
`eci2lla`	ECI	LLA	R2014a
`ecef2eci`	ECEF (pos/vel/acc)	ECI	R2019a
`eci2ecef`	ECI (pos/vel/acc)	ECEF	R2019a
`eci2aer`	ECI	AER (azimuth, elevation, range)	R2015a
`lla2flat`	LLA	Flat Earth	R2011a
`flat2lla`	Flat Earth	LLA	R2011a
`geod2geoc`	Geodetic latitude	Geocentric latitude	R2006b
`geoc2geod`	Geocentric latitude	Geodetic latitude	R2006b
`dcmeci2ecef`	—	ECI-to-ECEF DCM	R2013b
`dcmecef2ned`	—	ECEF-to-NED DCM	R2006b
`dcm2latlon`	ECEF-to-NED DCM	Lat/Lon	R2006b
`dcmbody2wind`	Alpha, Beta	Body-to-Wind DCM	R2006b
`dcm2alphabeta`	Body-to-Wind DCM	Alpha, Beta	R2006b
`dcmbody2stability`	Alpha	Body-to-Stability DCM	R2022a

Rotation Representations

Function	From	To	Since
`angle2dcm`	Euler angles	DCM	R2006b
`dcm2angle`	DCM	Euler angles	R2006b
`angle2quat`	Euler angles	Quaternion (1×4)	R2006b
`quat2angle`	Quaternion (1×4)	Euler angles	R2007b
`dcm2quat`	DCM	Quaternion (1×4)	R2006b
`quat2dcm`	Quaternion (1×4)	DCM	R2006b
`angle2rod`	Euler angles	Rodrigues vector	R2017a
`rod2angle`	Rodrigues vector	Euler angles	R2017a
`dcm2rod`	DCM	Rodrigues vector	R2017a
`rod2dcm`	Rodrigues vector	DCM	R2017a
`quat2rod`	Quaternion (1×4)	Rodrigues vector	R2017a
`rod2quat`	Rodrigues vector	Quaternion (1×4)	R2017a

Quaternion Object

Method	Purpose
`quaternion(E,'eulerd',RS,PF)`	Create from Euler angles (degrees)
`quaternion(E,'euler',RS,PF)`	Create from Euler angles (radians)
`quaternion(RM,'rotmat',PF)`	Create from rotation matrix
`quaternion(RV,'rotvec')`	Create from rotation vector (radians)
`compact(q)`	Extract [w x y z] array
`eulerd(q,RS,PF)`	Convert to Euler angles (degrees)
`euler(q,RS,PF)`	Convert to Euler angles (radians)
`rotmat(q,PF)`	Convert to rotation matrix
`rotvec(q)` / `rotvecd(q)`	Convert to rotation vector (rad/deg)
`rotatepoint(q,pts)`	Rotate points (active rotation)
`rotateframe(q,pts)`	Rotate frame (passive rotation)
`normalize(q)`	Normalize to unit quaternion
`slerp(q1,q2,t)`	Spherical linear interpolation
`meanrot(q)`	Mean rotation of array
`dist(q1,q2)`	Angular distance (radians)
`angvel(q,dt,PF)`	Angular velocity from quaternion array
`randrot(n)`	Uniform random rotations

Quaternion Math (Array-Based)

Function	Purpose	Since
`quatmultiply(q,r)`	Quaternion product (compose rotations)	R2006b
`quatconj(q)`	Conjugate (negate vector part)	R2006b
`quatinv(q)`	Inverse (conjugate / norm²)	R2006b
`quatnormalize(q)`	Normalize to unit quaternion	R2006b
`quatnorm(q)`	Squared norm (q·q)	R2006b
`quatmod(q)`	Modulus (sqrt of norm)	R2006b
`quatrotate(q,v)`	Rotate vector by quaternion	R2006b
`quatdivide(q,r)`	Divide quaternion by quaternion	R2006b
`quatinterp(p,q,f,method)`	Interpolate (`'slerp'`, `'lerp'`, `'nlerp'`)	R2016a
`quatexp(q)`	Exponential of quaternion	R2016a
`quatlog(q)`	Natural logarithm of quaternion	R2016a
`quatpower(q,pow)`	Quaternion raised to a power	R2016a

Patterns

Unit Conversion

matlab

% Always: convXXX(value, 'from', 'to')
alt_m = convlength(35000, 'ft', 'm');        % 10668.0 m
speed_ms = convvel(250, 'kts', 'm/s');       % 128.61 m/s
angle_rad = convang(45, 'deg', 'rad');       % 0.7854 rad
accel_g = convacc(9.81, 'm/s^2', 'G''s');   % 1.0 G
omega_rpm = convangvel(360, 'deg/s', 'rpm'); % 60 rpm

Time Conversion

matlab

% Julian Date from components (year, month, day, hour, min, sec)
jd = juliandate(2024, 6, 15, 12, 0, 0);     % 2460477.0
 
% Modified Julian Date
mjd = mjuliandate(2024, 6, 15, 12, 0, 0);   % 60476.5
 
% From datetime objects
dt = datetime(2024, 6, 15, 12, 0, 0);
jd = juliandate(dt);
 
% Decimal year
dy = decyear(2024, 6, 15);                   % 2024.4536
 
% TDB Julian Date from Terrestrial Time [yr mo day hr min sec]
tt = [2024 6 15 12 0 0];
jdTDB = tdbjuliandate(tt);

LLA ↔ ECEF

matlab

% LLA is [latitude_deg, longitude_deg, altitude_m]
lla = [40, -74, 0];
ecef = lla2ecef(lla);           % [1348613.0, -4703172.4, 4077985.6] m
 
% Round-trip verification
lla_check = ecef2lla(ecef);     % [40.0, -74.0, 0.0]

LLA ↔ ECI (time-dependent)

matlab

% ECI transforms require UTC time
lla = [40, -74, 1000];
utc = [2024 6 15 12 0 0];
posECI = lla2eci(lla, utc);
 
% Back to LLA
lla_check = eci2lla(posECI, utc);

Flat Earth Approximation

matlab

% Good for short-range simulations (< ~100 km from reference)
llo = [40, -74];    % reference lat/lon (deg)
psio = 0;           % angular direction of flat Earth x-axis (rad, 0=North)
href = 0;           % reference height (m)
 
lla_point = [40.01, -73.99, 100];
flatPos = lla2flat(lla_point, llo, psio, href);  % [x, y, z] in meters
lla_back = flat2lla(flatPos, llo, psio, href);

Euler Angles ↔ DCM

matlab

% CRITICAL: angle2dcm expects RADIANS, not degrees
yaw = deg2rad(30); pitch = deg2rad(10); roll = deg2rad(5);
dcm = angle2dcm(yaw, pitch, roll, 'ZYX');
 
% Extract angles back (returns radians)
[y, p, r] = dcm2angle(dcm, 'ZYX');
fprintf('Yaw=%.1f, Pitch=%.1f, Roll=%.1f deg\n', rad2deg(y), rad2deg(p), rad2deg(r));

DCM ↔ Quaternion (Aerospace Toolbox format)

matlab

% Aerospace Toolbox quaternion format: [q0 q1 q2 q3] = [scalar, vector]
q = dcm2quat(dcm);       % 1x4, scalar-first
dcm_back = quat2dcm(q);  % 3x3
 
% Verify orthogonality
err = max(abs(dcm' * dcm - eye(3)), [], 'all');
assert(err < 1e-14, 'DCM is not orthogonal');

Quaternion Object (Modern Approach)

matlab

% Create from Euler angles — specify sequence AND frame type
q = quaternion([30 10 5], 'eulerd', 'ZYX', 'frame');
 
% Rotate a point (active rotation)
pt = [1 0 0];
pt_rotated = rotatepoint(q, pt);   % [0.8529, 0.4924, -0.1736]
 
% Compose rotations by multiplication
q_total = q2 * q1;  % applies q1 first, then q2
 
% Always normalize after arithmetic accumulation
q = normalize(q);

Quaternion Interpolation (SLERP)

matlab

% Smooth interpolation between two orientations
q1 = quaternion([0 0 0], 'eulerd', 'ZYX', 'frame');
q2 = quaternion([90 0 0], 'eulerd', 'ZYX', 'frame');
t = linspace(0, 1, 5)';
q_interp = slerp(q1, q2, t);
 
% Verify: yaw progresses linearly for pure yaw rotation
e = eulerd(q_interp, 'ZYX', 'frame');
% e(:,1) = [0, 22.5, 45, 67.5, 90]

Quaternion Math (Array-Based Functions)

matlab

% Compose two rotations: q2 applied after q1
q1 = angle2quat(deg2rad(30), 0, 0, 'ZYX');
q2 = angle2quat(0, deg2rad(10), 0, 'ZYX');
q_total = quatmultiply(q2, q1);  % q2 * q1 (apply q1 first)
 
% Inverse rotation
q_inv = quatinv(q1);
q_identity = quatmultiply(q1, q_inv);  % [1 0 0 0]
 
% Rotate a vector
v = [1 0 0];
v_rot = quatrotate(q1, v);  % rotate v by q1
 
% Normalize after accumulation
q_accumulated = quatnormalize(q_total);
 
% Interpolation (slerp, lerp, nlerp)
q_mid = quatinterp(q1, q2, 0.5, 'slerp');
 
% Relative rotation: what rotation takes q1 to q2?
q_rel = quatdivide(q2, q1);  % q_rel such that q2 = q_rel * q1
 
% Exponential/logarithm (useful for angular velocity integration)
q_log = quatlog(q1);         % maps to tangent space
q_back = quatexp(q_log);     % back to quaternion
 
% Fractional rotation (half the rotation of q1)
q_half = quatpower(q1, 0.5);

Rodrigues Vector

matlab

% Compact 3-element representation (singular at 180 deg)
rod = dcm2rod(dcm);           % 1x3 vector
dcm_back = rod2dcm(rod);
 
% Convert between all representations
rod = quat2rod(q_array);      % q is M×4 (scalar-first)
q_back = rod2quat(rod);

Body-to-Wind and Stability Frames

matlab

% Body-to-wind DCM from angle of attack and sideslip
alpha = deg2rad(5);
beta = deg2rad(2);
dcm_bw = dcmbody2wind(alpha, beta);
 
% Extract alpha/beta from a DCM
[alpha_out, beta_out] = dcm2alphabeta(dcm_bw);
 
% Body-to-stability (alpha only, no sideslip)
dcm_bs = dcmbody2stability(alpha);

ECEF-to-NED Frame DCM

matlab

% Get the DCM to rotate vectors from ECEF to local NED
lat = deg2rad(40);
lon = deg2rad(-74);
dcm_ecef2ned = dcmecef2ned(lat, lon);
 
% Transform an ECEF velocity to NED
v_ecef = [10; 20; 30];
v_ned = dcm_ecef2ned * v_ecef;

ECI to AER (Azimuth, Elevation, Slant Range)

matlab

% Compute look angles from a ground station to a satellite in ECI
posECI = [-2981784, 5207055, 3161595];  % satellite ECI position (m)
utc = [2019 1 4 12 0 0];               % observation time
lla0 = [28.5, -80.5, 0];               % ground station [lat, lon, alt] (deg, deg, m)
 
aer = eci2aer(posECI, utc, lla0);
% aer = [azimuth_deg, elevation_deg, slant_range_m]

Conventions

`angle2dcm` and `angle2quat` expect radians — always convert with deg2rad() or convang first
Rotation sequence default is `'ZYX'` (yaw-pitch-roll) — always specify explicitly for clarity
Aerospace Toolbox quaternion format is `[scalar, i, j, k]` — the 1×4 array functions use scalar-first
`quaternion` object also uses scalar-first (compact returns [w x y z])
`rotatepoint` vs `rotateframe` — rotatepoint rotates the point (active); rotateframe rotates the frame (passive). They are inverses.
LLA ordering is `[lat, lon, alt]` in degrees and meters
`geod2geoc` and `geoc2geod` expect radians for latitude, meters for height/radius
ECI functions require UTC time — results change with Earth's rotation
Normalize quaternions after arithmetic operations to prevent drift
Rodrigues vector is singular at 180° — use quaternions for arbitrary rotations

Common Mistakes

Mistake	Why It's Wrong	Correct Approach
Pass degrees to `angle2dcm`	Function expects radians	Use `deg2rad()` or `convang(val,'deg','rad')`
Omit rotation sequence	Default ZYX may not match your convention	Always pass `'ZYX'`, `'XYZ'`, etc. explicitly
Assume quaternion is `[x y z w]`	Aerospace Toolbox uses `[w x y z]` (scalar-first)	Check format; use `compact(q)` to verify
Use `rotatepoint` when meaning `rotateframe`	They are inverses — wrong one flips the rotation	Active rotation = `rotatepoint`; passive = `rotateframe`
Skip normalization after quaternion math	Quaternion drift causes non-unit norm, distorted rotations	Call `normalize(q)` after accumulating rotations
Pass degrees to `geod2geoc`	Expects radians for latitude	Convert: `geod2geoc(deg2rad(lat), alt)`
Forget UTC for ECI transforms	ECI position depends on Earth rotation at that instant	Always provide `[yr mo day hr min sec]`
Use Rodrigues near 180° rotation	Rodrigues vector has a singularity at π	Use quaternion representation instead
Ignore gimbal lock near ±90° pitch	Euler angle extraction loses a degree of freedom	Use quaternion or DCM directly for computations

Gimbal Lock

Euler angle representations lose one degree of freedom when the second rotation reaches ±90° (for ZYX: pitch = ±90°). Symptoms:

dcm2angle returns unexpected yaw/roll values near pitch = ±90°
Interpolating Euler angles produces erratic paths near singularity

Solution: Use quaternions for computation and interpolation. Only convert to Euler angles for display or human interpretation.

matlab

% Detect gimbal lock risk
[~, pitch, ~] = dcm2angle(dcm, 'ZYX');
if abs(pitch) > deg2rad(85)
    warning('Near gimbal lock — use quaternion representation');
end

References

See references/coordinate-systems.md for detailed frame definitions (ECEF, ECI, NED, body, wind, stability), axis conventions, and a decision guide for choosing the right frame.
See references/rotation-conventions.md for detailed rotation math: DCM properties, quaternion algebra, gimbal lock theory, and conversion paths between all representations.
See references/unit-conversion-guide.md for complete unit string tables and chaining conversions.

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