Welcome to OS Net

National GPS web site specific terms
Active stations	A network of permanently recording, high quality, GPS receivers located across GB. Raw GPS data is downloaded hourly from each station to the National GPS Network web site.
Coordinate transformer	A utility to precisely transform EtrS89 coordinates to Ordnance Survey National Grid coordinates (OSGB36^®) and heights above mean sea level (MSL) (orthometric height) using the OSTN02^® and OSGM02^® models.
Ellipsoid	A 3-D geometric figure used to approximate the shape of the earth.
EtrS89	The European Terrestrial Reference System 1989, used as the standard precise GPS coordinate system throughout Europe. In 2000, the difference between the WGS84 and EtrS89 coordinates was about 25 cm, and increasing by about 2.5 cm per year. EtrS89 has been officially adopted as a standard coordinate system for precise GPS surveying by most national mapping agencies in Europe.
Galileo	Galileo is Europe's contribution to satellite navigation, due for completion in 2010.
Geoid	A model of the level surface which is closest to MSL over the oceans. This surface is continued under the land and acts as the fundamental reference surface for height measurement, as an approximation of MSL on land.
GLONASS	The Russian satellite navigation system.
GNSS	Global Navigation Satellite Systems.
GPS	The Global Positioning System. An outdoor positioning technique which uses a constellation of US Department of Defense satellites and a receiver to determine position.
ODN	Ordnance Datum Newlyn is the national standard vertical reference system for measuring height above MSL in GB. ODN is measured relative to a value taken at Newlyn, Cornwall.
OSGB36 (National Grid)	The national standard coordinate system for topographic mapping, including all Ordnance Survey mapping, and for geographically referencing many kinds of information in relation to Ordnance Survey mapping.
OSGM02	Ordnance Survey National Geoid Model 2002. A gravimetric model used to convert from EtrS89 to orthometric heights.
OSTN02	Ordnance Survey National Grid Transformation 2002.
Passive stations	A network of stations across GB that have been precisely positioned with GPS. EtrS89 coordinates, transformed OSGB36 coordinates and station location information is available from the National GPS Network web site.
Transformation	A procedure to change from one coordinate system to another.
WGS84	The World Geodetic System 1984 coordinate system, used by GPS. WGS84 coordinates are usually expressed as latitude, longitude and ellipsoid height.
General satellite navigation abbreviations
A-S	Anti-spoofing
C/A-code	coarse/acquisition-code
CEP	circular error probable
DGPS	differential GPS
DOP	dilution of precision
ECEF	Earth-centered-Earth-fixed
FOM	figure of merit
Galileo	European Global Navigation Satellite System under development
GDOP	geometric dilution of precision
GLONASS	Global Navigation satellite system
GMT	Greenwich Mean Time
GPS	Global Positioning System
HDOP	horizontal dilution of precision
HOW	hand over word
INS	Inertial Navigation System
L1	GPS primary frequency, 1575.42 MHz
L2	GPS secondary frequency, 1227.6 MHz
MSL	mean sea level
MTBF	mean time between failure
NMEA	National Marine Electronics Association
P-code	precise code
PDOP	position dilution of precision
PPM	parts per million
PPS	Precise Positioning Service
PRN	pseudo random noise
RF	radio frequency
RINEX	Receiver Independent Exchange Format
RMS	root mean square
RSS	root sum square
RTCA	Radio Technical Commission for Aeronautics
RTCM	Ratio Technical Commission for Maritime Services
RTK	Real-Time Kinematic
SA	Selective Availability
SEP	spherical error probable
SPS	Standard Positioning Service
TAI	International Atomic Time
tdOP	time dilution of precision
TTFF	time to first fix
UT	Universal Time
UTC	Universal Time Coordinated
VDOP	vertical dilution of precision
WGS84	World Geodetic System 1984
1 PPS	1 pulse per second

Glossary