Nomenclature

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.

Abbreviations

AACGM	Altitude-Adjusted Corrected Geomagnetic Coordinates
ACARE	Advisory Council for Aeronautics Research in Europe
ACAS	Airborne Collision Avoidance System
ACC	Adapttive Cruise Control
Ack	Acknowledge
ADC	Analog-to-Digital Converter
ADCS	Active Directed Control System
ADI	Attitude Director Indicator
ADIRS	Air Data Inertial Reference System
ADIRU	Air Data Inertial Reference Unit
ADEV	Allan deviation
AGNSSS	Assisted Global Navigation Satellite System
AGPS	Assisted Global Positioning System
AHRS	Attitude and Heading Reference System
AoA	Angle of Arrival
AoD	Angle of Departure
AR	Angular Rate
asl	above sea level
atan	Arctangent
atan2	Arctangent (four quadrants)
AQUA	Algebraic Quaternion Algorithm
AU	Astronomical Unit
AUV	Autonomous Underwater Vehicle
AVAR	Allan variance
AVCS	Autonomous Vehicle Control System
AVG	Autonomous Vehicle Guidance
AVLS	Autonomous Vehicle Localization System
AWGN	Additive White Gaussian Noise
az	Azimuth
B2B	Business to Business
BIH	Bureau International de l’Heure
BIIT	Built-in Integrity Test
BLE	Bluetooth Low Energy
BLUE	Best Linear Unbiased Estimate
BPF	Band-Pass Filter
CAS	Collision Avoidance System
CD	Centered Dipole
CDI	Course Deviation Indicator
CGM	Corrected Geomagnetic Coordinates
CIRAS	Coriolis Inertial Rate and Acceleration Sensor
CIRS	Conventional Inertial Reference System
CRS	Conventional Celestial Reference System
CTP	Conventional Terrestrial Pole
CTRS	Conventional Terrestrial Reference System
DCM	Direction Cosine Matrix
DGPS	Differential Global Positioning System
DME	Distance Measuring Equipment
DoD	Department of Defense
DoF	Degrees of Freedom
DR	Dead Reckoning
ECDIS	Electronic Chart Display and Information Systems
ECEF	Earth-Centered Earth-Fixed
ECIF	Earth-Centered Inertial Frame
EGM	Earth Gravitational Model
EGNOS	European Geostationary Navigation Overlay Service
EGNSS	Enhanced GNSS
EKF	Extended Kalman Filter
ENU	East-North-Up
ESA	European Space Agency
EU	European Union
FAA	Federal Aviation Administration
FMS	Flight Management System
GAGAN	GPS and Geo Augmented Navigation
GGF	Global Earth-fixed frame with one axis aligned with gravity
GIS	Geographic Information System
GNSS	Global Navigation Satellite System
GPS	Global Positioning System
GTRF	Galileo Terrestrial Reference Frame
gyro	Gyroscope
HP	High Performance
HPF	High-Pass Filter
IAG	International Association of Geodesy
IATA	International Air Transport Association
IAU	International Astronomical Union
IC	Integrated Circuit
ICAO	International Civil Aviation Organization
IERS	International Earth Rotation and Reference Systems Service
IFR	Instrument Flight Rules
IGRF	International Geomagnetic Reference Field
ILS	Instrument landing system
IMU	Inertial Measurement Unit
INS	Inertial Navigation System
INU	Inertial Navigation Unit
ITRF	International Terrestrial Reference Frame
JPALS	Joint Precision Approach and Landing System
KF	Kalman Filter
LAAS	Local Area Augmentation System
lat	Latitude
LERP	Linear Interpolation
LLF	Local-level Frame
LNAV	Lateral Navigation
lon	Longitude
LORAN	Long-range radio navigation
LPF	Low-Pass Filter
LPV	Localizer Performance with Vertical Guidance
MagCal	Magnetic Calibration
MARG	Magnetism, Angular Rate, and Gravity
MIMU	Magnetic and Inertial Measurement Unit
MIS	Management information system
MCU	Micro-Controller Unit
MEMS	Micro-electromechanical Systems
MLF	Marker-cluster Local Frame
MLS	Microwave landing system
MLT	Magnetic Local Time
mocap	Motion Capture
MOEMS	Micro-opto Electromechanical Systems
MST	Microsystem Technology
μC	Microcontroller
NASA	National Aeronautics and Space Administration
NDB	Non-directional beacon
NED	North-East-Down
NSE	Navigation System Error
PBN	Performance-Based Navigation
PDR	Pedestrian Dead Reckoning
PF	Particle Filtering
PIGA	Pendulous Integrating Gyroscopic Accelerometer
QD	Quasi-Dipole
QUEST	Quaternion Estimator
RHR	Right-Hand Rule
RNAV	Area Navigation
SBAS	Satellite-Based Augmentation System
SCI	Serial Communications Interface
SI	Système International d’unités
SLERP	Spherical Linear Interpolation
SOA	Silicon Oscillating Accelerometer
SVD	Singular Value Decomposition
TACAN	Tactical Air Navigation System
TAWS	Terrain Awareness and Warning System
TFG	Turning Fork Gyroscope
TLS	Transponder landing system
TSE	Total System Error
UART	Universal Asynchronous Receiver / Transmitter
UAV	Unmanned Aerial Vehicle
UKF	Unscented Kalman Filter
UT	Universal Time
VFR	Visual Flight Rules
VNAV	Vertical Navigation
VOR	Very High Frequency Omnidirectional Radio Range
WAAS	Wide Area Augmentation System
WGS	World Geodetic System
WMM	World Magnetic Model
ZUPT	Zero Velocity Update