Where am I? Where do I want to go? How can I get there? These are questions we've all asked at one time or another. Satellites for navigation were developed in the late 1950's as a direct result of ships needing to know exactly where they were at any given time. In the middle of the ocean or out of sight of land, one can't determine an accurate position by looking out the window.
The idea of using satellites for navigation began with the launch of Sputnik 1 on October 4, 1957. Scientists at Johns Hopkins University's Applied Physics Laboratory monitored that satellite. They noticed that when the transmitted radio frequency was plotted on a graph, a characteristic curve of Doppler shift appeared. By studying this apparent change of radio frequency as the satellite passed overhead, they were able to show that the Doppler curve, when properly used, described the orbit of the satellite.
Most navigation systems use time and distance to figure location. Early on, scientists recognized the principle that, given velocity and the time required for a radio signal to be transmitted between two points, the distance between the two points can be computed. In order to do this calculation, a precise, synchronized time of departure and measured time of arrival of the radio signal must be obtained. By synchronizing the signal transmission time to two precise clocks, one in a satellite and one at a ground-based receiver, the transit time could be measured and then multiplied by the exact speed of light to obtain the distance between the two positions.
This three-dimensional satellite navigational system (NAVSTAR) enables a traveler to obtain his/her position anywhere on or above the planet. Data transmitted from the satellite provides the user with time, the precise orbital position of the satellite and position of other satellites in the system. Currently, there is a full constellation of 24 orbiting satellites devoted to navigation.
Using a commercial Global Positioning System (GPS) locator, the user can calculate distance by measuring the time it takes for the satellite's radio transmissions, traveling at the speed of light, to reach the receiver. Once distance from four satellites is known, position in three dimensions (latitude, longitude, and altitude) can be calculated by triangulation, and velocity in three dimensions can be computed from Doppler shift in the received signal. Of course, the new GPS receivers do all of the work for you. You simply turn on the unit, make certain that it is locked onto at least 4 satellites, and the information is displayed automatically. One innovative application of GPS technology is to determine Earth movement after an earthquake. Referencing a network of these sensitive receivers often leads to remarkably accurate assessment of plate movement. GPS was also used to precisely locate drop points for airlifted Bosnian relief supplies.
There are two available radio signals that GPS receivers can use: the Standard Positioning Service (SPS) for civilians, and the Precise Positioning Service (PPS) for military and other authorized personnel. The most significant cause of errors in positioning is the deliberate effort by the Department of Defense to decrease the accuracy of user systems for reasons of national security. Selective Availability (SA) refers to the purposeful degradation of the information broadcast by the satellites. SA affects the accuracy of the SPS, but not PPS. With SA, you can expect your SPS system to be accurate to within 328 feet (100 meters) horizontally and 512 feet (156 meters) vertically 95% of the time.
For those who require positions with higher accuracy, Differential Global Positioning System (DGPS) adds a new element to GPS. DGPS places a GPS stationary receiver at a known location on or near the Earth's surface. This reference station receives satellite signals and adjusts for transmission delays and Selective Availability, using its own known latitude, longitude, and altitude. The stationary receiver sends out a correction message for any suitably-equipped local receiver. A DGPS-compatible receiver adjusts its position calculations using the correction message. DGPS reference stations are constructed, operated, and maintained by the United States Coast Guard.
