2 GLOBAL POSITIONING SYSTEM OVERVIEW
The NAVSTAR Global Positioning System (GPS), first launched in 1978, is designed by
the U.S. Department of Defense, demonstrates many benefits for positioning
requirements. The system is suitable for different type of applications and platforms. The
GPS navigation is used everywhere on land, at sea and in the air able to provide service
to an unlimited number of users. Many possible GPS receiver implementations are
possible and can range from external sensor coupling to a navigation system. The idea
behind an integration of GPS with a navigation system (INS, AHRS, or a Doppler Radar
Navigation Systems (DRNS)) is the ability of the overall combined system to provide a
solution by either GPS receiver, by the host navigation, or by the combination of the two.
Thus, the integration avoids the limitations of each system in stand-alone mode.
The GPS is a satellite-based radionavigation system. The system consists of a
constellation of at least 24 satellites in 6 orbital planes spaced by 60 degree see Figure 2.1
(4 satellites in each plane) and it is referred as the first segment or “the space segment”
(having around 5 to 7 spares ones to improve the system availability). Each satellite
orbits 11,000 nautical miles (20,200 kilometers) above Earth’s surface with an inclination
angle of 55 degree relative to the equatorial plane in Figure 2.2. One complete orbit is
approximately 11 hours and 58 minutes. Each satellite is equipped with an atomic clock
and transmits continuous synchronized signals with other satellites to ground stations and
receivers. The atomic clock keeps the time within three nanoseconds.
Therefore, these main functions of the satellites can be summarized to following: receive
and store data transmitted by the control segment stations, preserve precise time using
onboard atomic clocks, and broadcast data and signals to users on two L-band
frequencies. The GPS second segment referred as the user segment consists of receivers.
The main role and design of these receivers is to receive, decode, and process the GPS
satellite ranging codes and navigational data messages.