Absolute accuracy
(Geodetic or Geographic accuracy) 
The accuracy of a position with respect to the geographic or geodetic coordinates of the Earth. 
Accuracy 
The degree of conformance between the estimated or measured parameter of a craft at a given time and ts true parameter at that time (Parameters in this context may be position coordinates, velocity, time, angle, etc.) 
Aid to Navigation 
A device external to a craft designed to assist in determination of position of the craft, a safe course or to warn of dangers to navigation. 
Alongtrack 
In the direction of, or parallel to, the intended track of a craft. 
Alongtrack error 
A position error in the direction of the intended track. 
Ambiguity 
The condition obtaining when one set of measurements derived from a navigation system defines mere than one point, direction, line of position or surface of position. 

Base Line 
The line joining two points between which electrical phase or time is compared in determining navigation parameters (for ground stations this will be the line joining the two stations). 
Blunder 
A human mistake such as punching the wrong button on a keyboard, completely misreading an instrument, or wrongly identifying a target on a radar screen. 
Broadcast Availability 
The availablity of a radio signal at the transmitting antenna. 

Circular Error Probable
(CEP) 
The radius of a circle, centred on the measured position, inside which the true position lies with 50% probability. 
Cocked hat 
A triangle formed by the intersection of three position lines, adjusted to a common time, that do not meet at one point. 
Conditional accuracy 
An accuracy which is affected in a predictable way by the conditions under whkh the navigation system operates. 
Confidence interval 
The numerical range within which an unknown is estimated to be with a given probability. 
Confidence level 
The probability that a given statement is correct, or the probability that a stated confidence interval (numerical range) includes an unknown. 
Confidence limits 
The extremes of a confidence interval. 
Continuity 
Continuity is the ability of a system to function within specified performance limits without interruption during a specified period. 
Correction 
The numerical vaiue of a correction is the best estimate which can be made of the difference between the true and the measured value of a parameter. The sign is such that a correction which is to be added to an observed reading is taken as positive. 
Coverage 
The coverage provided by a radionavigation system is that surface area or space volume in which the signals are adequate to permit the user to determine position to a specified level of performance. 
Crosstrack 
At right angles to the intended track of a craft (in a horizontal plane). 
Crosstrack error 
A position error perpendicular to the intended track. 
Craft Autonomous Integrity Monitoring
(CAlM) 
This is a technique whereby all navigation sensor information available on the craft is autonomously processed to monitor the integrity of the navigation signals. (See also ¨Receiver Autonomous Integrity Monitoring¨.) 

Dead reckoning
(DR) 
Navigation based on speed, elapsed time and direction from a known position. 
Decca Navigator System 
A lowfrequency hyperbolic radionavigation system based on harmonically related continuous wave transmissions. 
Differential system 
A system whereby navigation signals are monitored at a known position and the corrections so determined are transmitted to users in the coverage area. 
Distance Root Mean Square (dRMS) 
The root mean square of the radial distances from the true position to the observed positions obtained from a number of trials. 

Electronic chart display and information system
(ECDIS) 
A navigational information system which, with adequate backup arrangements, can be accepted as complying with the uptodate chart required by regulation V/20 of the 1974 SOLAS Convention, by displaying selected information from a system electronic navigational chart (SENC) with positional information from navigation sensors to assist the mariner in route planning and route monitoring, and by displaying additional navigationrelated information if required. 

Failure 
The termination of the ability of a system, or part of a system to perform its required function. 
Failure Rate 
The average number of failures of a system, or part of a system per unit time. (See also ¨Mean Time Between Failures¨.) 
Fault 
That condition of a component of a system which may result in the failure of either the system or part of it. 
Fix 
A position established by processing information from a number of navigation observations. 
Fix rate 
The fix rate is the number of fixes per unit time. 

Gaussian distribution
(normal distribution) 
A theoretical error distribution, typically found in studies of the random errors which arise in the repeated measurement of the same quantity. The one dimensional error frequency curve is bell shaped and symmetncal about the mean. 
Geodetic or Geographic accuracy 
See ¨absolute accuracy¨. 
Geometric dilution of precision
(GDOP) 
The factor by which the accuracy of a fix is degraded by geometrical considerations. In GNSS it is the ratio of the dRMS of the three dimensional position and time error to the (equal) standard deviation of the pseudorange measurement errors. 
Global Navigation Satellite System
GLONASS) 
This is a spacebased radio positioning, navigation and timetransfer system operated by the Government of the Russian Federation. 
Global Navigation Satellite System
(GNSS) 
This is a spacebased radio positioning, navigation and time transfer system being developed internationally for civil purposes to supersede GPS and GLONASS which were designed primarily as military systems. It is expected that it will be compatible with, and at least intially make use of the signals from, GPS and GLONASS. 
Global Positioning System
(GPS) 
This is a spacebased, radio positioning, navigation and timetransfer system operated by the United States Government. 
Grid accuracy 
The accuracy of a position or position line relative to a grid system, for example a hyperbolic system such as LoranC, where the grid itself may be subject to fixed errors relative to geographical coordinates. (Sometimes referred to as repeatability.) 
Gross Errors 
Gross errors, or ” outliers “, are errors other than random errors or systematic errors. They are often large and, by definition, unpredictable. They are typically caused by sudden changes in the prevailing physical circumstances, by system faults or by blunders. 

Horizontal dilution of precision
(HDOP) 
The factor by which a two dimensional fix in a horizontal plane is degraded by geometrical considerations. (See also ¨GDOP¨.) 
Hyperbolic navigation system 
A system for determining the position of a craft by the intersection of hyperbolic position lines. Each such line is the locus of points of constant difference in the phase or arrival time of signals sent from a pair of synchronized transmitting stations. 

Integrated navigation system 
A system in which the information from two or more navigation aids is combined in a symbiotic manner to provide an output which is superior to any one of the component aids. 
Integrity 
Integrity is the ability of a system to provide timely warnings to users when the system should not be used for navigation. 


Kalman filtering 
A mathematical method by which imformation from a number of sources can be combined to provide estimates of navigation parameters which are optimal in terms of predefined criteria. 

Lane expansion factor 
The factor by which the lane width of a hyperbolic system increases compared with the lane width on the corresponding baseline. 
Linear error probable (LEP) 
Half of the interval containing half of the results of the trials in a series of one dimensional measurements. 
Line of Position
(LOP) 
A locus of points where a navigation parameter has a constant value. (Also called Position Line (PL).) 
LoranC 
A low frequency hyperbolic radionavgation system based on measurements of the differences of times of arrival of signals using pulse and phase comparison techniques. 

Marginally detectable bias
(MDB) 
The minimum size of the gross error in an observation that may be detected with given probabilities of type I (a) and type 2 (b). (Note that a type I error occurs when an observation without a gross error is wrongly rejected, and a type 2 error occurs when an observation with a gross error is wrongly accepted. The UK Offshore Operators Association recommends the adoption of a = 0.01 and b = 0.20.) 
Marginally detectable error
(MDE) 
The maximum positionoffset caused by a MDB in one of the LOPs or observations. 
Mean time between failures
(MTBF) 
The average time beween two successive failures of a system or part of a system. 

Navigation 
The process of planning, recording and controlling the movement of a craft from one place to another. 
Navigational Aid 
An instrument, device, chart, etc., carried on board and intended to assist the navigation of a craft. 
Nondirectional radio beacon
(NDB) 
A radio beacon with a fixed omnidirectional aerial (antenna), from which a bearing can be obtained using a directional aerial on the craft. 
Normal distribution 
See ¨Gaussian distribution¨. 

Omega 
A very low frequency (VLF) hyperbolic radionavigation system, based on phase cornparison techniques. 
Operational Technical Accuracy
(OTA) 
The accuracy with which the craft is controlled as measured by the indicated craft position wth respect to the indicated command or desired position. It does not include blunder errors. 
Outliers 
See ¨Gross errors¨. 

Positional dilution of position
(PDOP) 
The factor by which the accuracy of a three dimensional fix is degraded by geometrical considerations. (See also ¨Geometric dilution of precision¨.) 
Position line 
See ¨Line of position¨. 
Precision 
The accuracy of a measurement or a position with respect to randorn errors. 


Radar 
Radio detection and ranging. The use of radio waves, reflected, or automatically retransmitted, to gain information concerning a distant object. 
Radio beacon 
Radio transmitting stations which provide ground wave signals whose direction can be detected by suitable receivers. (See also ¨Nondirectional beacon¨.)
Radio beacon signals may also be used to carry differential corrections for satellite navigation systems. 
Radiodetermination 
The determination of position, or the obtaining of information relating to position, by means of the propagation properties of radio waves. 
Radio Direction Finding
(RDF) 
A system for measuring the direction of a radio wave as received by an observer. 
Radiolocation 
Radiodetermination used for purposes other than radionavigation. 
Radionavigation 
The use or radio waves in navigation for the determination of position or direction, or for obstruction warning. (Radionavigation is defined by ITU in the stricter sense of applying to lifesaving operations only.) 
Random error 
That error which can be predicted only on a statistical basis. 
Receiver Autonomous lntegrity Monitoring
(RAIM) 
This is a technique whereby all navigation sensor infcrmation available at a receiver is autonomously processed to monitor the integrity of the navigation signals. (See also ¨Craft autonomous integrity monitoring¨.) 
Redundancy 
The existence of more than one means for accomplishing a given function. The concept is such that a comptete failure can occur only when all means have failed. 
Relative accuracy 
The accuracy with which a user can determine position relative to that of another user of the same navigation system at the same time. 
Reliability (of a system) 
The probability of performing a specified function without failure under given conditions for a specified period of time. 
Reliability (of a LOP or observation) 
The reliabilty of a LOP or observation (¨internal¨ reliability) is a measure of the effectiveness with which gross errors may be detected. This reliability is usually expressed in terms of the marginally detectable bias (MDB). 
Reliability (of a position fix) 
A measure of the propagation of a nondetected gross error in a LOP, or observation, to the position fix. This ¨external¨ reliablity is usually expressed in terms of the marginally detectable error (MDE). 
Repeatable accuracy 
The accuracy with which a user can return to a position whose coordinates have been measured at a previous time with the same navigation system. (Not to be confused with repeatability, see ¨Grid accuracy¨.) 
Required navigational performance
(RNP) 
A statement of the navigational performance necessary for operation under specified conditions. 
Root mean square error
(RMS) 
RMS error refers to the variability of a measurement such as a single LOP in one dimension. In this one dimensional case the RMS error is also an estimate of the standard deviation of the errors. 

Signal availability 
The availablity of a radio signal in a specified coverage area. (Also known as usability.) 
Single point of failure 
That part of a navigation system which lacks redundancy so that a failure in that part would result in a failure of the whole system. 
Surveillance 
The observation of an area or space for the purpose of determining the position and movements of craft in that area or space. 
Systematic error 
An error which is nonrandom in the sense that it conforms to some kind of pattern. 
System Availability 
The availability of a radio system to a user, including signal availability and the performance of the user´s receiver. 
System capacity 
This is the number of users a system can accommodate simultaneously. 

Time dilution of precision
(TDOP) 
The factor by which the accuracy of receiver clock bias determination is degraded by geometrical considerations. 

Usability 
See ¨Signal availability¨. 

Vertical dilution of precision
(VDOP) 
The factor by which the one dimensional vertical accuracy of a fix is degraded by geometrical considerations. 

Waypoint 
A specified geographical location used to define a significant point on a planned navigational route. 
World Geodetic System
(WGS) 
A consistent set of parameters describing the size and shape of the Earth, positions of a network of points with respect to the centre and mass of the Earth, transformations from major geodetic datums and the potential of the Earth. 



