Global Positioning System
Global Positioning System (GPS)
The Global Positioning System or GPS is a great technological success story. Developed in the 1970s and 1980s by the U.S Department of Defense (DoD), the system was primarily intended for the U.S military. It was successfully used in the U.S-Iraq War (1990-1991) and U.S interventions in Kosovo (1999) and Afghanistan (2001-2002). Non-military use was a secondary objective, and throughout the 1990s civil users were limited to a purposefully degraded subset of the signals broadcast by GPS. Despite these limitations, civil applications of GPS grew at an astonishing rate. Applications unforeseen by the designers of the system are now thriving, and many more are on the way. Through applications in land transportation, civil aviation, maritime commerce, surveying and mapping, construction, mining, agriculture, earth science, electric power systems, telecommunications, and outdoor recreational activities, GPS is well on its way to becoming an essential part of the commercial and public infrastructure.
The baseline the GPS constellation comprises 24 satellites arranged in six orbital planes (lettered A-F). Each plane is inclined at 55° from the equatorial plane. The orbital period is 12 hours.
In order to measure the true transit time of a signal from a satellite to a receiver, it is critical that the clocks in the satellite and the receiver be in synchronism (that is “in sync”). Fortunately, this tough requirement is easily sidestepped, allowing use of inexpensive quartz oscillators in GPS receivers. The bias in the receiver clock, (that is, the amount by which the receiver clock is out of sync), at the instant of the measurements affects the transit times for all satellites equally. The corresponding measured ranges are thus all too short (or too long) by a common amount. These biased ranges are called pseudoranges. The receiver clock bias thus becomes the fourth unknown to be estimated, in addition to the three coordinates of position. A user, therefore, needs a minimum of four satellites in view to estimate his four-dimensional position: three coordinates of spatial position, plus time.
With GPS, you can determine your three-dimensional position instantaneously, continuously, and globally with an accuracy of several meters. Thanks to precise and ultra-stable clocks carried aboard the satellites, an inexpensive GPS receiver can also serve as a precise clock, keeping time with an accuracy of about 0.1 microsecond (that’s a tenth of one millionth of a second!). In fact, GPS has become a true global time reference for commercial and scientific activities. GPS users the world over have become accustomed to such accuracy in specifying position and time. The only requirement is for the receiver antenna to have a halfway clear view of the sky to be able to “see” a minimum of four satellites.
The GPS signals are extraordinarily faint and may be so attenuated in propagation (that is, weakened) through foliage that it becomes difficult for a hiker to navigate in the woods. Indoors, the signals are far too weak to be tracked independently. But GPS will provide position and time indoors in the near future with a little assistance from terrestrial radio signals transmitted specifically for this purpose.