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 CAN Controller Area Network CAS Collision Avoidance System CD Centered Dipole CDI Course Deviation Indicator CG Center of Gravity CGM Corrected Geomagnetic Coordinates CIRAS Coriolis Inertial Rate and Acceleration Sensor CIRS Conventional Inertial Reference System CLIK Closed-Loop Inverse Kinematics CRS Conventional Celestial Reference System CTP Conventional Terrestrial Pole CTRS Conventional Terrestrial Reference System DAAS Data as a Service DAC Digital-to-Analog Converter dB Decibel DCM Direction Cosine Matrix Dec Declination deg Degrees Dev Deviation DGPS Differential Global Positioning System DME Distance Measuring Equipment DoD Department of Defense DoF Degrees of Freedom dps Degrees per Second DR Dead Reckoning DRS Dead Reckoning System DSP Digital Signal Processing ECDIS Electronic Chart Display and Information Systems ECEF Earth-Centered Earth-Fixed ECIF Earth-Centered Inertial Frame ECU Electronic Control Unit EDA European Defence Agency EEA European Economic Area EFCS Electronic Flight Control System EGM Earth Gravitational Model EGNOS European Geostationary Navigation Overlay Service EGNSS Enhanced GNSS EGPS Enhanced Global Positioning System EGR Embedded Global Positioning System Receiver EIR Earth Inertial Reference EKF Extended Kalman Filter EMI Electromagnetic Interference ENU East-North-Up ENS Electronic Navigation System ESA European Space Agency ETRS European Terrestrial Reference System EU European Union FAA Federal Aviation Administration FADEC Full Authority Digital Engine Controller 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 HIL Hardware-in-the-Loop HMI Human-Machine Interface 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 ICRF International Celestial Reference Frame ICRS International Celestial Reference System IERS International Earth Rotation and Reference Systems Service IFR Instrument Flight Rules IGRF International Geomagnetic Reference Field ILS Instrument landing system IMC Intermodule Communication IMU Inertial Measurement Unit INS Inertial Navigation System INU Inertial Navigation Unit ITRF International Terrestrial Reference Frame ITRS International Terrestrial Reference System ITS Intelligent Transportation Systems 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 LQR Linear Quadratic Regulator MagCal Magnetic Calibration MANET Mobile ad hoc Network MARG Magnetism, Angular Rate, and Gravity MCU Micro-Controller Unit MEMS Micro-electromechanical Systems MHIL Micro Hardware-in-the-Loop MHW Mean High Water MIMU Magnetic and Inertial Measurement Unit MIS Management information system 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 NAD North American Datum 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 PLC Programmable Logic Controller PUMA Programmable Universal Manipulation Arm QD Quasi-Dipole QUEST Quaternion Estimator RHR Right-Hand Rule RMS Root Mean Square RNAV Area Navigation RTU Remote Terminal Unit SBAS Satellite-Based Augmentation System SBC Single Board Computer SCADA System Control and Data Acquisition SCI Serial Communications Interface SI Système International d'unités SLERP Spherical Linear Interpolation SOA Silicon Oscillating Accelerometer SPI Serial Peripheral Interface 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 V2V Vehicle-to-Vehicle VANET Vehicular ad hoc Network VFR Visual Flight Rules VNAV Vertical Navigation VOR Very High Frequency Omnidirectional Radio Range WAAS Wide Area Augmentation System WGS World Geodetic System WLAN Wireless Local Area Network WMM World Magnetic Model ZUPT Zero Velocity Update ====== =========