The Global Positioning System (GPS) is a space-based navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. The system provides critical capabilities to military, civil, and commercial users around the world. The United States government created the system, maintains it, and makes it freely accessible to anyone with a GPS receiver.
The US began the GPS project in 1973 to overcome the limitations of previous navigation systems, integrating ideas from several predecessors, including a number of classified engineering design studies from the 1960s. The U.S. Department of Defense (DoD) developed the system, which originally used 24 satellites. It became fully operational in 1995. Bradford Parkinson, Roger L. Easton, and Ivan A. Getting are credited with inventing it.
Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS and implement the next generation of GPS Block IIIA satellites and Next Generation Operational Control System (OCX). Announcements from Vice President Al Gore and the White House in 1998 initiated these changes. In 2000, the U.S. Congress authorized the modernization effort, GPS III.
In addition to GPS, other systems are in use or under development. The Russian Global Navigation Satellite System (GLONASS) was developed contemporaneously with GPS, but suffered from incomplete coverage of the globe until the mid-2000s. There are also the planned European Union Galileo positioning system, India’s Indian Regional Navigation Satellite System, China’s BeiDou Navigation Satellite System, and the Japanese Quasi-Zenith Satellite System.
More correctly, GPS (as the S implies) is a system composed of the receivers, the constellation of satellites that orbit the Earth and the control centers that monitor the velocity and shape of the satellites’ orbits. According to the US Naval Observatory, there are currently 32 satellites in the GPS constellation (2012). Out of all the satellites in orbit, 27 are in primary use (expanded from 24 in 2011), while the others serve as
backups in the event a primary satellite fails. We will discuss the importance of this number in the following section.
Together, the satellites, control centers, and users make up the three segments on which GPS relies: the space segment, the control segment, and the user segment. These segments communicate using radio signals.
Figure.1: Diagram of 3 segments of GPS. The Global Positioning System is based on the interoperation of three distinct segments. (Image source: www.e-education.psu.edu)
1.1.1 The Space Segment
The space segment consists of all the satellites in the GPS constellation, which undergoes continuous change as new satellites are launched and others are decommissioned on a periodic basis. Each satellite orbits the Earth following one of six orbital planes, and completes its orbit in 12 hours.
Figure.2: The six orbital planes of the GPS constellation. (Image source: www.e-education.psu.edu)
The orbital planes are arranged to ensure that at least four satellites are “in view” at any given time, anywhere on Earth (if obstructions intervene, the satellite's radio signal cannot be received). Three satellites are needed by the receivers to determine position, while the fourth enhances the measurement and provides the ability to calculate elevation. Since four satellites must be visible from any point on the planet and the satellites are arranged into six orbital planes, the minimum number of satellites needed to provide full coverage at any location on Earth is 24.
1.1.2 The Control Segment
Although the GPS satellites are examples of impressive engineering and design, they are not error free. Gravitational variations that result from the interaction between the Earth and Moon can affect the orbits of the satellites. Disturbances from radiation, electrical anomalies, space debris, and normal wear and tear can also degrade or disrupt a satellite’s orbit and functionality. From time to time, the satellites must receive instructions to correct these errors, based on data collected and analyzed by control centers on the ground. Two types of control centers exist: monitor stations and control stations.
Monitor Stations are very precise GPS receivers installed at known locations. They record discrepancies between known and calculated positions caused by slight variations in satellite orbits. Data describing the orbits are produced at the Master Control Station at Colorado Springs, uploaded to the satellites, and finally broadcast as part of the GPS positioning signal. GPS receivers use this satellite Navigation Message data to adjust the positions they measure.
If necessary, the Master Control Center can modify satellite orbits by radio signal commands transmitted via the control segment's ground antennas.
1.1.3 The User Segment
The U.S. Federal Aviation Administration (FAA) estimated in 2006 that some 500,000 GPS receivers were in use for many applications, including surveying, transportation, precision farming, geophysics, and recreation, not to mention military navigation. This was before in-vehicle GPS navigation gadgets emerged as one of the most popular consumer electronic gifts during the 2007 holiday season in North America. It is also before the first GPS-enabled consumer phone (the Nokia N95, released in 2007) and the first cameras with integrated GPS (which did not show up until 2010).
Today, more than one billion smartphones, tablets, cameras, and other GPS-enabled mobile devices have been activated.
These devices, and the operators who use them, make up the user segment of GPS.
1.2 Principle of GPS positioning
Every GPS satellite is equipped with an atomic clock that keeps time with exceptional accuracy. Similarly, every GPS receiver also includes a clock. The time kept by these clocks is used to determine how long it takes for the satellite’s signal to reach the receiver. More precisely, GPS satellites broadcast “pseudo-random codes” which contain the information about the time and orbital path of the satellite. The receiver then
interprets this code so that it can calculate the difference between its own clock and the time the signal was transmitted. When multiplied by the speed of the signal (which travels at the speed of light), the difference in times can be used to determine the distance between the satellite and receiver.
Figure.3: GPS receivers calculate distance as a function of the difference in time of broadcast and reception of a GPS signal. Distance = speed of light x time difference. (Image source: www.e-education.psu.edu)
As discussed above, the GPS constellation is configured so that a minimum of four satellites is always "in view" everywhere on Earth. If only one satellite signal was available to a receiver, the best that a receiver could do would be to use the signal time to determine its distance from that satellite, but the position of the receiver could be at any of the infinite number of points defined by an imaginary sphere with that radius surrounding the satellite (the “range” of that satellite). If two satellites are available, a receiver can tell that its position is somewhere along a circle formed by the intersection of the two spherical ranges. When distances from three satellites are known, the receiver's position must be one of two points at the intersection of three spherical ranges. GPS receivers are usually smart enough to choose the location nearest to the Earth's surface. At a minimum, three satellites are required for a two-dimensional
(horizontal) fix. Four ranges are needed for a three-dimensional fix (horizontal and elevation). The process of acquiring a two-dimensional fix is illustrated in Figure 4.
Figure.4: A 2-dimensional location fix requires three satellites. Adding a fourth satellite allows 3-dimensional location (horizontal + elevation). Here are shown the following: one satellite anywhere on surface of sphere, two anywhere spheres intersect, three where all three intersect. (Image source: www.e-education.psu.edu)
Satellite ranging is similar to an older technique called trilateration, which surveyors use to determine a horizontal location based on three known distances.
An alternative to triangulation is trilateration, which uses distances alone to determine positions. By eschewing angle measurements, trilateration is easier to perform, requires fewer tools, and is therefore less expensive. Having read this chapter so far, you have already been introduced to a practical application of trilateration, since it is the technique behind satellite ranging used in GPS.
1.3 GPS Error sources
Ranging errors are grouped into the six following classes
1.4 Positioning Method
The GPS positioning can be classified from different perspective. However a common classification approach is point and relative / differential positioning.
1.4.1 Point positioning Point
Positioning is the most basic and common technique for users who do not require high accuracy but require fast positioning. It is also called absolute positioning or autonomous positioning or single point positioning or navigation solution. This method uses single receiver for positioning solution where the receiver handles incoming signals only from satellite.
1.4.2 Relative / differential positioning
This technique, where we use two or more receivers makes different simultaneous measurements possible, known as relative positioning. It can attain higher accuracy
than point positioning because of extensive correction between observations taken to the same satellites at same time from separate station.
Bhatta, B. (2011), Remote Sensing and GIS, 7, 210-218.
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