Understanding the Global Positioning System

By Kal Kaur

Introduction
Satellite Segment
The GPS Control Segment
The GPS Receiver
GPS Accuracy
Applications
References

Introduction

Satellite-based navigation is commonly known as the global positioning system (GPS). The earliest use of GPS dates back to 1970s and was developed by the U.S. Department of Defense unit. The main purpose for developing GPS was to aid military groups for positioning and timing information.  The present day sees diverse application for GPS including commercial use, agriculture, robotics, and clock synchronization.
 
A basic definition of the GPS system is continuous positioning and timing of a moving object on Earth provided that there is no dense structure surrounding the moving object to obscure a GPS reading. The GPS system consists of three segments: the satellite segment, the ground control segment, and the GPS receiver. Figure 1 clearly illustrates the three fundamental segments to a GPS.



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

In order to understand the intricate signaling method involved with the GPS tracking device, let’s first pay attention to the cluster of satellites that orbit the globe and become the starting point for GPS communication. To monitor global coverage of GPS, three to four satellites are grouped in six orbitals in order to transmit a message on positioning and location of a moving device. Each of these four navigation satellites will transmit a signal at a different frequency. Data from all four satellites is interpreted to measure the altitude of a GPS receiver.

A functional principle to the space segment (an orbital plane consisting of four or five satellites) involves transmission of a signal by each of the four satellites as two sine waves, two digital codes, and a message on navigation (i.e., position and direction). The carrier frequency described as the two sine waves and the digital codes help 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 a circular orbit of a GPS, each one taking approximately 11 hours and one minute. Each orbital plane is positioned at a 55° tilt to the Earth’s equator.To visualize this system, no less than six satellites are always within electromagnetic transmission (i.e., line of sight) aided by a transmitting antenna and a receiving antenna.



The GPS Ground Control Segment

A global network tracking system with a master control station is based at the Schriever Air Force Base in Colorado United States of America. This control unit tracks the position of the GPS satellite, information that is then generated back to the GPS satellite through an S-band link (a microwave band). The Control segment will regularly updates on satellite signal transmission and orbital status; without this system the GPS cannot function.


The GPS Receiver

The GPS receiver is the third segment to GPS and its main purpose is to measure the position of the moving object or device. It is important that the receiver is aware of when a satellite signal was transmitted and received as a function of time. The video below is a discussion on a GPS receiver pathway.  



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

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, position of the receiver can be determined.

By determining the position of the receiver, a display map is presented along with information about direction, speed, and position 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 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

A continuous recalculation of exactly where a receiver is positioned is likely to be affected by many parameters. A GPS device is likely to provide the most accurate reading when in clear view of transmitting satellites, locations where there is high signal strength (e.g., mountains and islands). Circumstances that will affect signal strength to a GPS device includes dense buildings and trees surround the location where the device is positioned. Dense buildings and building shadows will interfere with the line-of-sight signal being transmitted from the satellite in view (i.e., signal from three of the four satellites will be blocked).

Applications

Global positioning systems have the following main applications:
  • Self-navigation for automated machines that work by integrating information on latitude, longitude, time, distance, and speed.
  • Aircraft tracking
  • Clock synchronization
  • Vehicle tracking systems
  • Military units: for tracking a target
  • Motion measurements of earthquakes and volcanoes as demonstrated in the video below.

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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