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K GPS Glossary of Terms
274 MiLLennium GPSCard SW Version 4.503/4.52 Command Descriptions Manual Rev 2
True Bearing Bearing relative to true north; compass bearing corrected for compass error.
True Heading Heading relative to true north.
Two-Dimensional The number of hours-per-day with three or more satellites visible. Three
Coverage visible satellites can be used to determine location if the GPS receiver is designed to
accept an external altitude input.
Two-Dimensional Navigation mode in which a fixed value of altitude is used for one or more
(2D) Navigation position calculations while horizontal (2D) position can vary freely based on satellite
range measurements.
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.
Update Rate The GPS receiver specification which indicates the solution rate
provided by the receiver
when operating normally.
UTC [See Coordinated Universal Time]
VDOP Vertical Dilution of Precision [See DOP]
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.
World Geodetic An ellipsoid designed to fit the shape of the entire Earth as well as possible
System 1984 with a single ellipsoid. It is often used as a reference on a worldwide basis,
(WGS84) 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.
Waypoint A reference point on a track.
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: Φ’ = Φ
1
- Φ
2
. The corresponding wavelength is 86.2 cm
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]