NavStar GPS (U.S.)

U.S. Answer to GNSS

More commonly known as just GPS, the Navstar GPS is currently the only fully functional GNSS that covers the world. However, there are plans for other systems to become operational in 2011 onwards.
The NavStar GPS system evolved from the original Transit system sponsored by the Navy to provide the Military with accurate position information for their submarines. The old Transit system relied of the receiving unit to be slow moving or the position to be very well known1.

Operational Principle

The system is considered to be made of three parts, a space segment (SS), a control segment (CS), and a user segment (US).

GPS in motion (embedded from Wiki)

Space Segment

The satellite network consists of 24 active satellites with an extra 4 spare. The satellites orbit the earth in 6 planes with 4 satellites need for each plane to work. Each satellite orbits the earth every 12 hours sending information about their position4.
The system has two modes. One mode is available to everyone, this mode is the Standard Positioning Service (SPS). This mode is provided on one frequency that sends position and timing. This service has no direct charge however, it can be reduced in accuracy whenever, the U.S. Military deem important. This reduction of service is known as Selective Availability. Selective Availability adjusts the timing in the clocks of the satellites by a random amount to reduce the accuracy.
The other mode is known as PPS, which is Precise Positioning Service. This service is only available to approved devices that can access the encrypted information on a second frequency sent by the satellite. As the name suggests this information can give far more accurate information and is generally used for military purposes. One of the main reasons for this control is the possible use of their own system against them5.

Control Segment

The control segment is composed of

  • a master control station (MCS),
  • an alternate master control station,
  • four dedicated ground antennas and
  • six dedicated monitor stations2

The control segment monitors, maintains and controls the satellite network. Using the station listed above which are in remote locations the Segment can make sure the system remains fully functional but also looks to improve how well it controls the satellite in better improve the service3.

User Segment

This segment refers to anymore who uses a GPS receiver to gather information. Whether it is for position purposes or even just for the time, they fall into this section. The technology of the receivers will determine the accuracy. Some GPS receivers include accurate clocks to help narrow down their position. The GPS receivers are often linked to some form of display for the user to use the required information. Some of these displays are directly link to the receiver, such as Moving Map GPS, and others are indirectly linked, such as Tracking Devices that can display the information to users connected to the web/

1. University of Colorado (n.d.). Lecture 2: A Brief History of Navigation. Retrieved from
2. Wiki, (Oct 2011). Global Positioning System. Retrieved from
3., (n.d.). GPS III Operational Control Segment (OCX). Retrieved from
4. spaceandtech, (2001). NAVSTAR GPS – Summary. Retrieved from
5., (n.d.). USNO NAVSTAR Global Positioning System. Retrieved from
+++ Footnotes +++
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Want to know more?

GNSS in Aviation
GPS History
GPS Control Segment

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