Global Positioning System (GPS)
The Global Positioning System (GPS) is a space-based global navigation satellite system that provides reliable location and time information in all weather and at all times and anywhere on or near the Earth when and where there is an unobstructed line of sight to four or more GPS satellites.
It is maintained by the United States government and is freely accessible by anyone with a GPS receiver.
GPS was created and realized by the U.S. Department of Defense (DOD) and was originally run with 24 satellites.
It was established in 1973 to overcome the limitations of previous navigation systems.
GPS consists of three parts: the space segment, the control segment, and the user segment.
The U.S. Air Force develops, maintains, and operates the space and control segments.
GPS satellites broadcast signals from space, which each GPS receiver uses to calculate its three-dimensional location (latitude, longitude, and altitude) plus the current time.
The space segment is composed of 24 to 32 satellites in medium Earth orbit and also includes the boosters required to launch them into orbit.
The control segment is composed of a master control station, an alternate master control station, and a host of dedicated and shared ground antennas and monitor stations.
User segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Service.
While originally a military project, GPS is considered a dual-use technology, meaning it has significant military and civilian applications.
GPS has become a widely used and a useful tool for commerce, scientific uses, tracking and surveillance.
GPS’ accurate timing facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids.
- Surveying: Surveyors use absolute locations to make maps and determine property boundaries
- Map-making: Both civilian and military cartographers use GPS extensively.
- Navigation: Navigators value digitally precise velocity and orientation measurements.
- Cellular telephony: Clock synchronization enables time transfer, which is critical for synchronizing its spreading codes with other base stations to facilitate inter-cell handoff and support hybrid GPS/cellular position detection for mobile emergency calls and other applications. The first handsets with integrated GPS launched in the late 1990s. The FCC mandated the feature in 2002 so emergency services could locate 911 callers. Third-party software developers later gained access to GPS APIs from Nextel upon launch, followed by Sprint in 2006, and Verizon soon thereafter.
- Disaster relief/emergency services: Depend upon GPS for location and timing capabilities.
- GPS tours: Location determines which content to display; for instance, information about an approaching point of interest is displayed.
- Geofencing: Vehicle tracking systems, person tracking systems, and pet tracking systems use GPS to locate a vehicle, person or pet. These devices attach to the vehicle, person, or the pet collar. The application provides 24/7 tracking and mobile or Internet updates should the trackee leave a designated area.
Position calculation introduction
To provide an introductory description of how a GPS receiver works, errors will be ignored in this section.
Using messages received from a minimum of four visible satellites, a GPS receiver is able to determine the times sent and then the satellite positions corresponding to these times sent.
The x, y, and z components of position, and the time sent, are designated as where the subscript i is the satellite number and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received , the GPS receiver can compute the transit time of the message as . Assuming the message traveled at the speed of light, c, the distance traveled or pseudorange, can be computed as .
A satellite’s position and pseudorange define a sphere, centered on the satellite with radius equal to the pseudorange.
The position of the receiver is somewhere on the surface of this sphere.
Thus with four satellites, the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres.
In the ideal case of no errors, the GPS receiver would be at a precise intersection of the four surfaces.
If the surfaces of two spheres intersect at more than one point, they intersect in a circle.
The article trilateration shows this mathematically.
A figure, Two Sphere Surfaces Intersecting in a Circle, is shown below.
The intersection of a third spherical surface with the first two will be its intersection with that circle; in most cases of practical interest, this means they intersect at two points.
Another figure, Surface of Sphere Intersecting a Circle (not disk) at Two Points, illustrates the intersection. The two intersections are marked with dots.
Again the article trilateration clearly shows this mathematically.
For space vehicles, the intersection farthest from Earth may be the correct one.
The correct position for the GPS receiver is also the intersection closest to the surface of the sphere corresponding to the fourth satellite.