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Describing the details of Ordnance Survey's national coordinate system (OS Net) can involve the some technical terminology which may not be familiar to you.

The table below explains these terms and provides further explanation to help your understanding of this topic.


Assisted GPS – provides supplementary information enabling reduced ‘time to first fix’ so that users get position very quickly at power on. This technique is used in low-end GPS equipment/applications (for example, to LBS market, tracking and so on).


Differential GNSS – a form of corrected GNSS giving enhanced accuracy in the few decimetre range (better than stand alone GNSS, not as good as RTK).

Several GNSS services are available:

General Lighthouse Authority beacons – Free service. External radio receiver (separate from GPS unit) usually required to obtain the corrections.

EGNOS European Geostationary Navigation Overlay Service. See below.

OS Net® partners offer GNSS services based on the data streams of the Ordnance Survey® OS Net permanent GNSS network.


European Geostationary Navigation Overlay Service – providing GNSS augmentation. Consisting of three geostationary satellites and a network of ground stations, EGNOS will achieve its aim by transmitting a signal containing information on the reliability and accuracy of the positioning signals sent out by the GPS. It will allow users in Europe and beyond to determine their position to better than 5 m compared with about 10 m at present. Free service. Now built into many GPS receivers, so no additional equipment required. Can be difficult to receive signal in harsh environments (tall buildings and so on). See http://www.esa.int/esaNA/egnos.html


The developing European GNSS.


Global'naya Navigatsionnaya Sputnikovaya Sistema – Russian GNS. Now being improved and expanded after years of falling into disrepair.


Global Navigation Satellite System – Generic term for systems such as GPS and Galileo.


General Packet Radio Service – a non-voice data service for the mobile telephone network.


Global Positioning System – US owned positioning satellite constellation. Gives 10 to 20 m accuracy as standard.


Global System for Mobile communications – a non-voice data service for the mobile telephone network. Predates GPRS and currently enjoys more than 71% of the world mobile non-voice market.

Inverse dGPS

Inverse dGPS takes a user’s coordinates (obtained by simple GPS) and improves them by using models based on data from permanent GPS receivers (for example, OS Net). Using this technique a user could be equipped with a simple (therefore cheap and power-efficient) GNSS receiver. The user’s position is sent to a central control hub where it is improved using Inverse dGNSS. A day’s worth of GNSS positioning data can be sent in at once, for example, negating the need for a permanent communication link to the user. An example application of this technique could be in road-user charging.


Ordnance Survey’s National Grid coordinate system.

OS Net

Ordnance Survey’s identity for its internal GPS correction network.


Receiver INdependent EXchange format – a set of standard definitions and formats that permits post-processing of GPS position information. Just as accurate as RTK but not real time.


Radio Technical Commission for Maritime Services an industry standard format for GPS data exchange based on real-time data.


Real Time Kinematic – a form of corrected GPS giving accuracy down around 20 mm.

CEPcircular error probable
DGPSdifferential GPS
DOPdilution of precision
FOMfigure of merit
GalileoEuropean Global Navigation Satellite System under development
GDOPgeometric dilution of precision
GLONASSGlobal Navigation satellite system
GMTGreenwich Mean Time
GPSGlobal Positioning System
HDOPhorizontal dilution of precision
HOWhand over word
INSInertial Navigation System
L1GPS primary frequency, 1575.42 MHz
L2GPS secondary frequency, 1227.6 MHz
MSLmean sea level
MTBFmean time between failure
NMEANational Marine Electronics Association
P-codeprecise code
PDOPposition dilution of precision
PPMparts per million
PPSPrecise Positioning Service
PRNpseudo random noise
RFradio frequency
RINEXReceiver Independent Exchange Format
RMSroot mean square
RSSroot sum square
RTCARadio Technical Commission for Aeronautics
RTCMRatio Technical Commission for Maritime Services
RTKReal-Time Kinematic
SASelective Availability
SEPspherical error probable
SPSStandard Positioning Service
TAIInternational Atomic Time
tdOPtime dilution of precision
TTFFtime to first fix
UTUniversal Time
UTCUniversal Time Coordinated
VDOPvertical dilution of precision
WGS84World Geodetic System 1984
1 PPS1 pulse per second

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