Glossary

Following is a list of terms and definitions, frequently used in Inertial Labs documents and product descriptions, associated with IMU, INS, AHRS, and GNSS technology.



2D DRMS, 2drms
Two-dimensional (or distance) root mean square analogous to a 2D 2-standard deviation statistic. 2DRMS represents a 2-dimensional ellipse containing 95% (or greater) of the independent, uncorrelated position points in a measurement distribution. It may also represent the 2D, 2σ accuracy of a navigation system.
Absolute Pressure Transducer
A pressure transducer or pressure sensor that has an internal reference chamber sealed at or close to 0 psia (full vacuum) and normally provides increasing output voltage for increases in pressure.
Acceleration-Insensitive Drift Rate (Gyroscope)
The component of environmentally sensitive drift rate not correlated with acceleration. NOTE—Acceleration-insensitive drift rate includes the effects of temperature, magnetic, and other external influences.
Acceleration-Sensitive Drift Rate (Gyroscope)
The components of systematic drift rate correlated with the first power of a linear acceleration component, typically expressed in (°/h)/g.
Accelerometer
An inertial sensor that measures linear or angular acceleration. Except where specifically stated, the term accelerometer refers to linear accelerometer.
Accumulated Delta Range (ADR)
Accumulated delta-range is also referred to as an “integrated Doppler” or “carrier phase” measurement which is a measure of the range between a satellite and receiver expressed in units of cycles of the carrier frequency. This measurement can be made with very high precision (of the order of millimeters), but the whole number of cycles between satellite and receiver is not measurable.
Accuracy
The combined error of nonlinearity, repeatability and hysteresis expressed as a percent of full scale output.
Acquisition
The process of locking onto a satellite's C/A code and P code. A receiver acquires all available satellites when it is first powered up, then acquires additional satellites as they become available and continues tracking them until they become unavailable.
Advanced Extremely High Frequency (AEHF)
Is a system composed of a series of four military communication satellites planned to replace the currently in-orbit Milstar system. It will provide extremely high-frequency (EHF) range uplink / crosslink capabilities and super high-frequency (SHF) range communications.
Advanced Relay Technology Mission (ARTEMIS)
Is ESA's first GEO data relay communication satellite with the objective to demonstrate new communication technologies, principally for data relay and mobile services.
Aerospace and Electronics Systems Society (AESS)
Is a society of the IEEE that focuses on the advancement of integrated electronic systems and large-scale integrated interoperable systems. The AESS is the only professional society dealing with total integrated electronic systems and the enabling technologies.
Air Force Command Post Terminal (AFCPT)
Air Force Command Post Terminals (AFCPTs) are designed to be rugged, reliable and to survive extreme environments, including modern conventional and nuclear warfare. They provide secure, jam resistant voice, data and teletype communications to both tactical and strategic commanders.
Air Force Geophysics Directorate (AFGD)
Is a scientific research organization operated by the United States Air Force Materiel Command dedicated to leading the discovery, development, and integration of affordable aerospace warfighting technologies, planning and executing the Air Force science and technology program, and providing warfighting capabilities to United States air, space, and cyberspace forces.
Air Traffic Control Radar Beacon System (ATCRBS)
Is a secondary surveillance radar system developed for use within the air traffic control system for more precise position reporting of planes. It is used in conjunction with the primary radar, which is used to determine the presence of planes in the airspace.
Aircraft Autonomous Integrity Monitoring (AAIM)
A technology used to augment GPS and GLONASS within the GNSS 1 framework that uses information from the aircraft inertial navigation systems to cross-check the integrity of the GPS signal."
Airlines Electronic Engineering Committee (AEEC)
Committee that creates value for airlines and the aviation industry by developing engineering standards and technical solutions for avionics, networks, and cabin systems that foster increased efficiency and reduced life cycle costs for the aviation community.
Airspace Information Management System (AIMS)
Is the "brains" of the Boeing 777 aircraft. It uses four ARINC 629 buses to transfer information. There are 2 cabinets on each plane (left and right).
Allan Variance
A characterization of the noise and other processes in a time series of data as a function of averaging time. It is one half the mean value of the square of the difference of adjacent time averages from a time series as a function of averaging time.
Altitude Direction Indicator (ADI)
An ADI is an AI integrated with a Flight Director System (FDS). The ADI incorporates a computer that receives information from the navigation system, such as the AHRS, and processes this information to provide the pilot with a 3-D flight trajectory cue to maintain a desired path.
American Congress of Surveying and Mapping (ACSM)
The American Congress on Surveying and Mapping (ACSM) was formed as an international professional association in 1941 to represent the interests of those engaged in measuring and communicating spatial data relating to the Earth's surface. It was founded as a non-profit organization to enhance education, technical and professional development in surveying, mapping, and related fields. It supported educational programs and publications to those interests. Members came from private practice, government, and academia.
Amplitude
The magnitude of the displacement of a wave from its mean value. In a pure sine wave, the amplitude is one-half the wave height.
Angular Acceleration Sensitivity (Accelerometer)
The change of output (divided by the scale factor) of a linear accelerometer that is produced per unit of angular acceleration input about a specified axis, excluding the response that is due to linear acceleration.
Angular acceleration sensitivity (gyroscope)
The ratio of drift rate due to angular acceleration about a Gyroscope axis to the angular acceleration causing it. NOTE—In single-degree-of-freedom Gyroscopes, it is nominally equal to the effective moment of inertia of the gimbal assembly divided by the angular momentum.
Angular Random Walk (ARW)
For gyroscope manufacturers this refers to the amount of error that is due to thermal mechanical noise surrounding the gyroscope that affects it's performance over time.
Antenna Reference Unit (ARU)
Normally used to calibrate other systems. Antenna Reference Units are built with particular care taken to make them simple, robust and repeatable.
Anti-Spoofing 
Denial of the P-code by the Control Segment is called Anti-Spoofing. It is normally replaced by encrypted Y-code, [see P-Code and Y-Code]
ASCII
A 7-bit wide serial code describing numbers, upper and lower case characters, special and non-printing characters. Typically used for textual data.
Attenuation
Reduction of signal strength; a reduction in wave amplitude.
Attitude and Heading Reference System (AHRS)
Common system used in aerospace industry to detect altitude and heading.
Automated Surface Observing System (ASOS)
These are units that are automated sensor suites that are designed to serve meteorological and aviation observing needs. There are currently more than 900 ASOS sites in the United States.
Automatic Dependent Surveillance (ADS)
A surveillance technique in which aircraft automatically provide, via a data link, data derived from on-board navigation and position-fixing systems, including aircraft identification, four-dimensional position and additional data as appropriate.
Automatic Direction Finder (ADF)
An aircraft radio navigation system which senses and indicates the direction to a L/MF nondirectional radio beacon (NDB) ground transmitter. Direction is indicated to the pilot as a magnetic bearing or as a relative bearing to the longitudinal axis of the aircraft depending on the type of indicator installed in the aircraft. In certain applications, such as military, ADF operations may be based on airborne and ground transmitters in the VHF/UHF frequency spectrum.
Axial Load
A load applied along or parallel to and concentric with the primary axis.
Azimuth
The horizontal direction of a celestial point from a terrestrial point, expressed as the angular distance from 000° (reference) clockwise through 360°. The reference point is generally True North, but may be Magnetic North, or Relative (ship's head).
Bandwith
The input signal frequency range from DC (zero frequency) up to the frequency where a -90 degree phase shift (between mechanical input and rate output) is observed.  This phase shift is determined by filters within the device.
Bearing
The horizontal direction of one terrestrial point from another terrestrial point, expressed as the angular distance from a reference direction, usually measured from 000° at the reference direction clockwise through 360°. The reference point may be True North, Magnetic North, or Relative (ship's head).
Bias (Accelerometer)
The average over a specified time of accelerometer output measured at specified operating conditions that have no correlation with input acceleration or rotation. Bias is expressed in [m/s2, g].
Bias (gyroscope)
The average over a specified time of Gyroscope output measured at specified operating conditions that have no correlation with input rotation or acceleration. Bias is typically expressed in degrees per hour (º/h). NOTE—Control of operating conditions may address sensitivities such as temperature, magnetic fields, and mechanical and electrical interfaces, as necessary.
Bias Stability
A means of monitoring stability in atomic clocks and deviation from a mean value of a sensor through the use of the Allan Variance curve.
Bias Stability (Drift)
The bounds within which the Rate Bias may vary over specified periods of time, typically 100 seconds at fixed conditions, including constant temperature (short term); or as long term stability, over 1 year, excluding outputs due to self-generating noise. 
Bias Temperature Stability
The bounds within which the Rate Bias may vary as the temperature varies across the operating temperature range, included in the “Over Operating Environments” specification.
Breaker
A wave that has reached maximum steepness and is breaking.
Bridge
A Wheatstone bridge configuration using four active strain gauges.
Bridge Resistance
The nominal value of the individual legs that make up a complete Wheatstone bridge.
Calibration
The comparison of transducer voltage outputs against the outputs of a reference standard.
Capillary Wave
A wave in which the velocity of propagation is a function of the surface tension of the water. Wind waves of wavelength less than about 2.5 cm (1") are considered capillary waves.
Carrier
The steady transmitted RF signal whose amplitude, frequency, or phase may be modulated to carry information.
Carrier Phase Ambiguity
The number of integer carrier phase cycles between the user and the satellite at the start of tracking. (Sometimes ambiguity for short)
Carrier Phase Measurements
These are "accumulated doppler range" (ADR) measurements. They contain Measurements the instantaneous phase of the signal (modulo 1 cycle) plus some arbitrary number of integer cycles. Once the receiver is tracking the satellite, the integer number of cycles correctly accumulates the change in range seen by the receiver. When a "lock break" occurs, this accumulated value can jump an arbitrary integer number of cycles (this is called a cycle slip).
Case (Gyroscope, accelerometer)
The housing or package that encloses the sensor, provides the mounting surface, and defines the reference axes.
Checksum
By NMEA standard, a validity check performed on the data contained in the sentences, calculated by the talker, appended to the message, then recalculated by the listener for comparison to determine if the message was received correctly. Required for some sentences, optional for all others.
Circular Error Probable (CEP)
Circular error probable; the radius of a circle such that 50% of a set of events occur inside the boundary.
Civil GPS Service Interface Committee (CGSIC)
The United States Department of Transportation established CGSIC to exchange information about GPS with the civil user community, respond to the needs of civil GPS users, and integrate GPS into civil sector applications.
Coarse Acquisition (C/A) Code 
A pseudorandom string of bits that is used primarily by commercial GPS receivers to determine the range to the transmitting GPS satellite. The 1023 chip C/A code repeats every 1 ms giving a code chip length of 300 m which, is very easy to lock onto.
Code Phase GPS
GPS measurements based on the pseudo random code [C/A or P(Y)] as opposed to the use of the carrier of the signal.
Command Post Terminal (CPT)
a post at which the commander of a unit in the field receives orders and exercises command
Commercial-Grade
Of the kind or quality used in commerce; average or inferior.
Common Geometry Module (CGM)
The Common Geometry Module (CGM) is a code library which provides geometry functionality used for mesh generation and other applications. This functionality includes that commonly found in solid modeling engines, like geometry creation, query and modification; CGM also includes capabilities not commonly found in solid modeling engines, like geometry decomposition tools and support for shared material interfaces.
Communication Protocol
A method established for message transfer between a talker and a listener which includes the message format and the sequence in which the messages are to be transferred. Also includes the signalling requirements such as bit rate, stop bits, parity, and bits per character.
Composite Error (Gyroscope, Accelerometer)
The maximum deviation of the output data from a specified output function. Composite error is due to the composite effects of hysteresis, resolution, nonlinearity, non-repeatability, and other uncertainties in the output data. It is generally expressed as a percentage of half the output span.
Continuous Tracking Reciever
A reciever design whivh includes four or more channels to simultaneously track four or more satellites.
Control Segment
The Master Control Station and the globally dispersed Reference Stations used to manage the GPS satellites, determine their precise orbital parameters, and synchronize their clocks.
Coordinated Universal Time (UTC) 
This time system uses the second-defined true angular rotation of the Earth measured as if the Earth rotated about its Conventional Terrestrial Pole. However, UTC is adjusted only in increments of one second. The time zone of UTC is that of Greenwich Mean Time (GMT).
Coriolis Acceleration
The acceleration of a particle in a coordinate frame rotating in inertial space, arising from its velocity with respect to that frame.
Coriolis vibratory Gyroscope (CVG)
A Gyroscope based on the coupling of a structural, driven, vibrating mode into at least one other structural mode (pickoff) via Coriolis acceleration. NOTE—CVGs may be designed to operate in open-loop, force-rebalance (i.e., closed-loop), and/or whole-angle modes.
Course
The horizontal direction in which a vessel is to be steered or is being steered; the direction of travel through the air or water. Expressed as angular distance from reference North (either true, magnetic, compass, or grid), usually 000° (north), clockwise through 360°. Strictly, the term applies to direction through the air or water, not the direction intended to be made good over the ground. Differs from heading.
Course Made Good (CMG)
The single resultant direction from a given point of departure to a subsequent position; the direction of the net movement from one point to the other. This often varies from the track caused by inaccuracies in steering, currents, cross-winds, etc. This term is often considered to be synonymous with Track Made Good, however, Course Made Good is the more correct term.
Course Over Ground (COG)
The actual path of a vessel with respect to the Earth (a misnomer in that courses are directions steered or intended to be steered through the water with respect to a reference meridian); this will not be a straight line if the vessel's heading yaws back and forth across the course.
Cross Acceleration (Accelerometer)
The acceleration applied in a plane normal to an accelerometer input reference axis.
Cross Track Error (XTE)
The distance from the vessel's present position to the closest point on a great Circle line connecting the current waypoint coordinates. If a track offset has been specified in the GPSCard SETNAV command, the cross track error will be relative to the offset track great circle line.
Cross-Axis Sensitivity (Accelerometer)
The proportionality constant that relates a variation of accelerometer output to cross acceleration. This sensitivity varies with the direction of cross acceleration and is primarily due to misalignment.
Cross-Coupling Errors (Gyroscope)
The errors in the Gyroscope output resulting from Gyroscope sensitivity to inputs about axes normal to an input reference axis.
Cycle Slip
When the carrier phase measurement jumps by an arbitrary number of integer cycles. It is generally caused by a break in the signal tracking due to shading or some similar occurrence.
Damping
The reduction of response at the resonant frequency through the use of a damping media, such as oil. This is usually specified as the ratio of critical damping.
Data Block
A continuous block of wave samples. Due to processing constraints, the number of wave samples in a block must be a power of two.
Data Rate
Refers to the time it takes for information to be exchanged between electronic devices.
Dead Reckoning (DR)
The process of determining a vessel's approximate position by applying from its last known position a vector or a series of consecutive vectors representing the run that has since been made, using only the courses being steered, and the distance run as determined by log, receiver rpm, or calculations from speed measurements.
Dead Volume
The volume inside the pressure port of a transducer at room temperature and barometric pressure.
Defense Advanced Research Projects Agency (DARPA)
United States Agency under the Department of Defense that was created in February 1958 by President Dwight D. Eisenhower in response to the Soviet launching of Sputnik 1 in 1957. By collaborating with academic, industry, and government partners, DARPA formulates and executes research and development projects to expand the frontiers of technology and science, often beyond immediate U.S. military requirements.
Deflection
The change in length along the primary axis or distance a diaphragm moves at the center between no-load and rated load conditions.
Degree-Of-Freedom (DOF) (Gyroscope)
An allowable mode of angular motion of the spin axis with respect to the case. The number of degrees-of-freedom is the number of orthogonal axes about which the spin axis is free to rotate.
Density
the ratio of the mass of any substance to the volume it occupies. Typical ocean water has densities in the approximate range of 1.020-1.028 g/cm3.
Destination
The immediate geographic point of interest to which a vessel is navigating. It may be the next waypoint along a route of waypoints or the final destination of a voyage.
Diaphragm
The sensing membrane that is deformed when pressure is applied.
Differential Pseudorange Correction (DPRC)
A correction method used for real-time corrections in precise three-dimensional coordinates of a differential GPS reference station.
Diffraction
A wave process in which energy is transmitted along wave crests. When a wave train passes a barrier, diffraction causes energy to propogate into sheltered regions behind the barrier.
Dilution of Precision (DOP) 
A numerical value expressing the confidence factor of the position solution based on current satellite geometry. The lower the value, the greater the confidence in the solution. DOP can be expressed in the following forms. GDOP - uncertainty of all parameters (latitude, longitude, height, clock offset); PDOP - uncertainty of 3D parameters (latitude, longitude, height); HTDOP - uncertainty of 2D and time parameters (latitude, longitude, time); HDOP - uncertainty of 2D parameters (latitude, longitude); VDOP - uncertainty of height parameter; TDOP - uncertainty of clock offset parameter
DIREC
A reciever manufactured by DATAWELL to receive digital data from a DATAWELL WAVEC Buoy and translate into RS232 digital data out.
Dithering
The deliberate introduction of digital noise. This is the process the DoD used to add inaccuracy to GPS signals to induce Selective Availability.
DIWAR
A receiver manufactured by DATAWELL to receive analogue tone shifting data from a DATAWELL WAVERIDER, and supply a digital output. Ie. DIgital WAverider Reciever
Doppler
The change in frequency of sound, light or other wave caused by movement of its source relative to the observer.
Doppler Aiding
A signal processing strategy, which uses a measured Doppler shift to help a receiver smoothly track the GPS signal, to allow more precise velocity and position measurement.
Doppler Velocity Log
Through the use of an accoustic sensor, a Dopler Velocity Log, typically used with a Kalman Filter, is used to calculate position. Most commonly used where location is needed relative to the sea floor.
Double-Difference
A mathematical technique comparing observations by differencing between receiver channels and then between the reference and rover receivers.
Double-Difference Carrier Phase Ambiguity
Carrier phase ambiguities which are differenced between receiver channels and between the reference and rover receivers. They are estimated when a double-difference mechanism is used for carrier phase positioning. (Sometimes double-difference ambiguity or ambiguity, for short)
Drift Rate (Gyroscope)
The component of Gyroscope output that is functionally independent of input rotation. It is expressed as an angular rate
Dynamic
Pertaining to or characterized by energy or effective action; vigorously active or forceful; energetic; a state of motion, linear or rotational.
Earth-Centred-Earth-Fixed (ECEF)
This is a coordinate-ordinate system which has the X-coordinate in the earth's equatorial plane pointing to the Greenwich prime meridian, the Z-axis pointing to the north pole, and the Y-axis in the equatorial plane 90° from the X-axis with an orientation which forms a right-handed XYZ system.
Eccentricity
A dimensionless parameter for an astronomical object that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptic orbit, 1 is a parabolic escape orbit, and greater than 1 is a hyperbola. The term derives its name from the parameters of conic sections, as every Kepler orbit is a conic section. It is normally used for the isolated two-body problem, but extensions exist for objects following a Klemperer rosette orbit through the galaxy.
Elevation
The angle from the horizon to the observed position of a satellite.
Elevation Mask Angle
A parameter which is modifiable in most GPS receivers, regardless of price. With this setting you can tweak to adjust your elevation "mask angle." This setting is important because you can use it to tell your GPS receiver to ignore satellites below a certain angle.
Ellipsoid
A smooth mathematical surface which represents the earth's shape and very closely approximates the geoid. It is used as a reference surface for geodetic surveys, refer to the MATCHEDPOS log in user manual Volume 2, Command and Log Reference.
Ellipsoidal Height
Height above a defined ellipsoid approximating the surface of the earth.
Environmentally Sensitive Drift Rate (Gyroscope)
The component of systematic drift rate that includes acceleration-sensitive, acceleration-squared-sensitive, and acceleration-insensitive drift rates.
Ephemeris
A set of satellite orbit parameters that are used by a GPS receiver to calculate precise GPS satellite positions and velocities. The ephemeris is used in the determination of the navigation solution and is updated periodically by the satellite to maintain the accuracy of GPS receivers.
Ephemeris
Is a table that provides information on the positions of celestial objects such as stars and planets on given dates in the past or future. It can also show the positions of man-made satellites.
Ephemeris Data
The data downlinked by a GPS satellite describing its own orbital position with respect to time.
Epheremis Parameters
Parameters that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky. These parameters may include: angular velocity, angular acceleration, mass, relative position, and geometric angles.
Epoch
Strictly a specific point in time. Typically when an observation is made.
Excitation, Electrical
The voltage or current applied to the input terminals of the transducer.
Factory Calibration
The accuracy of the measurement of a parameter when calibrated at the factory, usually at a specified reference temperature (typically +22°C).
Factory Setting (also called Initial Offset)
The maximum value, or tolerance of value, of a factory adjusted parameter, usually at a specified  temperature (typically +22°C).
Family of Advanced Beyond Line of Sight Terminals (FAB-T)
A secure, airborne communications network that allows the president of the United States to communicate with senior military leaders.
Fast-Switching Channel (GPS Receiver)
A single channel receiver that rapidly samples the pseudoranges of a number of GPS satellites. "Fast" usually means that the switching time is sufficiently fast (typically 2 to 5 milliseconds) to recover the data message.
Fault Detection and Exclusion (FDE)
An enhanced version of Receiver Autonomous Integrity Monitoring (RAIM) employed in some receivers is known as fault detection and exclusion (FDE). It uses a minimum of six satellites to not only detect a possible faulty satellite, but to exclude it from the navigation solution so the navigation function can continue without interruption.
Fault Detection and Isolation (FDI)
Is a sub-field of control engineering which concerns itself with monitoring a system, identifying when a fault has occurred, and pinpointing the type of fault and its location.
Federal Geodetic Control Committee (FGCC)
The Federal Geodetic Control Subcommittee helps coordinate the planning and execution of geodetic surveys, developing standards and specifications for these surveys, and exchanging geodetic survey data and technical information among Federal agencies.
Federal Highway Administration (FHWA)
The Federal Highway Administration is a division of the United States Department of Transportation that specializes in highway transportation. The agency's major activities are grouped into two programs, the Federal-aid Highway Program and the Federal Lands Highway Program. Its role had previously been performed by the Office of Road Inquiry, Office of Public Roads and the Bureau of Public Roads.
Federal Radionavigation Plan (FRP)
The Federal Radionavigation Plan (FRP) is the official source of radionavigation policy and planning for the federal government. It covers both terrestrial and space-based, common-use, federally operated radionavigation systems, including GPS and GPS augmentations.
Federal Railroad Administration (FRA)
The Federal Railroad Administration is an agency in the United States Department of Transportation. The agency was created by the Department of Transportation Act of 1966. The purpose of FRA is to promulgate and enforce rail safety regulations, administer railroad assistance programs, conduct research and development in support of improved railroad safety and national rail transportation policy, provide for the rehabilitation of Northeast Corridor rail passenger service, and consolidate government support of rail transportation activities.
Federal Transit Administration (FTA)
The Federal Transit Administration (FTA) provides financial and technical assistance to local public transit systems, including buses, subways, light rail, commuter rail, trolleys and ferries. FTA also oversees safety measures and helps develop next-generation technology research.Transit services supported by FTA span many groups and provide wide-ranging benefits.
Fiber Optic gyroscopes (FOG)
By using fiber optic technology, these gyroscopes can measure rotational movement of a body.
Field
A character or string of characters immediately preceded by a field delimiter.
Figure of Merit (FOM)
An indication of the navigational quality of a military receiver, represented by a digit between 0 and 9.
Fixed Ambiguity Estimates
Carrier phase ambiguity estimates which are set to a given number and held constant. Usually they are set to integers or values derived from linear combinations of integers.
Fixed Discrete Ambiguity Estimates
Carrier phase ambiguities which are set to values which are members of a predetermined set of discrete possibilities, and then held constant.
Fixed Field
A field in which the number of characters is fixed. For data fields, such fields are shown in the sentence definitions with no decimal point. Other fields which fall into this category are the address field and the checksum field (if present).
Fixed Integer Ambiguity Estimates
Carrier phase ambiguities which are set to integer values and then held constant.
Fixed Radiation Pattern Antenna - Ground Plane (FRPA-GP)
Antenna patterns are created by interference between radiation sources. The effect of this interference is described through pattern multiplication, where multiple radiation source fields add or subtract. Ground effects can be described by splitting power and applying the split to a second (or even more) imaginary antenna.
Flash ROM
Programmable read-only memory.
Fleet Ballistic Missle (FBM)
The nation's mix of strategic deterrent weapons known as the TRIAD has a purpose unique in the history of warfare: to prevent nuclear war. The Fleet Ballistic Missile (FBM) weapon system is the Navy's major contribution to this mix. The system consists of time-proven, operational, nuclear-powered submarines, each capable of carrying nuclear tipped ballistic missiles. Hidden, mobile, and ready, this formidable undersea deterrent force ranges the oceans of the world, assuring a potential enemy that a nuclear attack on the United States will be answered by a devastating nuclear blow.
Flexible Modem Interface (FMI)
A Flexible Modem Interface (FMI) enables secure communication across multiple operator networks, in multiple frequency bands, utilizing diverse waveforms and modems. This flexibility will allow a government end-user to rapidly roam among a range of satellite service providers and/or constellations, enabling more resilient SATCOM.
Floating Ambiguity Estimates
Ambiguity estimates which are not held to a constant value, but are allowed to gradually converge to the correct solution.
Flush Diaphragm
Sensing element is located on the very tip of the transducer (NO pressure port).
Fluxgate Magnetometer
A sensor capable of measuring the orientation and intensity of magnetic flux lines. Although traditionally used in geological prospecting, underwater navigation and land navigation, fluxgate magnetometers are also used in robotic space probes while analyzing magnetic fields of planets, including Earth.
Forward Error Correction Coding (FEC)
Is an error correction technique to detect and correct a limited number of errors in transmitted data without the need for retransmission. In this method, the sender sends a redundant error-correcting code along with the data frame. The receiver performs necessary checks based upon the additional redundant bits.
Frequency Band
A particular range of frequencies in a region of the electromagnetic spectrum.
Frequency Response
The range of frequencies over which the transducer voltage output will follow the sinusoidally varying mechanical input within specified limits.
Frequency Shift Keying (FSK)
Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as amateur radio, caller ID and emergency broadcasts. The simplest FSK is binary FSK (BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s).
Frequency Spectrum
A range of frequencies associated with a signal. Also, the distribution of signal amplitudes as a function of frequency of the constituent signal.
Frequency-Locked Loop (FLL)
A frequency-lock, or frequency-locked loop (FLL), is an electronic control system that generates a signal that is locked to the frequency of an input or "reference" signal.
Full Range Output
The nominal voltage output for the specified full range input.   The actual voltage output will be a complex compilation of several parameters including scale factor, bias, linearity and temperature characteristics.
Full Scale
See Rated Capacity.
Full Scale Output
The algebraic difference between the minimum output (normally zero) and the rated capacity.
Full-Scale Input (Gyroscope, Accelerometer)
The maximum magnitude of the two input limits.
G sensitivity (Gyroscope)
The change in rate bias due to g input from any direction.
GAGAN
GPS aided Geo Augmented Navigation
Gauge Pressure
The pressure above (or below) atmospheric. It represents positive difference between measured pressure and existing atmospheric pressure. It can be converted to absolute by adding actual atmospheric pressure value.
Gauge Pressure Transducer
A transducer that measures pressure relative to the atmospheric pressure.
Geodesy
Also known as geodetics or geodetics engineering — a branch of applied mathematics and earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal motion, tides, and polar motion.
Geodetic Datum
The reference ellipsoid surface that defines the coordinate system.
Geodetic Surveys
A land survey with corrections made to account for the curvature of the earth's surface.
Geoid
The shape of the earth if it were considered as a sea level surface extended continuously through the continents. The geoid is an equipotential surface coincident with mean sea level to which at every point the plumb line (direction in which gravity acts) is perpendicular. The geoid, affected by local gravity disturbances, has an irregular shape.
Geoid
The hypothetical shape of the earth, coinciding with mean sea level and its imagined extension under (or over) land areas.
Geometric Dilution of Precision (GDOP)
[See DOP]
Geostationary
A satellite orbit along the equator that results in a constant fixed position over a particular reference point on the earth's surface. (GPS satellites are not geostationary.)
Global Positioning System (GPS)
Full name is NAVSTAR Global Positioning System. A space-based radio Positioning system which provides suitably equipped users with accurate position, velocity and time data. GPS provides this data free of direct user charge worldwide, continuously, and under all weather conditions. The GPS constellation consists of 24 orbiting satellites, four equally spaced around each of six different orbital planes. The system is being developed by the Department of Defence under U.S. Air Force management.
GPRMC Log
Is a APRS GPS movile RMC sentence
Great Circle
The shortest distance between any two points along the surface of a sphere or ellipsoid, and therefore the shortest navigation distance between any two points on the Earth. Also called Geodesic Line.
Ground Control Point (GCP)
A point on the ground at which coordinates have been assigned that are of an agreed upon confidence level.
Ground Earth Station (GES)
An earth station in the fixed satellite service, or, in some cases, in the aeronautical mobile-satellite service, located at a specified fixed point on land to provide a feeder link for the aeronautical mobile-satellite service (ICAO).
Ground-Based Augmentation Services (GBAS)
A Ground Based Augmentation System (GBAS) augments the existing Global Positioning System (GPS) utilized in U.S. airspace by providing corrections to aircraft in the vicinity of an airport in order to improve the accuracy of, and provide integrity for, these aircrafts' GPS navigational position.
Gyroscope (gyroscope)
An inertial sensor that measures angular rotation with respect to inertial space about its input axis(es). NOTE 1—The sensing of such motion could utilize the angular momentum of a spinning rotor, the Coriolis effect on a vibrating mass, or the Sagnac effect on counter-propagating light beams in a ring laser or an optical fiber coil.
Handover Word, HOW Word
The Handover Word is a GPS (Global Positioning System) variable used to enable a GPS receiver to switch from the course acquisition code to the precision code.
Handshaking
Predetermined hardware or software activity designed to establish or maintain two machines or programs in synchronization. Handshaking concerns the exchange of messages or packets of data between two systems with limited buffers. Hardware handshaking uses voltage levels or pulses in wires to carry the handshaking signals. Software handshaking uses data units (e.g. ASCII characters) carried by some underlying communication medium.
Harmonic
a quantity whose frequency is an integral multiple of the frequency of a periodic quantity to which it is related.
Heading
The direction in which a vessel points or heads at any instant, expressed in degrees 000° clockwise through 360° and may be referenced to True North, Magnetic North, or Grid North. The heading of a vessel is also called the ship's head. Heading is a constantly changing value as the vessel oscillates or yaws across the course due to the effects of the air or sea, cross currents, and steering errors.
Horizontal and Time Dilution of Precision (HTDOP)
[See DOP]
Horizontal Dilution of Precision (HDOP)
[See DOP]
Hysteresis
The maximum difference between output readings for the same measured point, one point obtained while increasing from zero and the other while decreasing from full scale. The points are taken on the same continuous cycle. The deviation is expressed as a percent of full scale.
Hysteresis Error (Gyroscope, accelerometer)
The maximum separation due to hysteresis between upscale-going and down-scale-going indications of the measured variable (during a full-range traverse, unless otherwise specified) after transients have decayed. It is generally expressed as an equivalent input.
Inertial Measurement Unit (IMU)
Through the use of gyroscopes and accelerometers, IMU's can measure specific force, angular rate, and sometimes the orientation of a body.
Inertial Sensor
A position, attitude, or motion sensor whose references are completely internal, except possibly for initialization.
Initial Operating Capability (IOC)
Is the state achieved when a capability is available in its minimum usefully deployable form. The term is often used in government or military procurement.
Input Angle (Gyroscope)
The angular displacement of the case about an input axis.
Input Axis (IA) (Accelerometer)
The axis(es) along or about which a linear or angular acceleration input causes a maximum output.
Input Axis (IA) (gyroscope)
The axis(es) about which a rotation of the case causes a maximum output.
Input Impedance
The resistance measured across the excitation terminals of a transducer at room temperature, with no load applied, and with the output terminals open circuited.
Input Limits (Gyroscope, accelerometer)
The extreme values of the input, generally plus or minus, within which performance is of the specified accuracy.
Input Range (Gyroscope, accelerometer)
The region between the input limits within which a quantity is measured, expressed by stating the lower- and upper-range value. For example, a linear displacement input range of ±1.7g to ±12g.
Input Rate (Gyroscope)
The angular displacement per unit time of the case about an input axis. For example, an angular displacement input range of ±150°/sec to ±300°/sec.
Input Reference Axis (IRA) (Gyroscope, accelerometer)
The direction of an axis (nominally parallel to an input axis) as defined by the case mounting surfaces, or external case markings, or both.
Input-Axis Misalignment (Gyroscope, accelerometer)
The angle between an input axis and its associated input reference axis when the device is at a null condition.
Insulation (Isolation) Resistance
The DC resistance expressed in ohms measured between any electrical connector pin or lead wire and the transducer body or case. Normally measured at 50 VDC.
Integer Ambiguity Estimates
Carrier phase ambiguity estimates which are only allowed to take on integer values.
International Association of Aids to Navigation and Lighthouse Authorities (IALA)
Is a non-profit organization founded in 1957 to collect and provide nautical expertise and advice. Lateral marks indicate the edges of a channel. Cardinal marks indicate the direction of safe water at a dangerous spot.
International Association of Geodesy (IAG)
Is a scientific organization in the field of geodesy. It promotes scientific cooperation and research in geodesy on a global scale and contributes to it through its various research bodies. It is an active association of the International Union of Geodesy and Geophysics (IUGG).
International Civil Aviation Organization (ICAO)
is a UN specialized agency, established by States in 1944 to manage the administration and governance of the Convention on International Civil Aviation (Chicago Convention). ICAO works with the Convention’s 193 Member States and industry groups to reach consensus on international civil aviation Standards and Recommended Practices (SARPs) and policies in support of a safe, efficient, secure, economically sustainable and environmentally responsible civil aviation sector.
Iono-Free Carrier Phase Observation
A linear combination of L1 and L2 carrier phase measurements which provides an estimate of the carrier phase observation on one frequency with the effects of the ionosphere removed. It provides a different ambiguity value (non-integer) than a simple measurement on that frequency.
JONSWAP Spectrum
was established during a joint research project, the "JOint North Sea WAve Project", and is presented in literature by K.Hasselmann & al., in "Measurements of Wind-Wave Growth and Swell Decay during the Joint North Sea Wave Project (JONSWAP)" Deutsche
Kalman Filter
Is an advanced algorithm that uses input data in combination with error observed over time to continuously form estimates of unknown variables to be used to correct the current state of a measurement.
Kinematic
The user's GPS antenna is moving. In GPS, this term is typically used with precise carrier phase positioning, and the term dynamic is used with pseudorange positioning.
Knot
a unit of speed equal to 1 nautical mile per hour, approximately 51 centimetres per second.
L-Band
The range of radio frequencies that includes the GPS carrier frequencies L1 and L2 and the TerraStar satellite broadcast signal.
L1 Frequency
The 1575.42 MHz GPS carrier frequency which contains the course acquisition (C/A) code, as well as encrypted P-code, and navigation messages used by commercial GPS receivers.
L2 Frequency
The 1227.60 MHz. secondary GPS carrier frequency, containing only encrypted P-code, used primarily to calculate signal delays caused by the ionosphere.
LabVIEW
Laboratory Virtual Instrument Engineering Workbench: an visual interface used to monitor data.
Lane
A particular discrete ambiguity value on one carrier phase range measurement or double difference carrier phase observation. The type of measurement is not specified (L1, L2, L1-L2, iono-free)
Leeward
The direction toward which the wind and waves are going.
Line Pressure
The maximum pressure in the pressure vessel or pipe for differential pressure measurement.
Linear Accelerometer
An inertial sensor that measures the component of translational acceleration minus the component of gravitational acceleration along its input axis(es).
Linearity
The maximum deviation of the calibration curve from a straight line between zero and full scale, expressed as a percent of full scale output and measured on increasing measured only.
Linearity
Defines the upper and lower limit within which the output signal may vary or deviate from the Best-Fit-Straight-Line (BFSL) drawn through the data, expressed as a per cent (%) of the angular rate full range.
Linearity Error (Gyroscope, accelerometer)
The deviation of the output from a least-squares linear fit of the input-output data. It is generally expressed as a percentage of full scale, or percent of output, or both.
Load
The weight, torque or force applied to the transducer.
Load Buttons
A load button has a spherical-like shape on the top surface of a load cell where the load force is applied.
Local Observation Set
An observation set, as described below, taken by the receiver on which the software is operating as opposed to an observation taken at another receiver (the reference station) and transmitted through a radio link.
Local Tangent Plane
A coordinate system based on a plane tangent to the ellipsoid's surface at the user's location. The three coordinates are east, north and up. Latitude, longitude and height positions operate in this coordinate system.
Low-Latency Solution
A position solution which is based on a prediction. A model (based on previous reference station observations) is used to estimate what the observations will be at a given time epoch. These estimated reference station observations are combined with actual measurements taken at the remote station to provide a position solution.
Magnetic Bearing
Bearing relative to magnetic north; compass bearing corrected for deviation.
Magnetic Heading
Heading relative to magnetic north.
Magnetic Variation
The angle between the magnetic and geographic meridians at any place, expressed in degrees and minutes east or west to indicate the direction of magnetic north from true north.
Magneto-Inductive
Production of an electromotive force (i.e., voltage) across an electrical conductor in a changing magnetic field.
Magneto-Resistive
The dependence of the electrical resistance of a body on an external magnetic field.
Mask Angle
The minimum GPS satellite elevation angle permitted by a particular receiver design. Satellites below this angle will not be used in position solution.
Matched Observation Set Pair
Observations from both the reference station and the local receiver which have been matched by time epoch, contain the same satellites, and are corrected for any known offsets.
Measured Media
The physical quantity, property or condition that is measured (e.g., pressure, load, weight, acceleration).
Measurement Error Variance
The square of the standard deviation of a measurement quantity. The standard deviation is representative of the error typically expected in a measured value of that quantity.
Measurement Time Epoch
The point in time at which a GPSCard takes a measurement.
Mechanical Freedom (accelerometer)
The maximum linear or angular displacement of the accelerometer’s proof mass, relative to its case.
Microelectromechanical Systems (MEMS)
A very small mechanical system that is operated with electrical circuits.
Modeling
The technique of using a mathematical model of Scale Factor and/or Rate Bias vs. temperature to correct the output data and minimize such temperature-induced errors.  Such a model may make use of power-series equations, multiple-point data with interpolation between temperatures, for example.
Mounted Resonant Frequency
The frequency at which the internal spring/mass system of an accelerometer resonates, producing a 90º phase shift in output signal versus applied acceleration.
Multipath Errors
GPS positioning errors caused by the interaction of the GPS satellite signal and its reflections.
Natural Frequency (Gyroscope, accelerometer)
The frequency at which the output lags the input by 90°. It generally applies only to inertial sensors with approximate second-order response.
Non-Gravitational Acceleration (Accelerometer)
The component of the acceleration of a body that is caused by externally applied forces (excluding gravity) divided by the mass.
Non-Volatile Memory
A type of memory device that retains data in the absence of a power supply.
Nonlinearity (Gyroscope, Accelerometer)
The systematic deviation from the straight line that defines the nominal input-output relationship.
Null Field
By NMEA standard, indicates that data is not available for the field. Indicated by two ASCII commas, i.e., ",," (HEX 2C2C), or, for the last data field in a sentence, one comma followed by either the checksum delimiter "*" (HEX 2A) or the sentence delimiters (HEX 0D0A). [Note: the ASCII Null character (HEX 00) is not to be used for null fields.]
Nyquist Frequency
The highest resolvable frequency in a digital wave record. Frequencies above the Nyquist frequency appear as lower frequencies.
Obscuration
Term used to describe periods of time when a GPS receiver's line-of-sight to GPS satellites is blocked by natural or man-made objects.
Observation
Any measurement. The two observations used in NovAtel's RTK algorithms are the pseudorange measurement and the carrier phase measurement.
Observation Set
A set of GPSCard measurements taken at a given time which includes one time for all measurements, and the following for each satellite tracked: PRN number, pseudorange or carrier phase or both, lock time count, signal strength, and tracking status. Either L1 only or L1 and L2 measurements are included in the set. The observation set is assumed to contain information indicating how many satellites it contains and which ones have L1-only and which ones have L1/L2 pairs.
Open-Loop Mode (Coriolis Vibratory Gyroscope)
A mode in which the vibration amplitude of the pickoff is proportional to the rotation rate about the input axis(es).
Operating Life (Gyroscope, Accelerometer)
The accumulated time of operation throughout which a Gyroscope or accelerometer exhibits specified performance when maintained and calibrated in accordance with a specified schedule.
Operating Temperature (Gyroscope, Accelerometer)
The temperature at one or more Gyroscope or accelerometer elements when the device is in the specified operating environment.
Origin Waypoint
The starting point of the present navigation leg, expressed in latitude and longitude.
Output
The electrical signal measured at the output terminals that is produced by an applied input to a transducer.
Output Impedance
The resistance as measured on the output terminals of a transducer at standard temperature, with no measured applied and with the excitation terminals open-circuited.
Output Noise
Self-generated electrical noise is defined by its power-spectral-density (PSD) and given in volts-squared per Hz over a given range of frequency.  This defines the distribution of noise power in the output of the instrument, on a per-Hz basis.  For example, if one computes the area under the output response curve within any 1 Hz window, then take the square root of this area, one obtains a value of voltage.  Dividing this voltage by the Scale factor converts it to an equivalent rate reading.  This is the output noise of the instrument, measured in °/sec/Hz.
Output Range (Gyroscope, accelerometer)
The product of input range and scale factor.
Output Span (Gyroscope, accelerometer)
The algebraic difference between the upper and lower values of the output range.
Overrange, Safe
The maximum pressure or load that may be applied to the transducer without causing a permanent change in the performance specifications.
P-Code 
Precise code or protected code. A pseudorandom string of bits that is used by GPS receivers to determine the range to the transmitting GPS satellite. P-code is replaced by an encrypted Y-code when Anti-Spoofing is active. Y-code is intended to be available only to authorized (primarily military) users. [See Anti-Spoofing, C/A Code and Y-Code]
Parallel Receiver
A receiver that monitors four or more satellites simultaneously with independent channels.
Parity
The even or odd quality of the number of ones or zeroes in a binary code. Parity is often used to determine the integrity of data especially after transmission.
PDOP
Position Dilution of Precision [See DOP]
Perigee
The point in a body's orbit at which it is nearest the earth.
Phase Shift
The phase angle between the output signal and the applied acceleration.
Phase Velocity
Propogation velocity of an individual wave. In deep water it is proportional to the wave length, otherwise it depends on water depth.
Pickoff (mechanical Gyroscope, accelerometer)
A device that produces an output signal as a function of the relative linear or angular displacement between two elements.
Pitch
Refers to the direction of motion that is about the transverse axis.
Plumb Bob Gravity
The force per unit mass acting on a mass at rest at a point on the earth, not including any reaction force of the suspension. The plumb bob gravity includes the gravitational attraction of the earth, the effect of the centripetal acceleration due to the earth rotation, and tidal effects. The direction of the plumb bob gravity acceleration defines the local vertical down direction, and its magnitude defines a reference value of acceleration (g).
Point-To-Point Protocol (PPP)
Computer network protocol used to transfer a datagram between two directly connected (point-to-point) computers.
Power Spectral Density (PSD)
A characterization of the noise and other processes in a time series of data as a function of frequency. It is the mean squared amplitude per unit frequency of the time series. It is usually expressed in (º/h)2/Hz for gyroscope rate data or in (m/s2)2/Hz or g2/Hz for accelerometer acceleration data
Precise Positioning Service (PPS)
The GPS positioning, velocity, and time service which is available on a continuous, worldwide basis to users authorized by the U.S. Department of Defence (typically using P-Code).
Primary Axis
The axis along which the transducer is designed to be loaded; normally its geometric centerline.
Principal Axis of Compliance (Gyroscope, accelerometer)
An axis along which an applied force results in a displacement along that axis only.
PRN Number
A number assigned by the GPS system designers to a given set of pseudorandom codes. Typically, a particular satellite will keep its PRN (and hence its code assignment) indefinitely, or at least for a long period of time. It is commonly used as a way to label a particular satellite.
Proof Mass (accelerometer)
The effective mass whose inertia transforms an acceleration along, or about, an input axis into a force or torque. The effective mass takes into consideration rotation and contributing parts of the suspension.
Pseudolite
An Earth-based transmitter designed to mimic a satellite. May be used to transmit differential corrections.
Pseudorange
The calculated range from the GPS receiver to the satellite determined by taking the difference between the measured satellite transmit time and the receiver time of measurement, and multiplying by the speed of light. Contains several sources of error.
Pseudorange Measurements
Measurements made using one of the pseudorandom codes on the GPS signals. They provide an unambiguous measure of the range to the satellite including the effect of the satellite and user clock biases.
Pull Plate
Load cell attachment allows tension or compression force to be directed at the center line of a load cell through a threaded-center hole.
Quantization (Gyroscope, Accelerometer)
The analog-to-digital conversion of a Gyroscope or accelerometer output signal that gives an output that changes in discrete steps, as the input varies continuously.
Quantization Noise (Gyroscope, Accelerometer)
The random variation in the digitized output signal due to sampling and quantizing a continuous signal with a finite word length conversion. The resulting incremental error sequence is a uniformly distributed random variable over the interval 1/2 least significant bit (LSB).
Radio Technical Commission for Aeronautics (RTCA)
An organization which developed and defined a message format for differential positioning.
Radio Technical Commission for Maritime Services (RTCM)
An organization which developed and defined the SC-104 message format for differential positioning.
Random Drift Rate (Gyroscope)
The random time-varying component of drift rate.
Random Walk
A zero-mean Gaussian stochastic process with stationary independent increments and with standard deviation that grows as the square root of time.
Range
The measured values, over which a transducer is intended to measure, specified by their upper and lower limits.
Rated Capacity
The maximum measurand that a transducer is designed to measure within its specification.
Ratiometric Output
An output method where the representation of the measured output quantity (e.g., voltage, current, pulse rate, pulse width) varies in proportion to a reference quantity.
Real-Time Kinematic (RTK)
A type of differential positioning based on observations of carrier phase. In this document it is also used with reference to RT-2™ and RT-20™.
Receiver Channels
A GPS receiver specification which indicates the number of independent hardware signal processing channels included in the receiver design.
Rectification Error (accelerometer)
A steady-state error in the output while vibratory disturbances are acting on an accelerometer.
Reference Satellite
n a double difference implementation, measurements are differenced between different satellites on one receiver in order to cancel the correlated errors. Usually one satellite is chosen as the "reference, and all others are differenced with it.
Reference Station
The GPS receiver which is acting as the stationary reference. It has a known position and transmits messages for the rover receiver to use to calculate its position.
Relative Bearing
Bearing relative to heading or to the vessel.
Remote/ Rover Receiver
The GPS receiver which does not know its position and needs to receive measurements from a reference station to calculate differential GPS positions. (The terms remote and rover are interchangeable.)
Repeatability
The ability of a transducer to reproduce output readings when the same measured value is applied to it consecutively, under the same conditions, and in the same direction. Repeatability is expressed as the maximum difference between output readings as a percent of full scale.
Repeatability (Gyroscope, Accelerometer)
The closeness of agreement among repeated measurements of the same variable under the same operating conditions when changes in conditions or non-operating periods occur between measurements.
Residual
In the context of measurement, the residual is the misclosure between the calculated measurements, using the position solution and actual measurements.
Resolution
The smallest change in mechanical input that produces a detectable change in the output signal.
Resolution (Gyroscope, Accelerometer)
The largest value of the minimum change in input, for inputs greater than the noise level, that produces a change in output equal to some specified percentage (at least 50%) of the change in output expected using the nominal scale factor.
Roll
Refers to the direction of motion that is about the longitudinal axis.
Root Mean Square (RMS)
A probability level of 68%.
Route
A planned course of travel, usually composed of more than one navigation leg.
RT-20
NovAtel's Double Differencing Technology for real-time kinematic (RTK) carrier phase floating ambiguity resolution.
Satellite-Based Augmentation System (SBAS)
A type of geo-stationary satellite system that improves the accuracy, integrity, and availability of the basic GPS signals. This includes WAAS, EGNOS, and MSAS.
SBG
Industry competitor with innovative technology and professional services serving the federal government and private sectors.
Scale Factor (Gyroscope, Accelerometer)
The ratio of a change in output to a change in the input intended to be measured. Scale factor is generally evaluated as the slope of the straight line that can be fitted by the method of least squares to input-output data. Slope=Scale Factor
Scale Factor Temperature Sensitivity
The bounds within which the Scale Factor will lie as the temperature varies across the operating temperature range, usually referenced to a room temperature calibration value (+22°C typical).  Also can be defined as a temperature coefficient, usually the best fit straight line of Scale Factor change over the operating temperature range.
Second-Order Nonlinearity Coefficient (accelerometer)
The proportionality constant that relates a variation of the output to the square of the input, applied parallel to the input reference axis.
Selected Waypoint
he waypoint currently selected to be the point toward which the vessel is travelling. Also called "to waypoint, destination or destination waypoint.
Selective Availability (SA)
The method used by the United States Department of Defence to control access to the full accuracy achievable by civilian GPS equipment (generally by introducing timing and ephemeris errors).
Sensing Element
The part of the transducer that reacts directly in response to the measurand.
Sensitivity
The ratio of change in transducer output to a change in the value of the measurand.
Sensitivity (Gyroscope, Accelerometer)
The ratio of a change in output to a change in an undesirable or secondary input. For example: a scale factor temperature sensitivity of a Gyroscope or accelerometer is the ratio of change in scale factor to a change in temperature.
Sequential Receiver
A GPS receiver in which the number of satellite signals to be tracked exceeds the number of available hardware channels. Sequential receivers periodically reassign hardware channels to particular satellite signals in a predetermined sequence.
Shoaling
Changes in wave height as waves move into shallow water. Except for a limited depth region, shoaling increases wave heights. Shoaling occurs even if wave heights and directions do not change as a result of wave refraction.
Shock
The limit of shock which the device will withstand without damage. Typically, the GyroscopeChip® can withstand shocks without damage with a peak value of 200 g’s lasting 2 ms.
Shunt CAL (R-CAL)
The change in electrical output caused by placing a fixed resistor between the appropriate transducer terminals. Used "in the field" for quick calibration.
Space Vehicle ID (SV)
Sometimes used as SVID. A unique number assigned to each satellite for identification purposes. The 'space vehicle' is a GPS satellite.
Span
The algebraic difference between the limits of the range from zero to full scale.
Specifications
The group of error limits within which each device will operate.
Spherical Error Probable (SEP)
The radius of a sphere, centred at the user's true location, that contains 50 percent of the individual three-dimensional position measurements made using a particular navigation system.
Spheroid
Sometimes known as ellipsoid; a perfect mathematical figure which very closely approximates the geoid. Used as a surface of reference for geodetic surveys.
Stability (Gyroscope, Accelerometer)
A measure of the ability of a specific mechanism or performance coefficient to remain invariant when continuously exposed to a fixed operating condition.
Standard Positioning Service (SPS)
A positioning service made available by the United States Department of Defence which is available to all GPS civilian users on a continuous, worldwide basis (typically using C/A Code).
Start Up Time
The time required for the instrument to produce a usable rate output after power application.
Static
Lacking in movement, action, or change; still.
Stokes Wave Theory
A nonlinear wave theory in which higher order terms proportional to wave slope are not neglected as in linear wave theory.
Storage life (Gyroscope, Accelerometer)
The non-operating time interval under specified conditions, after which a device will still exhibit a specified operating life and performance.
Strain Gauge
A measuring element for converting force, pressure, tension, etc., into an electrical signal.
Strapdown (Gyroscope, Accelerometer)
Direct-mounting of inertial sensors (without gimbals) to a vehicle to sense the linear and angular motion of the vehicle.
Tactical-Grade
The qualifying metric for IMU grading is the In Run Bias Stability (IRBS) in their gyroscopes, which represents its stability during benign conditions, which include the following: ideal integration (averaging) time, stable temperature, no inertial motion. IEEE-STD-952-1997 (Appendix B) provides an industry-standard definition of IRBS, which defines it as the minima on the Allan Variance curve.
TDOP
Time Dilution of Precision [See DOP]
Temperature Compensation
The utilization of supplementary devices, materials or components within the bridge to minimize sources of error caused by changing temperature.
Temperature Effect On Span
The change in rated output due to a change in ambient temperature. This is usually expressed as ± a percentage change in rated output per degree F change in ambient temperature, over the compensated temperature range.
Temperature Effects On Zero
The change in zero balance due to a change in ambient temperature. This is usually expressed as ± a percentage change in rated output per degree F change in ambient temperature over the compensated temperature range.
Temperature, Compensated
The range of temperature over which a transducer can operate up to full scale and still meet all specifications.
Temperature, Operating
The range of temperature over which a transducer may be safely operated up to full scale without causing failure, but specifications may not be met.
Third-Order Nonlinearity Coefficient (accelerometer)
The proportionality constant that relates a variation of the output to the cube of the input, applied parallel to the input reference axis.
Three-Dimensional (3D) Navigation
Navigation mode in which altitude and horizontal position are determined from satellite range measurements.
Three-Dimensional Coverage
The number of hours-per-day when four or more satellites are available with acceptable positioning geometry. Four visible satellites are required to determine location and altitude.
Threshold (Gyroscope, Accelerometer)
The largest absolute value of the minimum input that produces an output equal to at least 50% of the output expected using the nominal scale factor.
Time-To-First-Fix (TTFF)
The actual time required by a GPS receiver to achieve a position solution. This specification will vary with the operating state of the receiver, the length of time since the last position fix, the location of the last fix, and the specific receiver design.
Track 
A planned or intended horizontal path of travel with respect to the Earth rather than the air or water. The track is expressed in degrees from 000° clockwise through 360° (true, magnetic, or grid).
Track Made Good
The single resultant direction from a point of departure to a point of arrival or subsequent position at any given time; may be considered synonymous with Course Made Good.
Transducer
A device (or medium) that converts energy from one form to another. The term is generally applied to devices that take physical phenomenon (pressure, temperature, humidity, flow, etc.) and convert it to an electrical signal.
Transverse Sensitivity
Signal output as a result of acceleration perpendicular to the sensitive axis. Specified as a percentage of sensitive axis output for equivalent right angle acceleration or as a decimal fraction.
True Bearing
Bearing relative to true north; compass bearing corrected for compass error.
True Heading
Heading relative to true north.
Turn-On Time (Gyroscope, Accelerometer)
The time from the initial application of power until a sensor produces a specified useful output, though not necessarily at the accuracy of full specification performance.
Two-Dimensional (2D) Navigation
Navigation mode in which a fixed value of altitude is used for one or more position calculations while horizontal (2D) position can vary freely based on satellite range measurements.
Two-Dimensional Coverage
The number of hours-per-day with three or more satellites visible. Three visible satellites can be used to determine location if the GPS receiver is designed to accept an external altitude input.
U.S. Federal Communications Commission (FCC)
The Federal Communications Commission is an independent agency of the United States government created by statute to regulate interstate communications by radio, television, wire, satellite, and cable.
Undulation
The distance of the geoid above (positive) or below (negative) the mathematical reference ellipsoid (spheroid). Also known as geoidal separation, geoidal undulation, geoidal height.
United States Strategic Command (USSC/USSTRATCOM)
Is one of ten unified combatant commands in the United States Department of Defense. Headquartered at Offutt Air Force Base, Nebraska, USSTRATCOM is responsible for strategic deterrence, global strike, and operating the Defense Department's Global Information Grid.
Update Rate
The GPS receiver specification which indicates the solution rate provided by the receiver when operating normally.
UTC
[See Coordinated Universal Time]
Variable Field
By NMEA standards, a data field which may or may not contain a decimal point and which may vary in precision following the decimal point depending on the requirements and the accuracy of the measuring device.
VDOP
Vertical Dilution of Precision [See DOP]
Velocity Random Walk
For accelerometer manufacturers this refers to the amount of error that is due to thermal mechanical noise surrounding the accelerometer that affects it's performance over time.
Vibration – Operating
The specified limit of random vibration in g-RMS, usually across the flat 20 Hz – 2,000 Hz input spectrum range, which the device will withstand while operating within its performance specifications.
Vibration – Survival
The specified limit of random vibration in g-RMS, usually within the flat 20 Hz – 2,000 Hz range, which the device will survive for a limited period of time without damage.
Vibration Error
The maximum change in output of a transducer when a specific amplitude and range of frequencies are applied to a specific axis at room temperature.
Virtual Reference Station (ViRS)
Common correction method used for the message type RTCM 3.2
Warm-Up Time (Gyroscope, Accelerometer)
The time from the initial application of power for a sensor to reach specified performance under specified operating conditions.
Waypoint
A reference point on a track.
Wet/Dry Differential
A differential pressure transducer or transmitter that uses a metal diaphragm at the wet port where fluids can be applied, and no diaphragm at the dry port. The dry port exposes the internal circuitry to the medium, so only clean dry gas can be applied to this port.
Wet/Wet Differential
A differential pressure transducer or transmitter that has a metal diaphragm in each pressure port to permit fluid into both ports.
Wetted Parts
The diaphragm and pressure port material that comes in direct contact with the medium (gas, liquid).
Wide Lane
A particular integer ambiguity value on one carrier phase range measurement or double difference carrier phase observation when the difference of the L1 and L2 measurements is used. It is a carrier phase observable formed by subtracting L2 from L1 carrier phase data. The corresponding wavelength is 86.2 cm
World Geodetic System 1984 (WGS84)
An ellipsoid designed to fit the shape of the entire Earth as well as possible with a single ellipsoid. It is often used as a reference on a worldwide basis, while other ellipsoids are used locally to provide a better fit to the Earth in a local region. GPS uses the centre of the WGS-84 ellipsoid as the centre of the GPS ECEF reference frame.
Y-Code 
An encrypted form of P-Code. Satellites transmit Y-Code in replace of P-Code when Anti-Spoofing is in effect. [See P-Code and Anti-Spoofing]
Yaw
Refers to the direction of motion that is about the vertical axis.
Zero Adjustments
Used when "setting up" a transducer to adjust the output signal to zero when zero load/pressure is applied.
Zero Balance
The output signal of the transducer with rated excitation and with no load applied, usually expressed as a percent of rated output.
Zero Offset (Restricted to Rate Gyroscopes)
The Gyroscope output when the input rate is zero, generally expressed as an equivalent input rate. It excludes outputs due to hysteresis and acceleration.
Zero Return
The difference in zero balance measured immediately before rated load application of specified duration and measured after removal of the load, and when the output has stabilized.