82 MPC User Manual Rev 0D
Appendix B GPS Overview
receiver with at least six satellites in view from any point on earth, at any particular time.
The GPS satellite signal identifies the satellite and provides the positioning, timing, ranging data,
satellite status and the corrected ephemerides (orbit parameters) of the satellite to the users. The
satellites can be identified either by the Space Vehicle Number (SVN) or the Pseudorandom Code
Number (PRN). The PRN is used by the NovAtel GPSCard.
The GPS satellites transmit on two L-band frequencies; one centered at 1575.42 MHz (L1) and the
other at 1227.60 MHz (L2). The L1 carrier is modulated by the C/A code (Coarse/Acquisition) and
the P code (Precision) which is encrypted for military and other authorized users. The L2 carrier is
modulated only with the P code.
B.1.2 The Control Segment
The control segment consists of a master control station, five reference stations and three data up-
loading stations in locations all around the globe.
The reference stations track and monitor the satellites via their broadcast signals. The broadcast
signals contain the ephemeris data of the satellites, the ranging signals, the clock data and the almanac
data. These signals are passed to the master control station where the ephemerides are re-computed.
The resulting ephemerides corrections and timing corrections are transmitted back to the satellites via
the data up-loading stations.
B.1.3 The User Segment
The user segment, such as the NovAtel GPSCard receiver, consists of equipment which tracks and
receives the satellite signals. The user equipment must be capable of simultaneously processing the
signals from a minimum of four satellites to obtain accurate position, velocity and timing
measurements.
B.2 Height Relationships
What is a geoid?
An equipotential surface is any surface where gravity is constant. This surface best represents mean
sea-level and not only covers the water but is projected throughout the continents. In North America
this surface is most commonly used at its zero value, i.e. all heights are referenced to this surface.
What is an ellipsoid?
An ellipsoid, also known as a spheroid, is a mathematical surface which is sometimes used to
represent the earth. Whenever you see latitudes and longitudes describing the location, this coordinate
is being referenced to a specific ellipsoid. GPS positions are referred to an ellipsoid known as WGS84
(World Geodetic System of 1984).
What is the relationship between a geoid and an ellipsoid?
The relationship between a geoid and an ellipsoid is shown in “Illustration of GPSCard Height
Measurements” on Page 83.
