Filter
Top Products
A GPS Overview
64 MiLLennium GPSCard SW Version 4.503/4.52 Command Descriptions Manual Rev 2
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.
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.
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. A user can also use the data provided
by the satellite signals to accomplish specific application requirements.
A.2 HEIGHT RELATIONSHIPS
What is a geoid?
The equipotential surface which best represents mean sea level where an equipotential surface is any surface where
gravity is constant. This surface not only covers the water but is projected throughout the continents. Most surfaces
in North America use this surface as 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 Figure A-2.