Editorial Feature

Understanding the Global Positioning System (GPS)

This article was updated on the 17th July 2019.

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The Global Positioning System (GPS) is a satellite-based navigation system, originally used to aid military groups in their positioning and timing. Now it is being used in many diverse applications; including agriculture, robotics, and clock synchronization. The earliest use of GPS dates back to the 1970s and the U.S. Department of Defense unit.

GPS is defined as the continuous positioning and timing of a moving object on the planet; provided that there is no dense structure surrounding it that could obscure a GPS reading. GPS consists of three segments – the satellite, ground control, and the GPS receiver.

Figure 1. Illustration of the three core segments to a Global Positioning System. Image adapted from Hinch, S.W. (2010). Outdoor Navigation with GPS. Birmingham, Alabama: Wilderness Press Keen Communications.

Satellite Segment

To understand the intricate signaling method involved in a GPS tracking device, it is essential to focus on the cluster of satellites that orbit the globe. This is the starting point for GPS communication.

To monitor global coverage of GPS, three to four satellites are grouped in six orbitals, which transmit messages on the positioning and location of a moving device. Each of these four navigation satellites will send a signal at a different frequency. Data from all four satellites is interpreted to measure the altitude of a GPS receiver.

A functional principle of the space segment (an orbital plane consisting of four or five satellites) involves the transmission of a signal by each of the four satellites. This signal is transmitted as two sine waves, two digital codes, and a message on navigation (i.e., position and direction).

The carrier frequency (the two sine waves and digital codes) helps measure the distance between the GPS receiver and the transmitting satellites. The navigation message compliments the tracking system by providing information on the exact location of a satellite-based on timing.

The movement of the space segment involves the circular orbit of a GPS, with each one taking approximately 11 hours and one minute. Each orbital plane is positioned at a 55° tilt to the Earth’s equator, and no less than six satellites are within electromagnetic transmission (i.e., line of sight); aided by a transmitting antenna and receiving antenna.

The GPS Ground Control Segment

To understand the intricate signaling method involved in a GPS tracking device, it is essential to focus on the cluster of satellites that orbit the globe. This is the starting point for GPS communication.

To monitor global coverage of GPS, three to four satellites are grouped in six orbitals, which transmit messages on the positioning and location of a moving device. Each of these four navigation satellites will send a signal at a different frequency. Data from all four satellites is interpreted to measure the altitude of a GPS receiver.

A functional principle of the space segment (an orbital plane consisting of four or five satellites) involves the transmission of a signal by each of the four satellites. This signal is transmitted as two sine waves, two digital codes, and a message on navigation (i.e., position and direction).

The carrier frequency (the two sine waves and digital codes) helps measure the distance between the GPS receiver and the transmitting satellites. The navigation message compliments the tracking system by providing information on the exact location of a satellite-based on timing.

The movement of the space segment involves the circular orbit of a GPS, with each one taking approximately 11 hours and one minute. Each orbital plane is positioned at a 55° tilt to the Earth’s equator, and no less than six satellites are within electromagnetic transmission (i.e., line of sight); aided by a transmitting antenna and receiving antenna.

The GPS Receiver

The main purpose of the GPS receiver is to measure the position of the moving object or device. It is crucial that the receiver knows when a satellite signal was transmitted and received as a function of time. The video below discusses GPS receiver pathways.  

The GPS receiver, attached to a moving object, contains a sensitive node that can trace signals to track the position of the fourth satellite and measure its location. The signaling pathway between the satellite and the GPS receiver involves the transmission of a signal from the satellite to the antenna (usually taking 0.007 seconds), which is connected to the front end of the receiver (made up of an amplifier, filter, mixer and A-D converter).

After receiving a signal from the antenna, the front end component transforms the radio frequency signal to an intermediate frequency signal (ranging from 2–20 MHz), using the mixer sub-component. The intermediate frequency will add noise to the transmitted signal which is then received by a correlator (a component of the receiver that amplifies the noise signal views as correlation peaks).

The oscillator in the receiver creates upper and lower sideband signals, which will give an idea on the distance to each satellite. By using trilateration, the position of the receiver can be determined.

By determining the position of the receiver, a display map is presented, along with information about the direction, speed, and place of the moving object. Figure 2 demonstrates exactly how the user segment works as part of a GPS network.

One satellite will only give limited information about the position of a moving object within a specific circumference. The application of two satellites will help to map out a precise location of a moving object as the receiver can only be in one of two predicted locations within an area.

By measuring the distance to three individual satellites, the GPS can provide information on the exact location of a moving object.

One satellite will only give limited information about the position of a moving object within a certain circumference.

Application of two satellites will help map out a precise location of a moving object because then receiver can only be in one of two predicted locations within an area.

By measuring the distance to three individual satellites, the GPS can provide information on the exact location of a moving object from all three satellites.


Figure 2. Application of a GPS receiver to identify the position of a moving object. Images adapted from Hinch, S.W. (2010). Outdoor Navigation with GPS. Birmingham, Alabama: Wilderness Press Keen Communications.

GPS Accuracy

Constant recalculation of exactly where a receiver is positioned is likely to be affected by many parameters. A GPS device is expected to provide the most accurate reading when in clear view of transmitting satellites or locations where there is high signal strength (e.g., mountains and islands).

Circumstances that will affect signal strength to a GPS device include dense buildings and trees that surround the location of the device. Dense buildings and building shadows will interfere with the line-of-sight signal being transmitted from the satellite in view (i.e., the signal from three of the four satellites will be blocked).

What are the Applications of GPS?

The global positioning system (GPS) can be used in various applications. For instance, in self-navigation for automated machines, which work by integrating information on latitude, longitude, time, distance and speed, aircraft tracking, clock synchronization, vehicle tracking systems, military unit and motion measurements of earthquakes and volcanoes.

In 2019, new trends in global positioning systems have evolved the way GPS is used. The agricultural sector has already used GPS in small scale applications, but in 2019 it is expected to go mainstream by combining both GPS and Geography Information Systems. Scientists have developed GPS-guided tractors to attain position accuracy of 10 centimeters. These tractors and other GPS powered farming equipment show promise in promoting accuracy in soil and field mapping, farm planning, crop scouting, yield mapping, and tractor guidance.

Over the past years, GPS has been utilized during natural disasters. For instance, GPS has been used during search and rescue operations, helping to save many lives. GPS enables rescuers to find victims or survivors more quickly.

For some parents, GPS tracking devices also help promote their kids’ safety and security. Many GPS-enabled items such as smartphones and watches enable the parents to track the location of their children. This helps ensure that their children are safe and locate their whereabouts.

There are many applications of GPS, and as time passes, more uses will emerge. Thanks to this technology, the lives of many people, such as farmers and the military units, are made easier.

References

  • Kaplan, E.D., Hegarty, C.J. (2006). Understanding GPS Principles and Applications. 2nd Edition. Norwood, Massachusetts: Artech House, Inc.
  • El-Rabbany, A. (2002). Introduction to GPS. The Global Positioning System. Norwood, Massachusetts: Artech House, Inc.
  • Chaplain, C. (2010). Global Positioning System (GPS): Challenged in Sustaining and Upgrading Capabilities Persist. United States Government Accountability Office. Pages 4–15.
  • Brawn, D.A. (2003). GPS: The Easy Way. Northampton, England: Discovery Walking Guides Ltd. Pages 5–11.
  • Diggelen, F.V. (2009). A-GPS: Assisted GPS, GNSS, and SBAS. Norwood, Massachusetts: Artech House, Inc. Pages 129-139.

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