Global Positioning System (GPS)

 


GPS (or, rather, The Global Positioning System) is a system operated by the US Department of Defense (DoD) that consists of a constellation of orbiting satellites and ground stations to determine positions on or above the surface of the earth with great accuracy. DoD set up the original system and launches and maintains the satellites at great expense but the system nonetheless is there for everyone to use--for free! All a user needs to determine his or her position on the planet is a GPS receiver and a map. Some of the more fancy receivers come with base maps built in.

The Global Positioning System consists of three segments--Space, Control, and User.

The Space Segment is the network of 24 satellites orbiting the earth in 6 different circular orbital planes--each with a 55° inclination, at a height of 20,200 km. The University of Colorado has a good graphic here. Each satellite orbits the earth every twelve hours, and passes over exactly the same ground every 24-hour period--tracing a path resembling the stitching on a baseball.. Slight gravitational anomalies in the earth and tidal influences from the moon and other bodies can cause the orbits to degrade. To solve the orbit degradation problem each satellite is equipped with small rockets that can be fired periodically to keep the position as exact as possible. Since each satellite has a limited amount of fuel for orbital adjustments, the expected lifetime for each is about 7.5 years.

The Control Segment consists of ground monitoring stations scattered around the globe. They are: five Monitor Stations (Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs), three Ground Antennas, (Ascension Island, Diego Garcia, Kwajalein), and a Master Control Station (MCS) at Schriever Air Force Base in Colorado.

The fact that the earth's surface is a jumble of crustal plates that are they themselves in motion, adds an interesting facet to the problem of determining the exact path of each satellite's orbit. In order to accurately assess the positions of satellite orbits, one would need some immobile reference point. Such a reference point, though, does not exist. Any GPS monitoring station on the face of the earth is riding on a tectonic plate and as such might have a velocity of several centimeters per year--not exactly even a snail's pace--but after several years this can be quite substantial. Remember, the Global Positioning System is not only used for navigation but, with technologies such as Differential GPS which can easily get measurements that are sub-meter in accuracy, is also used for surveying. The next best thing is being able to determine direction and velocities of crustal plates. This is exactly what JPL scientists are attempting.

The User Segment is the part of the system that should be familiar to most of us. This segment consists of a GPS receiver. Basically, the GPS receiver receives signals from satellites and through trilateration (similar to triangulation except the sides of the triangle are used instead of the angles) determines distances to those satellites. With a sufficient number of satellite signals, a unique position can be computed

GPS is not the only system in use on the planet. The Russian Glonass system has been called an elegantly designed satellite navigation system since the orbits are more robust, i.e., less susceptible to gravitational disturbances, than the American system. Unfortunately, Russian government funding is spotty for this system. One thing that could save the system would be for them to charge fees for its use but that would just cause users to go over to the The US system was set up with the military in mind but since the 1980s when civilian use was first permitted by government decree, the number of receivers being used for non-military purposes has increased exponentially. The accuracy of the civilian signal was once degraded under what was termed SA (Selective Availability). Ever since the Clinton administration, SA has been turned off, the civilian channel has reportedly been receiving the same grade data as the military and the benefits are manifold. A graph of position error plotted on the day SA was turned can be found here.


GPS Links:

Topo! mapping software: Topographical mapping software used for this web site
US Coast Guard GPS information page
University of Colorado's GPS page
A tutorial from Trimble on GPS
Printable guide from Garmin in pdf form

Other Related Links:

Wolfram's Science World An interesting site with information on astronomy, calendrical systems, mathematics and physics.