Global Positioning System ( GPS )

Abstract

Abstract The Global Positioning System, commonly referred to as GPS, is a worldwide, satellite‐based positioning and timing system that allows suitably equipped radio receivers to locate themselves in four dimensions, latitude, longitude, altitude, and time, anywhere there is a reasonably clear view of the sky. The system is also known as NAVSTAR, a convenient nickname that is not an acronym. The GPS system was developed, deployed, and is currently operated by the U.S. Air Force. GPS enables precision weapon delivery for all branches of the U.S. Department of Defense, as well as allied nations. Additionally, GPS supports civilian positioning and was always intended to support civil operations. The complete satellite constellation and ground support equipment that make up GPS was declared “operational” in December 1994, although civil use of the developmental signals started in the early 1980s. The fundamental operation is as follows: the 24 GPS satellites are uploaded from the ground with their current and predicted positions (called ephemeris or orbital parameters). Small corrections of their space‐borne atomic clocks are also uploaded. This information is broadcast to the user as a data modulation on an L‐band signal (1575 MHz for most civilian users) that doubles as a precise, one‐way ranging signal. Ranging is achieved by synchronizing the start time of a pseudorandom sequence of bits transmitted from the GPS satellites at an accuracy of about one nanosecond (10 −9 s). Three very important results are achieved by this implementation. First, this makes GPS ranging a one‐way signal that allows an infinite number of users to receive the signal and compute their position without saturating the GPS system. Additionally, this makes the GPS receiver passive, so that it does not radiate radio‐frequency (RF) energy. Last, by receiving four or more satellite signals, users can synchronize their local clocks to GPS time, obviating the need for a very high quality and very expensive atomic clock in the receivers. The design objectives of the GPS system were to provide a continuously available, worldwide, all‐weather, three‐dimensional precision navigation system for both military and civilian users on land, at sea, or in the air (or even in space). Selected application of the GPS system are discussed. Many topics presented are on the cating edge of research.