These are the most common definitions in AHRS:

Altitude is the distance along the ellipsoidal normal between the surface of the ellipsoid and a point of interest above it.

Attitude is the orientation of a vehicle or other object relative to a known reference, which could be a frame, the horizon, a direction of motion, or even another object. The heading of the object cn be omitted in the definition of its attitude.

Body Frame matches the frame of the platform the sensors are mounted on. The origin coincides with the center of gravity of the platform. The Y-axis points forward of the moving platform, while the Z-axis points upwards. The X-axis completes the right-hand system pointing in traverse direction. Some literature names it b-frame.

Earth-Centered Earth-Fixed Frame (ECEF) has its origin and z-axis aligned to the ECI frame, but rotates along with the Earth. Therefore is Earth-Fixed. Some literature names it e-frame.

Earth-Centered Inertial Frame (ECI) has the origin at the center of mass of the Earth. The X-axis points towards the vernal equinox in the equatorial plane. The Z-axis is along the rotation axis of the Earth. The y-axis completes with a right-hand system. Some literature names it i-frame.

East-North-Up Frame (ENU) is an LLF with the X-axis pointing East, Y-axis pointing to the true North, and the Z-axis completes a right-hand system pointing up (away from Earth.)

Elevation is the height above or below a fixed reference point. In Geography, it is most commonly the height of a terrain above (or below) the sea level.

Geodetic (also ellipsoidal or curvilinear) coordinates in the ECEF are defined for positioning elements on or near the Earth.

Inertial Measurement Unit is a device fitted with inertial sensors (accelerometers and gyroscopes) measuring a body’s specific force and angular rate.

Inertial Navigation System is a navigation architecture consisting of an IMU, and a computer to continuously estimate a device’s positoin and orientation using dead reckoning.

Latitude is the angle in the meridian plane from the equatorial plane to the ellipsoidal normal at he point of interest.

Local-Level Frame (LLF) is the local navigation frame, whose origin coincides with the sensor frame. Some literature names it l-frame.

Longitude is the angle in the equatorial plane from the prime meridian to the projection of the point of interest onto the equatorial plane.

Meridian is the half of a creat circle on Earth’s surface terminated by the poles.

North-East-Down Frame (NED) is an LLF with the X-axis pointing to the true North, Y-axis pointing East, and the Z-axis completing the right-hand system pointing Down.

Orientation is the rotation needed to place an object from a reference placement to its current placement, neglecting its position or location in space.

Rectangular coordinates in the ECEF represent position of a point with its x, y, and z vector components aligned parallel to the corresponding e-frame axes.

Strapdown INS has accelerometers rigidly mounted parallel to the body axes of the vehicle. In this application the gyroscopes do not provide a stable platform; they are instead used to sense the turning rates of the craft.


ADC Analog-to-Digital Converter
AHRS Attitude and Heading Reference System
AoA Angle of Arrival
AoD Angle of Departure
AR Angular Rate
atan Arctangent
atan2 Arctangent (four quadrants)
AUV Autonomous Underwater Vehicle
BIH Bureau International de l’Heure
BPF Band-Pass Filter
CIRS Conventional Inertial Reference System
CRS Conventional Celestial Reference System
CTP Conventional Terrestrial Pole
CTRS Conventional Terrestrial Reference System
DCM Direction Cosine Matrix
DoD Department of Defense
DoF Degrees of Freedom
DR Dead Reckoning
EGM Earth Gravitational Model
EKF Extended Kalman Filter
ENU East-North-Up
ESA European Space Agency
EU European Union
GNSS Global Navigation Satellite System
GPS Global Positioning System
GTRF Galileo Terrestrial Reference Frame
gyro Gyroscope
HP High Performance
HPF High-Pass Filter
IAU International Astronomical Union
IC Integrated Circuit
IERS International Earth Rotation and Reference Systems Service
IMU Inertial Measurement Unit
INS Inertial Navigation System
INU Inertial Navigation Unit
CIRAS Coriolis Inertial Rate and Acceleration Sensor
ITRF International Terrestrial Reference Frame
KF Kalman Filter
lat Latitude
LLF Local-level Frame
lon Longitude
LPF Low-Pass Filter
MARG Magnetism, Angular Rate, and Gravity
MEMS Micro-electromechanical Systems
MOEMS Micro-opto electromechanical Systems
MST Microsystem Technology
μC Microcontroller
NED North-East-Down
PF Particle Filtering
PIGA Pendulous Integrating Gyroscopic Accelerometer
QUEST Quaternion Estimator
SI Système International d’unités
SOA Silicon Oscillating Accelerometer
SVD Singular Value Decomposition
TFG Turning Fork Gyroscope
UAV Unmanned Aerial Vehicle
UKF Unscented Kalman Filter
WGS World Geodetic System
WMM World Magnetic Model
ZUPT Zero Velocity Update