K GPS Glossary of Terms
266 MiLLennium GP SCard Software Version 4.50 Command Descriptions Manual Rev 1
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).
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.
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. The geoid, affected by local gravity
disturbances, is irregular.
Standard Positioning Service (SPS) — a positioning service made available by the United States Department
of Defence which will be available to all GPS civilian users on a continuous, worldwide basis
(typically using C/A Code).
SV — Space Vehicle ID, sometimes used as SVID; also used interchangeably with Pseudo-Random Noise
Number (PRN).
Static — the user’s GPS antenna does not move.
TDOP — Time Dilution of Precision - A numerical value expressing the confidence factor of the position
solution based on current satellite geometry. The lower the TDOP value, the greater the confidence
factor.
Three-dimensional coverage (hours) — 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.
Three-dimensional (3D) navigation — navigation mode in which altitude and horizontal position are
determined from satellite range measurements.
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 depa rture to a poi nt of arrival or s ubsequent
position at any given time; may be considered synonymous with Course Made Good.
True bearing — bearing relative to true north; compass bearing corrected for compass error.
True heading — heading relative to true north.
Two-dimensional coverage (hours) — 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.
Two-dimensional (2D) navigation — navigation mode in which a fixed v alue of altitude is used for one or
more 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.